=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;

no warnings;
use strict;

use Socket qw(AF_INET SOCK_DGRAM SOCK_STREAM);

use AnyEvent ();
use AnyEvent::Handle ();
use AnyEvent::Util qw(AF_INET6);

our $VERSION = 4.411;

our @DNS_FALLBACK = (v208.67.220.220, v208.67.222.222);

=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.

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

Tries to resolve the given service, protocol and domain name into a list
of service records.

Each C<$srv_rr> is an array reference with the following contents: 
C<[$priority, $weight, $transport, $target]>.

They will be sorted with lowest priority first, then randomly
distributed by weight as per RFC 2782.

Example:

   AnyEvent::DNS::srv "sip", "udp", "schmorp.de", sub { ...
   # @_ = ( [10, 10, 5060, "sip1.schmorp.de" ] )

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

Tries to make a PTR lookup on the given domain. See C<reverse_lookup>
and C<reverse_verify> if you want to resolve an IP address to a hostname
instead.

=item AnyEvent::DNS::any $domain, $cb->(@rrs)

Tries to resolve the given domain and passes all resource records found to
the callback.

=item AnyEvent::DNS::reverse_lookup $ipv4_or_6, $cb->(@hostnames)

Tries to reverse-resolve the given IPv4 or IPv6 address (in textual form)
into it's hostname(s). Handles V4MAPPED and V4COMPAT IPv6 addresses
transparently.

=item AnyEvent::DNS::reverse_verify $ipv4_or_6, $cb->(@hostnames)

The same as C<reverse_lookup>, but does forward-lookups to verify that
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 {
      $cb->(map $_->[3], @_);
   });
}

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

   resolver->resolve ($domain => "*", $cb);
}

# convert textual ip address into reverse lookup form
sub _munge_ptr($) {
   my $ipn = $_[0]
      or return;

   my $ptr;

   my $af = AnyEvent::Socket::address_family ($ipn);

   if ($af == AF_INET6) {
      $ipn = substr $ipn, 0, 16; # anticipate future expansion

      # handle v4mapped and v4compat
      if ($ipn =~ s/^\x00{10}(?:\xff\xff|\x00\x00)//) {
         $af = AF_INET;
      } else {
         $ptr = join ".", (reverse split //, unpack "H32", $ipn), "ip6.arpa.";
      }
   }

   if ($af == AF_INET) {
      $ptr = join ".", (reverse unpack "C4", $ipn), "in-addr.arpa.";
   }

   $ptr
}

sub reverse_lookup($$) {
   my ($ip, $cb) = @_;

   $ip = _munge_ptr AnyEvent::Socket::parse_address ($ip)
      or return $cb->();

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

sub reverse_verify($$) {
   my ($ip, $cb) = @_;
   
   my $ipn = AnyEvent::Socket::parse_address ($ip)
      or return $cb->();

   my $af = AnyEvent::Socket::address_family ($ipn);

   my @res;
   my $cnt;

   my $ptr = _munge_ptr $ipn
      or return $cb->();

   $ip = AnyEvent::Socket::format_address ($ipn); # normalise into the same form

   ptr $ptr, sub {
      for my $name (@_) {
         ++$cnt;
         
         # () around AF_INET to work around bug in 5.8
         resolver->resolve ("$name." => ($af == (AF_INET) ? "a" : "aaaa"), sub {
            for (@_) {
               push @res, $name
                  if $_->[3] eq $ip;
            }
            $cb->(@res) unless --$cnt;
         });
      }

      $cb->() unless $cnt;
   };
}

#################################################################################

=back

=head2 LOW-LEVEL DNS EN-/DECODING FUNCTIONS

=over 4

=item $AnyEvent::DNS::EDNS0

This variable decides whether dns_pack automatically enables EDNS0
support. By default, this is disabled (C<0>), unless overridden by
C<$ENV{PERL_ANYEVENT_EDNS0}>, but when set to C<1>, AnyEvent::DNS will use
EDNS0 in all requests.

