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RESOLVER(3) Linux Programmer's Manual RESOLVER(3)
res_init, res_query, res_search, res_querydomain, res_mkquery, res_send,
dn_comp, dn_expand - resolver routines
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>
extern struct state _res;
int res_init(void);
int res_query(const char *dname, int class, int type,
unsigned char *answer, int anslen);
int res_search(const char *dname, int class, int type,
unsigned char *answer, int anslen);
int res_querydomain(const char *name, const char *domain,
int class, int type, unsigned char *answer,
int anslen);
int res_mkquery(int op, const char *dname, int class,
int type, char *data, int datalen, struct rrec *newrr,
char *buf, int buflen);
int res_send(const char *msg, int msglen, char *answer,
int anslen);
int dn_comp(unsigned char *exp_dn, unsigned char *comp_dn,
int length, unsigned char **dnptrs, unsigned char **lastdnptr);
int dn_expand(unsigned char *msg, unsigned char *eomorig,
unsigned char *comp_dn, char *exp_dn,
int length);
Link with -lresolv.
These functions make queries to and interpret the responses from Internet
domain name servers.
The res_init() function reads the configuration files (see resolv.conf(5)) to
get the default domain name, search order and name server address(es). If no
server is given, the local host is tried. If no domain is given, that
associated with the local host is used. It can be overridden with the
environment variable LOCALDOMAIN. res_init() is normally executed by the
first call to one of the other functions.
The res_query() function queries the name server for the fully qualified
domain name name of specified type and class. The reply is left in the buffer
answer of length anslen supplied by the caller.
The res_search() function makes a query and waits for the response like
res_query(), but in addition implements the default and search rules
controlled by RES_DEFNAMES and RES_DNSRCH (see description of _res options
below).
The res_querydomain() function makes a query using res_query() on the
concatenation of name and domain.
The following functions are lower-level routines used by res_query().
The res_mkquery() function constructs a query message in buf of length buflen
for the domain name dname. The query type op is usually QUERY, but can be any
of the types defined in <arpa/nameser.h>. newrr is currently unused.
The res_send() function sends a preformatted query given in msg of length
msglen and returns the answer in answer which is of length anslen. It will
call res_init(), if it has not already been called.
The dn_comp() function compresses the domain name exp_dn and stores it in the
buffer comp_dn of length length. The compression uses an array of pointers
dnptrs to previously compressed names in the current message. The first
pointer points to the beginning of the message and the list ends with NULL.
The limit of the array is specified by lastdnptr. If dnptr is NULL, domain
names are not compressed. If lastdnptr is NULL, the list of labels is not
updated.
The dn_expand() function expands the compressed domain name comp_dn to a full
domain name, which is placed in the buffer exp_dn of size length. The
compressed name is contained in a query or reply message, and msg points to
the beginning of the message.
The resolver routines use global configuration and state information contained
in the structure _res, which is defined in <resolv.h>. The only field that is
normally manipulated by the user is _res.options. This field can contain the
bitwise "OR" of the following options:
RES_INIT
True if res_init() has been called.
RES_DEBUG
Print debugging messages.
RES_AAONLY
Accept authoritative answers only. res_send() continues until it finds
an authoritative answer or returns an error. [Not currently
implemented].
RES_USEVC
Use TCP connections for queries rather than UDP datagrams.
RES_PRIMARY
Query primary domain name server only.
RES_IGNTC
Ignore truncation errors. Don't retry with TCP. [Not currently
implemented].
RES_RECURSE
Set the recursion desired bit in queries. Recursion is carried out by
the domain name server, not by res_send(). [Enabled by default].
RES_DEFNAMES
If set, res_search() will append the default domain name to single
component names, i.e., those that do not contain a dot. [Enabled by
default].
RES_STAYOPEN
Used with RES_USEVC to keep the TCP connection open between queries.
RES_DNSRCH
If set, res_search() will search for hostnames in the current domain
and in parent domains. This option is used by gethostbyname(3).
[Enabled by default].
The res_init() function returns 0 on success, or -1 if an error occurs.
The res_query(), res_search(), res_querydomain(), res_mkquery() and res_send()
functions return the length of the response, or -1 if an error occurs.
The dn_comp() and dn_expand() functions return the length of the compressed
name, or -1 if an error occurs.
/etc/resolv.conf resolver configuration file
/etc/host.conf resolver configuration file
4.3BSD.
gethostbyname(3), resolv.conf(5), resolver(5), hostname(7), named(8)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
GNU 2010-06-17 RESOLVER(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface