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=====
65USAGE
66=====
67
68To make use of this facility, one of the following functions that are
69implemented in the module can be called after doing:
70
71	#include <linux/dns_resolver.h>
72
73 (1) int dns_query(const char *type, const char *name, size_t namelen,
74		   const char *options, char **_result, time_t *_expiry);
75
76     This is the basic access function.  It looks for a cached DNS query and if
77     it doesn't find it, it upcalls to userspace to make a new DNS query, which
78     may then be cached.  The key description is constructed as a string of the
79     form:
80
81		[<type>:]<name>
82
83     where <type> optionally specifies the particular upcall program to invoke,
84     and thus the type of query to do, and <name> specifies the string to be
85     looked up.  The default query type is a straight hostname to IP address
86     set lookup.
87
88     The name parameter is not required to be a NUL-terminated string, and its
89     length should be given by the namelen argument.
90
91     The options parameter may be NULL or it may be a set of options
92     appropriate to the query type.
93
94     The return value is a string appropriate to the query type.  For instance,
95     for the default query type it is just a list of comma-separated IPv4 and
96     IPv6 addresses.  The caller must free the result.
97
98     The length of the result string is returned on success, and a negative
99     error code is returned otherwise.  -EKEYREJECTED will be returned if the
100     DNS lookup failed.
101
102     If _expiry is non-NULL, the expiry time (TTL) of the result will be
103     returned also.
104
105
106===============================
107READING DNS KEYS FROM USERSPACE
108===============================
109
110Keys of dns_resolver type can be read from userspace using keyctl_read() or
111"keyctl read/print/pipe".
112
113
114=========
115MECHANISM
116=========
117
118The dnsresolver module registers a key type called "dns_resolver".  Keys of
119this type are used to transport and cache DNS lookup results from userspace.
120
121When dns_query() is invoked, it calls request_key() to search the local
122keyrings for a cached DNS result.  If that fails to find one, it upcalls to
123userspace to get a new result.
124
125Upcalls to userspace are made through the request_key() upcall vector, and are
126directed by means of configuration lines in /etc/request-key.conf that tell
127/sbin/request-key what program to run to instantiate the key.
128
129The upcall handler program is responsible for querying the DNS, processing the
130result into a form suitable for passing to the keyctl_instantiate_key()
131routine.  This then passes the data to dns_resolver_instantiate() which strips
132off and processes any options included in the data, and then attaches the
133remainder of the string to the key as its payload.
134
135The upcall handler program should set the expiry time on the key to that of the
136lowest TTL of all the records it has extracted a result from.  This means that
137the key will be discarded and recreated when the data it holds has expired.
138
139dns_query() returns a copy of the value attached to the key, or an error if
140that is indicated instead.
141
142See <file:Documentation/security/keys-request-key.txt> for further
143information about request-key function.
144
145
146=========
147DEBUGGING
148=========
149
150Debugging messages can be turned on dynamically by writing a 1 into the
151following file:
152
153        /sys/module/dnsresolver/parameters/debug
154