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

NAME         top

       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         top

       #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);

       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         top

       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

       The following functions are lower-level routines used by

       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 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 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:

              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
              currently implemented].

              Use TCP connections for queries rather than UDP datagrams.

              Query primary domain name server only.  [Not currently

              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 default].

              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].

              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
              gethostbyname(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 purpose only).

              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).

              Disable usage of HOSTALIASES environment variable.

              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.

              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 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).

              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.

RETURN VALUE         top

       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         top

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

ATTRIBUTES         top

       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()                        │               │                │

CONFORMING TO         top


SEE ALSO         top

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

       The GNU C library source file resolv/README.

COLOPHON         top

       This page is part of release 4.09 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                              2016-10-08                      RESOLVER(3)