TOMOYO Linux Cross Reference
Linux/Documentation/networking/ip-sysctl.rst

   .. SPDX-License-Identifier: GPL-2.0
   
   =========
   IP Sysctl
   =========
   
   /proc/sys/net/ipv4/* Variables
   ==============================
   
  ip_forward - BOOLEAN
          - 0 - disabled (default)
          - not 0 - enabled
  
          Forward Packets between interfaces.
  
          This variable is special, its change resets all configuration
          parameters to their default state (RFC1122 for hosts, RFC1812
          for routers)
  
  ip_default_ttl - INTEGER
          Default value of TTL field (Time To Live) for outgoing (but not
          forwarded) IP packets. Should be between 1 and 255 inclusive.
          Default: 64 (as recommended by RFC1700)
  
  ip_no_pmtu_disc - INTEGER
          Disable Path MTU Discovery. If enabled in mode 1 and a
          fragmentation-required ICMP is received, the PMTU to this
          destination will be set to the smallest of the old MTU to
          this destination and min_pmtu (see below). You will need
          to raise min_pmtu to the smallest interface MTU on your system
          manually if you want to avoid locally generated fragments.
  
          In mode 2 incoming Path MTU Discovery messages will be
          discarded. Outgoing frames are handled the same as in mode 1,
          implicitly setting IP_PMTUDISC_DONT on every created socket.
  
          Mode 3 is a hardened pmtu discover mode. The kernel will only
          accept fragmentation-needed errors if the underlying protocol
          can verify them besides a plain socket lookup. Current
          protocols for which pmtu events will be honored are TCP, SCTP
          and DCCP as they verify e.g. the sequence number or the
          association. This mode should not be enabled globally but is
          only intended to secure e.g. name servers in namespaces where
          TCP path mtu must still work but path MTU information of other
          protocols should be discarded. If enabled globally this mode
          could break other protocols.
  
          Possible values: 0-3
  
          Default: FALSE
  
  min_pmtu - INTEGER
          default 552 - minimum Path MTU. Unless this is changed manually,
          each cached pmtu will never be lower than this setting.
  
  ip_forward_use_pmtu - BOOLEAN
          By default we don't trust protocol path MTUs while forwarding
          because they could be easily forged and can lead to unwanted
          fragmentation by the router.
          You only need to enable this if you have user-space software
          which tries to discover path mtus by itself and depends on the
          kernel honoring this information. This is normally not the
          case.
  
          Default: 0 (disabled)
  
          Possible values:
  
          - 0 - disabled
          - 1 - enabled
  
  fwmark_reflect - BOOLEAN
          Controls the fwmark of kernel-generated IPv4 reply packets that are not
          associated with a socket for example, TCP RSTs or ICMP echo replies).
          If unset, these packets have a fwmark of zero. If set, they have the
          fwmark of the packet they are replying to.
  
          Default: 0
  
  fib_multipath_use_neigh - BOOLEAN
          Use status of existing neighbor entry when determining nexthop for
          multipath routes. If disabled, neighbor information is not used and
          packets could be directed to a failed nexthop. Only valid for kernels
          built with CONFIG_IP_ROUTE_MULTIPATH enabled.
  
          Default: 0 (disabled)
  
          Possible values:
  
          - 0 - disabled
          - 1 - enabled
  
  fib_multipath_hash_policy - INTEGER
          Controls which hash policy to use for multipath routes. Only valid
          for kernels built with CONFIG_IP_ROUTE_MULTIPATH enabled.
  
          Default: 0 (Layer 3)
  
          Possible values:
 
         - 0 - Layer 3
         - 1 - Layer 4
         - 2 - Layer 3 or inner Layer 3 if present
         - 3 - Custom multipath hash. Fields used for multipath hash calculation
           are determined by fib_multipath_hash_fields sysctl
 
 fib_multipath_hash_fields - UNSIGNED INTEGER
         When fib_multipath_hash_policy is set to 3 (custom multipath hash), the
         fields used for multipath hash calculation are determined by this
         sysctl.
 
         This value is a bitmask which enables various fields for multipath hash
         calculation.
 
         Possible fields are:
 
         ====== ============================
         0x0001 Source IP address
         0x0002 Destination IP address
         0x0004 IP protocol
         0x0008 Unused (Flow Label)
         0x0010 Source port
         0x0020 Destination port
         0x0040 Inner source IP address
         0x0080 Inner destination IP address
         0x0100 Inner IP protocol
         0x0200 Inner Flow Label
         0x0400 Inner source port
         0x0800 Inner destination port
         ====== ============================
 
         Default: 0x0007 (source IP, destination IP and IP protocol)
 
 fib_multipath_hash_seed - UNSIGNED INTEGER
         The seed value used when calculating hash for multipath routes. Applies
         to both IPv4 and IPv6 datapath. Only present for kernels built with
         CONFIG_IP_ROUTE_MULTIPATH enabled.
 
         When set to 0, the seed value used for multipath routing defaults to an
         internal random-generated one.
 
         The actual hashing algorithm is not specified -- there is no guarantee
         that a next hop distribution effected by a given seed will keep stable
         across kernel versions.
 
         Default: 0 (random)
 
 fib_sync_mem - UNSIGNED INTEGER
         Amount of dirty memory from fib entries that can be backlogged before
         synchronize_rcu is forced.
 
         Default: 512kB   Minimum: 64kB   Maximum: 64MB
 
 ip_forward_update_priority - INTEGER
         Whether to update SKB priority from "TOS" field in IPv4 header after it
         is forwarded. The new SKB priority is mapped from TOS field value
         according to an rt_tos2priority table (see e.g. man tc-prio).
 
         Default: 1 (Update priority.)
 
         Possible values:
 
         - 0 - Do not update priority.
         - 1 - Update priority.
 
 route/max_size - INTEGER
         Maximum number of routes allowed in the kernel.  Increase
         this when using large numbers of interfaces and/or routes.
 
         From linux kernel 3.6 onwards, this is deprecated for ipv4
         as route cache is no longer used.
 
         From linux kernel 6.3 onwards, this is deprecated for ipv6
         as garbage collection manages cached route entries.
 
 neigh/default/gc_thresh1 - INTEGER
         Minimum number of entries to keep.  Garbage collector will not
         purge entries if there are fewer than this number.
 
         Default: 128
 
 neigh/default/gc_thresh2 - INTEGER
         Threshold when garbage collector becomes more aggressive about
         purging entries. Entries older than 5 seconds will be cleared
         when over this number.
 
         Default: 512
 
 neigh/default/gc_thresh3 - INTEGER
         Maximum number of non-PERMANENT neighbor entries allowed.  Increase
         this when using large numbers of interfaces and when communicating
         with large numbers of directly-connected peers.
 
         Default: 1024
 
 neigh/default/unres_qlen_bytes - INTEGER
         The maximum number of bytes which may be used by packets
         queued for each unresolved address by other network layers.
         (added in linux 3.3)
 
         Setting negative value is meaningless and will return error.
 
         Default: SK_WMEM_MAX, (same as net.core.wmem_default).
 
                 Exact value depends on architecture and kernel options,
                 but should be enough to allow queuing 256 packets
                 of medium size.
 
 neigh/default/unres_qlen - INTEGER
         The maximum number of packets which may be queued for each
         unresolved address by other network layers.
 
         (deprecated in linux 3.3) : use unres_qlen_bytes instead.
 
         Prior to linux 3.3, the default value is 3 which may cause
         unexpected packet loss. The current default value is calculated
         according to default value of unres_qlen_bytes and true size of
         packet.
 
         Default: 101
 
 neigh/default/interval_probe_time_ms - INTEGER
         The probe interval for neighbor entries with NTF_MANAGED flag,
         the min value is 1.
 
         Default: 5000
 
 mtu_expires - INTEGER
         Time, in seconds, that cached PMTU information is kept.
 
 min_adv_mss - INTEGER
         The advertised MSS depends on the first hop route MTU, but will
         never be lower than this setting.
 
 fib_notify_on_flag_change - INTEGER
         Whether to emit RTM_NEWROUTE notifications whenever RTM_F_OFFLOAD/
         RTM_F_TRAP/RTM_F_OFFLOAD_FAILED flags are changed.
 
         After installing a route to the kernel, user space receives an
         acknowledgment, which means the route was installed in the kernel,
         but not necessarily in hardware.
         It is also possible for a route already installed in hardware to change
         its action and therefore its flags. For example, a host route that is
         trapping packets can be "promoted" to perform decapsulation following
         the installation of an IPinIP/VXLAN tunnel.
         The notifications will indicate to user-space the state of the route.
 
         Default: 0 (Do not emit notifications.)
 
         Possible values:
 
         - 0 - Do not emit notifications.
         - 1 - Emit notifications.
         - 2 - Emit notifications only for RTM_F_OFFLOAD_FAILED flag change.
 
 IP Fragmentation:
 
 ipfrag_high_thresh - LONG INTEGER
         Maximum memory used to reassemble IP fragments.
 
 ipfrag_low_thresh - LONG INTEGER
         (Obsolete since linux-4.17)
         Maximum memory used to reassemble IP fragments before the kernel
         begins to remove incomplete fragment queues to free up resources.
         The kernel still accepts new fragments for defragmentation.
 
 ipfrag_time - INTEGER
         Time in seconds to keep an IP fragment in memory.
 
 ipfrag_max_dist - INTEGER
         ipfrag_max_dist is a non-negative integer value which defines the
         maximum "disorder" which is allowed among fragments which share a
         common IP source address. Note that reordering of packets is
         not unusual, but if a large number of fragments arrive from a source
         IP address while a particular fragment queue remains incomplete, it
         probably indicates that one or more fragments belonging to that queue
         have been lost. When ipfrag_max_dist is positive, an additional check
         is done on fragments before they are added to a reassembly queue - if
         ipfrag_max_dist (or more) fragments have arrived from a particular IP
         address between additions to any IP fragment queue using that source
         address, it's presumed that one or more fragments in the queue are
         lost. The existing fragment queue will be dropped, and a new one
         started. An ipfrag_max_dist value of zero disables this check.
 
         Using a very small value, e.g. 1 or 2, for ipfrag_max_dist can
         result in unnecessarily dropping fragment queues when normal
         reordering of packets occurs, which could lead to poor application
         performance. Using a very large value, e.g. 50000, increases the
         likelihood of incorrectly reassembling IP fragments that originate
         from different IP datagrams, which could result in data corruption.
         Default: 64
 
 bc_forwarding - INTEGER
         bc_forwarding enables the feature described in rfc1812#section-5.3.5.2
         and rfc2644. It allows the router to forward directed broadcast.
         To enable this feature, the 'all' entry and the input interface entry
         should be set to 1.
         Default: 0
 
 INET peer storage
 =================
 
 inet_peer_threshold - INTEGER
         The approximate size of the storage.  Starting from this threshold
         entries will be thrown aggressively.  This threshold also determines
         entries' time-to-live and time intervals between garbage collection
         passes.  More entries, less time-to-live, less GC interval.
 
 inet_peer_minttl - INTEGER
         Minimum time-to-live of entries.  Should be enough to cover fragment
         time-to-live on the reassembling side.  This minimum time-to-live  is
         guaranteed if the pool size is less than inet_peer_threshold.
         Measured in seconds.
 
 inet_peer_maxttl - INTEGER
         Maximum time-to-live of entries.  Unused entries will expire after
         this period of time if there is no memory pressure on the pool (i.e.
         when the number of entries in the pool is very small).
         Measured in seconds.
 
 TCP variables
 =============
 
 somaxconn - INTEGER
         Limit of socket listen() backlog, known in userspace as SOMAXCONN.
         Defaults to 4096. (Was 128 before linux-5.4)
         See also tcp_max_syn_backlog for additional tuning for TCP sockets.
 
 tcp_abort_on_overflow - BOOLEAN
         If listening service is too slow to accept new connections,
         reset them. Default state is FALSE. It means that if overflow
         occurred due to a burst, connection will recover. Enable this
         option _only_ if you are really sure that listening daemon
         cannot be tuned to accept connections faster. Enabling this
         option can harm clients of your server.
 
 tcp_adv_win_scale - INTEGER
         Obsolete since linux-6.6
         Count buffering overhead as bytes/2^tcp_adv_win_scale
         (if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale),
         if it is <= 0.
 
         Possible values are [-31, 31], inclusive.
 
         Default: 1
 
 tcp_allowed_congestion_control - STRING
         Show/set the congestion control choices available to non-privileged
         processes. The list is a subset of those listed in
         tcp_available_congestion_control.
 
         Default is "reno" and the default setting (tcp_congestion_control).
 
 tcp_app_win - INTEGER
         Reserve max(window/2^tcp_app_win, mss) of window for application
         buffer. Value 0 is special, it means that nothing is reserved.
 
         Possible values are [0, 31], inclusive.
 
         Default: 31
 
 tcp_autocorking - BOOLEAN
         Enable TCP auto corking :
         When applications do consecutive small write()/sendmsg() system calls,
         we try to coalesce these small writes as much as possible, to lower
         total amount of sent packets. This is done if at least one prior
         packet for the flow is waiting in Qdisc queues or device transmit
         queue. Applications can still use TCP_CORK for optimal behavior
         when they know how/when to uncork their sockets.
 
         Default : 1
 
 tcp_available_congestion_control - STRING
         Shows the available congestion control choices that are registered.
         More congestion control algorithms may be available as modules,
         but not loaded.
 
 tcp_base_mss - INTEGER
         The initial value of search_low to be used by the packetization layer
         Path MTU discovery (MTU probing).  If MTU probing is enabled,
         this is the initial MSS used by the connection.
 
 tcp_mtu_probe_floor - INTEGER
         If MTU probing is enabled this caps the minimum MSS used for search_low
         for the connection.
 
         Default : 48
 
 tcp_min_snd_mss - INTEGER
         TCP SYN and SYNACK messages usually advertise an ADVMSS option,
         as described in RFC 1122 and RFC 6691.
 
         If this ADVMSS option is smaller than tcp_min_snd_mss,
         it is silently capped to tcp_min_snd_mss.
 
         Default : 48 (at least 8 bytes of payload per segment)
 
 tcp_congestion_control - STRING
         Set the congestion control algorithm to be used for new
         connections. The algorithm "reno" is always available, but
         additional choices may be available based on kernel configuration.
         Default is set as part of kernel configuration.
         For passive connections, the listener congestion control choice
         is inherited.
 
