1 .. SPDX-License-Identifier: GPL-2.0
2
3 =========================
4 Transparent proxy support
5 =========================
6
7 This feature adds Linux 2.2-like transparent proxy support to current kernels.
8 To use it, enable the socket match and the TPROXY target in your kernel config.
9 You will need policy routing too, so be sure to enable that as well.
10
11 From Linux 4.18 transparent proxy support is also available in nf_tables.
12
13 1. Making non-local sockets work
14 ================================
15
16 The idea is that you identify packets with destination address matching a local
17 socket on your box, set the packet mark to a certain value::
18
19 # iptables -t mangle -N DIVERT
20 # iptables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT
21 # iptables -t mangle -A DIVERT -j MARK --set-mark 1
22 # iptables -t mangle -A DIVERT -j ACCEPT
23
24 Alternatively you can do this in nft with the following commands::
25
26 # nft add table filter
27 # nft add chain filter divert "{ type filter hook prerouting priority -150; }"
28 # nft add rule filter divert meta l4proto tcp socket transparent 1 meta mark set 1 accept
29
30 And then match on that value using policy routing to have those packets
31 delivered locally::
32
33 # ip rule add fwmark 1 lookup 100
34 # ip route add local 0.0.0.0/0 dev lo table 100
35
36 Because of certain restrictions in the IPv4 routing output code you'll have to
37 modify your application to allow it to send datagrams _from_ non-local IP
38 addresses. All you have to do is enable the (SOL_IP, IP_TRANSPARENT) socket
39 option before calling bind::
40
41 fd = socket(AF_INET, SOCK_STREAM, 0);
42 /* - 8< -*/
43 int value = 1;
44 setsockopt(fd, SOL_IP, IP_TRANSPARENT, &value, sizeof(value));
45 /* - 8< -*/
46 name.sin_family = AF_INET;
47 name.sin_port = htons(0xCAFE);
48 name.sin_addr.s_addr = htonl(0xDEADBEEF);
49 bind(fd, &name, sizeof(name));
50
51 A trivial patch for netcat is available here:
52 http://people.netfilter.org/hidden/tproxy/netcat-ip_transparent-support.patch
53
54
55 2. Redirecting traffic
56 ======================
57
58 Transparent proxying often involves "intercepting" traffic on a router. This is
59 usually done with the iptables REDIRECT target; however, there are serious
60 limitations of that method. One of the major issues is that it actually
61 modifies the packets to change the destination address -- which might not be
62 acceptable in certain situations. (Think of proxying UDP for example: you won't
63 be able to find out the original destination address. Even in case of TCP
64 getting the original destination address is racy.)
65
66 The 'TPROXY' target provides similar functionality without relying on NAT. Simply
67 add rules like this to the iptables ruleset above::
68
69 # iptables -t mangle -A PREROUTING -p tcp --dport 80 -j TPROXY \
70 --tproxy-mark 0x1/0x1 --on-port 50080
71
72 Or the following rule to nft:
73
74 # nft add rule filter divert tcp dport 80 tproxy to :50080 meta mark set 1 accept
75
76 Note that for this to work you'll have to modify the proxy to enable (SOL_IP,
77 IP_TRANSPARENT) for the listening socket.
78
79 As an example implementation, tcprdr is available here:
80 https://git.breakpoint.cc/cgit/fw/tcprdr.git/
81 This tool is written by Florian Westphal and it was used for testing during the
82 nf_tables implementation.
83
84 3. Iptables and nf_tables extensions
85 ====================================
86
87 To use tproxy you'll need to have the following modules compiled for iptables:
88
89 - NETFILTER_XT_MATCH_SOCKET
90 - NETFILTER_XT_TARGET_TPROXY
91
92 Or the floowing modules for nf_tables:
93
94 - NFT_SOCKET
95 - NFT_TPROXY
96
97 4. Application support
98 ======================
99
100 4.1. Squid
101 ----------
102
103 Squid 3.HEAD has support built-in. To use it, pass
104 '--enable-linux-netfilter' to configure and set the 'tproxy' option on
105 the HTTP listener you redirect traffic to with the TPROXY iptables
106 target.
107
108 For more information please consult the following page on the Squid
109 wiki: http://wiki.squid-cache.org/Features/Tproxy4
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