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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 message in buf of length buflen for the
       domain name dname.  The query type op is one of the following (typically QUERY):

       QUERY  Standard query.

       IQUERY Inverse query.  This option was removed in glibc 2.26, since it has  not  been  supported  by  DNS
              servers for a very long time.

       NS_NOTIFY_OP
              Notify secondary of SOA (Start of Authority) change.

       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
       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  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  functions).   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 single 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 gethostbyname(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 purpose
              only).

       RES_INSECURE2
              Accept a response which contains a wrong query.  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
              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  warning;  applications  should  use
              getaddrinfo(3), rather than gethostbyname(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 unimplemented in glibc until version 2.24;
              since glibc 2.25, it is deprecated, 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_nquerydomain(),  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 compressed 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).
       ┌────────────────────────────────────┬───────────────┬────────────────┐
       │ InterfaceAttributeValue          │
       ├────────────────────────────────────┼───────────────┼────────────────┤
       │ 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

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       information   about   reporting   bugs,   and   the  latest  version  of  this  page,  can  be  found  at
       https://www.kernel.org/doc/man-pages/.

GNU                                                2017-09-15                                        RESOLVER(3)