         [see setsockopt(listenfd, SOL_TCP, TCP_CONGESTION, "name" ...) ]
 
 tcp_dsack - BOOLEAN
         Allows TCP to send "duplicate" SACKs.
 
 tcp_early_retrans - INTEGER
         Tail loss probe (TLP) converts RTOs occurring due to tail
         losses into fast recovery (draft-ietf-tcpm-rack). Note that
         TLP requires RACK to function properly (see tcp_recovery below)
 
         Possible values:
 
                 - 0 disables TLP
                 - 3 or 4 enables TLP
 
         Default: 3
 
 tcp_ecn - INTEGER
         Control use of Explicit Congestion Notification (ECN) by TCP.
         ECN is used only when both ends of the TCP connection indicate
         support for it.  This feature is useful in avoiding losses due
         to congestion by allowing supporting routers to signal
         congestion before having to drop packets.
 
         Possible values are:
 
                 =  =====================================================
                 0  Disable ECN.  Neither initiate nor accept ECN.
                 1  Enable ECN when requested by incoming connections and
                    also request ECN on outgoing connection attempts.
                 2  Enable ECN when requested by incoming connections
                    but do not request ECN on outgoing connections.
                 =  =====================================================
 
         Default: 2
 
 tcp_ecn_fallback - BOOLEAN
         If the kernel detects that ECN connection misbehaves, enable fall
         back to non-ECN. Currently, this knob implements the fallback
         from RFC3168, section 6.1.1.1., but we reserve that in future,
         additional detection mechanisms could be implemented under this
         knob. The value is not used, if tcp_ecn or per route (or congestion
         control) ECN settings are disabled.
 
         Default: 1 (fallback enabled)
 
 tcp_fack - BOOLEAN
         This is a legacy option, it has no effect anymore.
 
 tcp_fin_timeout - INTEGER
         The length of time an orphaned (no longer referenced by any
         application) connection will remain in the FIN_WAIT_2 state
         before it is aborted at the local end.  While a perfectly
         valid "receive only" state for an un-orphaned connection, an
         orphaned connection in FIN_WAIT_2 state could otherwise wait
         forever for the remote to close its end of the connection.
 
         Cf. tcp_max_orphans
 
         Default: 60 seconds
 
 tcp_frto - INTEGER
         Enables Forward RTO-Recovery (F-RTO) defined in RFC5682.
         F-RTO is an enhanced recovery algorithm for TCP retransmission
         timeouts.  It is particularly beneficial in networks where the
         RTT fluctuates (e.g., wireless). F-RTO is sender-side only
         modification. It does not require any support from the peer.
 
         By default it's enabled with a non-zero value. 0 disables F-RTO.
 
 tcp_fwmark_accept - BOOLEAN
         If set, incoming connections to listening sockets that do not have a
         socket mark will set the mark of the accepting socket to the fwmark of
         the incoming SYN packet. This will cause all packets on that connection
         (starting from the first SYNACK) to be sent with that fwmark. The
         listening socket's mark is unchanged. Listening sockets that already
         have a fwmark set via setsockopt(SOL_SOCKET, SO_MARK, ...) are
         unaffected.
 
         Default: 0
 
 tcp_invalid_ratelimit - INTEGER
         Limit the maximal rate for sending duplicate acknowledgments
         in response to incoming TCP packets that are for an existing
         connection but that are invalid due to any of these reasons:
 
           (a) out-of-window sequence number,
           (b) out-of-window acknowledgment number, or
           (c) PAWS (Protection Against Wrapped Sequence numbers) check failure
 
         This can help mitigate simple "ack loop" DoS attacks, wherein
         a buggy or malicious middlebox or man-in-the-middle can
         rewrite TCP header fields in manner that causes each endpoint
         to think that the other is sending invalid TCP segments, thus
         causing each side to send an unterminating stream of duplicate
         acknowledgments for invalid segments.
 
         Using 0 disables rate-limiting of dupacks in response to
         invalid segments; otherwise this value specifies the minimal
         space between sending such dupacks, in milliseconds.
 
         Default: 500 (milliseconds).
 
 tcp_keepalive_time - INTEGER
         How often TCP sends out keepalive messages when keepalive is enabled.
         Default: 2hours.
 
 tcp_keepalive_probes - INTEGER
         How many keepalive probes TCP sends out, until it decides that the
         connection is broken. Default value: 9.
 
 tcp_keepalive_intvl - INTEGER
         How frequently the probes are send out. Multiplied by
         tcp_keepalive_probes it is time to kill not responding connection,
         after probes started. Default value: 75sec i.e. connection
         will be aborted after ~11 minutes of retries.
 
 tcp_l3mdev_accept - BOOLEAN
         Enables child sockets to inherit the L3 master device index.
         Enabling this option allows a "global" listen socket to work
         across L3 master domains (e.g., VRFs) with connected sockets
         derived from the listen socket to be bound to the L3 domain in
         which the packets originated. Only valid when the kernel was
         compiled with CONFIG_NET_L3_MASTER_DEV.
 
         Default: 0 (disabled)
 
 tcp_low_latency - BOOLEAN
         This is a legacy option, it has no effect anymore.
 
 tcp_max_orphans - INTEGER
         Maximal number of TCP sockets not attached to any user file handle,
         held by system. If this number is exceeded orphaned connections are
         reset immediately and warning is printed. This limit exists
         only to prevent simple DoS attacks, you _must_ not rely on this
         or lower the limit artificially, but rather increase it
         (probably, after increasing installed memory),
         if network conditions require more than default value,
         and tune network services to linger and kill such states
         more aggressively. Let me to remind again: each orphan eats
         up to ~64K of unswappable memory.
 
 tcp_max_syn_backlog - INTEGER
         Maximal number of remembered connection requests (SYN_RECV),
         which have not received an acknowledgment from connecting client.
 
         This is a per-listener limit.
 
         The minimal value is 128 for low memory machines, and it will
         increase in proportion to the memory of machine.
 
         If server suffers from overload, try increasing this number.
 
         Remember to also check /proc/sys/net/core/somaxconn
         A SYN_RECV request socket consumes about 304 bytes of memory.
 
 tcp_max_tw_buckets - INTEGER
         Maximal number of timewait sockets held by system simultaneously.
         If this number is exceeded time-wait socket is immediately destroyed
         and warning is printed. This limit exists only to prevent
         simple DoS attacks, you _must_ not lower the limit artificially,
         but rather increase it (probably, after increasing installed memory),
         if network conditions require more than default value.
 
 tcp_mem - vector of 3 INTEGERs: min, pressure, max
         min: below this number of pages TCP is not bothered about its
         memory appetite.
 
         pressure: when amount of memory allocated by TCP exceeds this number
         of pages, TCP moderates its memory consumption and enters memory
         pressure mode, which is exited when memory consumption falls
         under "min".
 
         max: number of pages allowed for queueing by all TCP sockets.
 
         Defaults are calculated at boot time from amount of available
         memory.
 
 tcp_min_rtt_wlen - INTEGER
         The window length of the windowed min filter to track the minimum RTT.
         A shorter window lets a flow more quickly pick up new (higher)
         minimum RTT when it is moved to a longer path (e.g., due to traffic
         engineering). A longer window makes the filter more resistant to RTT
         inflations such as transient congestion. The unit is seconds.
 
         Possible values: 0 - 86400 (1 day)
 
         Default: 300
 
 tcp_moderate_rcvbuf - BOOLEAN
         If set, TCP performs receive buffer auto-tuning, attempting to
         automatically size the buffer (no greater than tcp_rmem[2]) to
         match the size required by the path for full throughput.  Enabled by
         default.
 
 tcp_mtu_probing - INTEGER
         Controls TCP Packetization-Layer Path MTU Discovery.  Takes three
         values:
 
         - 0 - Disabled
         - 1 - Disabled by default, enabled when an ICMP black hole detected
         - 2 - Always enabled, use initial MSS of tcp_base_mss.
 
 tcp_probe_interval - UNSIGNED INTEGER
         Controls how often to start TCP Packetization-Layer Path MTU
         Discovery reprobe. The default is reprobing every 10 minutes as
         per RFC4821.
 
 tcp_probe_threshold - INTEGER
         Controls when TCP Packetization-Layer Path MTU Discovery probing
         will stop in respect to the width of search range in bytes. Default
         is 8 bytes.
 
 tcp_no_metrics_save - BOOLEAN
         By default, TCP saves various connection metrics in the route cache
         when the connection closes, so that connections established in the
         near future can use these to set initial conditions.  Usually, this
         increases overall performance, but may sometimes cause performance
         degradation.  If set, TCP will not cache metrics on closing
         connections.
 
 tcp_no_ssthresh_metrics_save - BOOLEAN
         Controls whether TCP saves ssthresh metrics in the route cache.
 
         Default is 1, which disables ssthresh metrics.
 
 tcp_orphan_retries - INTEGER
         This value influences the timeout of a locally closed TCP connection,
         when RTO retransmissions remain unacknowledged.
         See tcp_retries2 for more details.
 
         The default value is 8.
 
         If your machine is a loaded WEB server,
         you should think about lowering this value, such sockets
         may consume significant resources. Cf. tcp_max_orphans.
 
 tcp_recovery - INTEGER
         This value is a bitmap to enable various experimental loss recovery
         features.
 
         =========   =============================================================
         RACK: 0x1   enables the RACK loss detection for fast detection of lost
                     retransmissions and tail drops. It also subsumes and disables
                     RFC6675 recovery for SACK connections.
 
         RACK: 0x2   makes RACK's reordering window static (min_rtt/4).
 
         RACK: 0x4   disables RACK's DUPACK threshold heuristic
         =========   =============================================================
 
         Default: 0x1
 
 tcp_reflect_tos - BOOLEAN
         For listening sockets, reuse the DSCP value of the initial SYN message
         for outgoing packets. This allows to have both directions of a TCP
         stream to use the same DSCP value, assuming DSCP remains unchanged for
         the lifetime of the connection.
 
         This options affects both IPv4 and IPv6.
 
         Default: 0 (disabled)
 
 tcp_reordering - INTEGER
         Initial reordering level of packets in a TCP stream.
         TCP stack can then dynamically adjust flow reordering level
         between this initial value and tcp_max_reordering
 
         Default: 3
 
 tcp_max_reordering - INTEGER
         Maximal reordering level of packets in a TCP stream.
         300 is a fairly conservative value, but you might increase it
         if paths are using per packet load balancing (like bonding rr mode)
 
         Default: 300
 
 tcp_retrans_collapse - BOOLEAN
         Bug-to-bug compatibility with some broken printers.
         On retransmit try to send bigger packets to work around bugs in
         certain TCP stacks.
 
 tcp_retries1 - INTEGER
         This value influences the time, after which TCP decides, that
         something is wrong due to unacknowledged RTO retransmissions,
         and reports this suspicion to the network layer.
         See tcp_retries2 for more details.
 
         RFC 1122 recommends at least 3 retransmissions, which is the
         default.
 
 tcp_retries2 - INTEGER
         This value influences the timeout of an alive TCP connection,
         when RTO retransmissions remain unacknowledged.
         Given a value of N, a hypothetical TCP connection following
         exponential backoff with an initial RTO of TCP_RTO_MIN would
         retransmit N times before killing the connection at the (N+1)th RTO.
 
         The default value of 15 yields a hypothetical timeout of 924.6
         seconds and is a lower bound for the effective timeout.
         TCP will effectively time out at the first RTO which exceeds the
         hypothetical timeout.
 
         RFC 1122 recommends at least 100 seconds for the timeout,
         which corresponds to a value of at least 8.
 
 tcp_rfc1337 - BOOLEAN
         If set, the TCP stack behaves conforming to RFC1337. If unset,
         we are not conforming to RFC, but prevent TCP TIME_WAIT
         assassination.
 
         Default: 0
 
 tcp_rmem - vector of 3 INTEGERs: min, default, max
         min: Minimal size of receive buffer used by TCP sockets.
         It is guaranteed to each TCP socket, even under moderate memory
         pressure.
 
         Default: 4K
 
         default: initial size of receive buffer used by TCP sockets.
         This value overrides net.core.rmem_default used by other protocols.
         Default: 131072 bytes.
         This value results in initial window of 65535.
 
         max: maximal size of receive buffer allowed for automatically
         selected receiver buffers for TCP socket. This value does not override
         net.core.rmem_max.  Calling setsockopt() with SO_RCVBUF disables
         automatic tuning of that socket's receive buffer size, in which
         case this value is ignored.
         Default: between 131072 and 6MB, depending on RAM size.
 
 tcp_sack - BOOLEAN
         Enable select acknowledgments (SACKS).
 
 tcp_comp_sack_delay_ns - LONG INTEGER
         TCP tries to reduce number of SACK sent, using a timer
         based on 5% of SRTT, capped by this sysctl, in nano seconds.
         The default is 1ms, based on TSO autosizing period.
 
         Default : 1,000,000 ns (1 ms)
 
 tcp_comp_sack_slack_ns - LONG INTEGER
         This sysctl control the slack used when arming the
         timer used by SACK compression. This gives extra time
         for small RTT flows, and reduces system overhead by allowing
         opportunistic reduction of timer interrupts.
 
         Default : 100,000 ns (100 us)
 
 tcp_comp_sack_nr - INTEGER
         Max number of SACK that can be compressed.
         Using 0 disables SACK compression.
 
         Default : 44
 
 tcp_backlog_ack_defer - BOOLEAN
         If set, user thread processing socket backlog tries sending
         one ACK for the whole queue. This helps to avoid potential
         long latencies at end of a TCP socket syscall.
 
         Default : true
 
 tcp_slow_start_after_idle - BOOLEAN
         If set, provide RFC2861 behavior and time out the congestion
         window after an idle period.  An idle period is defined at
         the current RTO.  If unset, the congestion window will not
         be timed out after an idle period.
 
         Default: 1
 
 tcp_stdurg - BOOLEAN
         Use the Host requirements interpretation of the TCP urgent pointer field.
         Most hosts use the older BSD interpretation, so if you turn this on
         Linux might not communicate correctly with them.
 
