Net::DNS::Resolver(3pm)

1Net::DNS::Resolver(3) User Contributed Perl DocumentationNet::DNS::Resolver(3)
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NAME

6       Net::DNS::Resolver - DNS resolver class
7

SYNOPSIS

9           use Net::DNS;
10
11           $resolver = new Net::DNS::Resolver();
12
13           # Perform a lookup, using the searchlist if appropriate.
14           $reply = $resolver->search( 'example.com' );
15
16           # Perform a lookup, without the searchlist
17           $reply = $resolver->query( 'example.com', 'MX' );
18
19           # Perform a lookup, without pre or post-processing
20           $reply = $resolver->send( 'example.com', 'MX', 'IN' );
21
22           # Send a prebuilt query packet
23           $query = new Net::DNS::Packet( ... );
24           $reply = $resolver->send( $query );
25

DESCRIPTION

27       Instances of the Net::DNS::Resolver class represent resolver objects.
28       A program may have multiple resolver objects, each maintaining its own
29       state information such as the nameservers to be queried, whether
30       recursion is desired, etc.
31

METHODS

33   new
34           # Use the default configuration
35           $resolver = new Net::DNS::Resolver();
36
37           # Use my own configuration file
38           $resolver = new Net::DNS::Resolver( config_file => '/my/dns.conf' );
39
40           # Set options in the constructor
41           $resolver = new Net::DNS::Resolver(
42               nameservers => [ '2001:DB8::1', 'ns.example.com' ],
43               recurse     => 0,
44               debug       => 1
45               );
46
47       Returns a resolver object.  If no arguments are supplied, "new()"
48       returns an object having the default configuration.
49
50       On Unix and Linux systems, the default values are read from the
51       following files, in the order indicated:
52
53           /etc/resolv.conf, $HOME/.resolv.conf, ./.resolv.conf
54
55       The following keywords are recognised in resolver configuration files:
56
57           nameserver address
58
59           IP address of a name server that the resolver should query.
60
61           domain localdomain
62
63           The domain suffix to be appended to a short non-absolute name.
64
65           search domain ...
66
67           A space-separated list of domains in the desired search path.
68
69           options option:value ...
70
71           A space-separated list of key:value items.
72
73       Except for /etc/resolv.conf, files will only be read if owned by the
74       effective userid running the program.  In addition, several environment
75       variables may contain configuration information; see "ENVIRONMENT".
76
77       Note that the domain and searchlist keywords are mutually exclusive.
78       If both are present, the resulting behaviour is unspecified.  If
79       neither is present, the domain is determined from the local hostname.
80
81       On Windows systems, an attempt is made to determine the system defaults
82       using the registry.  Systems with many dynamically configured network
83       interfaces may confuse Net::DNS.
84
85           # Use my own configuration file
86           $resolver = new Net::DNS::Resolver( config_file => '/my/dns.conf' );
87
88       You can include a configuration file of your own when creating a
89       resolver object.  This is supported on both Unix and Windows.
90
91       If a custom configuration file is specified at first instantiation, all
92       other configuration files and environment variables are ignored.
93
94           # Set options in the constructor
95           $resolver = new Net::DNS::Resolver(
96               nameservers => [ '2001:DB8::1', 'ns.example.com' ],
97               recurse     => 0
98               );
99
100       Explicit arguments to "new()" override the corresponding configuration
101       variables.  The argument list consists of a sequence of (name=>value)
102       pairs, each interpreted as an invocation of the corresponding method.
103
104   print
105           $resolver->print;
106
107       Prints the resolver state on the standard output.
108
109   query
110           $packet = $resolver->query( 'mailhost' );
111           $packet = $resolver->query( 'mailhost.example.com' );
112           $packet = $resolver->query( '2001:DB8::1' );
113           $packet = $resolver->query( 'example.com', 'MX' );
114           $packet = $resolver->query( 'annotation.example.com', 'TXT', 'IN' );
115
116       Performs a DNS query for the given name; the search list is not
117       applied.  If "defnames" is true, the default domain will be appended to
118       unqualified names.
119
120       The record type and class can be omitted; they default to A and IN.  If
121       the name looks like an IP address (IPv4 or IPv6), then a query within
122       in-addr.arpa or ip6.arpa will be performed.
123
124       Returns a Net::DNS::Packet object, or "undef" if no answers were found.
125       The reason for failure may be determined using "errorstring()".
126
127       If you need to examine the response packet, whether it contains any
128       answers or not, use the "send()" method instead.
129
130   search
131           $packet = $resolver->search( 'mailhost' );
