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', 'CH' );
# 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.
- search
- A space-separated list of domains to put in the search list.
- nameserver
- A space-separated list of nameservers to query.
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".
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.
- searchlist
- A reference to an array of domains to search for unqualified names.
- domain
- Domain name suffix to be appended to queries of unqualified names.
- recurse
- debug
- 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 = $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', '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 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 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 = $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', 'HS' );
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), 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 = $resolver->send( $packet );
$packet = $resolver->send( 'mailhost.example.com' );
$packet = $resolver->send( 'example.com', 'MX' );
$packet = $resolver->send( 'annotation.example.com', 'TXT', 'HS' );
Performs a DNS query for the given name. Neither the searchlist nor the default
domain will be appended.
The argument list can be either a "Net::DNS::Packet" object or a list
of strings. The record type and class can be omitted; they default to A and
IN. If the name looks like an IP address (IPv4 or IPv6), an appropriate PTR
query will be performed.
Returns a "Net::DNS::Packet" object whether there were any answers or
not. Use "$packet->header->ancount" or
"$packet->answer" to find out if there were any records in the
answer section. Returns "undef" if no response was received.
axfr¶
@zone = $resolver->axfr();
@zone = $resolver->axfr( 'example.com' );
@zone = $resolver->axfr( 'example.com', 'HS' );
$iterator = $resolver->axfr();
$iterator = $resolver->axfr( 'example.com' );
$iterator = $resolver->axfr( 'example.com', 'HS' );
$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 or an empty list if the zone transfer failed.
The redundant SOA record that terminates the zone transfer is not returned to
the caller.
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' );
die 'Zone transfer failed: ', $resolver->errorstring unless @zone;
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. 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' );
die 'Zone transfer failed: ', $resolver->errorstring unless $iterator;
while ( $rr = $iterator->() ) {
$rr->print;
}
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
empty_nameservers¶
$resolver->empty_nameservers();
Empties the list of nameservers.
print¶
$resolver->print;
Prints the resolver state on the standard output.
string¶
print $resolver->string;
Returns a string representation of the resolver state.
searchlist¶
@searchlist = $resolver->searchlist;
$resolver->searchlist( 'a.example', 'b.example', 'c.example' );
Gets or sets the resolver search list.
empty_searchlist¶
$resolver->empty_searchlist();
Empties the searchlist.
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.
srcport¶
print 'sending queries from port ', $resolver->srcport, "\n";
$resolver->srcport(5353);
Gets or sets the port from which queries are sent. The default is 0, meaning any
port.
srcaddr¶
print 'sending queries from address ', $resolver->srcaddr, "\n";
$resolver->srcaddr('192.0.2.1');
Gets or 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 0.0.0.0, meaning any local address.
bgsend¶
$socket = $resolver->bgsend( $packet ) || die $resolver->errorstring;
$socket = $resolver->bgsend( 'mailhost.example.com' );
$socket = $resolver->bgsend( 'example.com', 'MX' );
$socket = $resolver->bgsend( 'annotation.example.com', 'TXT', 'HS' );
Performs a background DNS query for the given name, i.e., sends a query packet
to the first destination in the "nameservers" list and returns
immediately without waiting for a 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), 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
"bgread" to get the response packet. Either "bgisready" or
"IO::Select" may be used to find out if the socket is ready.
"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 = $resolver->bgread($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 = $resolver->bgsend( 'foo.example.com' );
until ($resolver->bgisready($socket)) {
# do some other processing
}
$packet = $resolver->bgread($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 "bgsend".
Returns true if the socket is ready, false if not.
tsig¶
$tsig = $resolver->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 );
Get or set the TSIG record used to automatically sign outgoing queries and
updates. Call with an undefined argument, 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 would like the resolver to sign 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.
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.
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.
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.
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.
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.
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.
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, Net::DNS will keep a single UDP
socket open for all queries. This is useful if you are using UDP and need to
make a lot of queries or updates.
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.
errorstring¶
print 'query status: ', $resolver->errorstring, "\n";
Returns a string containing the status of the most recent query.
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.
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.
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.
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 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".
IPv6 TRANSPORT¶
The Net::DNS::Resolver library will enable IPv6 transport if the appropriate
libraries (Socket6 and IO::Socket::INET6) are available and the destination
nameserver has at least one IPv6 address.
The
force_v4(),
force_v6() 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(@_);
}
BUGS¶
bgsend() does not honour 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.
Portions Copyright (c)2014 Dick Franks.
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