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resolver(3N)							  resolver(3N)

       resolver: dn_comp(), dn_expand(), get_resfield(), herror(), res_init(),
       res_mkquery(), res_query(), res_search(), res_send(), set_resfield()  -
       resolver routines

       These routines are used for making, sending, and interpreting query and
       reply messages with Internet domain name servers.

       Global configuration and state information used by  the	resolver  rou‐
       tines  are  kept in the structure and are defined in Most of the fields
       have reasonable defaults and can be ignored.  The resolver options  are
       stored  in  the field and are listed below. The options are stored as a
       simple bit mask containing the bitwise OR of the options enabled.

       In a multithreaded environment, a thread specific  structure  is	 allo‐
       cated for each thread.

	      True  if the initial name server address and default domain name
				  initialized (i.e., has been called).

	      Print debugging messages.

	      Accept authoritative answers only.
				  With this option, should continue  until  it
				  finds	 an  authoritative  answer or finds an
				  error.  Currently this is not implemented.

	      Query the primary server only.
				  Currently this is not implemented.

	      Use		  TCP connections for queries instead  of  UDP

	      Used with		  to  keep  the	 TCP  connection  open between
				  queries.  This is useful  only  in  programs
				  that	regularly do many queries.  UDP should
				  be the normal mode used.

	      The name server will set the truncation bit
				  if all of the data does  not	fit  into  the
				  response  datagram  packet.  If is set, will
				  not retry the query with TCP	(i.e.,	ignore
				  truncation errors).

	      Set the recursion-desired bit in queries.
				  This is the default.	(does not do iterative
				  queries and expects the name server to  han‐
				  dle recursion.)

	      If set,		  appends  the	default domain name to single-
				  component names (those that do not contain a
				  dot).	 This option is enabled by default.

	      If this option is set,
				  searches  for	 host  names  in  the  current
				  domain and  in  parent  domains;  see	 host‐
				  name(5).   This is used by the standard host
				  lookup routine (see  gethostent(3N)).	  This
				  option is enabled by default.

       Initialization  of  the resolver structure normally occurs on the first
       call to one of the resolver routines below.  If there are errors in the
       configuration file, they are silently ignored.

       The  values  for	 retransmission	 timeout  and  number of retries to be
       attempted can be configured.  These correspond to the retrans and retry
       fields  in  the	structure.  The following three options, listed in the
       order of precedence, have been provided for configuring the retransmis‐
       sion timeout and retry values.

	      1.     Environment Variables,

	      2.     Configuration file

	      3.     Through calls to API

       retrans and retry can be configured through the Environmental Variables
       and as follows:

       Alternatively in the following name-value pairs can be added :

       While the Environmental Variables and entries in the  file  are	inter‐
       preted when the API is called, the API has to be explicitly called from
       within the code.	 Setting the retrans and retry values through a	 lower
       precedence  option will be ignored if these values have been configured
       through higher precedence option.  A message  is	 flagged  in  when  an
       invalid	value  is specified in either or Environmental variables.  The
       retrans is to be specified in milliseconds, and its  default  value  is
       5000 milliseconds.  The default value for retry is 4.

   Primary Routines
       Reads the configuration file,
			   to  get  the	 default domain name, search list, the
			   Internet address of the local name  server(s),  the
			   values for retrans and retry.  If no server is con‐
			   figured, the host running the  resolver  is	tried.
			   The	current domain name is defined by the hostname
			   if not specified in the configuration file; it  can
			   be  overridden  by  the  environment	 variable This
			   environment variable may contain several blank sep‐
			   arated  tokens  and	overrides the search list on a
			   per process basis. This is similar to  the  command
			   in  the  configuration  file.   Another environment
			   variable such as, can be set	 to  override  certain
			   internal  resolver options which are set by calling
			   some of the configuration  routines	above,	or  by
			   using  the configuration file's command. The syntax
			   of  the  environment	 variable  is	explained   in
			   resolver(4).	 The entries for retrans and retry can
			   be  overridden  by  and   Environmental   Variables

       Provides an interface to the server query mechanism.
			   It  constructs  a  query,  sends  it	 to  the local
			   server, awaits a response,  and  makes  preliminary
			   checks  on  the reply.  The query requests informa‐
			   tion of the specified type and class for the speci‐
			   fied	 fully-qualified domain name dname.  The reply
			   message is left in the answer  buffer  with	length
			   anslen supplied by the caller.