132           $packet = $resolver->search( 'mailhost.example.com' );
133           $packet = $resolver->search( '2001:DB8::1' );
134           $packet = $resolver->search( 'example.com', 'MX' );
135           $packet = $resolver->search( 'annotation.example.com', 'TXT', 'IN' );
136
137       Performs a DNS query for the given name, applying the searchlist if
138       appropriate.  The search algorithm is as follows:
139
140       If the name contains one or more non-terminal dots, perform an initial
141       query using the unmodified name.
142
143       If the number of dots is less than "ndots", and there is no terminal
144       dot, try appending each suffix in the search list.
145
146       The record type and class can be omitted; they default to A and IN.  If
147       the name looks like an IP address (IPv4 or IPv6), then a query within
148       in-addr.arpa or ip6.arpa will be performed.
149
150       Returns a Net::DNS::Packet object, or "undef" if no answers were found.
151       The reason for failure may be determined using "errorstring()".
152
153       If you need to examine the response packet, whether it contains any
154       answers or not, use the "send()" method instead.
155
156   send
157           $packet = $resolver->send( $query );
158
159           $packet = $resolver->send( 'mailhost.example.com' );
160           $packet = $resolver->send( '2001:DB8::1' );
161           $packet = $resolver->send( 'example.com', 'MX' );
162           $packet = $resolver->send( 'annotation.example.com', 'TXT', 'IN' );
163
164       Performs a DNS query for the given name.  Neither the searchlist nor
165       the default domain will be appended.
166
167       The argument list can be either a pre-built query Net::DNS::Packet or a
168       list of strings.  The record type and class can be omitted; they
169       default to A and IN.  If the name looks like an IP address (IPv4 or
170       IPv6), then a query within in-addr.arpa or ip6.arpa will be performed.
171
172       Returns a Net::DNS::Packet object whether there were any answers or
173       not.  Use "$packet->header->ancount" or "$packet->answer" to find out
174       if there were any records in the answer section.  Returns "undef" if no
175       response was received.
176
177   axfr
178           @zone = $resolver->axfr();
179           @zone = $resolver->axfr( 'example.com' );
180           @zone = $resolver->axfr( 'example.com', 'IN' );
181
182           $iterator = $resolver->axfr();
183           $iterator = $resolver->axfr( 'example.com' );
184           $iterator = $resolver->axfr( 'example.com', 'IN' );
185
186           $rr = $iterator->();
187
188       Performs a zone transfer using the resolver nameservers list, attempted
189       in the order listed.
190
191       If the zone is omitted, it defaults to the first zone listed in the
192       resolver search list.
193
194       If the class is omitted, it defaults to IN.
195
196       When called in list context, "axfr()" returns a list of Net::DNS::RR
197       objects.  The redundant SOA record that terminates the zone transfer is
198       not returned to the caller.
199
200       In deferrence to RFC1035(6.3), a complete zone transfer is expected to
201       return all records in the zone or nothing at all.  When no resource
202       records are returned by "axfr()", the reason for failure may be
203       determined using "errorstring()".
204
205       Here is an example that uses a timeout and TSIG verification:
206
207           $resolver->tcp_timeout( 10 );
208           $resolver->tsig( 'Khmac-sha1.example.+161+24053.private' );
209           @zone = $resolver->axfr( 'example.com' );
210
211           foreach $rr (@zone) {
212               $rr->print;
213           }
214
215       When called in scalar context, "axfr()" returns an iterator object.
216       Each invocation of the iterator returns a single Net::DNS::RR or
217       "undef" when the zone is exhausted.
218
219       An exception is raised if the zone transfer can not be completed.
220
221       The redundant SOA record that terminates the zone transfer is not
222       returned to the caller.
223
224       Here is the example above, implemented using an iterator:
225
226           $resolver->tcp_timeout( 10 );
227           $resolver->tsig( 'Khmac-sha1.example.+161+24053.private' );
228           $iterator = $resolver->axfr( 'example.com' );
229
230           while ( $rr = $iterator->() ) {
231               $rr->print;
232           }
233
234   bgsend
235           $handle = $resolver->bgsend( $packet ) || die $resolver->errorstring;
236
237           $handle = $resolver->bgsend( 'mailhost.example.com' );
238           $handle = $resolver->bgsend( '2001:DB8::1' );
239           $handle = $resolver->bgsend( 'example.com', 'MX' );
240           $handle = $resolver->bgsend( 'annotation.example.com', 'TXT', 'IN' );
241
242       Performs a background DNS query for the given name and returns
243       immediately without waiting for the response. The program can then
244       perform other tasks while awaiting the response from the nameserver.
245
246       The argument list can be either a Net::DNS::Packet object or a list of