=cut

our $EDNS0 = $ENV{PERL_ANYEVENT_EDNS0}*1; # set to 1 to enable (partial) edns0

our %opcode_id = (
   query  => 0,
   iquery => 1,
   status => 2,
   notify => 4,
   update => 5,
   map +($_ => $_), 3, 6..15
);

our %opcode_str = reverse %opcode_id;

our %rcode_id = (
   noerror  =>  0,
   formerr  =>  1,
   servfail =>  2,
   nxdomain =>  3,
   notimp   =>  4,
   refused  =>  5,
   yxdomain =>  6, # Name Exists when it should not     [RFC 2136]
   yxrrset  =>  7, # RR Set Exists when it should not   [RFC 2136]
   nxrrset  =>  8, # RR Set that should exist does not  [RFC 2136]
   notauth  =>  9, # Server Not Authoritative for zone  [RFC 2136]
   notzone  => 10, # Name not contained in zone         [RFC 2136]
# EDNS0  16    BADVERS   Bad OPT Version                    [RFC 2671]
# EDNS0  16    BADSIG    TSIG Signature Failure             [RFC 2845]
# EDNS0  17    BADKEY    Key not recognized                 [RFC 2845]
# EDNS0  18    BADTIME   Signature out of time window       [RFC 2845]
# EDNS0  19    BADMODE   Bad TKEY Mode                      [RFC 2930]
# EDNS0  20    BADNAME   Duplicate key name                 [RFC 2930]
# EDNS0  21    BADALG    Algorithm not supported            [RFC 2930]
   map +($_ => $_), 11..15
);

our %rcode_str = reverse %rcode_id;

our %type_id = (
   a     =>   1,
   ns    =>   2,
   md    =>   3,
   mf    =>   4,
   cname =>   5,
   soa   =>   6,
   mb    =>   7,
   mg    =>   8,
   mr    =>   9,
   null  =>  10,
   wks   =>  11,
   ptr   =>  12,
   hinfo =>  13,
   minfo =>  14,
   mx    =>  15,
   txt   =>  16,
   aaaa  =>  28,
   srv   =>  33,
   naptr =>  35, # rfc2915
   dname =>  39, # rfc2672
   opt   =>  41,
   spf   =>  99,
   tkey  => 249,
   tsig  => 250,
   ixfr  => 251,
   axfr  => 252,
   mailb => 253,
   "*"   => 255,
);

our %type_str = reverse %type_id;

our %class_id = (
   in   =>   1,
   ch   =>   3,
   hs   =>   4,
   none => 254,
   "*"  => 255,
);

our %class_str = reverse %class_id;

sub _enc_name($) {
   pack "(C/a*)*", (split /\./, shift), ""
}

sub _enc_qd() {
   (_enc_name $_->[0]) . pack "nn",
     ($_->[1] > 0 ? $_->[1] : $type_id {$_->[1]}),
     ($_->[2] > 0 ? $_->[2] : $class_id{$_->[2] || "in"})
}

sub _enc_rr() {
   die "encoding of resource records is not supported";
}

=item $pkt = AnyEvent::DNS::dns_pack $dns

Packs a perl data structure into a DNS packet. Reading RFC 1035 is strongly
recommended, then everything will be totally clear. Or maybe not.

Resource records are not yet encodable.

Examples:

   # very simple request, using lots of default values:
   { rd => 1, qd => [ [ "host.domain", "a"] ] }
  
   # more complex example, showing how flags etc. are named:
  
   {
      id => 10000,
      op => "query",
      rc => "nxdomain",
  
      # flags
      qr => 1,
      aa => 0,
      tc => 0,
      rd => 0,
      ra => 0,
      ad => 0,
      cd => 0,
  
      qd => [@rr], # query section
      an => [@rr], # answer section
      ns => [@rr], # authority section
      ar => [@rr], # additional records section
   }

=cut

sub dns_pack($) {
   my ($req) = @_;

   pack "nn nnnn a* a* a* a* a*",
      $req->{id},

      ! !$req->{qr}   * 0x8000
      + $opcode_id{$req->{op}} * 0x0800
      + ! !$req->{aa} * 0x0400
      + ! !$req->{tc} * 0x0200
      + ! !$req->{rd} * 0x0100
      + ! !$req->{ra} * 0x0080
      + ! !$req->{ad} * 0x0020
      + ! !$req->{cd} * 0x0010
      + $rcode_id{$req->{rc}} * 0x0001,

      scalar @{ $req->{qd} || [] },
      scalar @{ $req->{an} || [] },
      scalar @{ $req->{ns} || [] },
      $EDNS0 + scalar @{ $req->{ar} || [] }, # EDNS0 option included here

      (join "", map _enc_qd, @{ $req->{qd} || [] }),
      (join "", map _enc_rr, @{ $req->{an} || [] }),
      (join "", map _enc_rr, @{ $req->{ns} || [] }),
      (join "", map _enc_rr, @{ $req->{ar} || [] }),

