1RESOLVER(3) Linux Programmer's Manual RESOLVER(3)
2
6 res_ninit, res_nclose, res_nquery, res_nsearch, res_nquerydomain,
7 res_nmkquery, res_nsend, res_init, res_query, res_search, res_querydo‐
8 main, res_mkquery, res_send, dn_comp, dn_expand - resolver routines
9
11 #include <netinet/in.h>
12 #include <arpa/nameser.h>
13 #include <resolv.h>
14 struct __res_state;
16 typedef struct __res_state *res_state;
17 int res_ninit(res_state statep);
19 void res_nclose(res_state statep);
21 int res_nquery(res_state statep,
23 const char *dname, int class, int type,
24 unsigned char *answer, int anslen);
25 int res_nsearch(res_state statep,
27 const char *dname, int class, int type,
28 unsigned char *answer, int anslen);
29 int res_nquerydomain(res_state statep,
31 const char *name, const char *domain,
32 int class, int type, unsigned char *answer,
33 int anslen);
34 int res_nmkquery(res_state statep,
36 int op, const char *dname, int class,
37 int type, const unsigned char *data, int datalen,
38 const unsigned char *newrr,
39 unsigned char *buf, int buflen);
40 int res_nsend(res_state statep,
42 const unsigned char *msg, int msglen,
43 unsigned char *answer, int anslen);
44 int dn_comp(const char *exp_dn, unsigned char *comp_dn,
46 int length, unsigned char **dnptrs,
47 unsigned char **lastdnptr);
48 int dn_expand(const unsigned char *msg,
50 const unsigned char *eomorig,
51 const unsigned char *comp_dn, char *exp_dn,
52 int length);
53 Deprecated
55 extern struct __res_state _res;
56 int res_init(void);
58 int res_query(const char *dname, int class, int type,
60 unsigned char *answer, int anslen);
61 int res_search(const char *dname, int class, int type,
63 unsigned char *answer, int anslen);
64 int res_querydomain(const char *name, const char *domain,
66 int class, int type, unsigned char *answer,
67 int anslen);
68 int res_mkquery(int op, const char *dname, int class,
70 int type, const unsigned char *data, int datalen,
71 const unsigned char *newrr,
72 unsigned char *buf, int buflen);
73 int res_send(const unsigned char *msg, int msglen,
75 unsigned char *answer, int anslen);
76 Link with -lresolv.
78
80 Note: This page is incomplete (various resolver functions provided by
81 glibc are not described) and likely out of date.
82 The functions described below make queries to and interpret the re‐
84 sponses from Internet domain name servers.
85 The API consists of a set of more modern, reentrant functions and an
87 older set of nonreentrant functions that have been superseded. The
88 traditional resolver interfaces such as res_init() and res_query() use
89 some static (global) state stored in the _res structure, rendering
90 these functions non-thread-safe. BIND 8.2 introduced a set of new in‐
91 terfaces res_ninit(), res_nquery(), and so on, which take a res_state
92 as their first argument, so you can use a per-thread resolver state.
93 The res_ninit() and res_init() functions read the configuration files
95 (see resolv.conf(5)) to get the default domain name and name server ad‐
96 dress(es). If no server is given, the local host is tried. If no do‐
97 main is given, that associated with the local host is used. It can be
98 overridden with the environment variable LOCALDOMAIN. res_ninit() or
99 res_init() is normally executed by the first call to one of the other
100 functions. Every call to res_ninit() requires a corresponding call to
101 res_nclose() to free memory allocated by res_ninit() and subsequent
102 calls to res_nquery().
103 The res_nquery() and res_query() functions query the name server for
105 the fully qualified domain name name of specified type and class. The
106 reply is left in the buffer answer of length anslen supplied by the
107 caller.
108 The res_nsearch() and res_search() functions make a query and waits for
110 the response like res_nquery() and res_query(), but in addition they
111 implement the default and search rules controlled by RES_DEFNAMES and
112 RES_DNSRCH (see description of _res options below).
113 The res_nquerydomain() and res_querydomain() functions make a query us‐
115 ing res_nquery()/res_query() on the concatenation of name and domain.
116 The following functions are lower-level routines used by
118 res_nquery()/res_query().
119 The res_nmkquery() and res_mkquery() functions construct a query mes‐
121 sage in buf of length buflen for the domain name dname. The query type
122 op is one of the following (typically QUERY):
123 QUERY Standard query.
125 IQUERY Inverse query. This option was removed in glibc 2.26, since it
127 has not been supported by DNS servers for a very long time.
