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linux
1 ===================
2 DNS Resolver Module
3 ===================
4
5Contents:
6
7 - Overview.
8 - Compilation.
9 - Setting up.
10 - Usage.
11 - Mechanism.
12 - Debugging.
13
14
15========
16OVERVIEW
17========
18
19The DNS resolver module provides a way for kernel services to make DNS queries
20by way of requesting a key of key type dns_resolver. These queries are
21upcalled to userspace through /sbin/request-key.
22
23These routines must be supported by userspace tools dns.upcall, cifs.upcall and
24request-key. It is under development and does not yet provide the full feature
25set. The features it does support include:
26
27 (*) Implements the dns_resolver key_type to contact userspace.
28
29It does not yet support the following AFS features:
30
31 (*) Dns query support for AFSDB resource record.
32
33This code is extracted from the CIFS filesystem.
34
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
45==========
46SETTING UP
47==========
48
49To set up this facility, the /etc/request-key.conf file must be altered so that
50/sbin/request-key can appropriately direct the upcalls. For example, to handle
51basic dname to IPv4/IPv6 address resolution, the following line should be
52added:
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
64
65=====
66USAGE
67=====
68
69To make use of this facility, one of the following functions that are
70implemented in the module can be called after doing:
71
72 #include <linux/dns_resolver.h>
73
74 (1) 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
107=========
108MECHANISM
109=========
110
111The dnsresolver module registers a key type called "dns_resolver". Keys of
112this type are used to transport and cache DNS lookup results from userspace.
113
114When dns_query() is invoked, it calls request_key() to search the local
115keyrings for a cached DNS result. If that fails to find one, it upcalls to
116userspace to get a new result.
117
118Upcalls to userspace are made through the request_key() upcall vector, and are
119directed by means of configuration lines in /etc/request-key.conf that tell
120/sbin/request-key what program to run to instantiate the key.
121
122The upcall handler program is responsible for querying the DNS, processing the
123result into a form suitable for passing to the keyctl_instantiate_key()
124routine. This then passes the data to dns_resolver_instantiate() which strips
125off and processes any options included in the data, and then attaches the
126remainder of the string to the key as its payload.
127
128The upcall handler program should set the expiry time on the key to that of the
129lowest TTL of all the records it has extracted a result from. This means that
130the key will be discarded and recreated when the data it holds has expired.
131
132dns_query() returns a copy of the value attached to the key, or an error if
133that is indicated instead.
134
135See <file:Documentation/keys-request-key.txt> for further information about
136request-key function.
137
138
139=========
140DEBUGGING
141=========
142
143Debugging messages can be turned on dynamically by writing a 1 into the
144following file:
145
146 /sys/module/dnsresolver/parameters/debug