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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 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 3.35 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/.