128 NS_NOTIFY_OP
130 Notify secondary of SOA (Start of Authority) change.
131 newrr is currently unused.
133 The res_nsend() and res_send() function send a preformatted query given
135 in msg of length msglen and returns the answer in answer which is of
136 length anslen. They will call res_ninit()/res_init() if it has not al‐
137 ready been called.
138 The dn_comp() function compresses the domain name exp_dn and stores it
140 in the buffer comp_dn of length length. The compression uses an array
141 of pointers dnptrs to previously compressed names in the current mes‐
142 sage. The first pointer points to the beginning of the message and the
143 list ends with NULL. The limit of the array is specified by lastdnptr.
144 If dnptr is NULL, domain names are not compressed. If lastdnptr is
145 NULL, the list of labels is not updated.
146 The dn_expand() function expands the compressed domain name comp_dn to
148 a full domain name, which is placed in the buffer exp_dn of size
149 length. The compressed name is contained in a query or reply message,
150 and msg points to the beginning of the message.
151 The resolver routines use configuration and state information contained
153 in a __res_state structure (either passed as the statep argument, or in
154 the global variable _res, in the case of the older nonreentrant func‐
155 tions). The only field of this structure that is normally manipulated
156 by the user is the options field. This field can contain the bitwise
157 "OR" of the following options:
158 RES_INIT
160 True if res_ninit() or res_init() has been called.
161 RES_DEBUG
163 Print debugging messages. This option is available only if
164 glibc was built with debugging enabled, which is not the de‐
165 fault.
166 RES_AAONLY (unimplemented; deprecated in glibc 2.25)
168 Accept authoritative answers only. res_send() continues until
169 it finds an authoritative answer or returns an error. This op‐
170 tion was present but unimplemented in glibc until version 2.24;
171 since glibc 2.25, it is deprecated, and its usage produces a
172 warning.
173 RES_USEVC
175 Use TCP connections for queries rather than UDP datagrams.
176 RES_PRIMARY (unimplemented; deprecated in glibc 2.25)
178 Query primary domain name server only. This option was present
179 but unimplemented in glibc until version 2.24; since glibc 2.25,
180 it is deprecated, and its usage produces a warning.
181 RES_IGNTC
183 Ignore truncation errors. Don't retry with TCP.
184 RES_RECURSE
186 Set the recursion desired bit in queries. Recursion is carried
187 out by the domain name server, not by res_send(). [Enabled by
188 default].
189 RES_DEFNAMES
191 If set, res_search() will append the default domain name to sin‐
192 gle component names—that is, those that do not contain a dot.
193 [Enabled by default].
194 RES_STAYOPEN
196 Used with RES_USEVC to keep the TCP connection open between
197 queries.
198 RES_DNSRCH
200 If set, res_search() will search for hostnames in the current
201 domain and in parent domains. This option is used by gethostby‐
202 name(3). [Enabled by default].
203 RES_INSECURE1
205 Accept a response from a wrong server. This can be used to de‐
206 tect potential security hazards, but you need to compile glibc
207 with debugging enabled and use RES_DEBUG option (for debug pur‐
208 pose only).
209 RES_INSECURE2
211 Accept a response which contains a wrong query. This can be
212 used to detect potential security hazards, but you need to com‐
213 pile glibc with debugging enabled and use RES_DEBUG option (for
214 debug purpose only).
215 RES_NOALIASES
217 Disable usage of HOSTALIASES environment variable.
218 RES_USE_INET6
220 Try an AAAA query before an A query inside the gethostbyname(3)
221 function, and map IPv4 responses in IPv6 "tunneled form" if no
222 AAAA records are found but an A record set exists. Since glibc
223 2.25, this option is deprecated, and its usage produces a warn‐
224 ing; applications should use getaddrinfo(3), rather than geth‐
225 ostbyname(3).
226 RES_ROTATE
228 Causes round-robin selection of name servers from among those
229 listed. This has the effect of spreading the query load among
230 all listed servers, rather than having all clients try the first
231 listed server first every time.
232 RES_NOCHECKNAME (unimplemented; deprecated in glibc 2.25)
234 Disable the modern BIND checking of incoming hostnames and mail
235 names for invalid characters such as underscore (_), non-ASCII,
236 or control characters. This option was present in glibc until
237 version 2.24; since glibc 2.25, it is deprecated, and its usage
238 produces a warning.
