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       ares_init, ares_init_options - Initialize a resolver channel


       #include <ares.h>

       int ares_init(ares_channel *channel)
       int ares_init_options(ares_channel *channel,
            struct ares_options *options, int optmask)

       cc file.c -lares


       The  ares_init  function  initializes a communications channel for name
       service lookups.  If it returns successfully, ares_init  will  set  the
       variable  pointed  to  by channel to a handle used to identify the name
       service channel.  The  caller  should  invoke  ares_destroy(3)  on  the
       handle when the channel is no longer needed.

       The ares_init_options function also initializes a name service channel,
       with additional options useful for applications requiring more  control
       over name service configuration.  The optmask parameter specifies which
       fields in the structure pointed to by options are set, as follows:

       ARES_OPT_FLAGS    int flags;
                         Flags controlling the behavior of the resolver.   See
                         below for a description of possible flag values.

       ARES_OPT_TIMEOUT  int timeout;
                         The  number  of  seconds each name server is given to
                         respond to a query on  the  first  try.   (After  the
                         first   try,   the  timeout  algorithm  becomes  more
                         complicated, but scales linearly with  the  value  of
                         timeout.)  The default is five seconds.

       ARES_OPT_TRIES    int tries;
                         The  number of tries the resolver will try contacting
                         each name server before giving up.   The  default  is
                         four tries.

       ARES_OPT_NDOTS    int ndots;
                         The  number of dots which must be present in a domain
                         name for it to  be  queried  for  "as  is"  prior  to
                         querying  for  it  with the default domain extensions
                         appended.   The  default  value  is  1   unless   set
                         otherwise   by   resolv.conf   or   the   RES_OPTIONS
                         environment variable.

       ARES_OPT_PORT     unsigned short port;
                         The port to use for queries (both TCP  and  UDP),  in
                         network  byte  order.   The  default  value is 53 (in
                         network byte order), the standard name service  port.

       ARES_OPT_SERVERS  struct in_addr *servers;
                         int nservers;
                         The  list  of  servers  to  contact,  instead  of the
                         servers specified in resolv.conf or the local  named.

       ARES_OPT_DOMAINS  char **domains;
                         int ndomains;
                         The   domains  to  search,  instead  of  the  domains
                         specified in resolv.conf or the domain  derived  from
                         the kernel hostname variable.

       ARES_OPT_LOOKUPS  char *lookups;
                         The  lookups  to  perform  for host queries.  lookups
                         should be set to a string of the  characters  "b"  or
                         "f",  where  "b"  indicates  a  DNS  lookup  and  "f"
                         indicates a lookup in the hosts file.

       The flags field should  be  the  bitwise  or  of  some  subset  of  the
       following values:

       ARES_FLAG_USEVC        Always  use  TCP queries (the "virtual circuit")
                              instead of UDP queries.  Normally, TCP  is  only
                              used if a UDP query yields a truncated result.

       ARES_FLAG_PRIMARY      Only  query  the  first  server  in  the list of
                              servers to query.

       ARES_FLAG_IGNTC        If a  truncated  response  to  a  UDP  query  is
                              received,  do  not  fall  back  to  TCP;  simply
                              continue on with the truncated response.

       ARES_FLAG_NORECURSE    Do  not  set  the  "recursion  desired"  bit  on
                              outgoing  queries, so that the name server being
                              contacted will not try to fetch the answer  from
                              other  servers  if  it  doesn’t  know the answer

       ARES_FLAG_STAYOPEN     Do not close  communciations  sockets  when  the
                              number of active queries drops to zero.

       ARES_FLAG_NOSEARCH     Do  not  use  the  default  search domains; only
                              query hostnames as-is or as aliases.

       ARES_FLAG_NOALIASES    Do  not  honor   the   HOSTALIASES   environment
                              variable,  which  normally  specifies  a file of
                              hostname translations.

       ARES_FLAG_NOCHECKRESP  Do not  discard  responses  with  the  SERVFAIL,
                              NOTIMP,  or  REFUSED  response code or responses
                              whose questions don’t match the questions in the
                              request.   Primarily  useful for writing clients
                              which might  be  used  to  test  or  debug  name


       ares_init or ares_init_options can return any of the following values:

       ARES_SUCCESS  Initialization succeeded.

       ARES_EFILE    A configuration file could not be read.

       ARES_ENOMEM   The process’s available memory was exhausted.




       Greg Hudson, MIT Information Systems
       Copyright 1998 by the Massachusetts Institute of Technology.

                                 21 July 1998                     ARES_INIT(3)