Net::DNS(3) User Contributed Perl Documentation Net::DNS(3)
NAME
Net::DNS - Perl interface to the Domain Name System
SYNOPSIS
use Net::DNS;
DESCRIPTION
Net::DNS is a collection of Perl modules that act as a Domain Name
System (DNS) resolver. It allows the programmer to perform DNS queries
that are beyond the capabilities of "gethostbyname" and
"gethostbyaddr".
The programmer should be somewhat familiar with the format of a DNS
packet and its various sections. See RFC 1035 or DNS and BIND (Albitz
& Liu) for details.
Resolver Objects
A resolver object is an instance of the Net::DNS::Resolver class. 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.
Packet Objects
Net::DNS::Resolver queries return Net::DNS::Packet objects. Packet
objects have five sections:
o The header section, a Net::DNS::Header object.
o The question section, a list of Net::DNS::Question objects.
o The answer section, a list of Net::DNS::RR objects.
o The authority section, a list of Net::DNS::RR objects.
o The additional section, a list of Net::DNS::RR objects.
Update Objects
The Net::DNS::Update package is a subclass of Net::DNS::Packet for
creating packet objects to be used in dynamic updates.
Header Objects
Net::DNS::Header objects represent the header section of a DNS packet.
Question Objects
Net::DNS::Question objects represent the question section of a DNS
packet.
RR Objects
Net::DNS::RR is the base class for DNS resource record (RR) objects in
the answer, authority, and additional sections of a DNS packet.
Don't assume that RR objects will be of the type you requested --
always check an RR object's type before calling any of its methods.
METHODS
See the manual pages listed above for other class-specific methods.
version
print Net::DNS->version, "\n";
Returns the version of Net::DNS.
mx
# Use a default resolver -- can't get an error string this way.
use Net::DNS;
my @mx = mx("example.com");
# Use your own resolver object.
use Net::DNS;
my $res = Net::DNS::Resolver->new;
my @mx = mx($res, "example.com");
Returns a list of Net::DNS::RR::MX objects representing the MX records
for the specified name; the list will be sorted by preference. Returns
an empty list if the query failed or no MX records were found.
This method does not look up A records -- it only performs MX queries.
See "EXAMPLES" for a more complete example.
yxrrset
Use this method to add an "RRset exists" prerequisite to a dynamic
update packet. There are two forms, value-independent and value-
dependent:
# RRset exists (value-independent)
$update->push(pre => yxrrset("host.example.com A"));
Meaning: At least one RR with the specified name and type must exist.
# RRset exists (value-dependent)
$packet->push(pre => yxrrset("host.example.com A 10.1.2.3"));
Meaning: At least one RR with the specified name and type must exist
and must have matching data.
Returns a "Net::DNS::RR" object or "undef" if the object couldn't be
created.
nxrrset
Use this method to add an "RRset does not exist" prerequisite to a
dynamic update packet.
$packet->push(pre => nxrrset("host.example.com A"));
Meaning: No RRs with the specified name and type can exist.
Returns a "Net::DNS::RR" object or "undef" if the object couldn't be
created.
yxdomain
Use this method to add a "name is in use" prerequisite to a dynamic
update packet.
$packet->push(pre => yxdomain("host.example.com"));
Meaning: At least one RR with the specified name must exist.
Returns a "Net::DNS::RR" object or "undef" if the object couldn't be
created.
nxdomain
Use this method to add a "name is not in use" prerequisite to a dynamic
update packet.
$packet->push(pre => nxdomain("host.example.com"));
Meaning: No RR with the specified name can exist.
Returns a "Net::DNS::RR" object or "undef" if the object couldn't be
created.
rr_add
Use this method to add RRs to a zone.
$packet->push(update => rr_add("host.example.com A 10.1.2.3"));
Meaning: Add this RR to the zone.
RR objects created by this method should be added to the "update"
section of a dynamic update packet. The TTL defaults to 86400 seconds
(24 hours) if not specified.
Returns a "Net::DNS::RR" object or "undef" if the object couldn't be
created.
rr_del
Use this method to delete RRs from a zone. There are three forms:
delete an RRset, delete all RRsets, and delete an RR.
# Delete an RRset.
$packet->push(update => rr_del("host.example.com A"));
Meaning: Delete all RRs having the specified name and type.
# Delete all RRsets.
$packet->push(update => rr_del("host.example.com"));
Meaning: Delete all RRs having the specified name.
# Delete an RR.
$packet->push(update => rr_del("host.example.com A 10.1.2.3"));
Meaning: Delete all RRs having the specified name, type, and data.
RR objects created by this method should be added to the "update"
section of a dynamic update packet.
Returns a "Net::DNS::RR" object or "undef" if the object couldn't be
created.
Zone Serial Number Management
The Net::DNS module provides auxiliary functions which support policy-
driven zone serial numbering regimes.
Strictly Sequential
$successor = $soa->serial( SEQUENTIAL );
The existing serial number is incremented modulo 2**32.
Time Encoded
$successor = $soa->serial( UNIXTIME );
The Unix time scale will be used as the basis for zone serial
numbering. The serial number will be incremented if the time elapsed
since the previous update is less than one second.
Date Encoded
$successor = $soa->serial( YYYYMMDDxx );
The 32 bit value returned by the auxiliary YYYYMMDDxx() function will
be used as the base for the date-coded zone serial number. Serial
number increments must be limited to 100 per day for the date
information to remain useful.