247       strings.  The record type and class can be omitted; they default to A
248       and IN.  If the name looks like an IP address (IPv4 or IPv6), then a
249       query within in-addr.arpa or ip6.arpa will be performed.
250
251       Returns an opaque handle which is passed to subsequent invocations of
252       the "bgbusy()" and "bgread()" methods.  Errors are indicated by
253       returning "undef" in which case the reason for failure may be
254       determined using "errorstring()".
255
256       The response Net::DNS::Packet object is obtained by calling "bgread()".
257
258       BEWARE: Programs should make no assumptions about the nature of the
259       handles returned by "bgsend()" which should be used strictly as
260       described here.
261
262   bgread
263           $handle = $resolver->bgsend( 'www.example.com' );
264           $packet = $resolver->bgread($handle);
265
266       Reads the response following a background query.  The argument is the
267       handle returned by "bgsend()".
268
269       Returns a Net::DNS::Packet object or "undef" if no response was
270       received before the timeout interval expired.
271
272   bgbusy
273           $handle = $resolver->bgsend( 'foo.example.com' );
274
275           while ($resolver->bgbusy($handle)) {
276               ...
277           }
278
279           $packet = $resolver->bgread($handle);
280
281       Returns true while awaiting the response or for the transaction to time
282       out.  The argument is the handle returned by "bgsend()".
283
284       Truncated UDP packets will be retried transparently using TCP while
285       continuing to assert busy to the caller.
286
287   debug
288           print 'debug flag: ', $resolver->debug, "\n";
289           $resolver->debug(1);
290
291       Get or set the debug flag.  If set, calls to "search()", "query()", and
292       "send()" will print debugging information on the standard output.  The
293       default is false.
294
295   defnames
296           print 'defnames flag: ', $resolver->defnames, "\n";
297           $resolver->defnames(0);
298
299       Get or set the defnames flag.  If true, calls to "query()" will append
300       the default domain to resolve names that are not fully qualified.  The
301       default is true.
302
303   dnsrch
304           print 'dnsrch flag: ', $resolver->dnsrch, "\n";
305           $resolver->dnsrch(0);
306
307       Get or set the dnsrch flag.  If true, calls to "search()" will apply
308       the search list to resolve names that are not fully qualified.  The
309       default is true.
310
311   domain
312           $domain = $resolver->domain;
313           $resolver->domain( 'domain.example' );
314
315       Gets or sets the resolver default domain.
316
317   igntc
318           print 'igntc flag: ', $resolver->igntc, "\n";
319           $resolver->igntc(1);
320
321       Get or set the igntc flag.  If true, truncated packets will be ignored.
322       If false, the query will be retried using TCP.  The default is false.
323
324   nameserver, nameservers
325           @nameservers = $resolver->nameservers();
326           $resolver->nameservers( '2001:DB8::1', '192.0.2.1' );
327           $resolver->nameservers( 'ns.domain.example.' );
328
329       Gets or sets the nameservers to be queried.
330
331       Also see the IPv6 transport notes below
332
333   persistent_tcp
334           print 'Persistent TCP flag: ', $resolver->persistent_tcp, "\n";
335           $resolver->persistent_tcp(1);
336
337       Get or set the persistent TCP setting.  If true, Net::DNS will keep a
338       TCP socket open for each host:port to which it connects.  This is
339       useful if you are using TCP and need to make a lot of queries or
340       updates to the same nameserver.
341
342       The default is false unless you are running a SOCKSified Perl, in which
343       case the default is true.
344
345   persistent_udp
346           print 'Persistent UDP flag: ', $resolver->persistent_udp, "\n";
347           $resolver->persistent_udp(1);
348
349       Get or set the persistent UDP setting.  If true, a Net::DNS resolver
350       will use the same UDP socket for all queries within each address
351       family.
352
353       This avoids the cost of creating and tearing down UDP sockets, but also
354       defeats source port randomisation.
355
356   port
357           print 'sending queries to port ', $resolver->port, "\n";
358           $resolver->port(9732);
359
360       Gets or sets the port to which queries are sent.  Convenient for
361       nameserver testing using a non-standard port.  The default is port 53.
362
363   recurse
364           print 'recursion flag: ', $resolver->recurse, "\n";
365           $resolver->recurse(0);
366
367       Get or set the recursion flag.  If true, this will direct nameservers
368       to perform a recursive query.  The default is true.
369
370   retrans
371           print 'retrans interval: ', $resolver->retrans, "\n";
372           $resolver->retrans(3);
373
374       Get or set the retransmission interval The default is 5 seconds.
375
376   retry
377           print 'number of tries: ', $resolver->retry, "\n";
378           $resolver->retry(2);
379
380       Get or set the number of times to try the query.  The default is 4.
