resolver(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | FILES | ATTRIBUTES | CONFORMING TO | SEE ALSO | COLOPHON

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

NAME         top

       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         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);

       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         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.  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_nquery()/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         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.

       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         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
       attributes(7).

       ┌───────────────────────────────┬───────────────┬────────────────┐
       │Interface                      Attribute     Value          │
       ├───────────────────────────────┼───────────────┼────────────────┤
       │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         top

       4.3BSD.

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 5.11 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                            2021-03-22                    RESOLVER(3)

Pages that refer to this page: hostname(1)pmdanetcheck(1)gethostbyname(3)hosts(5)resolv.conf(5)