         Default: FALSE
 
 tcp_synack_retries - INTEGER
         Number of times SYNACKs for a passive TCP connection attempt will
         be retransmitted. Should not be higher than 255. Default value
         is 5, which corresponds to 31seconds till the last retransmission
         with the current initial RTO of 1second. With this the final timeout
         for a passive TCP connection will happen after 63seconds.
 
 tcp_syncookies - INTEGER
         Only valid when the kernel was compiled with CONFIG_SYN_COOKIES
         Send out syncookies when the syn backlog queue of a socket
         overflows. This is to prevent against the common 'SYN flood attack'
         Default: 1
 
         Note, that syncookies is fallback facility.
         It MUST NOT be used to help highly loaded servers to stand
         against legal connection rate. If you see SYN flood warnings
         in your logs, but investigation shows that they occur
         because of overload with legal connections, you should tune
         another parameters until this warning disappear.
         See: tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow.
 
         syncookies seriously violate TCP protocol, do not allow
         to use TCP extensions, can result in serious degradation
         of some services (f.e. SMTP relaying), visible not by you,
         but your clients and relays, contacting you. While you see
         SYN flood warnings in logs not being really flooded, your server
         is seriously misconfigured.
 
         If you want to test which effects syncookies have to your
         network connections you can set this knob to 2 to enable
         unconditionally generation of syncookies.
 
 tcp_migrate_req - BOOLEAN
         The incoming connection is tied to a specific listening socket when
         the initial SYN packet is received during the three-way handshake.
         When a listener is closed, in-flight request sockets during the
         handshake and established sockets in the accept queue are aborted.
 
         If the listener has SO_REUSEPORT enabled, other listeners on the
         same port should have been able to accept such connections. This
         option makes it possible to migrate such child sockets to another
         listener after close() or shutdown().
 
         The BPF_SK_REUSEPORT_SELECT_OR_MIGRATE type of eBPF program should
         usually be used to define the policy to pick an alive listener.
         Otherwise, the kernel will randomly pick an alive listener only if
         this option is enabled.
 
         Note that migration between listeners with different settings may
         crash applications. Let's say migration happens from listener A to
         B, and only B has TCP_SAVE_SYN enabled. B cannot read SYN data from
         the requests migrated from A. To avoid such a situation, cancel
         migration by returning SK_DROP in the type of eBPF program, or
         disable this option.
 
         Default: 0
 
 tcp_fastopen - INTEGER
         Enable TCP Fast Open (RFC7413) to send and accept data in the opening
         SYN packet.
 
         The client support is enabled by flag 0x1 (on by default). The client
         then must use sendmsg() or sendto() with the MSG_FASTOPEN flag,
         rather than connect() to send data in SYN.
 
         The server support is enabled by flag 0x2 (off by default). Then
         either enable for all listeners with another flag (0x400) or
         enable individual listeners via TCP_FASTOPEN socket option with
         the option value being the length of the syn-data backlog.
 
         The values (bitmap) are
 
         =====  ======== ======================================================
           0x1  (client) enables sending data in the opening SYN on the client.
           0x2  (server) enables the server support, i.e., allowing data in
                         a SYN packet to be accepted and passed to the
                         application before 3-way handshake finishes.
           0x4  (client) send data in the opening SYN regardless of cookie
                         availability and without a cookie option.
         0x200  (server) accept data-in-SYN w/o any cookie option present.
         0x400  (server) enable all listeners to support Fast Open by
                         default without explicit TCP_FASTOPEN socket option.
         =====  ======== ======================================================
 
         Default: 0x1
 
         Note that additional client or server features are only
         effective if the basic support (0x1 and 0x2) are enabled respectively.
 
 tcp_fastopen_blackhole_timeout_sec - INTEGER
         Initial time period in second to disable Fastopen on active TCP sockets
         when a TFO firewall blackhole issue happens.
         This time period will grow exponentially when more blackhole issues
         get detected right after Fastopen is re-enabled and will reset to
         initial value when the blackhole issue goes away.
         0 to disable the blackhole detection.
 
         By default, it is set to 0 (feature is disabled).
 
 tcp_fastopen_key - list of comma separated 32-digit hexadecimal INTEGERs
         The list consists of a primary key and an optional backup key. The
         primary key is used for both creating and validating cookies, while the
         optional backup key is only used for validating cookies. The purpose of
         the backup key is to maximize TFO validation when keys are rotated.
 
         A randomly chosen primary key may be configured by the kernel if
         the tcp_fastopen sysctl is set to 0x400 (see above), or if the
         TCP_FASTOPEN setsockopt() optname is set and a key has not been
         previously configured via sysctl. If keys are configured via
         setsockopt() by using the TCP_FASTOPEN_KEY optname, then those
         per-socket keys will be used instead of any keys that are specified via
         sysctl.
 
         A key is specified as 4 8-digit hexadecimal integers which are separated
         by a '-' as: xxxxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxx. Leading zeros may be
         omitted. A primary and a backup key may be specified by separating them
         by a comma. If only one key is specified, it becomes the primary key and
         any previously configured backup keys are removed.
 
 tcp_syn_retries - INTEGER
         Number of times initial SYNs for an active TCP connection attempt
         will be retransmitted. Should not be higher than 127. Default value
         is 6, which corresponds to 67seconds (with tcp_syn_linear_timeouts = 4)
         till the last retransmission with the current initial RTO of 1second.
         With this the final timeout for an active TCP connection attempt
         will happen after 131seconds.
 
 tcp_timestamps - INTEGER
         Enable timestamps as defined in RFC1323.
 
         - 0: Disabled.
         - 1: Enable timestamps as defined in RFC1323 and use random offset for
           each connection rather than only using the current time.
         - 2: Like 1, but without random offsets.
 
         Default: 1
 
 tcp_min_tso_segs - INTEGER
         Minimal number of segments per TSO frame.
 
         Since linux-3.12, TCP does an automatic sizing of TSO frames,
         depending on flow rate, instead of filling 64Kbytes packets.
         For specific usages, it's possible to force TCP to build big
         TSO frames. Note that TCP stack might split too big TSO packets
         if available window is too small.
 
         Default: 2
 
 tcp_tso_rtt_log - INTEGER
         Adjustment of TSO packet sizes based on min_rtt
 
         Starting from linux-5.18, TCP autosizing can be tweaked
         for flows having small RTT.
 
         Old autosizing was splitting the pacing budget to send 1024 TSO
         per second.
 
         tso_packet_size = sk->sk_pacing_rate / 1024;
 
         With the new mechanism, we increase this TSO sizing using:
 
         distance = min_rtt_usec / (2^tcp_tso_rtt_log)
         tso_packet_size += gso_max_size >> distance;
 
         This means that flows between very close hosts can use bigger
         TSO packets, reducing their cpu costs.
 
         If you want to use the old autosizing, set this sysctl to 0.
 
         Default: 9  (2^9 = 512 usec)
 
 tcp_pacing_ss_ratio - INTEGER
         sk->sk_pacing_rate is set by TCP stack using a ratio applied
         to current rate. (current_rate = cwnd * mss / srtt)
         If TCP is in slow start, tcp_pacing_ss_ratio is applied
         to let TCP probe for bigger speeds, assuming cwnd can be
         doubled every other RTT.
 
         Default: 200
 
 tcp_pacing_ca_ratio - INTEGER
         sk->sk_pacing_rate is set by TCP stack using a ratio applied
         to current rate. (current_rate = cwnd * mss / srtt)
         If TCP is in congestion avoidance phase, tcp_pacing_ca_ratio
         is applied to conservatively probe for bigger throughput.
 
         Default: 120
 
 tcp_syn_linear_timeouts - INTEGER
         The number of times for an active TCP connection to retransmit SYNs with
         a linear backoff timeout before defaulting to an exponential backoff
         timeout. This has no effect on SYNACK at the passive TCP side.
 
         With an initial RTO of 1 and tcp_syn_linear_timeouts = 4 we would
         expect SYN RTOs to be: 1, 1, 1, 1, 1, 2, 4, ... (4 linear timeouts,
         and the first exponential backoff using 2^0 * initial_RTO).
         Default: 4
 
 tcp_tso_win_divisor - INTEGER
         This allows control over what percentage of the congestion window
         can be consumed by a single TSO frame.
         The setting of this parameter is a choice between burstiness and
         building larger TSO frames.
 
         Default: 3
 
 tcp_tw_reuse - INTEGER
         Enable reuse of TIME-WAIT sockets for new connections when it is
         safe from protocol viewpoint.
 
         - 0 - disable
         - 1 - global enable
         - 2 - enable for loopback traffic only
 
         It should not be changed without advice/request of technical
         experts.
 
         Default: 2
 
 tcp_window_scaling - BOOLEAN
         Enable window scaling as defined in RFC1323.
 
 tcp_shrink_window - BOOLEAN
         This changes how the TCP receive window is calculated.
 
         RFC 7323, section 2.4, says there are instances when a retracted
         window can be offered, and that TCP implementations MUST ensure
         that they handle a shrinking window, as specified in RFC 1122.
 
         - 0 - Disabled. The window is never shrunk.
         - 1 - Enabled.  The window is shrunk when necessary to remain within
                         the memory limit set by autotuning (sk_rcvbuf).
                         This only occurs if a non-zero receive window
                         scaling factor is also in effect.
 
         Default: 0
 
 tcp_wmem - vector of 3 INTEGERs: min, default, max
         min: Amount of memory reserved for send buffers for TCP sockets.
         Each TCP socket has rights to use it due to fact of its birth.
 
         Default: 4K
 
         default: initial size of send buffer used by TCP sockets.  This
         value overrides net.core.wmem_default used by other protocols.
 
         It is usually lower than net.core.wmem_default.
 
         Default: 16K
 
         max: Maximal amount of memory allowed for automatically tuned
         send buffers for TCP sockets. This value does not override
         net.core.wmem_max.  Calling setsockopt() with SO_SNDBUF disables
         automatic tuning of that socket's send buffer size, in which case
         this value is ignored.
 
         Default: between 64K and 4MB, depending on RAM size.
 
 tcp_notsent_lowat - UNSIGNED INTEGER
         A TCP socket can control the amount of unsent bytes in its write queue,
         thanks to TCP_NOTSENT_LOWAT socket option. poll()/select()/epoll()
         reports POLLOUT events if the amount of unsent bytes is below a per
         socket value, and if the write queue is not full. sendmsg() will
         also not add new buffers if the limit is hit.
 
         This global variable controls the amount of unsent data for
         sockets not using TCP_NOTSENT_LOWAT. For these sockets, a change
         to the global variable has immediate effect.
 
         Default: UINT_MAX (0xFFFFFFFF)
 
 tcp_workaround_signed_windows - BOOLEAN
         If set, assume no receipt of a window scaling option means the
         remote TCP is broken and treats the window as a signed quantity.
         If unset, assume the remote TCP is not broken even if we do
         not receive a window scaling option from them.
 
         Default: 0
 
 tcp_thin_linear_timeouts - BOOLEAN
         Enable dynamic triggering of linear timeouts for thin streams.
         If set, a check is performed upon retransmission by timeout to
         determine if the stream is thin (less than 4 packets in flight).
         As long as the stream is found to be thin, up to 6 linear
         timeouts may be performed before exponential backoff mode is
         initiated. This improves retransmission latency for
         non-aggressive thin streams, often found to be time-dependent.
         For more information on thin streams, see
         Documentation/networking/tcp-thin.rst
 
         Default: 0
 
 tcp_limit_output_bytes - INTEGER
         Controls TCP Small Queue limit per tcp socket.
         TCP bulk sender tends to increase packets in flight until it
         gets losses notifications. With SNDBUF autotuning, this can
         result in a large amount of packets queued on the local machine
         (e.g.: qdiscs, CPU backlog, or device) hurting latency of other
         flows, for typical pfifo_fast qdiscs.  tcp_limit_output_bytes
         limits the number of bytes on qdisc or device to reduce artificial
         RTT/cwnd and reduce bufferbloat.
 
         Default: 1048576 (16 * 65536)
 
 tcp_challenge_ack_limit - INTEGER
         Limits number of Challenge ACK sent per second, as recommended
         in RFC 5961 (Improving TCP's Robustness to Blind In-Window Attacks)
         Note that this per netns rate limit can allow some side channel
         attacks and probably should not be enabled.
         TCP stack implements per TCP socket limits anyway.
         Default: INT_MAX (unlimited)
 
 tcp_ehash_entries - INTEGER
         Show the number of hash buckets for TCP sockets in the current
         networking namespace.
 
         A negative value means the networking namespace does not own its
         hash buckets and shares the initial networking namespace's one.
 
 tcp_child_ehash_entries - INTEGER
         Control the number of hash buckets for TCP sockets in the child
         networking namespace, which must be set before clone() or unshare().
 
         If the value is not 0, the kernel uses a value rounded up to 2^n
         as the actual hash bucket size.  0 is a special value, meaning
         the child networking namespace will share the initial networking
         namespace's hash buckets.
 
         Note that the child will use the global one in case the kernel
         fails to allocate enough memory.  In addition, the global hash
         buckets are spread over available NUMA nodes, but the allocation
         of the child hash table depends on the current process's NUMA
         policy, which could result in performance differences.
 
         Note also that the default value of tcp_max_tw_buckets and
         tcp_max_syn_backlog depend on the hash bucket size.
 
         Possible values: 0, 2^n (n: 0 - 24 (16Mi))
 
         Default: 0
 
 tcp_plb_enabled - BOOLEAN
         If set and the underlying congestion control (e.g. DCTCP) supports
         and enables PLB feature, TCP PLB (Protective Load Balancing) is
         enabled. PLB is described in the following paper:
         https://doi.org/10.1145/3544216.3544226. Based on PLB parameters,
         upon sensing sustained congestion, TCP triggers a change in
         flow label field for outgoing IPv6 packets. A change in flow label
         field potentially changes the path of outgoing packets for switches
         that use ECMP/WCMP for routing.
 
         PLB changes socket txhash which results in a change in IPv6 Flow Label
         field, and currently no-op for IPv4 headers. It is possible
         to apply PLB for IPv4 with other network header fields (e.g. TCP
         or IPv4 options) or using encapsulation where outer header is used
         by switches to determine next hop. In either case, further host
         and switch side changes will be needed.
 
         When set, PLB assumes that congestion signal (e.g. ECN) is made
         available and used by congestion control module to estimate a
         congestion measure (e.g. ce_ratio). PLB needs a congestion measure to
         make repathing decisions.
 
         Default: FALSE
 
 tcp_plb_idle_rehash_rounds - INTEGER
         Number of consecutive congested rounds (RTT) seen after which
         a rehash can be performed, given there are no packets in flight.
         This is referred to as M in PLB paper:
         https://doi.org/10.1145/3544216.3544226.
 
         Possible Values: 0 - 31
 
         Default: 3
 
 tcp_plb_rehash_rounds - INTEGER
         Number of consecutive congested rounds (RTT) seen after which
         a forced rehash can be performed. Be careful when setting this
         parameter, as a small value increases the risk of retransmissions.
         This is referred to as N in PLB paper:
         https://doi.org/10.1145/3544216.3544226.
 