Sorting of RR arrays
As of version 0.55 there is functionality to help you sort RR arrays.
'rrsort()' is the function that is available to do the sorting. In most
cases rrsort will give you the answer that you want but you can specify
your own sorting method by using the
Net::DNS::RR::FOO->set_rrsort_func() class method. See Net::DNS::RR for
details.
rrsort()
use Net::DNS qw(rrsort);
my @prioritysorted=rrsort("SRV","priority",@rr_array);
rrsort() selects all RRs from the input array that are of the type that
are defined in the first argument. Those RRs are sorted based on the
attribute that is specified as second argument.
There are a number of RRs for which the sorting function is
specifically defined for certain attributes. If such sorting function
is defined in the code (it can be set or overwritten using the
set_rrsort_func() class method) that function is used.
For instance:
my @prioritysorted=rrsort("SRV","priority",@rr_array); returns the
SRV records sorted from lowest to heighest priority and for equal
priorities from heighes to lowes weight.
If the function does not exist then a numerical sort on the attribute
value is performed.
my @portsorted=rrsort("SRV","port",@rr_array);
If the attribute does not exist for a certain RR than the RRs are
sorted on string comparrisson of the rdata.
If the attribute is not defined than either the default_sort function
will be defined or "Canonical sorting" (as defined by DNSSEC) will be
used.
rrsort() returns a sorted array with only elements of the specified RR
type or undef.
rrsort() returns undef when arguments are incorrect.
EXAMPLES
The following examples show how to use the "Net::DNS" modules. See the
other manual pages and the demo scripts included with the source code
for additional examples.
See the "Net::DNS::Update" manual page for an example of performing
dynamic updates.
Look up a host's addresses.
use Net::DNS;
my $res = Net::DNS::Resolver->new;
my $query = $res->search("host.example.com");
if ($query) {
foreach my $rr ($query->answer) {
next unless $rr->type eq "A";
print $rr->address, "\n";
}
} else {
warn "query failed: ", $res->errorstring, "\n";
}
Find the nameservers for a domain.
use Net::DNS;
my $res = Net::DNS::Resolver->new;
my $query = $res->query("example.com", "NS");
if ($query) {
foreach $rr (grep { $_->type eq 'NS' } $query->answer) {
print $rr->nsdname, "\n";
}
}
else {
warn "query failed: ", $res->errorstring, "\n";
}
Find the MX records for a domain.
use Net::DNS;
my $name = "example.com";
my $res = Net::DNS::Resolver->new;
my @mx = mx($res, $name);
if (@mx) {
foreach $rr (@mx) {
print $rr->preference, " ", $rr->exchange, "\n";
}
} else {
warn "Can't find MX records for $name: ", $res->errorstring, "\n";
}
Print a domain's SOA record in zone file format.
use Net::DNS;
my $res = Net::DNS::Resolver->new;
my $query = $res->query("example.com", "SOA");
if ($query) {
($query->answer)[0]->print;
} else {
print "query failed: ", $res->errorstring, "\n";
}
Perform a zone transfer and print all the records.
use Net::DNS;
my $res = Net::DNS::Resolver->new;
$res->nameservers("ns.example.com");
my @zone = $res->axfr("example.com");
foreach $rr (@zone) {
$rr->print;
}
Perform a background query and do some other work while waiting for the
answer.
use Net::DNS;
my $res = Net::DNS::Resolver->new;
my $socket = $res->bgsend("host.example.com");
until ($res->bgisready($socket)) {
# do some work here while waiting for the answer
# ...and some more here
}
my $packet = $res->bgread($socket);
$packet->print;
Send a background query and use select to determine when the answer has
arrived.
use Net::DNS;
use IO::Select;
my $timeout = 5;
my $res = Net::DNS::Resolver->new;
my $bgsock = $res->bgsend("host.example.com");
my $sel = IO::Select->new($bgsock);
# Add more sockets to $sel if desired.
my @ready = $sel->can_read($timeout);
if (@ready) {
foreach my $sock (@ready) {
if ($sock == $bgsock) {
my $packet = $res->bgread($bgsock);
$packet->print;
$bgsock = undef;
}
# Check for the other sockets.
$sel->remove($sock);
$sock = undef;
}
} else {
warn "timed out after $timeout seconds\n";
}
BUGS
"Net::DNS" is slow.
For other items to be fixed, or if you discover a bug in this
distribution please use the CPAN bug reporting system.
COPYRIGHT
Copyright (c)1997-2002 Michael Fuhr. Portions Copyright(c)2002-2004
Chris Reinhardt. Portions Copyright(c)2005 Olaf Kolkman (RIPE NCC)
Portions Copyright(c)2006 Olaf 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.
AUTHOR INFORMATION
Net::DNS is currently maintained at NLnet Labs (www.nlnetlabs.nl) by:
Olaf Kolkman olaf AT net-dns.org
Between 2002 and 2004 Net::DNS was maintained by:
Chris Reinhardt
Net::DNS was created by: Michael Fuhr mike AT fuhr.org
For more information see:
http://www.net-dns.org/
Stay tuned and syndicate:
http://www.net-dns.org/blog/
SEE ALSO
perl, Net::DNS::Resolver, Net::DNS::Packet, Net::DNS::Update,
Net::DNS::Header, Net::DNS::Question, Net::DNS::RR, RFC 1035, DNS and
BIND by Paul Albitz & Cricket Liu
perl v5.16.3 2012-12-28 Net::DNS(3)
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