Unix/Linux Go Back    


CentOS 7.0 - man page for net::dns::resolver (centos section 3)

Linux & Unix Commands - Search Man Pages
Man Page or Keyword Search:   man
Select Man Page Set:       apropos Keyword Search (sections above)


Net::DNS::Resolver(3)	       User Contributed Perl Documentation	    Net::DNS::Resolver(3)

NAME
       Net::DNS::Resolver - DNS resolver class

SYNOPSIS
	 use Net::DNS;

	 my $res = Net::DNS::Resolver->new;

	 # Perform a lookup, using the searchlist if appropriate.
	 my $answer = $res->search('example.com');

	 # Perform a lookup, without the searchlist
	 my $answer = $res->query('example.com', 'MX');

	 # Perform a lookup, without pre or post-processing
	 my $answer = $res->send('example.com', 'MX', 'CH');

	 # Send a prebuilt packet
	 my $packet = Net::DNS::Packet->new(...);
	 my $answer = $res->send($packet);

DESCRIPTION
       Instances of the "Net::DNS::Resolver" class represent resolver objects.	A program can
       have multiple resolver objects, each maintaining its own state information such as the
       nameservers to be queried, whether recursion is desired, etc.

METHODS
   new
	 # Use the system defaults
	 my $res = Net::DNS::Resolver->new;

	 # Use my own configuration file
	 my $res = Net::DNS::Resolver->new(config_file => '/my/dns.conf');

	 # Set options in the constructor
	 my $res = Net::DNS::Resolver->new(
	       nameservers => [qw(10.1.1.128 10.1.2.128)],
	       recurse	   => 0,
	       debug	   => 1,
	 );

       Returns a resolver object.  If given no arguments, "new()" returns an object configured to
       your system's defaults.	On UNIX systems the defaults are read from the following files,
       in the order indicated:

	   /etc/resolv.conf
	   $HOME/.resolv.conf
	   ./.resolv.conf

       The following keywords are recognized in resolver configuration files:

       domain
	   The default domain.

       search
	   A space-separated list of domains to put in the search list.

       nameserver
	   A space-separated list of nameservers to query.

       Files except for /etc/resolv.conf must be owned by the effective userid running the
       program or they won't be read.  In addition, several environment variables can also
       contain configuration information; see "ENVIRONMENT".

       On Windows systems, an attempt is made to determine the system defaults using the
       registry.  This is still a work in progress; systems with many dynamically configured
       network interfaces may confuse Net::DNS.

       You can include a configuration file of your own when creating a resolver object:

	# Use my own configuration file
	my $res = Net::DNS::Resolver->new(config_file => '/my/dns.conf');

       This is supported on both UNIX and Windows.  Values pulled from a custom configuration
       file override the the system's defaults, but can still be overridden by the other
       arguments to new().

       Explicit arguments to new override both the system's defaults and the values of the custom
       configuration file, if any.  The following arguments to new() are supported:

       nameservers
	   An array reference of nameservers to query.

       searchlist
	   An array reference of domains.

       recurse
       debug
       domain
       port
       srcaddr
       srcport
       tcp_timeout
       udp_timeout
       retrans
       retry
       usevc
       stayopen
       igntc
       defnames
       dnsrch
       persistent_tcp
       persistent_udp
       dnssec

       For more information on any of these options, please consult the method of the same name.

   search
	   $packet = $res->search('mailhost');
	   $packet = $res->search('mailhost.example.com');
	   $packet = $res->search('192.168.1.1');
	   $packet = $res->search('example.com', 'MX');
	   $packet = $res->search('user.passwd.example.com', 'TXT', 'HS');

       Performs a DNS query for the given name, applying the searchlist if appropriate.  The
       search algorithm is as follows:

       1.  If the name contains at least one dot, try it as is.

       2.  If the name doesn't end in a dot then append each item in the search list to the name.
	   This is only done if dnsrch is true.

       3.  If the name doesn't contain any dots, try it as is.

       The record type and class can be omitted; they default to A and IN.  If the name looks
       like an IP address (4 dot-separated numbers), then an appropriate PTR query will be
       performed.