      ($EDNS0 ? pack "C nnNn", 0, 41, MAX_PKT, 0, 0 : "") # EDNS0 option
}

our $ofs;
our $pkt;

# bitches
sub _dec_name {
   my @res;
   my $redir;
   my $ptr = $ofs;
   my $cnt;

   while () {
      return undef if ++$cnt >= 256; # to avoid DoS attacks

      my $len = ord substr $pkt, $ptr++, 1;

      if ($len >= 0xc0) {
         $ptr++;
         $ofs = $ptr if $ptr > $ofs;
         $ptr = (unpack "n", substr $pkt, $ptr - 2, 2) & 0x3fff;
      } elsif ($len) {
         push @res, substr $pkt, $ptr, $len;
         $ptr += $len;
      } else {
         $ofs = $ptr if $ptr > $ofs;
         return join ".", @res;
      }
   }
}

sub _dec_qd {
   my $qname = _dec_name;
   my ($qt, $qc) = unpack "nn", substr $pkt, $ofs; $ofs += 4;
   [$qname, $type_str{$qt} || $qt, $class_str{$qc} || $qc]
}

our %dec_rr = (
     1 => sub { join ".", unpack "C4", $_ }, # a
     2 => sub { local $ofs = $ofs - length; _dec_name }, # ns
     5 => sub { local $ofs = $ofs - length; _dec_name }, # cname
     6 => sub { 
             local $ofs = $ofs - length;
             my $mname = _dec_name;
             my $rname = _dec_name;
             ($mname, $rname, unpack "NNNNN", substr $pkt, $ofs)
          }, # soa
    11 => sub { ((join ".", unpack "C4", $_), unpack "C a*", substr $_, 4) }, # wks
    12 => sub { local $ofs = $ofs - length; _dec_name }, # ptr
    13 => sub { unpack "C/a* C/a*", $_ }, # hinfo
    15 => sub { local $ofs = $ofs + 2 - length; ((unpack "n", $_), _dec_name) }, # mx
    16 => sub { unpack "(C/a*)*", $_ }, # txt
    28 => sub { AnyEvent::Socket::format_address ($_) }, # aaaa
    33 => sub { local $ofs = $ofs + 6 - length; ((unpack "nnn", $_), _dec_name) }, # srv
    35 => sub { # naptr
       # requires perl 5.10, sorry
       my ($order, $preference, $flags, $service, $regexp, $offset) = unpack "nn C/a* C/a* C/a* .", $_;
       local $ofs = $ofs + $offset - length;
       ($order, $preference, $flags, $service, $regexp, _dec_name)
    },
    39 => sub { local $ofs = $ofs - length; _dec_name }, # dname
    99 => sub { unpack "(C/a*)*", $_ }, # spf
);

sub _dec_rr {
   my $name = _dec_name;

   my ($rt, $rc, $ttl, $rdlen) = unpack "nn N n", substr $pkt, $ofs; $ofs += 10;
   local $_ = substr $pkt, $ofs, $rdlen; $ofs += $rdlen;

   [
      $name,
      $type_str{$rt}  || $rt,
      $class_str{$rc} || $rc,
      ($dec_rr{$rt} || sub { $_ })->(),
   ]
}

=item $dns = AnyEvent::DNS::dns_unpack $pkt

Unpacks a DNS packet into a perl data structure.

Examples:

   # an unsuccessful reply
   {
     'qd' => [
               [ 'ruth.plan9.de.mach.uni-karlsruhe.de', '*', 'in' ]
             ],
     'rc' => 'nxdomain',
     'ar' => [],
     'ns' => [
               [
                 'uni-karlsruhe.de',
                 'soa',
                 'in',
                 'netserv.rz.uni-karlsruhe.de',
                 'hostmaster.rz.uni-karlsruhe.de',
                 2008052201, 10800, 1800, 2592000, 86400
               ]
             ],
     'tc' => '',
     'ra' => 1,
     'qr' => 1,
     'id' => 45915,
     'aa' => '',
     'an' => [],
     'rd' => 1,
     'op' => 'query'
   }
   
