resolver,  res_query,  res_search,   res_mkquery,   res_send,   res_init,
     dn_comp, dn_expand - resolver routines

     #include <sys/types.h>
     #include <net/gen/in.h>
     #include <net/gen/nameser.h>
     #include <net/gen/resolv.h>

     res_query(dname, class, type, answer, anslen)
     char *dname;
     int class, type;
     u_char *answer;
     int anslen;

     res_search(dname, class, type, answer, anslen)
     char *dname;
     int class, type;
     u_char *answer;
     int anslen;

     res_mkquery(op, dname, class, type, data, datalen, newrr, buf, buflen)
     int op;
     char *dname;
     int class, type;
     char *data;
     int datalen;
     struct rrec *newrr;
     char *buf;
     int buflen;

     res_send(msg, msglen, answer, anslen)
     char *msg;
     int msglen;
     char *answer;
     int anslen;


     dn_comp(exp_dn, comp_dn, length, dnptrs, lastdnptr)
     char *exp_dn, *comp_dn;
     int length;
     char **dnptrs, **lastdnptr;

     dn_expand(msg, eomorig, comp_dn, exp_dn, length)
     char *msg, *eomorig, *comp_dn, exp_dn;
     int length;

     These routines are used for making, sending and  interpreting  query  and
     reply messages with Internet domain name servers.

     Global configuration and state information that is used by  the  resolver
     routines  is  kept  in  the  structure  _res.   Most  of  the values have
     reasonable defaults and can be ignored.  Options stored  in  _res.options
     are  defined  in  resolv.h  and  are as follows.  Options are stored as a
     simple bit mask containing the bitwise ``or'' of the options enabled.

          True if the initial name server address and default domain name  are
          initialized (i.e., res_init has been called).

          Print debugging messages.

          Accept authoritative  answers  only.   With  this  option,  res_send
          should  continue  until it finds an authoritative answer or finds an
          error.  Currently this is not implemented.

          Use TCP connections for queries instead of UDP datagrams.

          Used with RES_USEVC to keep the TCP connection open between queries.
          This is useful only in programs that regularly do many queries.  UDP
          should be the normal mode used.

          Unused currently (ignore truncation errors, i.e., don't  retry  with

          Set the recursion-desired bit in  queries.   This  is  the  default.
          (res_send  does not do iterative queries and expects the name server
          to handle recursion.)

          If set, res_search will append the default domain  name  to  single-
          component  names  (those that do not contain a dot).  This option is
          enabled by default.

          If this option is set, res_search will search for host names in  the
          current domain and in parent domains; see hostname(7).  This is used
          by the standard host lookup routine gethostbyname(3).   This  option
          is enabled by default.

     The  res_init  routine  reads  the  configuration  file  (if   any;   see
     resolver(5)) to get the default domain name, search list and the Internet
     address of the local name server(s).  If no  server  is  configured,  the
     host  running  the resolver is tried.  The current domain name is defined
     by the hostname if not specified in the configuration  file;  it  can  be
     overridden  by  the  environment  variable  LOCALDOMAIN.   Initialization
     normally occurs on the first call to one of the following routines.

     The  res_query  function  provides  an  interface  to  the  server  query
     mechanism.  It constructs a query, sends it to the local server, awaits a
     response, and makes preliminary checks on the reply.  The query  requests
     information  of  the  specified  type  and class for the specified fully-
     qualified domain name dname . The reply message is  left  in  the  answer
     buffer with length anslen supplied by the caller.

     The  res_search  routine  makes  a  query  and  awaits  a  response  like
     res_query,  but  in  addition, it implements the default and search rules
     controlled by the RES_DEFNAMES and RES_DNSRCH options.   It  returns  the
     first successful reply.

     The remaining routines are lower-level routines used by  res_query.   The
     res_mkquery function constructs a standard query message and places it in
     buf.  It returns the size of the query, or -1 if the query is larger than
     buflen.   The query type op is usually QUERY, but can be any of the query
     types defined in <arpa/nameser.h>.  The domain  name  for  the  query  is
     given  by  dname.   Newrr  is currently unused but is intended for making
     update messages.

     The res_send routine sends a pre-formatted query and returns  an  answer.
     It will call res_init if RES_INIT is not set, send the query to the local
     name server, and handle timeouts and retries.  The length  of  the  reply
     message is returned, or -1 if there were errors.

     The dn_comp function compresses the domain name exp_dn and stores  it  in
     comp_dn.  The size of the compressed name is returned or -1 if there were
     errors.  The size of the array pointed to by comp_dn is given by  length.
     The compression uses an array of pointers dnptrs to previously-compressed
     names in the current  message.   The  first  pointer  points  to  to  the
     beginning  of  the message and the list ends with NULL.  The limit to the
     array is specified by lastdnptr.  A side effect of dn_comp is  to  update
     the  list of pointers for labels inserted into the message as the name is
     compressed.  If dnptr is NULL, names are not compressed.  If lastdnptr is
     NULL, the list of labels is not updated.

     The dn_expand entry expands the compressed domain name comp_dn to a  full
     domain name The compressed name is contained in a query or reply message;
     msg is a pointer to the beginning of the message.  The uncompressed  name
     is placed in the buffer indicated by exp_dn which is of size length.  The
     size of compressed name is returned or -1 if there was an error.

     /etc/resolv.conf see resolver(5)

     gethostbyname(3), named(8), resolver(5), hostname(7),
     RFC1032, RFC1033, RFC1034, RFC1035, RFC974,
     SMM:11 Name Server Operations Guide for BIND