         Possible Values: 0 - 31
 
         Default: 12
 
 tcp_plb_suspend_rto_sec - INTEGER
         Time, in seconds, to suspend PLB in event of an RTO. In order to avoid
         having PLB repath onto a connectivity "black hole", after an RTO a TCP
         connection suspends PLB repathing for a random duration between 1x and
         2x of this parameter. Randomness is added to avoid concurrent rehashing
         of multiple TCP connections. This should be set corresponding to the
         amount of time it takes to repair a failed link.
 
         Possible Values: 0 - 255
 
         Default: 60
 
 tcp_plb_cong_thresh - INTEGER
         Fraction of packets marked with congestion over a round (RTT) to
         tag that round as congested. This is referred to as K in the PLB paper:
         https://doi.org/10.1145/3544216.3544226.
 
         The 0-1 fraction range is mapped to 0-256 range to avoid floating
         point operations. For example, 128 means that if at least 50% of
         the packets in a round were marked as congested then the round
         will be tagged as congested.
 
         Setting threshold to 0 means that PLB repaths every RTT regardless
         of congestion. This is not intended behavior for PLB and should be
         used only for experimentation purpose.
 
         Possible Values: 0 - 256
 
         Default: 128
 
 tcp_pingpong_thresh - INTEGER
         The number of estimated data replies sent for estimated incoming data
         requests that must happen before TCP considers that a connection is a
         "ping-pong" (request-response) connection for which delayed
         acknowledgments can provide benefits.
 
         This threshold is 1 by default, but some applications may need a higher
         threshold for optimal performance.
 
         Possible Values: 1 - 255
 
         Default: 1
 
 tcp_rto_min_us - INTEGER
         Minimal TCP retransmission timeout (in microseconds). Note that the
         rto_min route option has the highest precedence for configuring this
         setting, followed by the TCP_BPF_RTO_MIN socket option, followed by
         this tcp_rto_min_us sysctl.
 
         The recommended practice is to use a value less or equal to 200000
         microseconds.
 
         Possible Values: 1 - INT_MAX
 
         Default: 200000
 
 UDP variables
 =============
 
 udp_l3mdev_accept - BOOLEAN
         Enabling this option allows a "global" bound socket to work
         across L3 master domains (e.g., VRFs) with packets capable of
         being received regardless of the L3 domain in which they
         originated. Only valid when the kernel was compiled with
         CONFIG_NET_L3_MASTER_DEV.
 
         Default: 0 (disabled)
 
 udp_mem - vector of 3 INTEGERs: min, pressure, max
         Number of pages allowed for queueing by all UDP sockets.
 
         min: Number of pages allowed for queueing by all UDP sockets.
 
         pressure: This value was introduced to follow format of tcp_mem.
 
         max: This value was introduced to follow format of tcp_mem.
 
         Default is calculated at boot time from amount of available memory.
 
 udp_rmem_min - INTEGER
         Minimal size of receive buffer used by UDP sockets in moderation.
         Each UDP socket is able to use the size for receiving data, even if
         total pages of UDP sockets exceed udp_mem pressure. The unit is byte.
 
         Default: 4K
 
 udp_wmem_min - INTEGER
         UDP does not have tx memory accounting and this tunable has no effect.
 
 udp_hash_entries - INTEGER
         Show the number of hash buckets for UDP sockets in the current
         networking namespace.
 
         A negative value means the networking namespace does not own its
         hash buckets and shares the initial networking namespace's one.
 
 udp_child_ehash_entries - INTEGER
         Control the number of hash buckets for UDP sockets in the child
         networking namespace, which must be set before clone() or unshare().
 
         If the value is not 0, the kernel uses a value rounded up to 2^n
         as the actual hash bucket size.  0 is a special value, meaning
         the child networking namespace will share the initial networking
         namespace's hash buckets.
 
         Note that the child will use the global one in case the kernel
         fails to allocate enough memory.  In addition, the global hash
         buckets are spread over available NUMA nodes, but the allocation
         of the child hash table depends on the current process's NUMA
         policy, which could result in performance differences.
 
         Possible values: 0, 2^n (n: 7 (128) - 16 (64K))
 
         Default: 0
 
 
 RAW variables
 =============
 
 raw_l3mdev_accept - BOOLEAN
         Enabling this option allows a "global" bound socket to work
         across L3 master domains (e.g., VRFs) with packets capable of
         being received regardless of the L3 domain in which they
         originated. Only valid when the kernel was compiled with
         CONFIG_NET_L3_MASTER_DEV.
 
         Default: 1 (enabled)
 
 CIPSOv4 Variables
 =================
 
 cipso_cache_enable - BOOLEAN
         If set, enable additions to and lookups from the CIPSO label mapping
         cache.  If unset, additions are ignored and lookups always result in a
         miss.  However, regardless of the setting the cache is still
         invalidated when required when means you can safely toggle this on and
         off and the cache will always be "safe".
 
         Default: 1
 
 cipso_cache_bucket_size - INTEGER
         The CIPSO label cache consists of a fixed size hash table with each
         hash bucket containing a number of cache entries.  This variable limits
         the number of entries in each hash bucket; the larger the value is, the
         more CIPSO label mappings that can be cached.  When the number of
         entries in a given hash bucket reaches this limit adding new entries
         causes the oldest entry in the bucket to be removed to make room.
 
         Default: 10
 
 cipso_rbm_optfmt - BOOLEAN
         Enable the "Optimized Tag 1 Format" as defined in section 3.4.2.6 of
         the CIPSO draft specification (see Documentation/netlabel for details).
         This means that when set the CIPSO tag will be padded with empty
         categories in order to make the packet data 32-bit aligned.
 
         Default: 0
 
 cipso_rbm_structvalid - BOOLEAN
         If set, do a very strict check of the CIPSO option when
         ip_options_compile() is called.  If unset, relax the checks done during
         ip_options_compile().  Either way is "safe" as errors are caught else
         where in the CIPSO processing code but setting this to 0 (False) should
         result in less work (i.e. it should be faster) but could cause problems
         with other implementations that require strict checking.
 
         Default: 0
 
 IP Variables
 ============
 
 ip_local_port_range - 2 INTEGERS
         Defines the local port range that is used by TCP and UDP to
         choose the local port. The first number is the first, the
         second the last local port number.
         If possible, it is better these numbers have different parity
         (one even and one odd value).
         Must be greater than or equal to ip_unprivileged_port_start.
         The default values are 32768 and 60999 respectively.
 
 ip_local_reserved_ports - list of comma separated ranges
         Specify the ports which are reserved for known third-party
         applications. These ports will not be used by automatic port
         assignments (e.g. when calling connect() or bind() with port
         number 0). Explicit port allocation behavior is unchanged.
 
         The format used for both input and output is a comma separated
         list of ranges (e.g. "1,2-4,10-10" for ports 1, 2, 3, 4 and
         10). Writing to the file will clear all previously reserved
         ports and update the current list with the one given in the
         input.
 
         Note that ip_local_port_range and ip_local_reserved_ports
         settings are independent and both are considered by the kernel
         when determining which ports are available for automatic port
         assignments.
 
         You can reserve ports which are not in the current
         ip_local_port_range, e.g.::
 
             $ cat /proc/sys/net/ipv4/ip_local_port_range
             32000       60999
             $ cat /proc/sys/net/ipv4/ip_local_reserved_ports
             8080,9148
 
         although this is redundant. However such a setting is useful
         if later the port range is changed to a value that will
         include the reserved ports. Also keep in mind, that overlapping
         of these ranges may affect probability of selecting ephemeral
         ports which are right after block of reserved ports.
 
         Default: Empty
 
 ip_unprivileged_port_start - INTEGER
         This is a per-namespace sysctl.  It defines the first
         unprivileged port in the network namespace.  Privileged ports
         require root or CAP_NET_BIND_SERVICE in order to bind to them.
         To disable all privileged ports, set this to 0.  They must not
         overlap with the ip_local_port_range.
 
         Default: 1024
 
 ip_nonlocal_bind - BOOLEAN
         If set, allows processes to bind() to non-local IP addresses,
         which can be quite useful - but may break some applications.
 
         Default: 0
 
 ip_autobind_reuse - BOOLEAN
         By default, bind() does not select the ports automatically even if
         the new socket and all sockets bound to the port have SO_REUSEADDR.
         ip_autobind_reuse allows bind() to reuse the port and this is useful
         when you use bind()+connect(), but may break some applications.
         The preferred solution is to use IP_BIND_ADDRESS_NO_PORT and this
         option should only be set by experts.
         Default: 0
 
 ip_dynaddr - INTEGER
         If set non-zero, enables support for dynamic addresses.
         If set to a non-zero value larger than 1, a kernel log
         message will be printed when dynamic address rewriting
         occurs.
 
         Default: 0
 
 ip_early_demux - BOOLEAN
         Optimize input packet processing down to one demux for
         certain kinds of local sockets.  Currently we only do this
         for established TCP and connected UDP sockets.
 
         It may add an additional cost for pure routing workloads that
         reduces overall throughput, in such case you should disable it.
 
         Default: 1
 
 ping_group_range - 2 INTEGERS
         Restrict ICMP_PROTO datagram sockets to users in the group range.
         The default is "1 0", meaning, that nobody (not even root) may
         create ping sockets.  Setting it to "100 100" would grant permissions
         to the single group. "0 4294967294" would enable it for the world, "100
         4294967294" would enable it for the users, but not daemons.
 
 tcp_early_demux - BOOLEAN
         Enable early demux for established TCP sockets.
 
         Default: 1
 
 udp_early_demux - BOOLEAN
         Enable early demux for connected UDP sockets. Disable this if
         your system could experience more unconnected load.
 
         Default: 1
 
 icmp_echo_ignore_all - BOOLEAN
         If set non-zero, then the kernel will ignore all ICMP ECHO
         requests sent to it.
 
         Default: 0
 
 icmp_echo_enable_probe - BOOLEAN
         If set to one, then the kernel will respond to RFC 8335 PROBE
         requests sent to it.
 
         Default: 0
 
 icmp_echo_ignore_broadcasts - BOOLEAN
         If set non-zero, then the kernel will ignore all ICMP ECHO and
         TIMESTAMP requests sent to it via broadcast/multicast.
 
         Default: 1
 
 icmp_ratelimit - INTEGER
         Limit the maximal rates for sending ICMP packets whose type matches
         icmp_ratemask (see below) to specific targets.
         0 to disable any limiting,
         otherwise the minimal space between responses in milliseconds.
         Note that another sysctl, icmp_msgs_per_sec limits the number
         of ICMP packets sent on all targets.
 
         Default: 1000
 
 icmp_msgs_per_sec - INTEGER
         Limit maximal number of ICMP packets sent per second from this host.
         Only messages whose type matches icmp_ratemask (see below) are
         controlled by this limit. For security reasons, the precise count
         of messages per second is randomized.
 
         Default: 1000
 
 icmp_msgs_burst - INTEGER
         icmp_msgs_per_sec controls number of ICMP packets sent per second,
         while icmp_msgs_burst controls the burst size of these packets.
         For security reasons, the precise burst size is randomized.
 
         Default: 50
 
 icmp_ratemask - INTEGER
         Mask made of ICMP types for which rates are being limited.
 
         Significant bits: IHGFEDCBA9876543210
 
         Default mask:     0000001100000011000 (6168)
 
         Bit definitions (see include/linux/icmp.h):
 
                 = =========================
                 0 Echo Reply
                 3 Destination Unreachable [1]_
                 4 Source Quench [1]_
                 5 Redirect
                 8 Echo Request
                 B Time Exceeded [1]_
                 C Parameter Problem [1]_
                 D Timestamp Request
                 E Timestamp Reply
                 F Info Request
                 G Info Reply
                 H Address Mask Request
                 I Address Mask Reply
                 = =========================
 
         .. [1] These are rate limited by default (see default mask above)
 
 icmp_ignore_bogus_error_responses - BOOLEAN
         Some routers violate RFC1122 by sending bogus responses to broadcast
         frames.  Such violations are normally logged via a kernel warning.
         If this is set to TRUE, the kernel will not give such warnings, which
         will avoid log file clutter.
 
         Default: 1
 
 icmp_errors_use_inbound_ifaddr - BOOLEAN
 
         If zero, icmp error messages are sent with the primary address of
         the exiting interface.
 
         If non-zero, the message will be sent with the primary address of
         the interface that received the packet that caused the icmp error.
         This is the behaviour many network administrators will expect from
         a router. And it can make debugging complicated network layouts
         much easier.
 
         Note that if no primary address exists for the interface selected,
         then the primary address of the first non-loopback interface that
         has one will be used regardless of this setting.
 
         Default: 0
 
 igmp_max_memberships - INTEGER
         Change the maximum number of multicast groups we can subscribe to.
         Default: 20
 
         Theoretical maximum value is bounded by having to send a membership
         report in a single datagram (i.e. the report can't span multiple
         datagrams, or risk confusing the switch and leaving groups you don't
         intend to).
 
         The number of supported groups 'M' is bounded by the number of group
         report entries you can fit into a single datagram of 65535 bytes.
 
         M = 65536-sizeof (ip header)/(sizeof(Group record))
 
         Group records are variable length, with a minimum of 12 bytes.
         So net.ipv4.igmp_max_memberships should not be set higher than:
 
         (65536-24) / 12 = 5459
 
         The value 5459 assumes no IP header options, so in practice
         this number may be lower.
 
 igmp_max_msf - INTEGER
         Maximum number of addresses allowed in the source filter list for a
         multicast group.
 
         Default: 10
 
 igmp_qrv - INTEGER
         Controls the IGMP query robustness variable (see RFC2236 8.1).
 
         Default: 2 (as specified by RFC2236 8.1)
 
         Minimum: 1 (as specified by RFC6636 4.5)
 
 force_igmp_version - INTEGER
         - 0 - (default) No enforcement of a IGMP version, IGMPv1/v2 fallback
           allowed. Will back to IGMPv3 mode again if all IGMPv1/v2 Querier
           Present timer expires.
         - 1 - Enforce to use IGMP version 1. Will also reply IGMPv1 report if
           receive IGMPv2/v3 query.
         - 2 - Enforce to use IGMP version 2. Will fallback to IGMPv1 if receive
           IGMPv1 query message. Will reply report if receive IGMPv3 query.
         - 3 - Enforce to use IGMP version 3. The same react with default 0.
 