   # a successful reply
   
   {
     'qd' => [ [ 'www.google.de', 'a', 'in' ] ],
     'rc' => 0,
     'ar' => [
               [ 'a.l.google.com', 'a', 'in', '209.85.139.9' ],
               [ 'b.l.google.com', 'a', 'in', '64.233.179.9' ],
               [ 'c.l.google.com', 'a', 'in', '64.233.161.9' ],
             ],
     'ns' => [
               [ 'l.google.com', 'ns', 'in', 'a.l.google.com' ],
               [ 'l.google.com', 'ns', 'in', 'b.l.google.com' ],
             ],
     'tc' => '',
     'ra' => 1,
     'qr' => 1,
     'id' => 64265,
     'aa' => '',
     'an' => [
               [ 'www.google.de', 'cname', 'in', 'www.google.com' ],
               [ 'www.google.com', 'cname', 'in', 'www.l.google.com' ],
               [ 'www.l.google.com', 'a', 'in', '66.249.93.104' ],
               [ 'www.l.google.com', 'a', 'in', '66.249.93.147' ],
             ],
     'rd' => 1,
     'op' => 0
   }

=cut

sub dns_unpack($) {
   local $pkt = shift;
   my ($id, $flags, $qd, $an, $ns, $ar)
      = unpack "nn nnnn A*", $pkt;

   local $ofs = 6 * 2;

   {
      id => $id,
      qr => ! ! ($flags & 0x8000),
      aa => ! ! ($flags & 0x0400),
      tc => ! ! ($flags & 0x0200),
      rd => ! ! ($flags & 0x0100),
      ra => ! ! ($flags & 0x0080),
      ad => ! ! ($flags & 0x0020),
      cd => ! ! ($flags & 0x0010),
      op => $opcode_str{($flags & 0x001e) >> 11},
      rc => $rcode_str{($flags & 0x000f)},

      qd => [map _dec_qd, 1 .. $qd],
      an => [map _dec_rr, 1 .. $an],
      ns => [map _dec_rr, 1 .. $ns],
      ar => [map _dec_rr, 1 .. $ar],
   }
}

#############################################################################

=back

=head2 THE AnyEvent::DNS RESOLVER CLASS

This is the class which does the actual protocol work.

=over 4

=cut

use Carp ();
use Scalar::Util ();
use Socket ();

our $NOW;

=item AnyEvent::DNS::resolver

This function creates and returns a resolver that is ready to use and
should mimic the default resolver for your system as good as possible.

It only ever creates one resolver and returns this one on subsequent
calls.

Unless you have special needs, prefer this function over creating your own
resolver object.

=cut

our $RESOLVER;

sub resolver() {
   $RESOLVER || do {
      $RESOLVER = new AnyEvent::DNS untaint => 1;
      $RESOLVER->os_config;
      $RESOLVER
   }
}

=item $resolver = new AnyEvent::DNS key => value...

Creates and returns a new resolver.

The following options are supported:

=over 4

=item server => [...]

A list of server addresses (default: C<v127.0.0.1>) in network format
(i.e. as returned by C<AnyEvent::Socket::parse_address> - both IPv4 and
IPv6 are supported).

=item timeout => [...]

A list of timeouts to use (also determines the number of retries). To make
three retries with individual time-outs of 2, 5 and 5 seconds, use C<[2,
5, 5]>, which is also the default.

=item search => [...]

The default search list of suffixes to append to a domain name (default: none).

=item ndots => $integer

The number of dots (default: C<1>) that a name must have so that the resolver
tries to resolve the name without any suffixes first.

=item max_outstanding => $integer

Most name servers do not handle many parallel requests very well. This
option limits the number of outstanding requests to C<$integer>
(default: C<10>), that means if you request more than this many requests,
then the additional requests will be queued until some other requests have
been resolved.

=item reuse => $seconds

The number of seconds (default: C<300>) that a query id cannot be re-used
after a timeout. If there was no time-out then query ids can be reused
immediately.

=item untaint => $boolean

When true, then the resolver will automatically untaint results, and might
also ignore certain environment variables.

=back

=cut

sub new {
   my ($class, %arg) = @_;

   my $self = bless {
      server  => [],
      timeout => [2, 5, 5],
      search  => [],
      ndots   => 1,
      max_outstanding => 10,
      reuse   => 300,
      %arg,
      reuse_q => [],
   }, $class;

   # search should default to gethostname's domain
   # but perl lacks a good posix module

   # try to create an ipv4 and an ipv6 socket
   # only fail when we cannot create either
   my $got_socket;

   Scalar::Util::weaken (my $wself = $self);

   if (socket my $fh4, AF_INET , &Socket::SOCK_DGRAM, 0) {
      ++$got_socket;

