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

       res_init,    res_query,	 res_search,   res_querydomain,	  res_mkquery,
       res_send, dn_comp, dn_expand - resolver routines

       #include <netinet/in.h>
       #include <arpa/nameser.h>
       #include <resolv.h>
       extern struct 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, char *data, int datalen, struct rrec *newrr,
	      char *buf, int buflen);

       int res_send(const char *msg, int msglen, char *answer,
	      int anslen);

       int dn_comp(unsigned char *exp_dn, unsigned char *comp_dn,
	      int length, unsigned char **dnptrs, unsigned char *exp_dn,
	      unsigned char **lastdnptr);

       int dn_expand(unsigned char *msg, unsigned char *eomorig,
	      unsigned char *comp_dn, char *exp_dn,
	      int length);

       Link with -lresolv.

       These functions make queries to and interpret the responses from Inter‐
       net domain name servers.

       The   res_init()	  function   reads   the   configuration   files  (see
       resolv.conf(5)) to get the default domain name, search order  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_init() is normally executed by the first call to one of  the	 other

       The  res_query()	 function queries the name server for the fully quali‐
       fied 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_search() function makes a query and waits for the response like
       res_query(), but in addition implements the default  and	 search	 rules
       controlled  by  RES_DEFNAMES  and  RES_DNSRCH  (see description of _res
       options below).

       The res_querydomain() function makes a query using res_query()  on  the
       concatenation of name and domain.

       The following functions are lower-level routines used by res_query().

       The  res_mkquery() function constructs 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 cur‐
       rently unused.

       The res_send() function sends a pre-formatted query  given  in  msg  of
       length  msglen  and  returns  the  answer  in answer which is of length
       anslen.	It will call 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  mes‐
       sage.  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 global configuration and state information
       contained in the structure _res, which is defined in  <resolv.h>.   The
       only  field  that  is normally manipulated by the user is _res.options.
       This field can contain the bitwise "OR" of the following options:

	      True if res_init() has been called.

	      Print debugging messages.

	      Accept authoritative answers only.  res_send()  continues	 until
	      it finds an authoritative answer or returns an error.  [Not cur‐
	      rently implemented].

	      Use TCP connections for queries rather than UDP datagrams.

	      Query primary domain name server only.

	      Ignore truncation errors.	 Don't retry with TCP.	[Not currently

	      Set  the recursion desired bit in queries.  Recursion is carried
	      out by the domain name server, not by res_send().	  [Enabled  by

	      If set, res_search() will append the default domain name to sin‐
	      gle component names, i.e., 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 gethostby‐
	      name(3).	[Enabled by default].

       The res_init() function returns 0 on success, or -1 if an error occurs.

       The res_query(),	 res_search(),	res_querydomain(),  res_mkquery()  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  com‐
       pressed name, or -1 if an error occurs.

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


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

       This  page  is  part of release 3.22 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

GNU				  2008-11-07			   RESOLVER(3)

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