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NLnet Labs
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NAME

Net::DNS::Resolver - DNS resolver class

SYNOPSIS

    use Net::DNS;

    $resolver = new Net::DNS::Resolver();

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

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

    # Perform a lookup, without pre or post-processing
    $reply = $resolver->send( 'example.com', 'MX', 'IN' );

    # Send a prebuilt query packet
    $query = new Net::DNS::Packet( ... );
    $reply = $resolver->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 default configuration
    $resolver = new Net::DNS::Resolver();

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

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

Returns a resolver object. If no arguments are supplied, new() returns an object having the default configuration.

On Unix and Linux systems, the default values are read from the following files, in the order indicated:

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

The following keywords are recognised in resolver configuration files:

domain

The default domain.

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

nameserver

A space-separated list of nameservers to query.

options

A space-separated list of key:value items.

Except for /etc/resolv.conf, files will only be read if owned by the effective userid running the program. In addition, several environment variables may contain configuration information; see "ENVIRONMENT".

Note that the domain and searchlist keywords are mutually exclusive. If both are present, the resulting behaviour is unspecified.

On Windows systems, an attempt is made to determine the system defaults using the registry. 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
    $resolver = new Net::DNS::Resolver( config_file => '/my/dns.conf' );

This is supported on both Unix and Windows.

If a custom configuration file is specified at first instantiation, both the system configuration and environment variables are ignored.

Explicit arguments to new() override the corresponding configuration variables. The following arguments are supported:

nameservers

A reference to an array of nameservers to query.

domain

Domain name suffix to be appended to queries of unqualified names.

searchlist

A reference to an array of domains to search for unqualified names.

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

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

print

    $resolver->print;

Prints the resolver state on the standard output.

query

    $packet = $resolver->query( 'mailhost' );
    $packet = $resolver->query( 'mailhost.example.com' );
    $packet = $resolver->query( '192.0.2.1' );
    $packet = $resolver->query( 'example.com', 'MX' );
    $packet = $resolver->query( 'annotation.example.com', 'TXT', 'IN' );

Performs a DNS query for the given name; the search list is not applied. If the name does not contain any dots and defnames is true, 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 a query within in-addr.arpa or ip6.arpa will be performed.

Returns a Net::DNS::Packet object, or undef if no answers were found. The reason for failure may be determined using errorstring().

If you need to examine the response packet, whether it contains any answers or not, use the send() method instead.

search

    $packet = $resolver->search( 'mailhost' );
    $packet = $resolver->search( 'mailhost.example.com' );
    $packet = $resolver->search( '192.0.2.1' );
    $packet = $resolver->search( 'example.com', 'MX' );
    $packet = $resolver->search( 'annotation.example.com', 'TXT', 'IN' );

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 does not end in a dot, try appending each item in the search list to the name. This is only done if dnsrch is true.

  3. If the name does not 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 (IPv4 or IPv6), then a query within in-addr.arpa or ip6.arpa will be performed.

Returns a Net::DNS::Packet object, or undef if no answers were found. The reason for failure may be determined using errorstring().

If you need to examine the response packet, whether it contains any answers or not, use the send() method instead.

send

    $packet = $resolver->send( $packet );

    $packet = $resolver->send( 'mailhost.example.com' );
    $packet = $resolver->query( '192.0.2.1' );
    $packet = $resolver->send( 'example.com', 'MX' );
    $packet = $resolver->send( 'annotation.example.com', 'TXT', 'IN' );

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 a query within in-addr.arpa or ip6.arpa 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 no response was received.

axfr

    @zone = $resolver->axfr();
    @zone = $resolver->axfr( 'example.com' );
    @zone = $resolver->axfr( 'example.com', 'IN' );

    $iterator = $resolver->axfr();
    $iterator = $resolver->axfr( 'example.com' );
    $iterator = $resolver->axfr( 'example.com', 'IN' );

    $rr = $iterator->();

Performs a zone transfer using the resolver nameservers list, attempted in the order listed.

If the zone is omitted, it defaults to the first zone listed in the resolver search list.

If the class is omitted, it defaults to IN.

When called in list context, axfr() returns a list of Net::DNS::RR objects. The redundant SOA record that terminates the zone transfer is not returned to the caller.

In deferrence to RFC1035(6.3), a complete zone transfer is expected to return all records in the zone or nothing at all. When no resource records are returned by axfr(), the reason for failure may be determined using errorstring().