      AnyEvent::Util::fh_nonblocking $fh4, 1;
      $self->{fh4} = $fh4;
      $self->{rw4} = AnyEvent->io (fh => $fh4, poll => "r", cb => sub {
         if (my $peer = recv $fh4, my $pkt, MAX_PKT, 0) {
            $wself->_recv ($pkt, $peer);
         }
      });
   }

   if (AF_INET6 && socket my $fh6, AF_INET6, &Socket::SOCK_DGRAM, 0) {
      ++$got_socket;

      $self->{fh6} = $fh6;
      AnyEvent::Util::fh_nonblocking $fh6, 1;
      $self->{rw6} = AnyEvent->io (fh => $fh6, poll => "r", cb => sub {
         if (my $peer = recv $fh6, my $pkt, MAX_PKT, 0) {
            $wself->_recv ($pkt, $peer);
         }
      });
   }

   $got_socket
      or Carp::croak "unable to create either an IPv4 or an IPv6 socket";

   $self->_compile;

   $self
}

=item $resolver->parse_resolv_conv ($string)

Parses the given string as if it were a F<resolv.conf> file. The following
directives are supported (but not necessarily implemented).

C<#>-style comments, C<nameserver>, C<domain>, C<search>, C<sortlist>,
C<options> (C<timeout>, C<attempts>, C<ndots>).

Everything else is silently ignored.

=cut

sub parse_resolv_conf {
   my ($self, $resolvconf) = @_;

   $self->{server} = [];
   $self->{search} = [];

   my $attempts;

   for (split /\n/, $resolvconf) {
      if (/^\s*#/) {
         # comment
      } elsif (/^\s*nameserver\s+(\S+)\s*$/i) {
         my $ip = $1;
         if (my $ipn = AnyEvent::Socket::parse_address ($ip)) {
            push @{ $self->{server} }, $ipn;
         } else {
            warn "nameserver $ip invalid and ignored\n";
         }
      } elsif (/^\s*domain\s+(\S*)\s+$/i) {
         $self->{search} = [$1];
      } elsif (/^\s*search\s+(.*?)\s*$/i) {
         $self->{search} = [split /\s+/, $1];
      } elsif (/^\s*sortlist\s+(.*?)\s*$/i) {
         # ignored, NYI
      } elsif (/^\s*options\s+(.*?)\s*$/i) {
         for (split /\s+/, $1) {
            if (/^timeout:(\d+)$/) {
               $self->{timeout} = [$1];
            } elsif (/^attempts:(\d+)$/) {
               $attempts = $1;
            } elsif (/^ndots:(\d+)$/) {
               $self->{ndots} = $1;
            } else {
               # debug, rotate, no-check-names, inet6
            }
         }
      }
   }

   $self->{timeout} = [($self->{timeout}[0]) x $attempts]
      if $attempts;

   $self->_compile;
}

=item $resolver->os_config

Tries so load and parse F</etc/resolv.conf> on portable operating
systems. Tries various egregious hacks on windows to force the DNS servers
and searchlist out of the system.

=cut

sub os_config {
   my ($self) = @_;

   $self->{server} = [];
   $self->{search} = [];

   if (AnyEvent::WIN32 || $^O =~ /cygwin/i) {
      no strict 'refs';

      # there are many options to find the current nameservers etc. on windows
      # all of them don't work consistently:
      # - the registry thing needs separate code on win32 native vs. cygwin
      # - the registry layout differs between windows versions
      # - calling windows api functions doesn't work on cygwin
      # - ipconfig uses locale-specific messages

      # we use ipconfig parsing because, despite all its brokenness,
      # it seems most stable in practise.
      # for good measure, we append a fallback nameserver to our list.

      if (open my $fh, "ipconfig /all |") {
         # parsing strategy: we go through the output and look for
         # :-lines with DNS in them. everything in those is regarded as
         # either a nameserver (if it parses as an ip address), or a suffix
         # (all else).

         my $dns;
         while (<$fh>) {
            if (s/^\s.*\bdns\b.*://i) {
               $dns = 1;
            } elsif (/^\S/ || /^\s[^:]{16,}: /) {
               $dns = 0;
            }
            if ($dns && /^\s*(\S+)\s*$/) {
               my $s = $1;
               $s =~ s/%\d+(?!\S)//; # get rid of ipv6 scope id
               if (my $ipn = AnyEvent::Socket::parse_address ($s)) {
                  push @{ $self->{server} }, $ipn;
               } else {
                  push @{ $self->{search} }, $s;
               }
            }
         }

         # always add one fallback server
         push @{ $self->{server} }, $DNS_FALLBACK[rand @DNS_FALLBACK];

         $self->_compile;
      }
   } else {
      # try resolv.conf everywhere

      if (open my $fh, "</etc/resolv.conf") {
         local $/;
         $self->parse_resolv_conf (<$fh>);
      }
   }
}

=item $resolver->timeout ($timeout, ...)