         .. note::
 
            this is not the same with force_mld_version because IGMPv3 RFC3376
            Security Considerations does not have clear description that we could
            ignore other version messages completely as MLDv2 RFC3810. So make
            this value as default 0 is recommended.
 
 ``conf/interface/*``
         changes special settings per interface (where
         interface" is the name of your network interface)
 
 ``conf/all/*``
           is special, changes the settings for all interfaces
 
 log_martians - BOOLEAN
         Log packets with impossible addresses to kernel log.
         log_martians for the interface will be enabled if at least one of
         conf/{all,interface}/log_martians is set to TRUE,
         it will be disabled otherwise
 
 accept_redirects - BOOLEAN
         Accept ICMP redirect messages.
         accept_redirects for the interface will be enabled if:
 
         - both conf/{all,interface}/accept_redirects are TRUE in the case
           forwarding for the interface is enabled
 
         or
 
         - at least one of conf/{all,interface}/accept_redirects is TRUE in the
           case forwarding for the interface is disabled
 
         accept_redirects for the interface will be disabled otherwise
 
         default:
 
                 - TRUE (host)
                 - FALSE (router)
 
 forwarding - BOOLEAN
         Enable IP forwarding on this interface.  This controls whether packets
         received _on_ this interface can be forwarded.
 
 mc_forwarding - BOOLEAN
         Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE
         and a multicast routing daemon is required.
         conf/all/mc_forwarding must also be set to TRUE to enable multicast
         routing for the interface
 
 medium_id - INTEGER
         Integer value used to differentiate the devices by the medium they
         are attached to. Two devices can have different id values when
         the broadcast packets are received only on one of them.
         The default value 0 means that the device is the only interface
         to its medium, value of -1 means that medium is not known.
 
         Currently, it is used to change the proxy_arp behavior:
         the proxy_arp feature is enabled for packets forwarded between
         two devices attached to different media.
 
 proxy_arp - BOOLEAN
         Do proxy arp.
 
         proxy_arp for the interface will be enabled if at least one of
         conf/{all,interface}/proxy_arp is set to TRUE,
         it will be disabled otherwise
 
 proxy_arp_pvlan - BOOLEAN
         Private VLAN proxy arp.
 
         Basically allow proxy arp replies back to the same interface
         (from which the ARP request/solicitation was received).
 
         This is done to support (ethernet) switch features, like RFC
         3069, where the individual ports are NOT allowed to
         communicate with each other, but they are allowed to talk to
         the upstream router.  As described in RFC 3069, it is possible
         to allow these hosts to communicate through the upstream
         router by proxy_arp'ing. Don't need to be used together with
         proxy_arp.
 
         This technology is known by different names:
 
           In RFC 3069 it is called VLAN Aggregation.
           Cisco and Allied Telesyn call it Private VLAN.
           Hewlett-Packard call it Source-Port filtering or port-isolation.
           Ericsson call it MAC-Forced Forwarding (RFC Draft).
 
 proxy_delay - INTEGER
         Delay proxy response.
 
         Delay response to a neighbor solicitation when proxy_arp
         or proxy_ndp is enabled. A random value between [0, proxy_delay)
         will be chosen, setting to zero means reply with no delay.
         Value in jiffies. Defaults to 80.
 
 shared_media - BOOLEAN
         Send(router) or accept(host) RFC1620 shared media redirects.
         Overrides secure_redirects.
 
         shared_media for the interface will be enabled if at least one of
         conf/{all,interface}/shared_media is set to TRUE,
         it will be disabled otherwise
 
         default TRUE
 
 secure_redirects - BOOLEAN
         Accept ICMP redirect messages only to gateways listed in the
         interface's current gateway list. Even if disabled, RFC1122 redirect
         rules still apply.
 
         Overridden by shared_media.
 
         secure_redirects for the interface will be enabled if at least one of
         conf/{all,interface}/secure_redirects is set to TRUE,
         it will be disabled otherwise
 
         default TRUE
 
 send_redirects - BOOLEAN
         Send redirects, if router.
 
         send_redirects for the interface will be enabled if at least one of
         conf/{all,interface}/send_redirects is set to TRUE,
         it will be disabled otherwise
 
         Default: TRUE
 
 bootp_relay - BOOLEAN
         Accept packets with source address 0.b.c.d destined
         not to this host as local ones. It is supposed, that
         BOOTP relay daemon will catch and forward such packets.
         conf/all/bootp_relay must also be set to TRUE to enable BOOTP relay
         for the interface
 
         default FALSE
 
         Not Implemented Yet.
 
 accept_source_route - BOOLEAN
         Accept packets with SRR option.
         conf/all/accept_source_route must also be set to TRUE to accept packets
         with SRR option on the interface
 
         default
 
                 - TRUE (router)
                 - FALSE (host)
 
 accept_local - BOOLEAN
         Accept packets with local source addresses. In combination with
         suitable routing, this can be used to direct packets between two
         local interfaces over the wire and have them accepted properly.
         default FALSE
 
 route_localnet - BOOLEAN
         Do not consider loopback addresses as martian source or destination
         while routing. This enables the use of 127/8 for local routing purposes.
 
         default FALSE
 
 rp_filter - INTEGER
         - 0 - No source validation.
         - 1 - Strict mode as defined in RFC3704 Strict Reverse Path
           Each incoming packet is tested against the FIB and if the interface
           is not the best reverse path the packet check will fail.
           By default failed packets are discarded.
         - 2 - Loose mode as defined in RFC3704 Loose Reverse Path
           Each incoming packet's source address is also tested against the FIB
           and if the source address is not reachable via any interface
           the packet check will fail.
 
         Current recommended practice in RFC3704 is to enable strict mode
         to prevent IP spoofing from DDos attacks. If using asymmetric routing
         or other complicated routing, then loose mode is recommended.
 
         The max value from conf/{all,interface}/rp_filter is used
         when doing source validation on the {interface}.
 
         Default value is 0. Note that some distributions enable it
         in startup scripts.
 
 src_valid_mark - BOOLEAN
         - 0 - The fwmark of the packet is not included in reverse path
           route lookup.  This allows for asymmetric routing configurations
           utilizing the fwmark in only one direction, e.g., transparent
           proxying.
 
         - 1 - The fwmark of the packet is included in reverse path route
           lookup.  This permits rp_filter to function when the fwmark is
           used for routing traffic in both directions.
 
         This setting also affects the utilization of fmwark when
         performing source address selection for ICMP replies, or
         determining addresses stored for the IPOPT_TS_TSANDADDR and
         IPOPT_RR IP options.
 
         The max value from conf/{all,interface}/src_valid_mark is used.
 
         Default value is 0.
 
 arp_filter - BOOLEAN
         - 1 - Allows you to have multiple network interfaces on the same
           subnet, and have the ARPs for each interface be answered
           based on whether or not the kernel would route a packet from
           the ARP'd IP out that interface (therefore you must use source
           based routing for this to work). In other words it allows control
           of which cards (usually 1) will respond to an arp request.
 
         - 0 - (default) The kernel can respond to arp requests with addresses
           from other interfaces. This may seem wrong but it usually makes
           sense, because it increases the chance of successful communication.
           IP addresses are owned by the complete host on Linux, not by
           particular interfaces. Only for more complex setups like load-
           balancing, does this behaviour cause problems.
 
         arp_filter for the interface will be enabled if at least one of
         conf/{all,interface}/arp_filter is set to TRUE,
         it will be disabled otherwise
 
 arp_announce - INTEGER
         Define different restriction levels for announcing the local
         source IP address from IP packets in ARP requests sent on
         interface:
 
         - 0 - (default) Use any local address, configured on any interface
         - 1 - Try to avoid local addresses that are not in the target's
           subnet for this interface. This mode is useful when target
           hosts reachable via this interface require the source IP
           address in ARP requests to be part of their logical network
           configured on the receiving interface. When we generate the
           request we will check all our subnets that include the
           target IP and will preserve the source address if it is from
           such subnet. If there is no such subnet we select source
           address according to the rules for level 2.
         - 2 - Always use the best local address for this target.
           In this mode we ignore the source address in the IP packet
           and try to select local address that we prefer for talks with
           the target host. Such local address is selected by looking
           for primary IP addresses on all our subnets on the outgoing
           interface that include the target IP address. If no suitable
           local address is found we select the first local address
           we have on the outgoing interface or on all other interfaces,
           with the hope we will receive reply for our request and
           even sometimes no matter the source IP address we announce.
 
         The max value from conf/{all,interface}/arp_announce is used.
 
         Increasing the restriction level gives more chance for
         receiving answer from the resolved target while decreasing
         the level announces more valid sender's information.
 
 arp_ignore - INTEGER
         Define different modes for sending replies in response to
         received ARP requests that resolve local target IP addresses:
 
         - 0 - (default): reply for any local target IP address, configured
           on any interface
         - 1 - reply only if the target IP address is local address
           configured on the incoming interface
         - 2 - reply only if the target IP address is local address
           configured on the incoming interface and both with the
           sender's IP address are part from same subnet on this interface
         - 3 - do not reply for local addresses configured with scope host,
           only resolutions for global and link addresses are replied
         - 4-7 - reserved
         - 8 - do not reply for all local addresses
 
         The max value from conf/{all,interface}/arp_ignore is used
         when ARP request is received on the {interface}
 
 arp_notify - BOOLEAN
         Define mode for notification of address and device changes.
 
          ==  ==========================================================
           0  (default): do nothing
           1  Generate gratuitous arp requests when device is brought up
              or hardware address changes.
          ==  ==========================================================
 
 arp_accept - INTEGER
         Define behavior for accepting gratuitous ARP (garp) frames from devices
         that are not already present in the ARP table:
 
         - 0 - don't create new entries in the ARP table
         - 1 - create new entries in the ARP table
         - 2 - create new entries only if the source IP address is in the same
           subnet as an address configured on the interface that received the
           garp message.
 
         Both replies and requests type gratuitous arp will trigger the
         ARP table to be updated, if this setting is on.
 
         If the ARP table already contains the IP address of the
         gratuitous arp frame, the arp table will be updated regardless
         if this setting is on or off.
 
 arp_evict_nocarrier - BOOLEAN
         Clears the ARP cache on NOCARRIER events. This option is important for
         wireless devices where the ARP cache should not be cleared when roaming
         between access points on the same network. In most cases this should
         remain as the default (1).
 
         - 1 - (default): Clear the ARP cache on NOCARRIER events
         - 0 - Do not clear ARP cache on NOCARRIER events
 
 mcast_solicit - INTEGER
         The maximum number of multicast probes in INCOMPLETE state,
         when the associated hardware address is unknown.  Defaults
         to 3.
 
 ucast_solicit - INTEGER
         The maximum number of unicast probes in PROBE state, when
         the hardware address is being reconfirmed.  Defaults to 3.
 
 app_solicit - INTEGER
         The maximum number of probes to send to the user space ARP daemon
         via netlink before dropping back to multicast probes (see
         mcast_resolicit).  Defaults to 0.
 
 mcast_resolicit - INTEGER
         The maximum number of multicast probes after unicast and
         app probes in PROBE state.  Defaults to 0.
 
 disable_policy - BOOLEAN
         Disable IPSEC policy (SPD) for this interface
 
 disable_xfrm - BOOLEAN
         Disable IPSEC encryption on this interface, whatever the policy
 
 igmpv2_unsolicited_report_interval - INTEGER
         The interval in milliseconds in which the next unsolicited
         IGMPv1 or IGMPv2 report retransmit will take place.
 
         Default: 10000 (10 seconds)
 
 igmpv3_unsolicited_report_interval - INTEGER
         The interval in milliseconds in which the next unsolicited
         IGMPv3 report retransmit will take place.
 
         Default: 1000 (1 seconds)
 
 ignore_routes_with_linkdown - BOOLEAN
         Ignore routes whose link is down when performing a FIB lookup.
 
 promote_secondaries - BOOLEAN
         When a primary IP address is removed from this interface
         promote a corresponding secondary IP address instead of
         removing all the corresponding secondary IP addresses.
 
 drop_unicast_in_l2_multicast - BOOLEAN
         Drop any unicast IP packets that are received in link-layer
         multicast (or broadcast) frames.
 
         This behavior (for multicast) is actually a SHOULD in RFC
         1122, but is disabled by default for compatibility reasons.
 
         Default: off (0)
 
 drop_gratuitous_arp - BOOLEAN
         Drop all gratuitous ARP frames, for example if there's a known
         good ARP proxy on the network and such frames need not be used
         (or in the case of 802.11, must not be used to prevent attacks.)
 
         Default: off (0)
 
 
 tag - INTEGER
         Allows you to write a number, which can be used as required.
 
         Default value is 0.
 
 xfrm4_gc_thresh - INTEGER
         (Obsolete since linux-4.14)
         The threshold at which we will start garbage collecting for IPv4
         destination cache entries.  At twice this value the system will
         refuse new allocations.
 
 igmp_link_local_mcast_reports - BOOLEAN
         Enable IGMP reports for link local multicast groups in the
         224.0.0.X range.
 
         Default TRUE
 
 Alexey Kuznetsov.
 kuznet@ms2.inr.ac.ru
 
 Updated by:
 
 - Andi Kleen
   ak@muc.de
 - Nicolas Delon
   delon.nicolas@wanadoo.fr
 
 
 
 
 /proc/sys/net/ipv6/* Variables
 ==============================
 
 IPv6 has no global variables such as tcp_*.  tcp_* settings under ipv4/ also
 apply to IPv6 [XXX?].
 
 bindv6only - BOOLEAN
         Default value for IPV6_V6ONLY socket option,
         which restricts use of the IPv6 socket to IPv6 communication
         only.
 
                 - TRUE: disable IPv4-mapped address feature
                 - FALSE: enable IPv4-mapped address feature
 
         Default: FALSE (as specified in RFC3493)
 
 flowlabel_consistency - BOOLEAN
         Protect the consistency (and unicity) of flow label.
         You have to disable it to use IPV6_FL_F_REFLECT flag on the
         flow label manager.
 
         - TRUE: enabled
         - FALSE: disabled
 
         Default: TRUE
 
 auto_flowlabels - INTEGER
         Automatically generate flow labels based on a flow hash of the
         packet. This allows intermediate devices, such as routers, to
         identify packet flows for mechanisms like Equal Cost Multipath
         Routing (see RFC 6438).
 