Here is an example that uses a timeout and TSIG verification:

    $resolver->tcp_timeout( 10 );
    $resolver->tsig( 'Khmac-sha1.example.+161+24053.private' );
    @zone = $resolver->axfr( 'example.com' );

    foreach $rr (@zone) {
        $rr->print;
    }

When called in scalar context, axfr() returns an iterator object. Each invocation of the iterator returns a single Net::DNS::RR or undef when the zone is exhausted.

An exception is raised if the zone transfer can not be completed.

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

Here is the example above, implemented using an iterator:

    $resolver->tcp_timeout( 10 );
    $resolver->tsig( 'Khmac-sha1.example.+161+24053.private' );
    $iterator = $resolver->axfr( 'example.com' );

    while ( $rr = $iterator->() ) {
        $rr->print;
    }

bgsend

    $handle = $resolver->bgsend( $packet ) || die $resolver->errorstring;

    $handle = $resolver->bgsend( 'mailhost.example.com' );
    $handle = $resolver->bgsend( '192.0.2.1' );
    $handle = $resolver->bgsend( 'example.com', 'MX' );
    $handle = $resolver->bgsend( 'annotation.example.com', 'TXT', 'IN' );

Performs a background DNS query for the given name and returns immediately without waiting for the response. The program can then perform other tasks while awaiting the 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 (IPv4 or IPv6), then a query within in-addr.arpa or ip6.arpa will be performed.

Returns an opaque handle which is passed to subsequent invocations of the bgbusy and bgread methods. Errors are indicated by returning undef in which case the reason for failure may be determined using errorstring().

The program may determine when the handle is ready for reading by calling bgbusy.

The response Net::DNS::Packet object is obtained by calling bgread.

BEWARE: Programs should make no assumptions about the nature of the handles returned by bgsend which should be used strictly as described here.

bgread

    $packet = $resolver->bgread($handle);

Reads the answer from a background query. The argument is the handle returned by bgsend.

Returns a Net::DNS::Packet object or undef if no response was received or timeout occurred.

bgbusy

    $handle = $resolver->bgsend( 'foo.example.com' );

    while ($resolver->bgbusy($handle)) {
        ...
    }

    $packet = $resolver->bgread($handle);

Returns true while awaiting the response or for the transaction to time out. The argument is the handle returned by bgsend.

Truncated UDP packets will be retried over TCP transparently while continuing to assert busy to the caller.

bgisready

    until ($resolver->bgisready($handle)) {
        ...
    }

bgisready is the logical complement of bgbusy which is retained for backward compatibility.

debug

    print 'debug flag: ', $resolver->debug, "\n";
    $resolver->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.

defnames

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

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

dnsrch

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

Get or set the dnsrch flag. If true, calls to search will apply the search list to resolve names that are not fully qualified. The default is true.

igntc

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

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

nameservers

    @nameservers = $resolver->nameservers();
    $resolver->nameservers( '192.0.2.1', '192.0.2.2', '2001:DB8::3' );

Gets or sets the nameservers to be queried.

Also see the IPv6 transport notes below

persistent_tcp

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

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

The default is false unless you are running a SOCKSified Perl, in which case the default is true.

persistent_udp

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

Get or set the persistent UDP setting. If true, a Net::DNS resolver will use the same UDP socket for all queries within each address family.

This avoids the cost of creating and tearing down UDP sockets, but also defeats source port randomisation.

port

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

Gets or sets the port to which queries are sent. Convenient for nameserver testing using a non-standard port. The default is port 53.

recurse

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

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

retrans

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

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

retry

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

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

searchlist

    @searchlist = $resolver->searchlist;
    $resolver->searchlist( 'a.example', 'b.example', 'c.example' );

Gets or sets the resolver search list.

srcaddr

    $resolver->srcaddr('192.0.2.1');

Sets the source address from which queries are sent. Convenient for forcing queries from a specific interface on a multi-homed host. The default is to use any local address.

srcport

    $resolver->srcport(5353);

Sets the port from which queries are sent. The default is 0, meaning any port.

tcp_timeout

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

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

udp_timeout

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

Get or set the UDP timeout in seconds. The default is undef, which means that the retry and retrans settings will be used to perform the retries until they exhausted.

udppacketsize

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

udppacketsize will set or get the packet size. If set to a value greater than the default DNS packet size, an EDNS extension will be added indicating support for UDP fragment reassembly.

usevc

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

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

answerfrom

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

Returns the IP address from which the most recent packet was received in response to a query.

answersize

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

Returns the size in bytes of the most recent packet received in response to a query.

errorstring

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

Returns a string containing error information from the most recent method call. errorstring() is meaningful only when interrogated immediately after an error.

dnssec

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

The dnssec flag causes the resolver to transmit DNSSEC queries and to add a EDNS0 record as required by RFC2671 and RFC3225. The actions of, and response from, the remote nameserver is determined by the settings of the AD and CD flags.

