1 .. SPDX-License-Identifier: GPL-2.0 2 3 =================== 4 DNS Resolver Module 5 =================== 6 7 .. Contents: 8 9 - Overview. 10 - Compilation. 11 - Setting up. 12 - Usage. 13 - Mechanism. 14 - Debugging. 15 16 17 Overview 18 ======== 19 20 The DNS resolver module provides a way for kernel services to make DNS queries 21 by way of requesting a key of key type dns_resolver. These queries are 22 upcalled to userspace through /sbin/request-key. 23 24 These routines must be supported by userspace tools dns.upcall, cifs.upcall and 25 request-key. It is under development and does not yet provide the full feature 26 set. The features it does support include: 27 28 (*) Implements the dns_resolver key_type to contact userspace. 29 30 It does not yet support the following AFS features: 31 32 (*) Dns query support for AFSDB resource record. 33 34 This code is extracted from the CIFS filesystem. 35 36 37 Compilation 38 =========== 39 40 The module should be enabled by turning on the kernel configuration options:: 41 42 CONFIG_DNS_RESOLVER - tristate "DNS Resolver support" 43 44 45 Setting up 46 ========== 47 48 To set up this facility, the /etc/request-key.conf file must be altered so that 49 /sbin/request-key can appropriately direct the upcalls. For example, to handle 50 basic dname to IPv4/IPv6 address resolution, the following line should be 51 added:: 52 53 54 #OP TYPE DESC CO-INFO PROGRAM ARG1 ARG2 ARG3 ... 55 #====== ============ ======= ======= ========================== 56 create dns_resolver * * /usr/sbin/cifs.upcall %k 57 58 To direct a query for query type 'foo', a line of the following should be added 59 before the more general line given above as the first match is the one taken:: 60 61 create dns_resolver foo:* * /usr/sbin/dns.foo %k 62 63 64 Usage 65 ===== 66 67 To make use of this facility, one of the following functions that are 68 implemented in the module can be called after doing:: 69 70 #include <linux/dns_resolver.h> 71 72 :: 73 74 int dns_query(const char *type, const char *name, size_t namelen, 75 const char *options, char **_result, time_t *_expiry); 76 77 This is the basic access function. It looks for a cached DNS query and if 78 it doesn't find it, it upcalls to userspace to make a new DNS query, which 79 may then be cached. The key description is constructed as a string of the 80 form:: 81 82 [<type>:]<name> 83 84 where <type> optionally specifies the particular upcall program to invoke, 85 and thus the type of query to do, and <name> specifies the string to be 86 looked up. The default query type is a straight hostname to IP address 87 set lookup. 88 89 The name parameter is not required to be a NUL-terminated string, and its 90 length should be given by the namelen argument. 91 92 The options parameter may be NULL or it may be a set of options 93 appropriate to the query type. 94 95 The return value is a string appropriate to the query type. For instance, 96 for the default query type it is just a list of comma-separated IPv4 and 97 IPv6 addresses. The caller must free the result. 98 99 The length of the result string is returned on success, and a negative 100 error code is returned otherwise. -EKEYREJECTED will be returned if the 101 DNS lookup failed. 102 103 If _expiry is non-NULL, the expiry time (TTL) of the result will be 104 returned also. 105 106 The kernel maintains an internal keyring in which it caches looked up keys. 107 This can be cleared by any process that has the CAP_SYS_ADMIN capability by 108 the use of KEYCTL_KEYRING_CLEAR on the keyring ID. 109 110 111 Reading DNS Keys from Userspace 112 =============================== 113 114 Keys of dns_resolver type can be read from userspace using keyctl_read() or 115 "keyctl read/print/pipe". 116 117 118 Mechanism 119 ========= 120 121 The dns_resolver module registers a key type called "dns_resolver". Keys of 122 this type are used to transport and cache DNS lookup results from userspace. 123 124 When dns_query() is invoked, it calls request_key() to search the local 125 keyrings for a cached DNS result. If that fails to find one, it upcalls to 126 userspace to get a new result. 127 128 Upcalls to userspace are made through the request_key() upcall vector, and are 129 directed by means of configuration lines in /etc/request-key.conf that tell 130 /sbin/request-key what program to run to instantiate the key. 131 132 The upcall handler program is responsible for querying the DNS, processing the 133 result into a form suitable for passing to the keyctl_instantiate_key() 134 routine. This then passes the data to dns_resolver_instantiate() which strips 135 off and processes any options included in the data, and then attaches the 136 remainder of the string to the key as its payload. 137 138 The upcall handler program should set the expiry time on the key to that of the 139 lowest TTL of all the records it has extracted a result from. This means that 140 the key will be discarded and recreated when the data it holds has expired. 141 142 dns_query() returns a copy of the value attached to the key, or an error if 143 that is indicated instead. 144 145 See <file:Documentation/security/keys/request-key.rst> for further 146 information about request-key function. 147 148 149 Debugging 150 ========= 151 152 Debugging messages can be turned on dynamically by writing a 1 into the 153 following file:: 154 155 /sys/module/dns_resolver/parameters/debug
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