         =  ===========================================================
         0  automatic flow labels are completely disabled
         1  automatic flow labels are enabled by default, they can be
            disabled on a per socket basis using the IPV6_AUTOFLOWLABEL
            socket option
         2  automatic flow labels are allowed, they may be enabled on a
            per socket basis using the IPV6_AUTOFLOWLABEL socket option
         3  automatic flow labels are enabled and enforced, they cannot
            be disabled by the socket option
         =  ===========================================================
 
         Default: 1
 
 flowlabel_state_ranges - BOOLEAN
         Split the flow label number space into two ranges. 0-0x7FFFF is
         reserved for the IPv6 flow manager facility, 0x80000-0xFFFFF
         is reserved for stateless flow labels as described in RFC6437.
 
         - TRUE: enabled
         - FALSE: disabled
 
         Default: true
 
 flowlabel_reflect - INTEGER
         Control flow label reflection. Needed for Path MTU
         Discovery to work with Equal Cost Multipath Routing in anycast
         environments. See RFC 7690 and:
         https://tools.ietf.org/html/draft-wang-6man-flow-label-reflection-01
 
         This is a bitmask.
 
         - 1: enabled for established flows
 
           Note that this prevents automatic flowlabel changes, as done
           in "tcp: change IPv6 flow-label upon receiving spurious retransmission"
           and "tcp: Change txhash on every SYN and RTO retransmit"
 
         - 2: enabled for TCP RESET packets (no active listener)
           If set, a RST packet sent in response to a SYN packet on a closed
           port will reflect the incoming flow label.
 
         - 4: enabled for ICMPv6 echo reply messages.
 
         Default: 0
 
 fib_multipath_hash_policy - INTEGER
         Controls which hash policy to use for multipath routes.
 
         Default: 0 (Layer 3)
 
         Possible values:
 
         - 0 - Layer 3 (source and destination addresses plus flow label)
         - 1 - Layer 4 (standard 5-tuple)
         - 2 - Layer 3 or inner Layer 3 if present
         - 3 - Custom multipath hash. Fields used for multipath hash calculation
           are determined by fib_multipath_hash_fields sysctl
 
 fib_multipath_hash_fields - UNSIGNED INTEGER
         When fib_multipath_hash_policy is set to 3 (custom multipath hash), the
         fields used for multipath hash calculation are determined by this
         sysctl.
 
         This value is a bitmask which enables various fields for multipath hash
         calculation.
 
         Possible fields are:
 
         ====== ============================
         0x0001 Source IP address
         0x0002 Destination IP address
         0x0004 IP protocol
         0x0008 Flow Label
         0x0010 Source port
         0x0020 Destination port
         0x0040 Inner source IP address
         0x0080 Inner destination IP address
         0x0100 Inner IP protocol
         0x0200 Inner Flow Label
         0x0400 Inner source port
         0x0800 Inner destination port
         ====== ============================
 
         Default: 0x0007 (source IP, destination IP and IP protocol)
 
 anycast_src_echo_reply - BOOLEAN
         Controls the use of anycast addresses as source addresses for ICMPv6
         echo reply
 
         - TRUE:  enabled
         - FALSE: disabled
 
         Default: FALSE
 
 idgen_delay - INTEGER
         Controls the delay in seconds after which time to retry
         privacy stable address generation if a DAD conflict is
         detected.
 
         Default: 1 (as specified in RFC7217)
 
 idgen_retries - INTEGER
         Controls the number of retries to generate a stable privacy
         address if a DAD conflict is detected.
 
         Default: 3 (as specified in RFC7217)
 
 mld_qrv - INTEGER
         Controls the MLD query robustness variable (see RFC3810 9.1).
 
         Default: 2 (as specified by RFC3810 9.1)
 
         Minimum: 1 (as specified by RFC6636 4.5)
 
 max_dst_opts_number - INTEGER
         Maximum number of non-padding TLVs allowed in a Destination
         options extension header. If this value is less than zero
         then unknown options are disallowed and the number of known
         TLVs allowed is the absolute value of this number.
 
         Default: 8
 
 max_hbh_opts_number - INTEGER
         Maximum number of non-padding TLVs allowed in a Hop-by-Hop
         options extension header. If this value is less than zero
         then unknown options are disallowed and the number of known
         TLVs allowed is the absolute value of this number.
 
         Default: 8
 
 max_dst_opts_length - INTEGER
         Maximum length allowed for a Destination options extension
         header.
 
         Default: INT_MAX (unlimited)
 
 max_hbh_length - INTEGER
         Maximum length allowed for a Hop-by-Hop options extension
         header.
 
         Default: INT_MAX (unlimited)
 
 skip_notify_on_dev_down - BOOLEAN
         Controls whether an RTM_DELROUTE message is generated for routes
         removed when a device is taken down or deleted. IPv4 does not
         generate this message; IPv6 does by default. Setting this sysctl
         to true skips the message, making IPv4 and IPv6 on par in relying
         on userspace caches to track link events and evict routes.
 
         Default: false (generate message)
 
 nexthop_compat_mode - BOOLEAN
         New nexthop API provides a means for managing nexthops independent of
         prefixes. Backwards compatibility with old route format is enabled by
         default which means route dumps and notifications contain the new
         nexthop attribute but also the full, expanded nexthop definition.
         Further, updates or deletes of a nexthop configuration generate route
         notifications for each fib entry using the nexthop. Once a system
         understands the new API, this sysctl can be disabled to achieve full
         performance benefits of the new API by disabling the nexthop expansion
         and extraneous notifications.
         Default: true (backward compat mode)
 
 fib_notify_on_flag_change - INTEGER
         Whether to emit RTM_NEWROUTE notifications whenever RTM_F_OFFLOAD/
         RTM_F_TRAP/RTM_F_OFFLOAD_FAILED flags are changed.
 
         After installing a route to the kernel, user space receives an
         acknowledgment, which means the route was installed in the kernel,
         but not necessarily in hardware.
         It is also possible for a route already installed in hardware to change
         its action and therefore its flags. For example, a host route that is
         trapping packets can be "promoted" to perform decapsulation following
         the installation of an IPinIP/VXLAN tunnel.
         The notifications will indicate to user-space the state of the route.
 
         Default: 0 (Do not emit notifications.)
 
         Possible values:
 
         - 0 - Do not emit notifications.
         - 1 - Emit notifications.
         - 2 - Emit notifications only for RTM_F_OFFLOAD_FAILED flag change.
 
 ioam6_id - INTEGER
         Define the IOAM id of this node. Uses only 24 bits out of 32 in total.
 
         Min: 0
         Max: 0xFFFFFF
 
         Default: 0xFFFFFF
 
 ioam6_id_wide - LONG INTEGER
         Define the wide IOAM id of this node. Uses only 56 bits out of 64 in
         total. Can be different from ioam6_id.
 
         Min: 0
         Max: 0xFFFFFFFFFFFFFF
 
         Default: 0xFFFFFFFFFFFFFF
 
 IPv6 Fragmentation:
 
 ip6frag_high_thresh - INTEGER
         Maximum memory used to reassemble IPv6 fragments. When
         ip6frag_high_thresh bytes of memory is allocated for this purpose,
         the fragment handler will toss packets until ip6frag_low_thresh
         is reached.
 
 ip6frag_low_thresh - INTEGER
         See ip6frag_high_thresh
 
 ip6frag_time - INTEGER
         Time in seconds to keep an IPv6 fragment in memory.
 
 ``conf/default/*``:
         Change the interface-specific default settings.
 
         These settings would be used during creating new interfaces.
 
 
 ``conf/all/*``:
         Change all the interface-specific settings.
 
         [XXX:  Other special features than forwarding?]
 
 conf/all/disable_ipv6 - BOOLEAN
         Changing this value is same as changing ``conf/default/disable_ipv6``
         setting and also all per-interface ``disable_ipv6`` settings to the same
         value.
 
         Reading this value does not have any particular meaning. It does not say
         whether IPv6 support is enabled or disabled. Returned value can be 1
         also in the case when some interface has ``disable_ipv6`` set to 0 and
         has configured IPv6 addresses.
 
 conf/all/forwarding - BOOLEAN
         Enable global IPv6 forwarding between all interfaces.
 
         IPv4 and IPv6 work differently here; e.g. netfilter must be used
         to control which interfaces may forward packets and which not.
 
         This also sets all interfaces' Host/Router setting
         'forwarding' to the specified value.  See below for details.
 
         This referred to as global forwarding.
 
 proxy_ndp - BOOLEAN
         Do proxy ndp.
 
 fwmark_reflect - BOOLEAN
         Controls the fwmark of kernel-generated IPv6 reply packets that are not
         associated with a socket for example, TCP RSTs or ICMPv6 echo replies).
         If unset, these packets have a fwmark of zero. If set, they have the
         fwmark of the packet they are replying to.
 
         Default: 0
 
 ``conf/interface/*``:
         Change special settings per interface.
 
         The functional behaviour for certain settings is different
         depending on whether local forwarding is enabled or not.
 
 accept_ra - INTEGER
         Accept Router Advertisements; autoconfigure using them.
 
         It also determines whether or not to transmit Router
         Solicitations. If and only if the functional setting is to
         accept Router Advertisements, Router Solicitations will be
         transmitted.
 
         Possible values are:
 
                 ==  ===========================================================
                  0  Do not accept Router Advertisements.
                  1  Accept Router Advertisements if forwarding is disabled.
                  2  Overrule forwarding behaviour. Accept Router Advertisements
                     even if forwarding is enabled.
                 ==  ===========================================================
 
         Functional default:
 
                 - enabled if local forwarding is disabled.
                 - disabled if local forwarding is enabled.
 
 accept_ra_defrtr - BOOLEAN
         Learn default router in Router Advertisement.
 
         Functional default:
 
                 - enabled if accept_ra is enabled.
                 - disabled if accept_ra is disabled.
 
 ra_defrtr_metric - UNSIGNED INTEGER
         Route metric for default route learned in Router Advertisement. This value
         will be assigned as metric for the default route learned via IPv6 Router
         Advertisement. Takes affect only if accept_ra_defrtr is enabled.
 
         Possible values:
                 1 to 0xFFFFFFFF
 
                 Default: IP6_RT_PRIO_USER i.e. 1024.
 
 accept_ra_from_local - BOOLEAN
         Accept RA with source-address that is found on local machine
         if the RA is otherwise proper and able to be accepted.
 
         Default is to NOT accept these as it may be an un-intended
         network loop.
 
         Functional default:
 
            - enabled if accept_ra_from_local is enabled
              on a specific interface.
            - disabled if accept_ra_from_local is disabled
              on a specific interface.
 
 accept_ra_min_hop_limit - INTEGER
         Minimum hop limit Information in Router Advertisement.
 
         Hop limit Information in Router Advertisement less than this
         variable shall be ignored.
 
         Default: 1
 
 accept_ra_min_lft - INTEGER
         Minimum acceptable lifetime value in Router Advertisement.
 
         RA sections with a lifetime less than this value shall be
         ignored. Zero lifetimes stay unaffected.
 
         Default: 0
 
 accept_ra_pinfo - BOOLEAN
         Learn Prefix Information in Router Advertisement.
 
         Functional default:
 
                 - enabled if accept_ra is enabled.
                 - disabled if accept_ra is disabled.
 
 ra_honor_pio_life - BOOLEAN
         Whether to use RFC4862 Section 5.5.3e to determine the valid
         lifetime of an address matching a prefix sent in a Router
         Advertisement Prefix Information Option.
 
         - If enabled, the PIO valid lifetime will always be honored.
         - If disabled, RFC4862 section 5.5.3e is used to determine
           the valid lifetime of the address.
 
         Default: 0 (disabled)
 
 ra_honor_pio_pflag - BOOLEAN
         The Prefix Information Option P-flag indicates the network can
         allocate a unique IPv6 prefix per client using DHCPv6-PD.
         This sysctl can be enabled when a userspace DHCPv6-PD client
         is running to cause the P-flag to take effect: i.e. the
         P-flag suppresses any effects of the A-flag within the same
         PIO. For a given PIO, P=1 and A=1 is treated as A=0.
 
         - If disabled, the P-flag is ignored.
         - If enabled, the P-flag will disable SLAAC autoconfiguration
           for the given Prefix Information Option.
 
         Default: 0 (disabled)
 
 accept_ra_rt_info_min_plen - INTEGER
         Minimum prefix length of Route Information in RA.
 
         Route Information w/ prefix smaller than this variable shall
         be ignored.
 
         Functional default:
 
                 * 0 if accept_ra_rtr_pref is enabled.
                 * -1 if accept_ra_rtr_pref is disabled.
 
 accept_ra_rt_info_max_plen - INTEGER
         Maximum prefix length of Route Information in RA.
 
         Route Information w/ prefix larger than this variable shall
         be ignored.
 
         Functional default:
 
                 * 0 if accept_ra_rtr_pref is enabled.
                 * -1 if accept_ra_rtr_pref is disabled.
 
 accept_ra_rtr_pref - BOOLEAN
         Accept Router Preference in RA.
 
         Functional default:
 
                 - enabled if accept_ra is enabled.
                 - disabled if accept_ra is disabled.
 
 accept_ra_mtu - BOOLEAN
         Apply the MTU value specified in RA option 5 (RFC4861). If
         disabled, the MTU specified in the RA will be ignored.
 
         Functional default:
 
                 - enabled if accept_ra is enabled.
                 - disabled if accept_ra is disabled.
 
 accept_redirects - BOOLEAN
         Accept Redirects.
 
         Functional default:
 
                 - enabled if local forwarding is disabled.
                 - disabled if local forwarding is enabled.
 
 accept_source_route - INTEGER
         Accept source routing (routing extension header).
 
         - >= 0: Accept only routing header type 2.
         - < 0: Do not accept routing header.
 
         Default: 0
 
 autoconf - BOOLEAN
         Autoconfigure addresses using Prefix Information in Router
         Advertisements.
 
         Functional default:
 
                 - enabled if accept_ra_pinfo is enabled.
                 - disabled if accept_ra_pinfo is disabled.
 
 dad_transmits - INTEGER
         The amount of Duplicate Address Detection probes to send.
 
         Default: 1
 
 forwarding - INTEGER
         Configure interface-specific Host/Router behaviour.
 
         .. note::
 
            It is recommended to have the same setting on all
            interfaces; mixed router/host scenarios are rather uncommon.
 
