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

       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

       #include <netinet/in.h>
       #include <arpa/nameser.h>
       #include <resolv.h>

       struct __res_state;
       typedef struct __res_state *res_state;

       int res_ninit(void);

       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);

       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.

       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

       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

       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:

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

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

              Accept  authoritative  answers only.  res_send() continues until
              it finds an authoritative answer or returns an error.  [Not cur-
              rently implemented].

              Use TCP connections for queries rather than UDP datagrams.

              Query  primary  domain  name server only.  [Not currently imple-

              Ignore truncation errors.  Don't retry with TCP.

              Set the recursion desired bit in queries.  Recursion is  carried
              out  by  the domain name server, not by res_send().  [Enabled by

              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].

              Used with RES_USEVC to keep  the  TCP  connection  open  between

              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].

              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).

              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).

              Disable usage of HOSTALIASES environment variable.

              Try  an  AAAA query before an A query inside the gethostbyname()
              function, and map IPv4 responses in IPv6 "tunneled form"  if  no
              AAAA records are found but an A record set exists.

              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.

              Disable the modern BIND checking of incoming hostnames and  mail
              names  for invalid characters such as underscore (_), non-ASCII,
              or control characters.  [Not currently implemented].

              Do not strip TSIG records.  [Not currently implemented].

              Send each query simultaneously and recursively to  all  servers.
              Note this option overrides RES_ROTATE.

       RES_USEBSTRING (since glibc 2.3.4)
              Make  reverse  IPv6 lookups using the bit-label format described
              in RFC 2673; if this option is not set, then  nibble  format  is

              Use  zone  in  IPv6 reverse lookup instead of,
              which is deprecated since glibc 2.3.4.  [Enabled by default].

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

       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).

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

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

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

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

       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.

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

       For  an  explanation  of  the  terms  used   in   this   section,   see

       |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()                        |               |                |


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

       The GNU C library source file resolv/README.

       This page is part of release 4.04 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

GNU                               2015-05-07                       RESOLVER(3)
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