381
382   searchlist
383           @searchlist = $resolver->searchlist;
384           $resolver->searchlist( 'a.example', 'b.example', 'c.example' );
385
386       Gets or sets the resolver search list.
387
388   srcaddr
389           $resolver->srcaddr('2001::DB8::1');
390
391       Sets the source address from which queries are sent.  Convenient for
392       forcing queries from a specific interface on a multi-homed host.  The
393       default is to use any local address.
394
395   srcport
396           $resolver->srcport(5353);
397
398       Sets the port from which queries are sent.  The default is 0, meaning
399       any port.
400
401   tcp_timeout
402           print 'TCP timeout: ', $resolver->tcp_timeout, "\n";
403           $resolver->tcp_timeout(10);
404
405       Get or set the TCP timeout in seconds.  The default is 120 seconds (2
406       minutes).
407
408   udp_timeout
409           print 'UDP timeout: ', $resolver->udp_timeout, "\n";
410           $resolver->udp_timeout(10);
411
412       Get or set the bgsend() UDP timeout in seconds.  The default is 30
413       seconds.
414
415   udppacketsize
416           print "udppacketsize: ", $resolver->udppacketsize, "\n";
417           $resolver->udppacketsize(2048);
418
419       Get or set the UDP packet size.  If set to a value not less than the
420       default DNS packet size, an EDNS extension will be added indicating
421       support for large UDP datagrams.
422
423   usevc
424           print 'usevc flag: ', $resolver->usevc, "\n";
425           $resolver->usevc(1);
426
427       Get or set the usevc flag.  If true, queries will be performed using
428       virtual circuits (TCP) instead of datagrams (UDP).  The default is
429       false.
430
431   replyfrom
432           print 'last response was from: ', $resolver->replyfrom, "\n";
433
434       Returns the IP address from which the most recent packet was received
435       in response to a query.
436
437   errorstring
438           print 'query status: ', $resolver->errorstring, "\n";
439
440       Returns a string containing error information from the most recent DNS
441       protocol interaction.  "errorstring()" is meaningful only when
442       interrogated immediately after the corresponding method call.
443
444   dnssec
445           print "dnssec flag: ", $resolver->dnssec, "\n";
446           $resolver->dnssec(0);
447
448       The dnssec flag causes the resolver to transmit DNSSEC queries and to
449       add a EDNS0 record as required by RFC2671 and RFC3225.  The actions of,
450       and response from, the remote nameserver is determined by the settings
451       of the AD and CD flags.
452
453       Calling the "dnssec()" method with a non-zero value will also set the
454       UDP packet size to the default value of 2048. If that is too small or
455       too big for your environment, you should call the "udppacketsize()"
456       method immediately after.
457
458          $resolver->dnssec(1);                # DNSSEC using default packetsize
459          $resolver->udppacketsize(1250);      # lower the UDP packet size
460
461       A fatal exception will be raised if the "dnssec()" method is called but
462       the Net::DNS::SEC library has not been installed.
463
464   adflag
465           $resolver->dnssec(1);
466           $resolver->adflag(1);
467           print "authentication desired flag: ", $resolver->adflag, "\n";
468
469       Gets or sets the AD bit for dnssec queries.  This bit indicates that
470       the caller is interested in the returned AD (authentic data) bit but
471       does not require any dnssec RRs to be included in the response.  The
472       default value is false.
473
474   cdflag
475           $resolver->dnssec(1);
476           $resolver->cdflag(1);
477           print "checking disabled flag: ", $resolver->cdflag, "\n";
478
479       Gets or sets the CD bit for dnssec queries.  This bit indicates that
480       authentication by upstream nameservers should be suppressed.  Any
481       dnssec RRs required to execute the authentication procedure should be
482       included in the response.  The default value is false.
483
484   tsig
485           $resolver->tsig( $tsig );
486
487           $resolver->tsig( 'Khmac-sha1.example.+161+24053.private' );
488
489           $resolver->tsig( 'Khmac-sha1.example.+161+24053.key' );
490
491           $resolver->tsig( 'Khmac-sha1.example.+161+24053.key',
492                       fudge => 60
493                       );
494
495           $resolver->tsig( $key_name, $key );
496
497           $resolver->tsig( undef );
498
499       Set the TSIG record used to automatically sign outgoing queries, zone
500       transfers and updates. Automatic signing is disabled if called with
501       undefined arguments.
502
503       The default resolver behaviour is not to sign any packets.  You must
504       call this method to set the key if you would like the resolver to sign
505       and verify packets automatically.
506
507       Packets can also be signed manually; see the Net::DNS::Packet and
508       Net::DNS::Update manual pages for examples.  TSIG records in manually-
509       signed packets take precedence over those that the resolver would add
510       automatically.
511