Sets the timeout values. See the C<timeout> constructor argument (and note
that this method uses the values itself, not an array-reference).

=cut

sub timeout {
   my ($self, @timeout) = @_;

   $self->{timeout} = \@timeout;
   $self->_compile;
}

=item $resolver->max_outstanding ($nrequests)

Sets the maximum number of outstanding requests to C<$nrequests>. See the
C<max_outstanding> constructor argument.

=cut

sub max_outstanding {
   my ($self, $max) = @_;

   $self->{max_outstanding} = $max;
   $self->_scheduler;
}

sub _compile {
   my $self = shift;

   my %search; $self->{search} = [grep 0 < length, grep !$search{$_}++, @{ $self->{search} }];
   my %server; $self->{server} = [grep 0 < length, grep !$server{$_}++, @{ $self->{server} }];

   unless (@{ $self->{server} }) {
      # use 127.0.0.1 by default, and one opendns nameserver as fallback
      $self->{server} = [v127.0.0.1, $DNS_FALLBACK[rand @DNS_FALLBACK]];
   }

   my @retry;

   for my $timeout (@{ $self->{timeout} }) {
      for my $server (@{ $self->{server} }) {
         push @retry, [$server, $timeout];
      }
   }

   $self->{retry} = \@retry;
}

sub _feed {
   my ($self, $res) = @_;

   ($res) = $res =~ /^(.*)$/s
      if AnyEvent::TAINT && $self->{untaint};

   $res = dns_unpack $res
      or return;

   my $id = $self->{id}{$res->{id}};

   return unless ref $id;

   $NOW = time;
   $id->[1]->($res);
}

sub _recv {
   my ($self, $pkt, $peer) = @_;

   # we ignore errors (often one gets port unreachable, but there is
   # no good way to take advantage of that.

   my ($port, $host) = AnyEvent::Socket::unpack_sockaddr ($peer);

   return unless $port == 53 && grep $_ eq $host, @{ $self->{server} };

   $self->_feed ($pkt);
}

sub _free_id {
   my ($self, $id, $timeout) = @_;

   if ($timeout) {
      # we need to block the id for a while
      $self->{id}{$id} = 1;
      push @{ $self->{reuse_q} }, [$NOW + $self->{reuse}, $id];
   } else {
      # we can quickly recycle the id
      delete $self->{id}{$id};
   }

   --$self->{outstanding};
   $self->_scheduler;
}

# execute a single request, involves sending it with timeouts to multiple servers
sub _exec {
   my ($self, $req) = @_;

   my $retry; # of retries
   my $do_retry;

   $do_retry = sub {
      my $retry_cfg = $self->{retry}[$retry++]
         or do {
            # failure
            $self->_free_id ($req->[2], $retry > 1);
            undef $do_retry; return $req->[1]->();
         };

      my ($server, $timeout) = @$retry_cfg;
      
      $self->{id}{$req->[2]} = [AnyEvent->timer (after => $timeout, cb => sub {
         $NOW = time;

         # timeout, try next
         &$do_retry if $do_retry;
      }), sub {
         my ($res) = @_;

         if ($res->{tc}) {
            # success, but truncated, so use tcp
            AnyEvent::Socket::tcp_connect (AnyEvent::Socket::format_address ($server), DOMAIN_PORT, sub {
               return unless $do_retry; # some other request could have invalidated us already

               my ($fh) = @_
                  or return &$do_retry;

               my $handle; $handle = new AnyEvent::Handle
                  fh       => $fh,
                  timeout  => $timeout,
                  on_error => sub {
                     undef $handle;
                     return unless $do_retry; # some other request could have invalidated us already
                     # failure, try next
                     &$do_retry;
                  };

               $handle->push_write (pack "n/a", $req->[0]);
               $handle->push_read (chunk => 2, sub {
                  $handle->unshift_read (chunk => (unpack "n", $_[1]), sub {
                     undef $handle;
                     $self->_feed ($_[1]);
                  });
               });

            }, sub { $timeout });