       Makes a query and awaits a response much like
			   but	in  addition,  it  implements  the default and
			   search rules controlled by  the  and	 options.   It
			   returns the first successful reply.

       Sets the value for the
			   retry  and  retrans	fields	in  the structure. The
			   value for the retrans option must be	 specified  in
			   milliseconds.  This routine also validates the val‐
			   ues which the user tries to set for the  retry  and
			   retrans  options.  returns on success, and on fail‐
			   ure.	 Calls to fails	 when  values  for  the	 field
			   passed  as  argument	 is  already set by any higher
			   precedence option like entering name-value pairs in
			   the file or setting the Environmental Variables and

       Get the value for the
			   retry and retrans fields in the structure.  returns
			   the value of the field requested on success and  if
			   on  failure.	  It  fails  when the arguments do not
			   refer to retrans or retry.

   Other Routines
       Routines described here are lower-level routines used by

       Constructs a standard query message and places it in
			   buf.	 It returns the size of the query,  or	−1  if
			   the query is larger than buflen.  The query type op
			   is usually but  can	be  any	 of  the  query	 types
			   defined  in	The domain name for the query is given
			   by dname.  class can be any of  the	query  classes
			   defined  in	type  can  be  any  of the query types
			   defined in data is the data for  an	inverse	 query
			   newrr  is currently unused but is intended for mak‐
			   ing update messages.

       Sends a pre-formatted query and returns an answer.
			   It calls if is not set,  sends  the	query  to  the
			   local   name	  server,  and	handles	 timeouts  and
			   retries.  returns the length of the reply  message,
			   or −1 if there were errors.

       Compresses the domain name
			   exp_dn  and	stores it in comp_dn.  The size of the
			   compressed name is returned or  −1  if  there  were
			   errors.  length is the size of the array pointed to
			   by comp_dn.	 The  compression  uses	 an  array  of
			   pointers  dnptrs  to previously compressed names in
			   the current message.	 The first pointer  points  to
			   to  the  beginning of the message and the list ends
			   with NULL.  The limit to the array is specified  by
			   lastdnptr.	A side effect of is to update the list
			   of pointers for labels inserted into the message as
			   the	name  is  compressed.  If dnptr is NULL, names
			   are not compressed.	If lastdnptr is NULL, the list
			   of labels is not updated.

       Expands the compressed domain name
			   comp_dn to a full domain name.  The compressed name
			   is contained in a query or reply message; msg is  a
			   pointer  to	the  beginning	of  the	 message.  The
			   uncompressed name is placed in the buffer indicated
			   by  exp_dn  which  is  of size length.  The size of
			   compressed name is returned or −1 if there  was  an

   Obsolescent Routine
	      supports	 existing  applications	 to  print  an	error  message
				  a failure.  ANSI applications	 must  specify
				  the following definition for

				  The argument string s is printed first, fol‐
				  lowed by a colon, a blank, the message,  and
				  a   new-line.	  may  be  removed  in	future

       Error return status from is indicated by a return  value	 of  −1.   The
       external integer can then be checked to see whether this is a temporary
       failure or an invalid or unknown host.

       In a multithreaded application using kernel thread, a  thread  specific
       is allocated for each thread.

       can have the following values:

	      No such host is known.

	      This  is	usually	 a  temporary  error  and means that the local
				  did not receive a response from an  authori‐
				  tative  server.   A retry at some later time
				  may succeed.

	      Some unexpected server failure was encountered.
				  This is a non-recoverable error.

	      The name is known to the name server,
				  but there is no data of the  requested  type
				  associated  with  this  name;	 this is not a
				  temporary error.  Another type of request to
				  the  name server using this domain name will
				  result in an answer.

       The field should be modified only by using  call,  and  should  not  be
       manipulated  directly.	is  referenced as an for non-threaded applica‐
       tions and is defined as function call macro for multithreaded  applica‐
       tions in file Applications which reference should include

       is  referenced  as  an  for non-threaded applications and is defined as
       function call macro for multithreaded application in file  Applications
       which reference should include

       These resolver routines were developed by the University of California,

       Resolver configuration file.

       named(1M), gethostent(3N), resolver(4), hostname(5),  thread_safety(5),
       RFC1034, RFC1035, RFC1535.


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