Linux manual pages Section 3  

Name

res_init, res_query, res_search, res_querydomain, res_mkquery, res_send, dn_comp, dn_expand — resolver routines

Synopsis

#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);
 
[Note] Note

Link with −lresolv.

DESCRIPTION

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].

RETURN VALUE

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.

FILES

/etc/resolv.conf          resolver configuration file
/etc/host.conf            resolver configuration file

CONFORMING TO

4.3BSD.

SEE ALSO

gethostbyname(3), resolv.conf(5), resolver(5), hostname(7), named(8)

COLOPHON

This page is part of release 3.33 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://man7.org/linux/man-pages/.


  Copyright 1993 David Metcalfe (davidprism.demon.co.uk)

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
professionally.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.

References consulted:
    Linux libc source code
    Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
    386BSD man pages
Modified 1993-07-25 by Rik Faith (faithcs.unc.edu)
Modified 2004-10-31 by aeb