         Possible values are:
 
                 - 0 Forwarding disabled
                 - 1 Forwarding enabled
 
         **FALSE (0)**:
 
         By default, Host behaviour is assumed.  This means:
 
         1. IsRouter flag is not set in Neighbour Advertisements.
         2. If accept_ra is TRUE (default), transmit Router
            Solicitations.
         3. If accept_ra is TRUE (default), accept Router
            Advertisements (and do autoconfiguration).
         4. If accept_redirects is TRUE (default), accept Redirects.
 
         **TRUE (1)**:
 
         If local forwarding is enabled, Router behaviour is assumed.
         This means exactly the reverse from the above:
 
         1. IsRouter flag is set in Neighbour Advertisements.
         2. Router Solicitations are not sent unless accept_ra is 2.
         3. Router Advertisements are ignored unless accept_ra is 2.
         4. Redirects are ignored.
 
         Default: 0 (disabled) if global forwarding is disabled (default),
         otherwise 1 (enabled).
 
 hop_limit - INTEGER
         Default Hop Limit to set.
 
         Default: 64
 
 mtu - INTEGER
         Default Maximum Transfer Unit
 
         Default: 1280 (IPv6 required minimum)
 
 ip_nonlocal_bind - BOOLEAN
         If set, allows processes to bind() to non-local IPv6 addresses,
         which can be quite useful - but may break some applications.
 
         Default: 0
 
 router_probe_interval - INTEGER
         Minimum interval (in seconds) between Router Probing described
         in RFC4191.
 
         Default: 60
 
 router_solicitation_delay - INTEGER
         Number of seconds to wait after interface is brought up
         before sending Router Solicitations.
 
         Default: 1
 
 router_solicitation_interval - INTEGER
         Number of seconds to wait between Router Solicitations.
 
         Default: 4
 
 router_solicitations - INTEGER
         Number of Router Solicitations to send until assuming no
         routers are present.
 
         Default: 3
 
 use_oif_addrs_only - BOOLEAN
         When enabled, the candidate source addresses for destinations
         routed via this interface are restricted to the set of addresses
         configured on this interface (vis. RFC 6724, section 4).
 
         Default: false
 
 use_tempaddr - INTEGER
         Preference for Privacy Extensions (RFC3041).
 
           * <= 0 : disable Privacy Extensions
           * == 1 : enable Privacy Extensions, but prefer public
             addresses over temporary addresses.
           * >  1 : enable Privacy Extensions and prefer temporary
             addresses over public addresses.
 
         Default:
 
                 * 0 (for most devices)
                 * -1 (for point-to-point devices and loopback devices)
 
 temp_valid_lft - INTEGER
         valid lifetime (in seconds) for temporary addresses. If less than the
         minimum required lifetime (typically 5-7 seconds), temporary addresses
         will not be created.
 
         Default: 172800 (2 days)
 
 temp_prefered_lft - INTEGER
         Preferred lifetime (in seconds) for temporary addresses. If
         temp_prefered_lft is less than the minimum required lifetime (typically
         5-7 seconds), the preferred lifetime is the minimum required. If
         temp_prefered_lft is greater than temp_valid_lft, the preferred lifetime
         is temp_valid_lft.
 
         Default: 86400 (1 day)
 
 keep_addr_on_down - INTEGER
         Keep all IPv6 addresses on an interface down event. If set static
         global addresses with no expiration time are not flushed.
 
         *   >0 : enabled
         *    0 : system default
         *   <0 : disabled
 
         Default: 0 (addresses are removed)
 
 max_desync_factor - INTEGER
         Maximum value for DESYNC_FACTOR, which is a random value
         that ensures that clients don't synchronize with each
         other and generate new addresses at exactly the same time.
         value is in seconds.
 
         Default: 600
 
 regen_min_advance - INTEGER
         How far in advance (in seconds), at minimum, to create a new temporary
         address before the current one is deprecated. This value is added to
         the amount of time that may be required for duplicate address detection
         to determine when to create a new address. Linux permits setting this
         value to less than the default of 2 seconds, but a value less than 2
         does not conform to RFC 8981.
 
         Default: 2
 
 regen_max_retry - INTEGER
         Number of attempts before give up attempting to generate
         valid temporary addresses.
 
         Default: 5
 
 max_addresses - INTEGER
         Maximum number of autoconfigured addresses per interface.  Setting
         to zero disables the limitation.  It is not recommended to set this
         value too large (or to zero) because it would be an easy way to
         crash the kernel by allowing too many addresses to be created.
 
         Default: 16
 
 disable_ipv6 - BOOLEAN
         Disable IPv6 operation.  If accept_dad is set to 2, this value
         will be dynamically set to TRUE if DAD fails for the link-local
         address.
 
         Default: FALSE (enable IPv6 operation)
 
         When this value is changed from 1 to 0 (IPv6 is being enabled),
         it will dynamically create a link-local address on the given
         interface and start Duplicate Address Detection, if necessary.
 
         When this value is changed from 0 to 1 (IPv6 is being disabled),
         it will dynamically delete all addresses and routes on the given
         interface. From now on it will not possible to add addresses/routes
         to the selected interface.
 
 accept_dad - INTEGER
         Whether to accept DAD (Duplicate Address Detection).
 
          == ==============================================================
           0  Disable DAD
           1  Enable DAD (default)
           2  Enable DAD, and disable IPv6 operation if MAC-based duplicate
              link-local address has been found.
          == ==============================================================
 
         DAD operation and mode on a given interface will be selected according
         to the maximum value of conf/{all,interface}/accept_dad.
 
 force_tllao - BOOLEAN
         Enable sending the target link-layer address option even when
         responding to a unicast neighbor solicitation.
 
         Default: FALSE
 
         Quoting from RFC 2461, section 4.4, Target link-layer address:
 
         "The option MUST be included for multicast solicitations in order to
         avoid infinite Neighbor Solicitation "recursion" when the peer node
         does not have a cache entry to return a Neighbor Advertisements
         message.  When responding to unicast solicitations, the option can be
         omitted since the sender of the solicitation has the correct link-
         layer address; otherwise it would not have be able to send the unicast
         solicitation in the first place. However, including the link-layer
         address in this case adds little overhead and eliminates a potential
         race condition where the sender deletes the cached link-layer address
         prior to receiving a response to a previous solicitation."
 
 ndisc_notify - BOOLEAN
         Define mode for notification of address and device changes.
 
         * 0 - (default): do nothing
         * 1 - Generate unsolicited neighbour advertisements when device is brought
           up or hardware address changes.
 
 ndisc_tclass - INTEGER
         The IPv6 Traffic Class to use by default when sending IPv6 Neighbor
         Discovery (Router Solicitation, Router Advertisement, Neighbor
         Solicitation, Neighbor Advertisement, Redirect) messages.
         These 8 bits can be interpreted as 6 high order bits holding the DSCP
         value and 2 low order bits representing ECN (which you probably want
         to leave cleared).
 
         * 0 - (default)
 
 ndisc_evict_nocarrier - BOOLEAN
         Clears the neighbor discovery table on NOCARRIER events. This option is
         important for wireless devices where the neighbor discovery cache should
         not be cleared when roaming between access points on the same network.
         In most cases this should remain as the default (1).
 
         - 1 - (default): Clear neighbor discover cache on NOCARRIER events.
         - 0 - Do not clear neighbor discovery cache on NOCARRIER events.
 
 mldv1_unsolicited_report_interval - INTEGER
         The interval in milliseconds in which the next unsolicited
         MLDv1 report retransmit will take place.
 
         Default: 10000 (10 seconds)
 
 mldv2_unsolicited_report_interval - INTEGER
         The interval in milliseconds in which the next unsolicited
         MLDv2 report retransmit will take place.
 
         Default: 1000 (1 second)
 
 force_mld_version - INTEGER
         * 0 - (default) No enforcement of a MLD version, MLDv1 fallback allowed
         * 1 - Enforce to use MLD version 1
         * 2 - Enforce to use MLD version 2
 
 suppress_frag_ndisc - INTEGER
         Control RFC 6980 (Security Implications of IPv6 Fragmentation
         with IPv6 Neighbor Discovery) behavior:
 
         * 1 - (default) discard fragmented neighbor discovery packets
         * 0 - allow fragmented neighbor discovery packets
 
 optimistic_dad - BOOLEAN
         Whether to perform Optimistic Duplicate Address Detection (RFC 4429).
 
         * 0: disabled (default)
         * 1: enabled
 
         Optimistic Duplicate Address Detection for the interface will be enabled
         if at least one of conf/{all,interface}/optimistic_dad is set to 1,
         it will be disabled otherwise.
 
 use_optimistic - BOOLEAN
         If enabled, do not classify optimistic addresses as deprecated during
         source address selection.  Preferred addresses will still be chosen
         before optimistic addresses, subject to other ranking in the source
         address selection algorithm.
 
         * 0: disabled (default)
         * 1: enabled
 
         This will be enabled if at least one of
         conf/{all,interface}/use_optimistic is set to 1, disabled otherwise.
 
 stable_secret - IPv6 address
         This IPv6 address will be used as a secret to generate IPv6
         addresses for link-local addresses and autoconfigured
         ones. All addresses generated after setting this secret will
         be stable privacy ones by default. This can be changed via the
         addrgenmode ip-link. conf/default/stable_secret is used as the
         secret for the namespace, the interface specific ones can
         overwrite that. Writes to conf/all/stable_secret are refused.
 
         It is recommended to generate this secret during installation
         of a system and keep it stable after that.
 
         By default the stable secret is unset.
 
 addr_gen_mode - INTEGER
         Defines how link-local and autoconf addresses are generated.
 
         =  =================================================================
         0  generate address based on EUI64 (default)
         1  do no generate a link-local address, use EUI64 for addresses
            generated from autoconf
         2  generate stable privacy addresses, using the secret from
            stable_secret (RFC7217)
         3  generate stable privacy addresses, using a random secret if unset
         =  =================================================================
 
 drop_unicast_in_l2_multicast - BOOLEAN
         Drop any unicast IPv6 packets that are received in link-layer
         multicast (or broadcast) frames.
 
         By default this is turned off.
 
 drop_unsolicited_na - BOOLEAN
         Drop all unsolicited neighbor advertisements, for example if there's
         a known good NA proxy on the network and such frames need not be used
         (or in the case of 802.11, must not be used to prevent attacks.)
 
         By default this is turned off.
 
 accept_untracked_na - INTEGER
         Define behavior for accepting neighbor advertisements from devices that
         are absent in the neighbor cache:
 
         - 0 - (default) Do not accept unsolicited and untracked neighbor
           advertisements.
 
         - 1 - Add a new neighbor cache entry in STALE state for routers on
           receiving a neighbor advertisement (either solicited or unsolicited)
           with target link-layer address option specified if no neighbor entry
           is already present for the advertised IPv6 address. Without this knob,
           NAs received for untracked addresses (absent in neighbor cache) are
           silently ignored.
 
           This is as per router-side behavior documented in RFC9131.
 
           This has lower precedence than drop_unsolicited_na.
 
           This will optimize the return path for the initial off-link
           communication that is initiated by a directly connected host, by
           ensuring that the first-hop router which turns on this setting doesn't
           have to buffer the initial return packets to do neighbor-solicitation.
           The prerequisite is that the host is configured to send unsolicited
           neighbor advertisements on interface bringup. This setting should be
           used in conjunction with the ndisc_notify setting on the host to
           satisfy this prerequisite.
 
         - 2 - Extend option (1) to add a new neighbor cache entry only if the
           source IP address is in the same subnet as an address configured on
           the interface that received the neighbor advertisement.
 
 enhanced_dad - BOOLEAN
         Include a nonce option in the IPv6 neighbor solicitation messages used for
         duplicate address detection per RFC7527. A received DAD NS will only signal
         a duplicate address if the nonce is different. This avoids any false
         detection of duplicates due to loopback of the NS messages that we send.
         The nonce option will be sent on an interface unless both of
         conf/{all,interface}/enhanced_dad are set to FALSE.
 
         Default: TRUE
 
 ``icmp/*``:
 ===========
 
 ratelimit - INTEGER
         Limit the maximal rates for sending ICMPv6 messages.
 
         0 to disable any limiting,
         otherwise the minimal space between responses in milliseconds.
 
         Default: 1000
 
 ratemask - list of comma separated ranges
         For ICMPv6 message types matching the ranges in the ratemask, limit
         the sending of the message according to ratelimit parameter.
 
         The format used for both input and output is a comma separated
         list of ranges (e.g. "0-127,129" for ICMPv6 message type 0 to 127 and
         129). Writing to the file will clear all previous ranges of ICMPv6
         message types and update the current list with the input.
 
         Refer to: https://www.iana.org/assignments/icmpv6-parameters/icmpv6-parameters.xhtml
         for numerical values of ICMPv6 message types, e.g. echo request is 128
         and echo reply is 129.
 
         Default: 0-1,3-127 (rate limit ICMPv6 errors except Packet Too Big)
 
 echo_ignore_all - BOOLEAN
         If set non-zero, then the kernel will ignore all ICMP ECHO
         requests sent to it over the IPv6 protocol.
 
         Default: 0
 
 echo_ignore_multicast - BOOLEAN
         If set non-zero, then the kernel will ignore all ICMP ECHO
         requests sent to it over the IPv6 protocol via multicast.
 
         Default: 0
 
 echo_ignore_anycast - BOOLEAN
         If set non-zero, then the kernel will ignore all ICMP ECHO
         requests sent to it over the IPv6 protocol destined to anycast address.
 
         Default: 0
 
 error_anycast_as_unicast - BOOLEAN
         If set to 1, then the kernel will respond with ICMP Errors
         resulting from requests sent to it over the IPv6 protocol destined
         to anycast address essentially treating anycast as unicast.
 
         Default: 0
 
 xfrm6_gc_thresh - INTEGER
         (Obsolete since linux-4.14)
         The threshold at which we will start garbage collecting for IPv6
         destination cache entries.  At twice this value the system will
         refuse new allocations.
 
 
 IPv6 Update by:
 Pekka Savola <pekkas@netcore.fi>
 YOSHIFUJI Hideaki / USAGI Project <yoshfuji@linux-ipv6.org>
 
 
 /proc/sys/net/bridge/* Variables:
 =================================
 
 bridge-nf-call-arptables - BOOLEAN
         - 1 : pass bridged ARP traffic to arptables' FORWARD chain.
         - 0 : disable this.
 
         Default: 1
 
 bridge-nf-call-iptables - BOOLEAN
         - 1 : pass bridged IPv4 traffic to iptables' chains.
         - 0 : disable this.
 
         Default: 1
 
 bridge-nf-call-ip6tables - BOOLEAN
         - 1 : pass bridged IPv6 traffic to ip6tables' chains.
         - 0 : disable this.
 
