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NAME

       res_ninit, res_nclose, 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);

       void res_nclose(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.   Every  call  to
       res_ninit()  requires  a  corresponding  call  to res_nclose() to free memory allocated by
       res_ninit() and subsequent calls to res_nquery().

       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.

       In   the   case   of  an  error  return  from  res_nquery(),  res_query(),  res_nsearch(),
       res_search(), res_nquerydomain(), or res_querydomain(), the global variable  h_errno  (see
       gethostbyname(3)) can be consulted to determine the cause of the error.

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_nclose(), │ Thread safety │ MT-Safe locale │
       │res_nquery(),                      │               │                │
       │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 5.01 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/.