239 RES_KEEPTSIG (unimplemented; deprecated in glibc 2.25)
241 Do not strip TSIG records. This option was present but unimple‐
242 mented in glibc until version 2.24; since glibc 2.25, it is dep‐
243 recated, and its usage produces a warning.
244 RES_BLAST (unimplemented; deprecated in glibc 2.25)
246 Send each query simultaneously and recursively to all servers.
247 This option was present but unimplemented in glibc until version
248 2.24; since glibc 2.25, it is deprecated, and its usage produces
249 a warning.
250 RES_USEBSTRING (glibc 2.3.4 to 2.24)
252 Make reverse IPv6 lookups using the bit-label format described
253 in RFC 2673; if this option is not set (which is the default),
254 then nibble format is used. This option was removed in glibc
255 2.25, since it relied on a backward-incompatible DNS extension
256 that was never deployed on the Internet.
257 RES_NOIP6DOTINT (glibc 2.24 and earlier)
259 Use ip6.arpa zone in IPv6 reverse lookup instead of ip6.int,
260 which is deprecated since glibc 2.3.4. This option is present
261 in glibc up to and including version 2.24, where it is enabled
262 by default. In glibc 2.25, this option was removed.
263 RES_USE_EDNS0 (since glibc 2.6)
265 Enables support for the DNS extensions (EDNS0) described in RFC
266 2671.
267 RES_SNGLKUP (since glibc 2.10)
269 By default, glibc performs IPv4 and IPv6 lookups in parallel
270 since version 2.9. Some appliance DNS servers cannot handle
271 these queries properly and make the requests time out. This op‐
272 tion disables the behavior and makes glibc perform the IPv6 and
273 IPv4 requests sequentially (at the cost of some slowdown of the
274 resolving process).
275 RES_SNGLKUPREOP
277 When RES_SNGLKUP option is enabled, opens a new socket for the
278 each request.
279 RES_USE_DNSSEC
281 Use DNSSEC with OK bit in OPT record. This option implies
282 RES_USE_EDNS0.
283 RES_NOTLDQUERY
285 Do not look up unqualified name as a top-level domain (TLD).
286 RES_DEFAULT
288 Default option which implies: RES_RECURSE, RES_DEFNAMES,
289 RES_DNSRCH, and RES_NOIP6DOTINT.
290
292 The res_ninit() and res_init() functions return 0 on success, or -1 if
293 an error occurs.
294 The res_nquery(), res_query(), res_nsearch(), res_search(), res_nquery‐
296 domain(), res_querydomain(), res_nmkquery(), res_mkquery(),
297 res_nsend(), and res_send() functions return the length of the re‐
298 sponse, or -1 if an error occurs.
299 The dn_comp() and dn_expand() functions return the length of the com‐
301 pressed name, or -1 if an error occurs.
302 In the case of an error return from res_nquery(), res_query(),
304 res_nsearch(), res_search(), res_nquerydomain(), or res_querydomain(),
305 the global variable h_errno (see gethostbyname(3)) can be consulted to
306 determine the cause of the error.
307
309 /etc/resolv.conf
310 resolver configuration file
311 /etc/host.conf
313 resolver configuration file
314
316 For an explanation of the terms used in this section, see at‐
317 tributes(7).
318 ┌─────────────────────────────────────┬───────────────┬────────────────┐
320 │Interface │ Attribute │ Value │
321 ├─────────────────────────────────────┼───────────────┼────────────────┤
322 │res_ninit(), res_nclose(), │ Thread safety │ MT-Safe locale │
323 │res_nquery(), res_nsearch(), │ │ │
324 │res_nquerydomain(), res_nsend() │ │ │
325 ├─────────────────────────────────────┼───────────────┼────────────────┤
326 │res_nmkquery(), dn_comp(), │ Thread safety │ MT-Safe │
327 │dn_expand() │ │ │
328 └─────────────────────────────────────┴───────────────┴────────────────┘
329
331 4.3BSD.
332
334 gethostbyname(3), resolv.conf(5), resolver(5), hostname(7), named(8)
335 The GNU C library source file resolv/README.
337
339 This page is part of release 5.12 of the Linux man-pages project. A
340 description of the project, information about reporting bugs, and the
341 latest version of this page, can be found at
342 https://www.kernel.org/doc/man-pages/.
343GNU 2021-03-22 RESOLVER(3)