         } else {
            # success
            $self->_free_id ($req->[2], $retry > 1);
            undef $do_retry; return $req->[1]->($res);
         }
      }];
      
      my $sa = AnyEvent::Socket::pack_sockaddr (DOMAIN_PORT, $server);

      my $fh = AF_INET == Socket::sockaddr_family ($sa)
               ? $self->{fh4} : $self->{fh6}
         or return &$do_retry;

      send $fh, $req->[0], 0, $sa;
   };

   &$do_retry;
}

sub _scheduler {
   my ($self) = @_;

   no strict 'refs';

   $NOW = time;

   # first clear id reuse queue
   delete $self->{id}{ (shift @{ $self->{reuse_q} })->[1] }
      while @{ $self->{reuse_q} } && $self->{reuse_q}[0][0] <= $NOW;

   while ($self->{outstanding} < $self->{max_outstanding}) {

      if (@{ $self->{reuse_q} } >= 30000) {
         # we ran out of ID's, wait a bit
         $self->{reuse_to} ||= AnyEvent->timer (after => $self->{reuse_q}[0][0] - $NOW, cb => sub {
            delete $self->{reuse_to};
            $self->_scheduler;
         });
         last;
      }

      if (my $req = shift @{ $self->{queue} }) {
         # found a request in the queue, execute it
         while () {
            $req->[2] = int rand 65536;
            last unless exists $self->{id}{$req->[2]};
         }

         ++$self->{outstanding};
         $self->{id}{$req->[2]} = 1;
         substr $req->[0], 0, 2, pack "n", $req->[2];

         $self->_exec ($req);

      } elsif (my $cb = shift @{ $self->{wait} }) {
         # found a wait_for_slot callback, call that one first
         $cb->($self);

      } else {
         # nothing to do, just exit
         last;
      }
   }
}

=item $resolver->request ($req, $cb->($res))

This is the main low-level workhorse for sending DNS requests.

This function sends a single request (a hash-ref formated as specified
for C<dns_pack>) to the configured nameservers in turn until it gets a
response. It handles timeouts, retries and automatically falls back to
virtual circuit mode (TCP) when it receives a truncated reply.

Calls the callback with the decoded response packet if a reply was
received, or no arguments in case none of the servers answered.

=cut

sub request($$) {
   my ($self, $req, $cb) = @_;

   push @{ $self->{queue} }, [dns_pack $req, $cb];
   $self->_scheduler;
}

=item $resolver->resolve ($qname, $qtype, %options, $cb->(@rr))

Queries the DNS for the given domain name C<$qname> of type C<$qtype>.

A C<$qtype> is either a numerical query type (e.g. C<1> for A records) or
a lowercase name (you have to look at the source to see which aliases are
supported, but all types from RFC 1035, C<aaaa>, C<srv>, C<spf> and a few
more are known to this module). A C<$qtype> of "*" is supported and means
"any" record type.

The callback will be invoked with a list of matching result records or
none on any error or if the name could not be found.

CNAME chains (although illegal) are followed up to a length of 10.

The callback will be invoked with arraryefs of the form C<[$name, $type,
$class, @data>], where C<$name> is the domain name, C<$type> a type string
or number, C<$class> a class name and @data is resource-record-dependent
data. For C<a> records, this will be the textual IPv4 addresses, for C<ns>
or C<cname> records this will be a domain name, for C<txt> records these
are all the strings and so on.

All types mentioned in RFC 1035, C<aaaa>, C<srv>, C<naptr> and C<spf> are
decoded. All resource records not known to this module will have
the raw C<rdata> field as fourth entry.

Note that this resolver is just a stub resolver: it requires a name server
supporting recursive queries, will not do any recursive queries itself and
is not secure when used against an untrusted name server.

The following options are supported:

=over 4

=item search => [$suffix...]

Use the given search list (which might be empty), by appending each one
in turn to the C<$qname>. If this option is missing then the configured
C<ndots> and C<search> values define its value (depending on C<ndots>, the
empty suffix will be prepended or appended to that C<search> value). If
the C<$qname> ends in a dot, then the searchlist will be ignored.

=item accept => [$type...]

Lists the acceptable result types: only result types in this set will be
accepted and returned. The default includes the C<$qtype> and nothing
else. If this list includes C<cname>, then CNAME-chains will not be
followed (because you asked for the CNAME record).

=item class => "class"

Specify the query class ("in" for internet, "ch" for chaosnet and "hs" for
hesiod are the only ones making sense). The default is "in", of course.