       Returns a "Net::DNS::Packet" object, or "undef" if no answers were found.  If you need to
       examine the response packet whether it contains any answers or not, use the send() method
       instead.

   query
	   $packet = $res->query('mailhost');
	   $packet = $res->query('mailhost.example.com');
	   $packet = $res->query('192.168.1.1');
	   $packet = $res->query('example.com', 'MX');
	   $packet = $res->query('user.passwd.example.com', 'TXT', 'HS');

       Performs a DNS query for the given name; the search list is not applied.  If the name
       doesn't contain any dots and defnames is true then the default domain will be appended.

       The record type and class can be omitted; they default to A and IN.  If the name looks
       like an IP address (IPv4 or IPv6), then an appropriate PTR query will be performed.

       Returns a "Net::DNS::Packet" object, or "undef" if no answers were found.  If you need to
       examine the response packet whether it contains any answers or not, use the send() method
       instead.

   send
	   $packet = $res->send($packet_object);
	   $packet = $res->send('mailhost.example.com');
	   $packet = $res->send('example.com', 'MX');
	   $packet = $res->send('user.passwd.example.com', 'TXT', 'HS');

       Performs a DNS query for the given name.  Neither the searchlist nor the default domain
       will be appended.

       The argument list can be either a "Net::DNS::Packet" object or a list of strings.  The
       record type and class can be omitted; they default to A and IN.	If the name looks like an
       IP address (Ipv4 or IPv6), then an appropriate PTR query will be performed.

       Returns a "Net::DNS::Packet" object whether there were any answers or not.  Use
       "$packet->header->ancount" or "$packet->answer" to find out if there were any records in
       the answer section.  Returns "undef" if there was an error.

   axfr
	   @zone = $res->axfr;
	   @zone = $res->axfr('example.com');
	   @zone = $res->axfr('passwd.example.com', 'HS');

       Performs a zone transfer from the first nameserver listed in "nameservers".  If the zone
       is omitted, it defaults to the first zone listed in the resolver's search list.	If the
       class is omitted, it defaults to IN.

       Returns a list of "Net::DNS::RR" objects, or "undef" if the zone transfer failed.

       The redundant SOA record that terminates the zone transfer is not returned to the caller.

       See also "axfr_start" and "axfr_next".

       Here's an example that uses a timeout:

	   $res->tcp_timeout(10);
	   my @zone = $res->axfr('example.com');

	   if (@zone) {
	       foreach my $rr (@zone) {
		   $rr->print;
	       }
	   } else {
	       print 'Zone transfer failed: ', $res->errorstring, "\n";
	   }

   axfr_start
	   $res->axfr_start;
	   $res->axfr_start('example.com');
	   $res->axfr_start('example.com', 'HS');

       Starts a zone transfer from the first nameserver listed in "nameservers".  If the zone is
       omitted, it defaults to the first zone listed in the resolver's search list.  If the class
       is omitted, it defaults to IN.

       IMPORTANT:

       This method currently returns the "IO::Socket::INET" object that will be used for reading,
       or "undef" on error.  DO NOT DEPEND ON "axfr_start()" returning a socket object.  THIS
       MIGHT CHANGE in future releases.

       Use "axfr_next" to read the zone records one at a time.

   axfr_next
	   $res->axfr_start('example.com');

	   while (my $rr = $res->axfr_next) {
		   $rr->print;
	   }

       Reads records from a zone transfer one at a time.

       Returns "undef" at the end of the zone transfer.  The redundant SOA record that terminates
       the zone transfer is not returned.

       See also "axfr".

   nameservers
	   @nameservers = $res->nameservers;
	   $res->nameservers('192.168.1.1', '192.168.2.2', '192.168.3.3');

       Gets or sets the nameservers to be queried.

