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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 *exp_dn,
              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  LOCALDO-
       MAIN.   res_init() is normally executed by the first call to one of the other func-

       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 pre-formatted 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  nor-
       mally  manipulated by the user is _res.options.  This field can contain the bitwise
       "OR" of the following options:

              True if res_init() has been called.

              Print debugging messages.

              Accept authoritative answers only.  res_send() continues until it  finds  an
              authoritative answer or returns an error.  [Not currently implemented].

              Use TCP connections for queries rather than UDP datagrams.

              Query primary domain name server only.

              Ignore  truncation  errors.   Don't  retry  with TCP.  [Not currently imple-

              Set the recursion desired bit in queries.  Recursion is carried out  by  the
              domain name server, not by res_send().  [Enabled by default].

              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].

              Used with RES_USEVC to keep the TCP connection open between queries.

              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

       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


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

       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-

GNU                               2008-11-07                       RESOLVER(3)

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