Net::DNS::Resolver(3) User Contributed Perl Documentation Net::DNS::Resolver(3)
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.
item 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, "
";
$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, "
";
$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, "
";
$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 name-
server.
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, "
";
}
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 manu-
ally-signed packets take precedence over those that the resolver would add automatically.
retrans
print 'retrans interval: ', $res->retrans, "
";
$res->retrans(3);
Get or set the retransmission interval. The default is 5.
retry
print 'number of tries: ', $res->retry, "
";
$res->retry(2);
Get or set the number of times to try the query. The default is 4.
recurse
print 'recursion flag: ', $res->recurse, "
";
$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, "
";
$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, "
";
$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, "
";
$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, "
";
$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, "
";
$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, "
";
$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, "
";
$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, "
";
$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, "
";
Returns a string containing the status of the most recent query.
answerfrom
print 'last answer was from: ', $res->answerfrom, "
";
Returns the IP address from which we received the last answer in response to a query.
answersize
print 'size of last answer: ', $res->answersize, "
";
Returns the size in bytes of the last answer we received in response to a query.
dnssec
print "dnssec flag: ", $res->dnssec, "
";
$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 KEY, NXT and SIG records.
cdflag
print "checking disabled flag: ", $res->dnssec, "
";
$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 can be set
to 1.
udppacketsize
print "udppacketsize: ", $res->udppacketsize, "
";
$res->udppacketsize(2048);
udppacketsize will set or get the packet size. If set to a value greater than &Net::DNS::PACKETSZ an EDNS extention will be added indicat-
ing suppport for MTU path recovery.
Default udppacketsize is &Net::DNS::PACKETSZ(512) =cut
sub AUTOLOAD { my ($self) = @_;
my $name = $AUTOLOAD;
$name =~ s/.*://;
Carp::croak "$name: no such method" unless exists $self->{$name};
no strict q/refs/;
*{$AUTOLOAD} = sub {
my ($self, $new_val) = @_;
if (defined $new_val) {
$self->{"$name"} = $new_val;
}
return $self->{"$name"};
};
goto &{$AUTOLOAD};
}
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
perl v5.8.0 2002-11-15 Net::DNS::Resolver(3)