       Also see the IPv6 transport notes below

   empty_nameservers
	   $res->empty_nameservers();

       Empties the list of nameservers.

   print
	   $res->print;

       Prints the resolver state on the standard output.

   string
	   print $res->string;

       Returns a string representation of the resolver state.

   searchlist
	   @searchlist = $res->searchlist;
	   $res->searchlist('example.com', 'a.example.com', 'b.example.com');

       Gets or sets the resolver search list.

   empty_searchlist
	   $res->empty_searchlist();

       Empties the searchlist.

   port
	   print 'sending queries to port ', $res->port, "\n";
	   $res->port(9732);

       Gets or sets the port to which we send queries.	This can be useful for testing a
       nameserver running on a non-standard port.  The default is port 53.

   srcport
	   print 'sending queries from port ', $res->srcport, "\n";
	   $res->srcport(5353);

       Gets or sets the port from which we send queries.  The default is 0, meaning any port.

   srcaddr
	   print 'sending queries from address ', $res->srcaddr, "\n";
	   $res->srcaddr('192.168.1.1');

       Gets or sets the source address from which we send queries.  Convenient for forcing
       queries out a specific interfaces on a multi-homed host.  The default is 0.0.0.0, meaning
       any local address.

   bgsend
	   $socket = $res->bgsend($packet_object) || die " $res->errorstring";

	   $socket = $res->bgsend('mailhost.example.com');
	   $socket = $res->bgsend('example.com', 'MX');
	   $socket = $res->bgsend('user.passwd.example.com', 'TXT', 'HS');

       Performs a background DNS query for the given name, i.e., sends a query packet to the
       first nameserver listed in "$res->nameservers" and returns immediately without waiting for
       a response.  The program can then perform other tasks while waiting for a response from
       the nameserver.

       The argument list can be either a "Net::DNS::Packet" object or a list of strings.  The
       record type and class can be omitted; they default to A and IN.	If the name looks like an
       IP address (4 dot-separated numbers), then an appropriate PTR query will be performed.

       Returns an "IO::Socket::INET" object or "undef" on error in which case the reason for
       failure can be found through a call to the errorstring method.

       The program must determine when the socket is ready for reading and call "$res->bgread" to
       get the response packet.  You can use "$res->bgisready" or "IO::Select" to find out if the
       socket is ready before reading it.

       bgsend does not support persistent sockets.

       BEWARE: bgsend does not support the usevc option (TCP) and operates on UDP only; Answers
       may not fit in an UDP packet and might be truncated. Truncated packets will not be retried
       over TCP automatically and should be handled by the caller.

   bgread
	   $packet = $res->bgread($socket);
	   if ($packet->header->tc) {
	       # Retry over TCP (blocking).
	   }
	   undef $socket;

       Reads the answer from a background query (see "bgsend").  The argument is an "IO::Socket"
       object returned by "bgsend".

       Returns a "Net::DNS::Packet" object or "undef" on error.

       The programmer should close or destroy the socket object after reading it.

   bgisready
	   $socket = $res->bgsend('foo.example.com');
	   until ($res->bgisready($socket)) {
	       # do some other processing
	   }
	   $packet = $res->bgread($socket);
	   if ($packet->header->tc) {
	       # Retry over TCP (blocking).
	   }
	   $socket = undef;

       Determines whether a socket is ready for reading.  The argument is an "IO::Socket" object
       returned by "$res->bgsend".

       Returns true if the socket is ready, false if not.

   tsig
	   my $tsig = $res->tsig;

	   $res->tsig(Net::DNS::RR->new("$key_name TSIG $key"));

	   $tsig = Net::DNS::RR->new("$key_name TSIG $key");
	   $tsig->fudge(60);
	   $res->tsig($tsig);

	   $res->tsig($key_name, $key);

	   $res->tsig(0);

       Get or set the TSIG record used to automatically sign outgoing queries and updates.  Call
       with an argument of 0 or '' to turn off automatic signing.

       The default resolver behavior is not to sign any packets.  You must call this method to
       set the key if you'd like the resolver to sign packets automatically.

       You can also sign packets manually -- see the "Net::DNS::Packet" and "Net::DNS::Update"
       manual pages for examples.  TSIG records in manually-signed packets take precedence over
       those that the resolver would add automatically.

   retrans
	   print 'retrans interval: ', $res->retrans, "\n";
	   $res->retrans(3);

       Get or set the retransmission interval.	The default is 5.

   retry
	   print 'number of tries: ', $res->retry, "\n";
	   $res->retry(2);

       Get or set the number of times to try the query.  The default is 4.

   recurse
	   print 'recursion flag: ', $res->recurse, "\n";
	   $res->recurse(0);

       Get or set the recursion flag.  If this is true, nameservers will be requested to perform
       a recursive query.  The default is true.