Calling the dnssec() method with a non-zero value will also 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.

   $resolver->dnssec(1);                # DNSSEC using default packetsize
   $resolver->udppacketsize(1250);      # lower the UDP packet size

A fatal exception will be raised if the dnssec() method is called but the Net::DNS::SEC library has not been installed.

adflag

    $resolver->dnssec(1);
    $resolver->adflag(1);
    print "authentication desired flag: ", $resolver->adflag, "\n";

Gets or sets the AD bit for dnssec queries. This bit indicates that the caller is interested in the returned AD (authentic data) bit but does not require any dnssec RRs to be included in the response. The default value is 0.

cdflag

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

Gets or sets the CD bit for dnssec queries. This bit indicates that authentication by upstream nameservers should be suppressed. Any dnssec RRs required to execute the authentication procedure should be included in the response. The default value is 0.

tsig

    $resolver->tsig( $tsig );

    $resolver->tsig( 'Khmac-sha1.example.+161+24053.private' );

    $resolver->tsig( 'Khmac-sha1.example.+161+24053.key' );

    $resolver->tsig( 'Khmac-sha1.example.+161+24053.key',
                fudge => 60
                );

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

    $resolver->tsig( undef );

Set the TSIG record used to automatically sign outgoing queries, zone transfers and updates. Automatic signing is disabled if called with undefined arguments.

The default resolver behaviour is not to sign any packets. You must call this method to set the key if you would like the resolver to sign and verify packets automatically.

Packets can also be signed 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.

ENVIRONMENT

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

RES_NAMESERVERS

    # Bourne Shell
    RES_NAMESERVERS="192.0.2.1 192.0.2.2 2001:DB8::3"
    export RES_NAMESERVERS

    # C Shell
    setenv RES_NAMESERVERS "192.0.2.1 192.0.2.2 2001:DB8::3"

A space-separated list of nameservers to query.

RES_SEARCHLIST

    # Bourne Shell
    RES_SEARCHLIST="a.example.com b.example.com c.example.com"
    export RES_SEARCHLIST

    # C Shell
    setenv RES_SEARCHLIST "a.example.com b.example.com c.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 inet6"
    export RES_OPTIONS

    # C Shell
    setenv RES_OPTIONS "retrans:3 retry:2 inet6"

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

IPv6 TRANSPORT

The Net::DNS::Resolver library will enable IPv6 transport if the appropriate libraries (IO::Socket::INET6 or IO::Socket::IP) are available and the destination nameserver has an IPv6 address.

The force_v4(), force_v6(),prefer_v4 and prefer_v6() methods with a non-zero argument may be used to configure transport selection.

The behaviour of the nameserver() method illustrates the transport selection mechanism. If, for example, IPv6 is not available or IPv4 transport has been forced, the nameserver() method will only return IPv4 addresses:

    $resolver->nameservers( '192.0.2.1', '192.0.2.2', '2001:DB8::3' );
    $resolver->force_v4(1);
    print join ' ', $resolver->nameservers();

will print

    192.0.2.1 192.0.2.2

CUSTOMISED RESOLVERS

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 can simply add a method to the namespace!

For example, if we wanted to cache lookups:

    package Net::DNS::Resolver;

    my %cache;

    sub search {
        $self = shift;

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

COPYRIGHT

Copyright (c)1997-2000 Michael Fuhr.

Portions Copyright (c)2002-2004 Chris Reinhardt.

Portions Copyright (c)2005 Olaf M. Kolkman, NLnet Labs.

Portions Copyright (c)2014,2015 Dick Franks.

All rights reserved.

LICENSE

Permission to use, copy, modify, and distribute this software and its documentation for any purpose and without fee is hereby granted, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation, and that the name of the author not be used in advertising or publicity pertaining to distribution of the software without specific prior written permission.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

SEE ALSO

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