NAME¶
Net::DNS::Resolver - DNS resolver class
SYNOPSIS¶
use Net::DNS;
my $res = Net::DNS::Resolver->new;
# Perform a lookup, using the searchlist if appropriate.
my $answer = $res->search('example.com');
# Perform a lookup, without the searchlist
my $answer = $res->query('example.com', 'MX');
# Perform a lookup, without pre or post-processing
my $answer = $res->send('example.com', 'MX', 'CH');
# Send a prebuilt packet
my $packet = Net::DNS::Packet->new(...);
my $answer = $res->send($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 system defaults
my $res = Net::DNS::Resolver->new;
# Use my own configuration file
my $res = Net::DNS::Resolver->new(config_file => '/my/dns.conf');
# Set options in the constructor
my $res = Net::DNS::Resolver->new(
nameservers => [qw(10.1.1.128 10.1.2.128)],
recurse => 0,
debug => 1,
);
Returns a resolver object. If given no arguments, "new()" returns an
object configured to your system's defaults. On UNIX systems the defaults are
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".
On Windows systems, an attempt is made to determine the system defaults using
the registry. This is still a work in progress; 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
my $res = Net::DNS::Resolver->new(config_file => '/my/dns.conf');
This is supported on both UNIX and Windows. Values pulled from a custom
configuration file override the the system's defaults, but can still be
overridden by the other arguments to
new().
Explicit arguments to new override both the system's defaults and the values of
the custom configuration file, if any. The following arguments to
new()
are supported:
- nameservers
- An array reference of nameservers to query.
- searchlist
- An array reference of domains.
- recurse
- debug
- domain
- 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 = $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. If you need to examine the response packet whether it
contains any answers or not, use the
send() method instead.
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 (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.
send¶
$packet = $res->send($packet_object);
$packet = $res->send('mailhost.example.com');
$packet = $res->send('example.com', 'MX');
$packet = $res->send('user.passwd.example.com', 'TXT', 'HS');
Performs a DNS query for the given name. Neither the searchlist nor the default
domain will be appended.
The argument list can be either a "Net::DNS::Packet" object or a list
of strings. The record type and class can be omitted; they default to A and
IN. If the name looks like an IP address (Ipv4 or IPv6), then an appropriate
PTR query will be performed.
Returns a "Net::DNS::Packet" object whether there were any answers or
not. Use "$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.
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);
my @zone = $res->axfr('example.com');
if (@zone) {
foreach my $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.
IMPORTANT:
This method currently returns the "IO::Socket::INET" object that will
be used for reading, or "undef" on error. DO NOT DEPEND ON
"axfr_start()" returning a socket object. THIS MIGHT CHANGE in
future releases.
Use "axfr_next" to read the zone records one at a time.
axfr_next¶
$res->axfr_start('example.com');
while (my $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".
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.
Also see the IPv6 transport notes below
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', 'a.example.com', 'b.example.com');
Gets or sets the resolver search list.
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.
bgsend¶
$socket = $res->bgsend($packet_object) || die " $res->errorstring";
$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 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
"$res->bgread" to get the response packet. You can use
"$res->bgisready" or "IO::Select" to find out if the
socket is ready before reading it.
bgsend does not support persistent sockets.
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.
tsig¶
my $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).
udp_timeout¶
print 'UDP timeout: ', $res->udp_timeout, "\n";
$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, "\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.
persistent_udp¶
print 'Persistent UDP flag: ', $res->persistent_udp, "\n";
$res->persistent_udp(1);
Get or set the persistent UDP setting. If set to true, Net::DNS will keep a
single UDP socket open for all queries. This is useful if you're using UDP and
need to make a lot of queries or updates.
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.
dnssec¶
print "dnssec flag: ", $res->dnssec, "\n";
$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 DNSKEY, NSEC and RRSIG records.
Setting calling the dnssec method with a non-zero value will set the UDP packet
size to the default value of 2048. If that is to small or to big for your
environment you should call the
udppacketsize() method immeditatly
after.
$res->dnssec(1); # turns on DNSSEC and sets udp packetsize to 2048
$res->udppacketsize(1028); # lowers the UDP pakcet size
The method will Croak::croak with the message "You called the
Net::DNS::Resolver::dnssec() method but do not have Net::DNS::SEC
installed at ..." if you call it without Net::DNS::SEC being in your @INC
path.
cdflag¶
print "checking disabled flag: ", $res->dnssec, "\n";
$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 defaults to 0 can be set
to 1.
adflag¶
print "checking disabled flag: ", $res->dnssec, "\n";
$res->dnssec(1);
$res->adflag(1);
Sets or gets the AD bit for a dnssec query. This bit is always zero for non
dnssec queries. When the dnssec is enabled the flag defaults to 1.
udppacketsize¶
print "udppacketsize: ", $res->udppacketsize, "\n";
$res->udppacketsize(2048);
udppacketsize will set or get the packet size. If set to a value greater than
Net::DNS::PACKETSZ() an EDNS extension will be added indicating support
for MTU path recovery.
Default udppacketsize is
Net::DNS::PACKETSZ() (512)
CUSTOMIZING¶
Net::DNS::Resolver is actually an empty subclass. At compile time a super class
is chosen based on the current platform. A side benefit of this allows for
easy modification of the methods in Net::DNS::Resolver. You simply add a
method to the namespace!
For example, if we wanted to cache lookups:
package Net::DNS::Resolver;
my %cache;
sub search {
my ($self, @args) = @_;
return $cache{@args} ||= $self->SUPER::search(@args);
}
IPv6 transport¶
The Net::DNS::Resolver library will use IPv6 transport if the appropriate
libraries (Socket6 and IO::Socket::INET6) are available and the address the
server tries to connect to is an IPv6 address.
The
print() will method will report if IPv6 transport is available.
You can use the
force_v4() method with a non-zero argument to force IPv4
transport.
The
nameserver() method has IPv6 dependend behavior. If IPv6 is not
available or IPv4 transport has been forced the
nameserver() method
will only return IPv4 addresses.
For example
$res->nameservers('192.168.1.1', '192.168.2.2', '2001:610:240:0:53:0:0:3');
$res->force_v4(1);
print join (" ",$res->nameserver());
Will print: 192.168.1.1 192.168.2.2
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.
bgsend does not honor 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.
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(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