   defnames
	   print 'defnames flag: ', $res->defnames, "\n";
	   $res->defnames(0);

       Get or set the defnames flag.  If this is true, calls to query will append the default
       domain to names that contain no dots.  The default is true.

   dnsrch
	   print 'dnsrch flag: ', $res->dnsrch, "\n";
	   $res->dnsrch(0);

       Get or set the dnsrch flag.  If this is true, calls to search will apply the search list.
       The default is true.

   debug
	   print 'debug flag: ', $res->debug, "\n";
	   $res->debug(1);

       Get or set the debug flag.  If set, calls to search, query, and send will print debugging
       information on the standard output.  The default is false.

   usevc
	   print 'usevc flag: ', $res->usevc, "\n";
	   $res->usevc(1);

       Get or set the usevc flag.  If true, then queries will be performed using virtual circuits
       (TCP) instead of datagrams (UDP).  The default is false.

   tcp_timeout
	   print 'TCP timeout: ', $res->tcp_timeout, "\n";
	   $res->tcp_timeout(10);

       Get or set the TCP timeout in seconds.  A timeout of "undef" means indefinite.  The
       default is 120 seconds (2 minutes).

   udp_timeout
	   print 'UDP timeout: ', $res->udp_timeout, "\n";
	   $res->udp_timeout(10);

       Get or set the UDP timeout in seconds.  A timeout of "undef" means the retry and retrans
       settings will be just utilized to perform the retries until they are exhausted.	The
       default is "undef".

   persistent_tcp
	   print 'Persistent TCP flag: ', $res->persistent_tcp, "\n";
	   $res->persistent_tcp(1);

       Get or set the persistent TCP setting.  If set to true, Net::DNS will keep a TCP socket
       open for each host:port to which it connects.  This is useful if you're using TCP and need
       to make a lot of queries or updates to the same nameserver.

       This option defaults to false unless you're running under a SOCKSified Perl, in which case
       it defaults to true.

   persistent_udp
	   print 'Persistent UDP flag: ', $res->persistent_udp, "\n";
	   $res->persistent_udp(1);

       Get or set the persistent UDP setting.  If set to true, Net::DNS will keep a single UDP
       socket open for all queries.  This is useful if you're using UDP and need to make a lot of
       queries or updates.

   igntc
	   print 'igntc flag: ', $res->igntc, "\n";
	   $res->igntc(1);

       Get or set the igntc flag.  If true, truncated packets will be ignored.	If false,
       truncated packets will cause the query to be retried using TCP.	The default is false.

   errorstring
	   print 'query status: ', $res->errorstring, "\n";

       Returns a string containing the status of the most recent query.

   answerfrom
	   print 'last answer was from: ', $res->answerfrom, "\n";

       Returns the IP address from which we received the last answer in response to a query.

   answersize
	   print 'size of last answer: ', $res->answersize, "\n";

       Returns the size in bytes of the last answer we received in response to a query.

   dnssec
	   print "dnssec flag: ", $res->dnssec, "\n";
	   $res->dnssec(0);

       Enabled DNSSEC this will set the checking disabled flag in the query header and add EDNS0
       data as in RFC2671 and RFC3225

       When set to true the answer and additional section of queries from secured zones will
       contain DNSKEY, NSEC and RRSIG records.

       Setting calling the dnssec method with a non-zero value will set the UDP packet size to
       the default value of 2048. If that is too small or too big for your environment you should
       call the udppacketsize() method immediately after.

	  $res->dnssec(1);    # turns on DNSSEC and sets udp packetsize to 2048
	  $res->udppacketsize(1028);   # lowers the UDP pakcet size

       The method will Croak::croak with the message "You called the Net::DNS::Resolver::dnssec()
       method but do not have Net::DNS::SEC installed at ..." if you call it without
       Net::DNS::SEC being in your @INC path.

   cdflag
	   print "checking disabled flag: ", $res->dnssec, "\n";
	   $res->dnssec(1);
	   $res->cdflag(1);

       Sets or gets the CD bit for a dnssec query.  This bit is always zero for non dnssec
       queries. When the dnssec is enabled the flag defaults to 0 can be set to 1.

   adflag
	   print "checking disabled flag: ", $res->dnssec, "\n";
	   $res->dnssec(1);
	   $res->adflag(1);

       Sets or gets the AD bit for a dnssec query.  This bit is always zero for non dnssec
       queries. When the dnssec is enabled the flag defaults to 1.

   udppacketsize
	   print "udppacketsize: ", $res->udppacketsize, "\n";
	   $res->udppacketsize(2048);

       udppacketsize will set or get the packet size. If set to a value greater than
       Net::DNS::PACKETSZ() an EDNS extension will be added indicating support for MTU path
       recovery.