ENVIRONMENT

513       The following environment variables can also be used to configure the
514       resolver:
515
516   RES_NAMESERVERS
517           # Bourne Shell
518           RES_NAMESERVERS="2001:DB8::1 192.0.2.1"
519           export RES_NAMESERVERS
520
521           # C Shell
522           setenv RES_NAMESERVERS "2001:DB8::1 192.0.2.1"
523
524       A space-separated list of nameservers to query.
525
526   RES_SEARCHLIST
527           # Bourne Shell
528           RES_SEARCHLIST="a.example.com b.example.com c.example.com"
529           export RES_SEARCHLIST
530
531           # C Shell
532           setenv RES_SEARCHLIST "a.example.com b.example.com c.example.com"
533
534       A space-separated list of domains to put in the search list.
535
536   LOCALDOMAIN
537           # Bourne Shell
538           LOCALDOMAIN=example.com
539           export LOCALDOMAIN
540
541           # C Shell
542           setenv LOCALDOMAIN example.com
543
544       The default domain.
545
546   RES_OPTIONS
547           # Bourne Shell
548           RES_OPTIONS="retrans:3 retry:2 inet6"
549           export RES_OPTIONS
550
551           # C Shell
552           setenv RES_OPTIONS "retrans:3 retry:2 inet6"
553
554       A space-separated list of resolver options to set.  Options that take
555       values are specified as "option:value".
556

IPv4 TRANSPORT

558       The "force_v4()", "force_v6()", "prefer_v4()", and "prefer_v6()"
559       methods with non-zero argument may be used to configure transport
560       selection.
561
562       The behaviour of the "nameserver()" method illustrates the transport
563       selection mechanism.  If, for example, IPv4 transport has been forced,
564       the "nameserver()" method will only return IPv4 addresses:
565
566           $resolver->nameservers( '192.0.2.1', '192.0.2.2', '2001:DB8::3' );
567           $resolver->force_v4(1);
568           print join ' ', $resolver->nameservers();
569
570       will print
571
572           192.0.2.1 192.0.2.2
573

CUSTOMISED RESOLVERS

575       Net::DNS::Resolver is actually an empty subclass.  At compile time a
576       super class is chosen based on the current platform.  A side benefit of
577       this allows for easy modification of the methods in Net::DNS::Resolver.
578       You can simply add a method to the namespace!
579
580       For example, if we wanted to cache lookups:
581
582           package Net::DNS::Resolver;
583
584           my %cache;
585
586           sub search {
587               $self = shift;
588
589               $cache{"@_"} ||= $self->SUPER::search(@_);
590           }
591

COPYRIGHT

593       Copyright (c)1997-2000 Michael Fuhr.
594
595       Portions Copyright (c)2002-2004 Chris Reinhardt.
596
597       Portions Copyright (c)2005 Olaf M. Kolkman, NLnet Labs.
598
599       Portions Copyright (c)2014,2015 Dick Franks.
600
601       All rights reserved.
602

LICENSE

604       Permission to use, copy, modify, and distribute this software and its
605       documentation for any purpose and without fee is hereby granted,
606       provided that the above copyright notice appear in all copies and that
607       both that copyright notice and this permission notice appear in
608       supporting documentation, and that the name of the author not be used
609       in advertising or publicity pertaining to distribution of the software
610       without specific prior written permission.
611
612       THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
613       OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
614       MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
615       IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
616       CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
617       TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
618       SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
619