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=head1 NAME

AnyEvent::DNS - fully asynchronous DNS resolution

=head1 SYNOPSIS

   use AnyEvent::DNS;
   
   my $cv = AnyEvent->condvar;
   AnyEvent::DNS::a "www.google.de", $cv;
   # ... later
   my @addrs = $cv->recv;

=head1 DESCRIPTION

This module offers both a number of DNS convenience functions as well
as a fully asynchronous and high-performance pure-perl stub resolver.

The stub resolver supports DNS over IPv4 and IPv6, UDP and TCP, optional
EDNS0 support for up to 4kiB datagrams and automatically falls back to
virtual circuit mode for large responses.

=head2 CONVENIENCE FUNCTIONS

=over 4

=cut

package AnyEvent::DNS;

use Carp ();
use Socket qw(AF_INET SOCK_DGRAM SOCK_STREAM);

use AnyEvent (); BEGIN { AnyEvent::common_sense }
use AnyEvent::Util qw(AF_INET6);

our $VERSION = $AnyEvent::VERSION;
our @DNS_FALLBACK; # some public dns servers as fallback

{
   my $prep = sub {
      $_ = $_->[rand @$_] for @_;
      push @_, splice @_, rand $_, 1 for reverse 1..@_; # shuffle
      $_ = pack "H*", $_ for @_;
      \@_
   };

   my $ipv4 = $prep->(
      ["08080808", "08080404"], # 8.8.8.8, 8.8.4.4 - google public dns
      ["01010101", "01000001"], # 1.1.1.1, 1.0.0.1 - cloudflare public dns
      ["50505050", "50505151"], # 80.80.80.80, 80.80.81.81 - freenom.world
##      ["d1f40003", "d1f30004"], # v209.244.0.3/4 - resolver1/2.level3.net - status unknown
##      ["04020201", "04020203", "04020204", "04020205", "04020206"], # v4.2.2.1/3/4/5/6 - vnsc-pri.sys.gtei.net - effectively public
##      ["cdd22ad2", "4044c8c8"], # 205.210.42.205, 64.68.200.200 - cache1/2.dnsresolvers.com - verified public
#      ["8d010101"], # 141.1.1.1 - cable&wireless, now vodafone - status unknown
# 84.200.69.80      # dns.watch
# 84.200.70.40      # dns.watch
# 37.235.1.174      # freedns.zone
# 37.235.1.177      # freedns.zone
# 213.73.91.35      # dnscache.berlin.ccc.de
# 194.150.168.168   # dns.as250.net; Berlin/Frankfurt
# 85.214.20.141     # FoeBud (digitalcourage.de)
# 77.109.148.136    # privacyfoundation.ch
# 77.109.148.137    # privacyfoundation.ch
# 91.239.100.100    # anycast.censurfridns.dk
# 89.233.43.71      # ns1.censurfridns.dk
# 204.152.184.76    # f.6to4-servers.net, ISC, USA
   );

   my $ipv6 = $prep->(
      ["20014860486000000000000000008888", "20014860486000000000000000008844"], # 2001:4860:4860::8888/8844 - google ipv6
      ["26064700470000000000000000001111", "26064700470000000000000000001001"], # 2606:4700:4700::1111/1001 - cloudflare dns
   );

   undef $ipv4 unless $AnyEvent::PROTOCOL{ipv4};
   undef $ipv6 unless $AnyEvent::PROTOCOL{ipv6};

   ($ipv6, $ipv4) = ($ipv4, $ipv6)
      if $AnyEvent::PROTOCOL{ipv6} > $AnyEvent::PROTOCOL{ipv4};

   @DNS_FALLBACK = (@$ipv4, @$ipv6);
}

=item AnyEvent::DNS::a $domain, $cb->(@addrs)

Tries to resolve the given domain to IPv4 address(es).

=item AnyEvent::DNS::aaaa $domain, $cb->(@addrs)

Tries to resolve the given domain to IPv6 address(es).

=item AnyEvent::DNS::mx $domain, $cb->(@hostnames)

Tries to resolve the given domain into a sorted (lower preference value
first) list of domain names.

=item AnyEvent::DNS::ns $domain, $cb->(@hostnames)

Tries to resolve the given domain name into a list of name servers.

=item AnyEvent::DNS::txt $domain, $cb->(@hostnames)

Tries to resolve the given domain name into a list of text records. Only
the first text string per record will be returned. If you want all
strings, you need to call the resolver manually:

   resolver->resolve ($domain => "txt", sub {
      for my $record (@_) {
         my (undef, undef, undef, @txt) = @$record;
         # strings now in @txt
      }
   });

=item AnyEvent::DNS::srv $service, $proto, $domain, $cb->(@srv_rr)