RESOLVER(3)		   Linux Programmer's Manual		   RESOLVER(3)



NAME
       res_ninit,  res_nquery,	res_nsearch,  res_nquerydomain,	 res_nmkquery,
       res_nsend,   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>

       struct __res_state;
       typedef struct __res_state *res_state;

       int res_ninit(res_state statep);

       int res_nquery(res_state statep,
		  const char *dname, int class, int type,
		  unsigned char *answer, int anslen);

       int res_nsearch(res_state statep,
		  const char *dname, int class, int type,
		  unsigned char *answer, int anslen);

       int res_nquerydomain(res_state statep,
		  const char *name, const char *domain,
		  int class, int type, unsigned char *answer,
		  int anslen);

       int res_nmkquery(res_state statep,
		  int op, const char *dname, int class,
		  int type, const unsigned char *data, int datalen,
		  const unsigned char *newrr,
		  unsigned char *buf, int buflen);

       int res_nsend(res_state statep,
		  const unsigned char *msg, int msglen,
		  unsigned char *answer, int anslen);

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

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

   Deprecated
       extern struct __res_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, const unsigned char *data, int datalen,
		  const unsigned char *newrr,
		  unsigned char *buf, int buflen);

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

       Link with -lresolv.

DESCRIPTION
       Note:  This  page is incomplete (various resolver functions provided by
       glibc are not described) and likely out of date.

       The functions  described	 below	make  queries  to  and	interpret  the
       responses from Internet domain name servers.

       The  API	 consists  of a set of more modern, reentrant functions and an
       older set of nonreentrant functions that	 have  been  superseded.   The
       traditional  resolver interfaces such as res_init() and res_query() use
       some static (global) state stored  in  the  _res	 structure,  rendering
       these  functions	 non-thread-safe.   BIND  8.2  introduced a set of new
       interfaces res_ninit(), res_nquery(), and so on, which take a res_state
       as their first argument, so you can use a per-thread resolver state.

       The  res_ninit()	 and res_init() functions read the configuration files
       (see resolv.conf(5)) to get the default domain  name  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_ninit()
       or res_init() is normally executed by the first	call  to  one  of  the
       other functions.

       The  res_nquery()  and  res_query() functions query 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_nsearch() and res_search() functions make a query and waits for
       the  response  like  res_nquery() and res_query(), but in addition they
       implement the default and search rules controlled by  RES_DEFNAMES  and
       RES_DNSRCH (see description of _res options below).

       The  res_nquerydomain()	and  res_querydomain()	functions make a query
       using res_nquery()/res_query() on the concatenation of name and domain.

       The   following	 functions   are   lower-level	 routines   used    by
       res_query()/res_query().

       The  res_nmkquery()  and res_mkquery() functions construct a query mes-
       sage 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_nsend() and res_send() function send a preformatted query given
       in  msg	of  length msglen and returns the answer in answer which is of
       length anslen.  They will call res_ninit()/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 mes-
       sage.  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 configuration and state information contained
       in a __res_state structure (either passed as the statep argument, or in
       the global variable _res, in the case of the older  nonreentrant	 func-
       tions).	 The only field of this structure that is normally manipulated
       by the user is the options field.  This field can contain  the  bitwise
       "OR" of the following options:

       RES_INIT
	      True if res_ninit() or res_init() has been called.

       RES_DEBUG
	      Print  debugging	messages.   This  option  is available only if
	      glibc was	 built	with  debugging	 enabled,  which  is  not  the
	      default.

       RES_AAONLY (unimplemented; deprecated in glibc 2.25)
	      Accept  authoritative  answers only.  res_send() continues until
	      it finds an authoritative answer	or  returns  an	 error.	  This
	      option  was  present  but	 unimplemented	in glibc until version
	      2.24; since glibc 2.25, it is deprecated, and its usage produces
	      a warning.

       RES_USEVC
	      Use TCP connections for queries rather than UDP datagrams.

       RES_PRIMARY (unimplemented; deprecated in glibc 2.25)
	      Query  primary domain name server only.  This option was present
	      but unimplemented in glibc until version 2.24; since glibc 2.25,
	      it is deprecated, and its usage produces a warning.

       RES_IGNTC
	      Ignore truncation errors.	 Don't retry with TCP.

       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 sin-
	      gle component names--that is, 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 gethostby-
	      name(3).	[Enabled by default].

       RES_INSECURE1
	      Accept a response from a wrong server.   This  can  be  used  to
	      detect potential security hazards, but you need to compile glibc
	      with debugging enabled and use RES_DEBUG option (for debug  pur-
	      pose only).

       RES_INSECURE2
	      Accept  a	 response  which  contains a wrong query.  This can be
	      used to detect potential security hazards, but you need to  com-
	      pile  glibc with debugging enabled and use RES_DEBUG option (for
	      debug purpose only).

       RES_NOALIASES
	      Disable usage of HOSTALIASES environment variable.

