RESOLVER(3)                                         Linux Programmer's Manual                                        RESOLVER(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);

       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  com-
       pression  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 con-
              tain 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 3.25 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)