Rob Brown > Net-DNS-0.19 > Net::DNS::Resolver

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NAME ^

Net::DNS::Resolver - DNS resolver class

SYNOPSIS ^

use Net::DNS::Resolver;

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.

Resolver configuration is 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".

METHODS ^

new

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

Creates a new DNS resolver object.

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", "sub1.example.com", "sub2.example.com");

Gets or sets the resolver search list.

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.

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.

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.

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 (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.

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 (4 dot-separated numbers), 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.

bgsend

    $socket = $res->bgsend($packet_object);
    $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. 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.

bgread

    $packet = $res->bgread($socket);
    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);
    $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.

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);
    @zone = $res->axfr("example.com");
    if (@zone) {
        foreach $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.

Returns the IO::Socket::INET object that will be used for reading, or undef on error.

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

axfr_next

    $res->axfr_start("example.com");
    while ($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".

tsig

    $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).

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.

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.

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.

COPYRIGHT ^

Copyright (c) 1997-2000 Michael Fuhr. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

SEE ALSO ^

perl(1), 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

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