RESOLVER(3)               (2020-12-21)                RESOLVER(3)

     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);

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        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 func-
          tions 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 inter-
          faces 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 configura-
          tion 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 asso-
          ciated 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().

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          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 com-
          pression uses an array of pointers dnptrs to previously com-
          pressed 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 lastd-
          nptr.  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

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          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 informa-
          tion 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() contin-
               ues 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 dep-
               recated, and its usage produces a warning.

          RES_USEVC
               Use TCP connections for queries rather than UDP data-
               grams.

          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 pro-
               duces 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

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               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; appli-
               cations 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 under-
               score (_), 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 warn-
               ing.

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          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 dep-
               recated, 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

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          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 vari-
          able 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).  allbox; lbw34 lb lb l l l.
          Interface Attribute Value T{ res_ninit(), res_nclose(),
          res_nquery(),
          res_nsearch(), res_nquerydomain(), res_nsend() T}   Thread
          safety  MT-Safe locale T{ res_nmkquery(), dn_comp(),
          dn_expand() T}   Thread safety  MT-Safe

     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 5.10 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/.

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