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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/.