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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 *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.

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  LOCAL-
       DOMAIN.  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 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 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 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



GNU                               2008-11-07                       RESOLVER(3)