       RES_USE_INET6
	      Try an AAAA query before an A query inside the  gethostbyname(3)
	      function,	 and  map IPv4 responses in IPv6 "tunneled form" if no
	      AAAA records are found but an A record set exists.  Since	 glibc
	      2.25,  this option is deprecated, and its usage produces a warn-
	      ing; applications should use getaddrinfo(3), rather  than	 geth-
	      ostbyname(3).

       RES_ROTATE
	      Causes  round-robin  selection  of name servers from among those
	      listed.  This has the effect of spreading the query  load	 among
	      all listed servers, rather than having all clients try the first
	      listed server first every time.

       RES_NOCHECKNAME (unimplemented; deprecated in glibc 2.25)
	      Disable the modern BIND checking of incoming hostnames and  mail
	      names  for invalid characters such as underscore (_), non-ASCII,
	      or control characters.  This option was present in  glibc	 until
	      version  2.24; since glibc 2.25, it is deprecated, and its usage
	      produces a warning.

       RES_KEEPTSIG (unimplemented; deprecated in glibc 2.25)
	      Do not strip TSIG records.  This option was present but unimple-
	      mented in glibc until version 2.24; since glibc 2.25, it is dep-
	      recated, and its usage produces a warning.

       RES_BLAST  (unimplemented; deprecated in glibc 2.25)
	      Send each query simultaneously and recursively to	 all  servers.
	      This option was present but unimplemented in glibc until version
	      2.24; since glibc 2.25, it is deprecated, and its usage produces
	      a warning.

       RES_USEBSTRING (glibc 2.3.4 to 2.24)
	      Make  reverse  IPv6 lookups using the bit-label format described
	      in RFC 2673; if this option is not set (which is	the  default),
	      then  nibble  format  is used.  This option was removed in glibc
	      2.25, since it relied on a backward-incompatible	DNS  extension
	      that was never deployed on the Internet.

       RES_NOIP6DOTINT (glibc 2.24 and earlier)
	      Use  ip6.arpa  zone  in  IPv6 reverse lookup instead of ip6.int,
	      which is deprecated since glibc 2.3.4.  This option  is  present
	      in  glibc	 up to and including version 2.24, where it is enabled
	      by default.  In glibc 2.25, this option was removed.

       RES_USE_EDNS0 (since glibc 2.6)
	      Enables support for the DNS extensions (EDNS0) described in  RFC
	      2671.

       RES_SNGLKUP (since glibc 2.10)
	      By  default,  glibc  performs  IPv4 and IPv6 lookups in parallel
	      since version 2.9.  Some appliance  DNS  servers	cannot	handle
	      these  queries  properly	and  make the requests time out.  This
	      option disables the behavior and makes glibc  perform  the  IPv6
	      and  IPv4 requests sequentially (at the cost of some slowdown of
	      the resolving process).

       RES_SNGLKUPREOP
	      When RES_SNGLKUP option is enabled, opens a new socket  for  the
	      each request.

       RES_USE_DNSSEC
	      Use  DNSSEC  with	 OK  bit  in  OPT record.  This option implies
	      RES_USE_EDNS0.

       RES_NOTLDQUERY
	      Do not look up unqualified name as a top-level domain (TLD).

       RES_DEFAULT
	      Default  option  which   implies:	  RES_RECURSE,	 RES_DEFNAMES,
	      RES_DNSRCH and RES_NOIP6DOTINT.

RETURN VALUE
       The  res_ninit() and res_init() functions return 0 on success, or -1 if
       an error occurs.

       The res_nquery(), res_query(), res_nsearch(), res_search(), res_nquery-
       domain(),     res_querydomain(),	    res_nmkquery(),	res_mkquery(),
       res_nsend(),  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 com-
       pressed name, or -1 if an error occurs.

FILES
       /etc/resolv.conf		 resolver configuration file
       /etc/host.conf		 resolver configuration file

ATTRIBUTES
       For  an	explanation  of	 the  terms  used   in	 this	section,   see
       attributes(7).

       +-----------------------------------+---------------+----------------+
       |Interface			   | Attribute	   | Value	    |
       +-----------------------------------+---------------+----------------+
       |res_ninit(), res_nquery(),	   | Thread safety | MT-Safe locale |
       |res_nsearch(), res_nquerydomain(), |		   |		    |
       |res_nsend()			   |		   |		    |
       +-----------------------------------+---------------+----------------+
       |res_nmkquery(), dn_comp(),	   | Thread safety | MT-Safe	    |
       |dn_expand()			   |		   |		    |
       +-----------------------------------+---------------+----------------+

CONFORMING TO
       4.3BSD.

SEE ALSO
       gethostbyname(3), resolv.conf(5), resolver(5), hostname(7), named(8)

       The GNU C library source file resolv/README.

COLOPHON
       This page is part of release 4.10 of the Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       https://www.kernel.org/doc/man-pages/.



GNU				  2017-03-13			   RESOLVER(3)