         Default: 1
 
 bridge-nf-filter-vlan-tagged - BOOLEAN
         - 1 : pass bridged vlan-tagged ARP/IP/IPv6 traffic to {arp,ip,ip6}tables.
         - 0 : disable this.
 
         Default: 0
 
 bridge-nf-filter-pppoe-tagged - BOOLEAN
         - 1 : pass bridged pppoe-tagged IP/IPv6 traffic to {ip,ip6}tables.
         - 0 : disable this.
 
         Default: 0
 
 bridge-nf-pass-vlan-input-dev - BOOLEAN
         - 1: if bridge-nf-filter-vlan-tagged is enabled, try to find a vlan
           interface on the bridge and set the netfilter input device to the
           vlan. This allows use of e.g. "iptables -i br0.1" and makes the
           REDIRECT target work with vlan-on-top-of-bridge interfaces.  When no
           matching vlan interface is found, or this switch is off, the input
           device is set to the bridge interface.
 
         - 0: disable bridge netfilter vlan interface lookup.
 
         Default: 0
 
 ``proc/sys/net/sctp/*`` Variables:
 ==================================
 
 addip_enable - BOOLEAN
         Enable or disable extension of  Dynamic Address Reconfiguration
         (ADD-IP) functionality specified in RFC5061.  This extension provides
         the ability to dynamically add and remove new addresses for the SCTP
         associations.
 
         1: Enable extension.
 
         0: Disable extension.
 
         Default: 0
 
 pf_enable - INTEGER
         Enable or disable pf (pf is short for potentially failed) state. A value
         of pf_retrans > path_max_retrans also disables pf state. That is, one of
         both pf_enable and pf_retrans > path_max_retrans can disable pf state.
         Since pf_retrans and path_max_retrans can be changed by userspace
         application, sometimes user expects to disable pf state by the value of
         pf_retrans > path_max_retrans, but occasionally the value of pf_retrans
         or path_max_retrans is changed by the user application, this pf state is
         enabled. As such, it is necessary to add this to dynamically enable
         and disable pf state. See:
         https://datatracker.ietf.org/doc/draft-ietf-tsvwg-sctp-failover for
         details.
 
         1: Enable pf.
 
         0: Disable pf.
 
         Default: 1
 
 pf_expose - INTEGER
         Unset or enable/disable pf (pf is short for potentially failed) state
         exposure.  Applications can control the exposure of the PF path state
         in the SCTP_PEER_ADDR_CHANGE event and the SCTP_GET_PEER_ADDR_INFO
         sockopt.   When it's unset, no SCTP_PEER_ADDR_CHANGE event with
         SCTP_ADDR_PF state will be sent and a SCTP_PF-state transport info
         can be got via SCTP_GET_PEER_ADDR_INFO sockopt;  When it's enabled,
         a SCTP_PEER_ADDR_CHANGE event will be sent for a transport becoming
         SCTP_PF state and a SCTP_PF-state transport info can be got via
         SCTP_GET_PEER_ADDR_INFO sockopt;  When it's disabled, no
         SCTP_PEER_ADDR_CHANGE event will be sent and it returns -EACCES when
         trying to get a SCTP_PF-state transport info via SCTP_GET_PEER_ADDR_INFO
         sockopt.
 
         0: Unset pf state exposure, Compatible with old applications.
 
         1: Disable pf state exposure.
 
         2: Enable pf state exposure.
 
         Default: 0
 
 addip_noauth_enable - BOOLEAN
         Dynamic Address Reconfiguration (ADD-IP) requires the use of
         authentication to protect the operations of adding or removing new
         addresses.  This requirement is mandated so that unauthorized hosts
         would not be able to hijack associations.  However, older
         implementations may not have implemented this requirement while
         allowing the ADD-IP extension.  For reasons of interoperability,
         we provide this variable to control the enforcement of the
         authentication requirement.
 
         == ===============================================================
         1  Allow ADD-IP extension to be used without authentication.  This
            should only be set in a closed environment for interoperability
            with older implementations.
 
         0  Enforce the authentication requirement
         == ===============================================================
 
         Default: 0
 
 auth_enable - BOOLEAN
         Enable or disable Authenticated Chunks extension.  This extension
         provides the ability to send and receive authenticated chunks and is
         required for secure operation of Dynamic Address Reconfiguration
         (ADD-IP) extension.
 
         - 1: Enable this extension.
         - 0: Disable this extension.
 
         Default: 0
 
 prsctp_enable - BOOLEAN
         Enable or disable the Partial Reliability extension (RFC3758) which
         is used to notify peers that a given DATA should no longer be expected.
 
         - 1: Enable extension
         - 0: Disable
 
         Default: 1
 
 max_burst - INTEGER
         The limit of the number of new packets that can be initially sent.  It
         controls how bursty the generated traffic can be.
 
         Default: 4
 
 association_max_retrans - INTEGER
         Set the maximum number for retransmissions that an association can
         attempt deciding that the remote end is unreachable.  If this value
         is exceeded, the association is terminated.
 
         Default: 10
 
 max_init_retransmits - INTEGER
         The maximum number of retransmissions of INIT and COOKIE-ECHO chunks
         that an association will attempt before declaring the destination
         unreachable and terminating.
 
         Default: 8
 
 path_max_retrans - INTEGER
         The maximum number of retransmissions that will be attempted on a given
         path.  Once this threshold is exceeded, the path is considered
         unreachable, and new traffic will use a different path when the
         association is multihomed.
 
         Default: 5
 
 pf_retrans - INTEGER
         The number of retransmissions that will be attempted on a given path
         before traffic is redirected to an alternate transport (should one
         exist).  Note this is distinct from path_max_retrans, as a path that
         passes the pf_retrans threshold can still be used.  Its only
         deprioritized when a transmission path is selected by the stack.  This
         setting is primarily used to enable fast failover mechanisms without
         having to reduce path_max_retrans to a very low value.  See:
         http://www.ietf.org/id/draft-nishida-tsvwg-sctp-failover-05.txt
         for details.  Note also that a value of pf_retrans > path_max_retrans
         disables this feature. Since both pf_retrans and path_max_retrans can
         be changed by userspace application, a variable pf_enable is used to
         disable pf state.
 
         Default: 0
 
 ps_retrans - INTEGER
         Primary.Switchover.Max.Retrans (PSMR), it's a tunable parameter coming
         from section-5 "Primary Path Switchover" in rfc7829.  The primary path
         will be changed to another active path when the path error counter on
         the old primary path exceeds PSMR, so that "the SCTP sender is allowed
         to continue data transmission on a new working path even when the old
         primary destination address becomes active again".   Note this feature
         is disabled by initializing 'ps_retrans' per netns as 0xffff by default,
         and its value can't be less than 'pf_retrans' when changing by sysctl.
 
         Default: 0xffff
 
 rto_initial - INTEGER
         The initial round trip timeout value in milliseconds that will be used
         in calculating round trip times.  This is the initial time interval
         for retransmissions.
 
         Default: 3000
 
 rto_max - INTEGER
         The maximum value (in milliseconds) of the round trip timeout.  This
         is the largest time interval that can elapse between retransmissions.
 
         Default: 60000
 
 rto_min - INTEGER
         The minimum value (in milliseconds) of the round trip timeout.  This
         is the smallest time interval the can elapse between retransmissions.
 
         Default: 1000
 
 hb_interval - INTEGER
         The interval (in milliseconds) between HEARTBEAT chunks.  These chunks
         are sent at the specified interval on idle paths to probe the state of
         a given path between 2 associations.
 
         Default: 30000
 
 sack_timeout - INTEGER
         The amount of time (in milliseconds) that the implementation will wait
         to send a SACK.
 
         Default: 200
 
 valid_cookie_life - INTEGER
         The default lifetime of the SCTP cookie (in milliseconds).  The cookie
         is used during association establishment.
 
         Default: 60000
 
 cookie_preserve_enable - BOOLEAN
         Enable or disable the ability to extend the lifetime of the SCTP cookie
         that is used during the establishment phase of SCTP association
 
         - 1: Enable cookie lifetime extension.
         - 0: Disable
 
         Default: 1
 
 cookie_hmac_alg - STRING
         Select the hmac algorithm used when generating the cookie value sent by
         a listening sctp socket to a connecting client in the INIT-ACK chunk.
         Valid values are:
 
         * md5
         * sha1
         * none
 
         Ability to assign md5 or sha1 as the selected alg is predicated on the
         configuration of those algorithms at build time (CONFIG_CRYPTO_MD5 and
         CONFIG_CRYPTO_SHA1).
 
         Default: Dependent on configuration.  MD5 if available, else SHA1 if
         available, else none.
 
 rcvbuf_policy - INTEGER
         Determines if the receive buffer is attributed to the socket or to
         association.   SCTP supports the capability to create multiple
         associations on a single socket.  When using this capability, it is
         possible that a single stalled association that's buffering a lot
         of data may block other associations from delivering their data by
         consuming all of the receive buffer space.  To work around this,
         the rcvbuf_policy could be set to attribute the receiver buffer space
         to each association instead of the socket.  This prevents the described
         blocking.
 
         - 1: rcvbuf space is per association
         - 0: rcvbuf space is per socket
 
         Default: 0
 
 sndbuf_policy - INTEGER
         Similar to rcvbuf_policy above, this applies to send buffer space.
 
         - 1: Send buffer is tracked per association
         - 0: Send buffer is tracked per socket.
 
         Default: 0
 
 sctp_mem - vector of 3 INTEGERs: min, pressure, max
         Number of pages allowed for queueing by all SCTP sockets.
 
         min: Below this number of pages SCTP is not bothered about its
         memory appetite. When amount of memory allocated by SCTP exceeds
         this number, SCTP starts to moderate memory usage.
 
         pressure: This value was introduced to follow format of tcp_mem.
 
         max: Number of pages allowed for queueing by all SCTP sockets.
 
         Default is calculated at boot time from amount of available memory.
 
 sctp_rmem - vector of 3 INTEGERs: min, default, max
         Only the first value ("min") is used, "default" and "max" are
         ignored.
 
         min: Minimal size of receive buffer used by SCTP socket.
         It is guaranteed to each SCTP socket (but not association) even
         under moderate memory pressure.
 
         Default: 4K
 
 sctp_wmem  - vector of 3 INTEGERs: min, default, max
         Only the first value ("min") is used, "default" and "max" are
         ignored.
 
         min: Minimum size of send buffer that can be used by SCTP sockets.
         It is guaranteed to each SCTP socket (but not association) even
         under moderate memory pressure.
 
         Default: 4K
 
 addr_scope_policy - INTEGER
         Control IPv4 address scoping - draft-stewart-tsvwg-sctp-ipv4-00
 
         - 0   - Disable IPv4 address scoping
         - 1   - Enable IPv4 address scoping
         - 2   - Follow draft but allow IPv4 private addresses
         - 3   - Follow draft but allow IPv4 link local addresses
 
         Default: 1
 
 udp_port - INTEGER
         The listening port for the local UDP tunneling sock. Normally it's
         using the IANA-assigned UDP port number 9899 (sctp-tunneling).
 
         This UDP sock is used for processing the incoming UDP-encapsulated
         SCTP packets (from RFC6951), and shared by all applications in the
         same net namespace. This UDP sock will be closed when the value is
         set to 0.
 
         The value will also be used to set the src port of the UDP header
         for the outgoing UDP-encapsulated SCTP packets. For the dest port,
         please refer to 'encap_port' below.
 
         Default: 0
 
 encap_port - INTEGER
         The default remote UDP encapsulation port.
 
         This value is used to set the dest port of the UDP header for the
         outgoing UDP-encapsulated SCTP packets by default. Users can also
         change the value for each sock/asoc/transport by using setsockopt.
         For further information, please refer to RFC6951.
 
         Note that when connecting to a remote server, the client should set
         this to the port that the UDP tunneling sock on the peer server is
         listening to and the local UDP tunneling sock on the client also
         must be started. On the server, it would get the encap_port from
         the incoming packet's source port.
 
         Default: 0
 
 plpmtud_probe_interval - INTEGER
         The time interval (in milliseconds) for the PLPMTUD probe timer,
         which is configured to expire after this period to receive an
         acknowledgment to a probe packet. This is also the time interval
         between the probes for the current pmtu when the probe search
         is done.
 
         PLPMTUD will be disabled when 0 is set, and other values for it
         must be >= 5000.
 
         Default: 0
 
 reconf_enable - BOOLEAN
         Enable or disable extension of Stream Reconfiguration functionality
         specified in RFC6525. This extension provides the ability to "reset"
         a stream, and it includes the Parameters of "Outgoing/Incoming SSN
         Reset", "SSN/TSN Reset" and "Add Outgoing/Incoming Streams".
 
         - 1: Enable extension.
         - 0: Disable extension.
 
         Default: 0
 
 intl_enable - BOOLEAN
         Enable or disable extension of User Message Interleaving functionality
         specified in RFC8260. This extension allows the interleaving of user
         messages sent on different streams. With this feature enabled, I-DATA
         chunk will replace DATA chunk to carry user messages if also supported
         by the peer. Note that to use this feature, one needs to set this option
         to 1 and also needs to set socket options SCTP_FRAGMENT_INTERLEAVE to 2
         and SCTP_INTERLEAVING_SUPPORTED to 1.
 
         - 1: Enable extension.
         - 0: Disable extension.
 
         Default: 0
 
 ecn_enable - BOOLEAN
         Control use of Explicit Congestion Notification (ECN) by SCTP.
         Like in TCP, ECN is used only when both ends of the SCTP connection
         indicate support for it. This feature is useful in avoiding losses
         due to congestion by allowing supporting routers to signal congestion
         before having to drop packets.
 
         1: Enable ecn.
         0: Disable ecn.
 
         Default: 1
 
 l3mdev_accept - BOOLEAN
         Enabling this option allows a "global" bound socket to work
         across L3 master domains (e.g., VRFs) with packets capable of
         being received regardless of the L3 domain in which they
         originated. Only valid when the kernel was compiled with
         CONFIG_NET_L3_MASTER_DEV.
 
         Default: 1 (enabled)
 
 
 ``/proc/sys/net/core/*``
 ========================
 
         Please see: Documentation/admin-guide/sysctl/net.rst for descriptions of these entries.
 
 
 ``/proc/sys/net/unix/*``
 ========================
 
 max_dgram_qlen - INTEGER
         The maximum length of dgram socket receive queue
 
         Default: 10
 

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