       Default udppacketsize is Net::DNS::PACKETSZ() (512)

CUSTOMIZING
       Net::DNS::Resolver is actually an empty subclass.  At compile time a super class is chosen
       based on the current platform.  A side benefit of this allows for easy modification of the
       methods in Net::DNS::Resolver.  You simply add a method to the namespace!

       For example, if we wanted to cache lookups:

	package Net::DNS::Resolver;

	my %cache;

	sub search {
	       my ($self, @args) = @_;

	       return $cache{@args} ||= $self->SUPER::search(@args);
	}

IPv6 transport
       The Net::DNS::Resolver library will use IPv6 transport if the appropriate libraries
       (Socket6 and IO::Socket::INET6) are available and the address the server tries to connect
       to is an IPv6 address.

       The print() will method will report if IPv6 transport is available.

       You can use the force_v4() method with a non-zero argument to force IPv4 transport.

       The nameserver() method has IPv6 dependend behavior. If IPv6 is not available or IPv4
       transport has been forced the nameserver() method will only return IPv4 addresses.

       For example

	   $res->nameservers('192.168.1.1', '192.168.2.2', '2001:610:240:0:53:0:0:3');
	   $res->force_v4(1);
	   print join (" ",$res->nameserver());

       Will print: 192.168.1.1 192.168.2.2

ENVIRONMENT
       The following environment variables can also be used to configure the resolver:

   RES_NAMESERVERS
	   # Bourne Shell
	   RES_NAMESERVERS="192.168.1.1 192.168.2.2 192.168.3.3"
	   export RES_NAMESERVERS

	   # C Shell
	   setenv RES_NAMESERVERS "192.168.1.1 192.168.2.2 192.168.3.3"

       A space-separated list of nameservers to query.

   RES_SEARCHLIST
	   # Bourne Shell
	   RES_SEARCHLIST="example.com sub1.example.com sub2.example.com"
	   export RES_SEARCHLIST

	   # C Shell
	   setenv RES_SEARCHLIST "example.com sub1.example.com sub2.example.com"

       A space-separated list of domains to put in the search list.

   LOCALDOMAIN
	   # Bourne Shell
	   LOCALDOMAIN=example.com
	   export LOCALDOMAIN

	   # C Shell
	   setenv LOCALDOMAIN example.com

       The default domain.

   RES_OPTIONS
	   # Bourne Shell
	   RES_OPTIONS="retrans:3 retry:2 debug"
	   export RES_OPTIONS

	   # C Shell
	   setenv RES_OPTIONS "retrans:3 retry:2 debug"

       A space-separated list of resolver options to set.  Options that take values are specified
       as option:value.

BUGS
       Error reporting and handling needs to be improved.

       The current implementation supports TSIG only on outgoing packets.  No validation of
       server replies is performed.

       bgsend does not honor the usevc flag and only uses UDP for transport.

COPYRIGHT
       Copyright (c) 1997-2002 Michael Fuhr.

       Portions Copyright (c) 2002-2004 Chris Reinhardt.  Portions Copyright (c) 2005 Olaf M.
       Kolkman, NLnet Labs.

       All rights reserved.  This program is free software; you may redistribute it and/or modify
       it under the same terms as Perl itself.

SEE ALSO
       perl, Net::DNS, Net::DNS::Packet, Net::DNS::Update, Net::DNS::Header, Net::DNS::Question,
       Net::DNS::RR, resolver(5), RFC 1035, RFC 1034 Section 4.3.5

perl v5.16.3				    2012-12-28			    Net::DNS::Resolver(3)
Unix & Linux Commands & Man Pages : ©2000 - 2018 Unix and Linux Forums


All times are GMT -4. The time now is 12:25 PM.