resolver

resolver(3) Library Functions Manual resolver(3)

NAME

   res_ninit,  res_nquery, res_nsearch, res_nquerydomain, res_nmkquery, res_nsend, res_nclose, res_init, res_query, res_search, res_querydomain, res_mkquery, res_send, dn_comp, dn_expand
   - resolver routines

LIBRARY

   Resolver library (libresolv, -lresolv)

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[.anslen], int anslen);

   int res_nsearch(res_state statep,
              const char *dname, int class, int type,
              unsigned char answer[.anslen], int anslen);

   int res_nquerydomain(res_state statep,
              const char *name, const char *domain,
              int class, int type, unsigned char answer[.anslen],
              int anslen);

   int res_nmkquery(res_state statep,
              int op, const char *dname, int class,
              int type, const unsigned char data[.datalen], int datalen,
              const unsigned char *newrr,
              unsigned char buf[.buflen], int buflen);

   int res_nsend(res_state statep,
              const unsigned char msg[.msglen], int msglen,
              unsigned char answer[.anslen], int anslen);

   int dn_comp(const char *exp_dn, unsigned char comp_dn[.length],
              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[.length],
              int length);

   [[deprecated]] extern struct __res_state _res;

   [[deprecated]] int res_init(void);

   [[deprecated]]
   int res_query(const char *dname, int class, int type,
              unsigned char answer[.anslen], int anslen);

   [[deprecated]]
   int res_search(const char *dname, int class, int type,
              unsigned char answer[.anslen], int anslen);

   [[deprecated]]
   int res_querydomain(const char *name, const char *domain,
              int class, int type, unsigned char answer[.anslen],
              int anslen);

   [[deprecated]]
   int res_mkquery(int op, const char *dname, int class,
              int type, const unsigned char data[.datalen], int datalen,
              const unsigned char *newrr,
              unsigned char buf[.buflen], int buflen);

   [[deprecated]]
   int res_send(const unsigned char msg[.msglen], int msglen,
              unsigned char answer[.anslen], int anslen);

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  lo
   cal  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  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 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 until glibc
          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 until glibc 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 namesthat 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  ex
          ists.  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
          until glibc 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 until glibc 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 until glibc 2.24; since glibc 2.25, it is deprecated, and its usage
          produces a warning.

   RES_USEBSTRING (glibc 2.3.4 to glibc 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  re
          moved 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 up to and including glibc 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 glibc 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  gethostby
   name(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).
   
    Interface                                                                                                                                           Attribute      Value          
   
    res_ninit(), res_nclose(), res_nquery(), res_nsearch(), res_nquerydomain(), res_nsend()                                                             Thread safety  MT-Safe locale 
   
    res_nmkquery(), dn_comp(), dn_expand()                                                                                                              Thread safety  MT-Safe        
   

STANDARDS

   None.

HISTORY

   4.3BSD.

SEE ALSO

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

   The GNU C library source file resolv/README.

Linux man-pages 6.9.1 2024-05-02 resolver(3)