the resolved hostnames indeed point to the address, which makes spoofing
harder.

If you want to resolve an address into a hostname, this is the preferred
method: The DNS records could still change, but at least this function
verified that the hostname, at one point in the past, pointed at the IP
address you originally resolved.

Example:

   AnyEvent::DNS::ptr "2001:500:2f::f", sub { print shift };
   # => f.root-servers.net

=cut

sub MAX_PKT() { 4096 } # max packet size we advertise and accept

sub DOMAIN_PORT() { 53 } # if this changes drop me a note

sub resolver;

sub a($$) {
   my ($domain, $cb) = @_;

   resolver->resolve ($domain => "a", sub {
      $cb->(map $_->[3], @_);
   });
}

sub aaaa($$) {
   my ($domain, $cb) = @_;

   resolver->resolve ($domain => "aaaa", sub {
      $cb->(map $_->[3], @_);
   });
}

sub mx($$) {
   my ($domain, $cb) = @_;

   resolver->resolve ($domain => "mx", sub {
      $cb->(map $_->[4], sort { $a->[3] <=> $b->[3] } @_);
   });
}

sub ns($$) {
   my ($domain, $cb) = @_;

   resolver->resolve ($domain => "ns", sub {
      $cb->(map $_->[3], @_);
   });
}

sub txt($$) {
   my ($domain, $cb) = @_;

   resolver->resolve ($domain => "txt", sub {
      $cb->(map $_->[3], @_);
   });
}

sub srv($$$$) {
   my ($service, $proto, $domain, $cb) = @_;

   # todo, ask for any and check glue records
   resolver->resolve ("_$service._$proto.$domain" => "srv", sub {
      my @res;

      # classify by priority
      my %pri;
      push @{ $pri{$_->[3]} }, [ @$_[3,4,5,6] ]
         for @_;

      # order by priority
      for my $pri (sort { $a <=> $b } keys %pri) {
         # order by weight
         my @rr = sort { $a->[1] <=> $b->[1] } @{ delete $pri{$pri} };

         my $sum; $sum += $_->[1] for @rr;

         while (@rr) {
            my $w = int rand $sum + 1;
            for (0 .. $#rr) {
               if (($w -= $rr[$_][1]) <= 0) {
                  $sum -= $rr[$_][1];
                  push @res, splice @rr, $_, 1, ();
                  last;
               }
            }
         }
      }

      $cb->(@res);
   });
}

sub ptr($$) {
   my ($domain, $cb) = @_;

   resolver->resolve ($domain => "ptr", sub {