1RESOLVER(3) Linux Programmer's Manual RESOLVER(3)
2
6 res_init, res_query, res_search, res_querydomain, res_mkquery,
7 res_send, dn_comp, dn_expand - resolver routines
8
10 #include <netinet/in.h>
11 #include <arpa/nameser.h>
12 #include <resolv.h>
13 extern struct state _res;
14 int res_init(void);
16 int res_query(const char *dname, int class, int type,
18 unsigned char *answer, int anslen);
19 int res_search(const char *dname, int class, int type,
21 unsigned char *answer, int anslen);
22 int res_querydomain(const char *name, const char *domain,
24 int class, int type, unsigned char *answer,
25 int anslen);
26 int res_mkquery(int op, const char *dname, int class,
28 int type, char *data, int datalen, struct rrec *newrr,
29 char *buf, int buflen);
30 int res_send(const char *msg, int msglen, char *answer,
32 int anslen);
33 int dn_comp(unsigned char *exp_dn, unsigned char *comp_dn,
35 int length, unsigned char **dnptrs, unsigned char **lastdnptr);
36 int dn_expand(unsigned char *msg, unsigned char *eomorig,
38 unsigned char *comp_dn, char *exp_dn,
39 int length);
40 Link with -lresolv.
42
44 These functions make queries to and interpret the responses from Inter‐
45 net domain name servers.
46 The res_init() function reads the configuration files (see
48 resolv.conf(5)) to get the default domain name, search order and name
49 server address(es). If no server is given, the local host is tried.
50 If no domain is given, that associated with the local host is used. It
51 can be overridden with the environment variable LOCALDOMAIN.
52 res_init() is normally executed by the first call to one of the other
53 functions.
54 The res_query() function queries the name server for the fully quali‐
56 fied domain name name of specified type and class. The reply is left
57 in the buffer answer of length anslen supplied by the caller.
58 The res_search() function makes a query and waits for the response like
60 res_query(), but in addition implements the default and search rules
61 controlled by RES_DEFNAMES and RES_DNSRCH (see description of _res
62 options below).
63 The res_querydomain() function makes a query using res_query() on the
65 concatenation of name and domain.
66 The following functions are lower-level routines used by res_query().
68 The res_mkquery() function constructs a query message in buf of length
70 buflen for the domain name dname. The query type op is usually QUERY,
71 but can be any of the types defined in <arpa/nameser.h>. newrr is cur‐
72 rently unused.
73 The res_send() function sends a preformatted query given in msg of
75 length msglen and returns the answer in answer which is of length
76 anslen. It will call res_init(), if it has not already been called.
77 The dn_comp() function compresses the domain name exp_dn and stores it
79 in the buffer comp_dn of length length. The compression uses an array
80 of pointers dnptrs to previously compressed names in the current mes‐
81 sage. The first pointer points to the beginning of the message and the
82 list ends with NULL. The limit of the array is specified by lastdnptr.
83 If dnptr is NULL, domain names are not compressed. If lastdnptr is
84 NULL, the list of labels is not updated.
85 The dn_expand() function expands the compressed domain name comp_dn to
87 a full domain name, which is placed in the buffer exp_dn of size
88 length. The compressed name is contained in a query or reply message,
89 and msg points to the beginning of the message.
90 The resolver routines use global configuration and state information
92 contained in the structure _res, which is defined in <resolv.h>. The
93 only field that is normally manipulated by the user is _res.options.
94 This field can contain the bitwise "OR" of the following options:
95 RES_INIT
97 True if res_init() has been called.
98 RES_DEBUG
100 Print debugging messages.
101 RES_AAONLY
103 Accept authoritative answers only. res_send() continues until
104 it finds an authoritative answer or returns an error. [Not cur‐
105 rently implemented].
106 RES_USEVC
108 Use TCP connections for queries rather than UDP datagrams.
109 RES_PRIMARY
111 Query primary domain name server only.
112 RES_IGNTC
114 Ignore truncation errors. Don't retry with TCP. [Not currently
115 implemented].
116 RES_RECURSE
118 Set the recursion desired bit in queries. Recursion is carried
119 out by the domain name server, not by res_send(). [Enabled by
120 default].
121 RES_DEFNAMES
123 If set, res_search() will append the default domain name to sin‐
124 gle component names, i.e., those that do not contain a dot.
125 [Enabled by default].
126 RES_STAYOPEN
128 Used with RES_USEVC to keep the TCP connection open between
129 queries.
130 RES_DNSRCH
132 If set, res_search() will search for hostnames in the current
133 domain and in parent domains. This option is used by gethostby‐
134 name(3). [Enabled by default].
135
137 The res_init() function returns 0 on success, or -1 if an error occurs.
138 The res_query(), res_search(), res_querydomain(), res_mkquery() and
140 res_send() functions return the length of the response, or -1 if an
141 error occurs.
142 The dn_comp() and dn_expand() functions return the length of the com‐
144 pressed name, or -1 if an error occurs.
145
147 /etc/resolv.conf resolver configuration file
148 /etc/host.conf resolver configuration file
149
151 4.3BSD.
152
154 gethostbyname(3), resolv.conf(5), resolver(5), hostname(7), named(8)
155
157 This page is part of release 3.25 of the Linux man-pages project. A
158 description of the project, and information about reporting bugs, can
159 be found at http://www.kernel.org/doc/man-pages/.
160GNU 2010-06-17 RESOLVER(3)