=back

Examples:

   # full example, you can paste this into perl:
   use Data::Dumper;
   use AnyEvent::DNS;
   AnyEvent::DNS::resolver->resolve (
      "google.com", "*", my $cv = AnyEvent->condvar);
   warn Dumper [$cv->recv];

   # shortened result:
   # [
   #   [ 'google.com', 'soa', 'in', 'ns1.google.com', 'dns-admin.google.com',
   #     2008052701, 7200, 1800, 1209600, 300 ],
   #   [
   #     'google.com', 'txt', 'in',
   #     'v=spf1 include:_netblocks.google.com ~all'
   #   ],
   #   [ 'google.com', 'a', 'in', '64.233.187.99' ],
   #   [ 'google.com', 'mx', 'in', 10, 'smtp2.google.com' ],
   #   [ 'google.com', 'ns', 'in', 'ns2.google.com' ],
   # ]

   # resolve a records:
   $res->resolve ("ruth.plan9.de", "a", sub { warn Dumper [@_] });

   # result:
   # [
   #   [ 'ruth.schmorp.de', 'a', 'in', '129.13.162.95' ]
   # ]

   # resolve any records, but return only a and aaaa records:
   $res->resolve ("test1.laendle", "*",
      accept => ["a", "aaaa"],
      sub {
         warn Dumper [@_];
      }
   );

   # result:
   # [
   #   [ 'test1.laendle', 'a', 'in', '10.0.0.255' ],
   #   [ 'test1.laendle', 'aaaa', 'in', '3ffe:1900:4545:0002:0240:0000:0000:f7e1' ]
   # ]

=cut

sub resolve($%) {
   my $cb = pop;
   my ($self, $qname, $qtype, %opt) = @_;

   my @search = $qname =~ s/\.$//
      ? ""
      : $opt{search}
        ? @{ $opt{search} }
        : ($qname =~ y/.//) >= $self->{ndots}
          ? ("", @{ $self->{search} })
          : (@{ $self->{search} }, "");

   my $class = $opt{class} || "in";

   my %atype = $opt{accept}
      ? map +($_ => 1), @{ $opt{accept} }
      : ($qtype => 1);

   # advance in searchlist
   my ($do_search, $do_req);
   
   $do_search = sub {
      @search
         or (undef $do_search), (undef $do_req), return $cb->();

      (my $name = lc "$qname." . shift @search) =~ s/\.$//;
      my $depth = 10;

      # advance in cname-chain
      $do_req = sub {
         $self->request ({
            rd => 1,
            qd => [[$name, $qtype, $class]],
         }, sub {
            my ($res) = @_
               or return $do_search->();

            my $cname;

            while () {
               # results found?
               my @rr = grep $name eq lc $_->[0] && ($atype{"*"} || $atype{$_->[1]}), @{ $res->{an} };

               (undef $do_search), (undef $do_req), return $cb->(@rr)
                  if @rr;

               # see if there is a cname we can follow
               my @rr = grep $name eq lc $_->[0] && $_->[1] eq "cname", @{ $res->{an} };

               if (@rr) {
                  $depth--
                     or return $do_search->(); # cname chain too long

                  $cname = 1;
                  $name = $rr[0][3];

               } elsif ($cname) {
                  # follow the cname
                  return $do_req->();

               } else {
                  # no, not found anything
                  return $do_search->();
               }
             }
         });
      };

      $do_req->();
   };

   $do_search->();
}

=item $resolver->wait_for_slot ($cb->($resolver))

Wait until a free request slot is available and call the callback with the
resolver object.

A request slot is used each time a request is actually sent to the
nameservers: There are never more than C<max_outstanding> of them.

Although you can submit more requests (they will simply be queued until
a request slot becomes available), sometimes, usually for rate-limiting
purposes, it is useful to instead wait for a slot before generating the
request (or simply to know when the request load is low enough so one can
submit requests again).

This is what this method does: The callback will be called when submitting
a DNS request will not result in that request being queued. The callback
may or may not generate any requests in response.

Note that the callback will only be invoked when the request queue is
empty, so this does not play well if somebody else keeps the request queue
full at all times.

=cut

sub wait_for_slot {
   my ($self, $cb) = @_;

   push @{ $self->{wait} }, $cb;
   $self->_scheduler;
}

use AnyEvent::Socket (); # circular dependency, so do not import anything and do it at the end

1;

=back

=head1 AUTHOR

   Marc Lehmann <schmorp@schmorp.de>
   http://home.schmorp.de/

=cut