1.. SPDX-License-Identifier: GPL-2.0 2 3=================== 4DNS Resolver Module 5=================== 6 7.. Contents: 8 9 - Overview. 10 - Compilation. 11 - Setting up. 12 - Usage. 13 - Mechanism. 14 - Debugging. 15 16 17Overview 18======== 19 20The DNS resolver module provides a way for kernel services to make DNS queries 21by way of requesting a key of key type dns_resolver. These queries are 22upcalled to userspace through /sbin/request-key. 23 24These routines must be supported by userspace tools dns.upcall, cifs.upcall and 25request-key. It is under development and does not yet provide the full feature 26set. The features it does support include: 27 28 (*) Implements the dns_resolver key_type to contact userspace. 29 30It does not yet support the following AFS features: 31 32 (*) Dns query support for AFSDB resource record. 33 34This code is extracted from the CIFS filesystem. 35 36 37Compilation 38=========== 39 40The module should be enabled by turning on the kernel configuration options:: 41 42 CONFIG_DNS_RESOLVER - tristate "DNS Resolver support" 43 44 45Setting up 46========== 47 48To 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 50basic dname to IPv4/IPv6 address resolution, the following line should be 51added:: 52 53 54 #OP TYPE DESC CO-INFO PROGRAM ARG1 ARG2 ARG3 ... 55 #====== ============ ======= ======= ========================== 56 create dns_resolver * * /usr/sbin/cifs.upcall %k 57 58To direct a query for query type 'foo', a line of the following should be added 59before 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 64Usage 65===== 66 67To make use of this facility, one of the following functions that are 68implemented 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 106The kernel maintains an internal keyring in which it caches looked up keys. 107This can be cleared by any process that has the CAP_SYS_ADMIN capability by 108the use of KEYCTL_KEYRING_CLEAR on the keyring ID. 109 110 111Reading DNS Keys from Userspace 112=============================== 113 114Keys of dns_resolver type can be read from userspace using keyctl_read() or 115"keyctl read/print/pipe". 116 117 118Mechanism 119========= 120 121The dns_resolver module registers a key type called "dns_resolver". Keys of 122this type are used to transport and cache DNS lookup results from userspace. 123 124When dns_query() is invoked, it calls request_key() to search the local 125keyrings for a cached DNS result. If that fails to find one, it upcalls to 126userspace to get a new result. 127 128Upcalls to userspace are made through the request_key() upcall vector, and are 129directed 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 132The upcall handler program is responsible for querying the DNS, processing the 133result into a form suitable for passing to the keyctl_instantiate_key() 134routine. This then passes the data to dns_resolver_instantiate() which strips 135off and processes any options included in the data, and then attaches the 136remainder of the string to the key as its payload. 137 138The upcall handler program should set the expiry time on the key to that of the 139lowest TTL of all the records it has extracted a result from. This means that 140the key will be discarded and recreated when the data it holds has expired. 141 142dns_query() returns a copy of the value attached to the key, or an error if 143that is indicated instead. 144 145See <file:Documentation/security/keys/request-key.rst> for further 146information about request-key function. 147 148 149Debugging 150========= 151 152Debugging messages can be turned on dynamically by writing a 1 into the 153following file:: 154 155 /sys/module/dns_resolver/parameters/debug 156