qpsmtpd/plugins/dnsbl

#!perl -w

=head1 NAME

dnsbl - handle DNS BlackList lookups

=head1 DESCRIPTION

Plugin that checks the IP address of the incoming connection against
a configurable set of RBL services.

=head1 USAGE

Add the following line to the config/plugins file:

  dnsbl [ reject_type disconnect ] [loglevel -1]

=head2 reject_type [ temp | perm ]

To immediately drop the connection (since some blacklisted servers attempt
multiple sends per session), set I<reject_type disconnect>. In most cases,
an IP address that is listed should not be given the opportunity to begin a
new transaction, since even the most volatile blacklists will return the same
answer for a short period of time (the minimum DNS cache period).

Default: perm

=head2 loglevel

Adjust the quantity of logging for this plugin. See docs/logging.pod

  dnsbl [loglevel -1]

=head1 CONFIG FILES

This plugin uses the following configuration files. All are optional. Not
specifying dnsbl_zones is like not using the plugin at all.

=head2 dnsbl_zones

Normal ip based dns blocking lists ("RBLs") which contain TXT records are
specified simply as:

  relays.ordb.org
  spamsources.fabel.dk

To configure RBL services which do not contain TXT records in the DNS,
but only A records (e.g. the RBL+ at http://www.mail-abuse.org), specify your
own error message to return in the SMTP conversation after a colon e.g.

  rbl-plus.mail-abuse.org:You are listed at - http://http://www.mail-abuse.org/cgi-bin/lookup?%IP%

The string %IP% will be replaced with the IP address of incoming connection.
Thus a fully specified file could be:

  sbl-xbl.spamhaus.org
  list.dsbl.org
  rbl-plus.mail-abuse.ja.net:Listed by rbl-plus.mail-abuse.ja.net - see <URL:http://www.mail-abuse.org/cgi-bin/lookup?%IP%>
  relays.ordb.org

=head2 dnsbl_allow

List of allowed ip addresses that bypass RBL checking. Format is one entry per line,
with either a full IP address or a truncated IP address with a period at the end.
For example:

  192.168.1.1
  172.16.33.

NB the environment variable RBLSMTPD is considered before this file is
referenced. See below.

=head2 dnsbl_rejectmsg

A textual message that is sent to the sender on an RBL failure. The TXT record
from the RBL list is also sent, but this file can be used to indicate what
action the sender should take.

For example:

   If you think you have been blocked in error, then please forward
   this entire error message to your ISP so that they can fix their problems.
   The next line often contains a URL that can be visited for more information.

=head1 Environment Variables

=head2 RBLSMTPD

The environment variable RBLSMTPD is supported and mimics the behaviour of
Dan Bernstein's rblsmtpd. The exception to this is the '-' char at the
start of RBLSMTPD which is used to force a hard error in Dan's rblsmtpd.
NB I don't really see the benefit
of using a soft error for a site in an RBL list. This just complicates
things as it takes 7 days (or whatever default period) before a user
gets an error email back. In the meantime they are complaining that their
emails are being "lost" :(

=over 4

=item RBLSMTPD is set and non-empty

The contents are used as the SMTP conversation error.
Use this for forcibly blocking sites you don't like

=item RBLSMTPD is set, but empty

In this case no RBL checks are made.
This can be used for local addresses.

=item RBLSMTPD is not set

All RBL checks will be made.
This is the setting for remote sites that you want to check against RBL.

=back

=head1 Revisions

See: https://github.com/smtpd/qpsmtpd/commits/master/plugins/dnsbl

=cut

sub register {
    my ($self, $qp) = shift, shift;

    if ( @_ % 2 ) {
        $self->{_args}{reject_type} = shift;   # backwards compatibility
    }
    else {
        $self->{_args} = { @_ };
    };
}

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

    # perform RBLSMTPD checks to mimic Dan Bernstein's rblsmtpd
    return DECLINED if $self->is_set_rblsmtpd();
    return DECLINED if $self->is_immune();
    return DECLINED if $self->ip_whitelisted();

  my %dnsbl_zones = map { (split /:/, $_, 2)[0,1] } $self->qp->config('dnsbl_zones');
    if ( ! %dnsbl_zones ) {
        $self->log( LOGDEBUG, "skip: no list configured");
        return DECLINED;
    };

  my $remote_ip = $self->qp->connection->remote_ip;
  my $reversed_ip = join('.', reverse(split(/\./, $remote_ip)));

  # we queue these lookups in the background and fetch the
  # results in the first rcpt handler

  my $res = new Net::DNS::Resolver;
  $res->tcp_timeout(30);
  $res->udp_timeout(30);

  my $sel = IO::Select->new();

  my $dom;
  for my $dnsbl (keys %dnsbl_zones) {
    # fix to find A records, if the dnsbl_zones line has a second field 20/1/04 ++msp
    $dom->{"$reversed_ip.$dnsbl"} = 1;
    if (defined($dnsbl_zones{$dnsbl})) {
      $self->log(LOGDEBUG, "Checking $reversed_ip.$dnsbl for A record in the background");
      $sel->add($res->bgsend("$reversed_ip.$dnsbl"));
    }
    else {
      $self->log(LOGDEBUG, "Checking $reversed_ip.$dnsbl for TXT record in the background");
      $sel->add($res->bgsend("$reversed_ip.$dnsbl", "TXT"));
    }
  }

  $self->connection->notes('dnsbl_sockets', $sel);
  $self->connection->notes('dnsbl_domains', $dom);

  return DECLINED;
}

sub is_set_rblsmtpd {
    my $self = shift;

    my $remote_ip = $self->qp->connection->remote_ip;

    if ( ! defined $ENV{'RBLSMTPD'} ) {
        $self->log(LOGDEBUG, "RBLSMTPD not set for $remote_ip");
        return;
    };

    if ($ENV{'RBLSMTPD'} ne '') {
        $self->log(LOGINFO, "RBLSMTPD=\"$ENV{'RBLSMTPD'}\" for $remote_ip");
        return $ENV{'RBLSMTPD'};
    }

    $self->log(LOGINFO, "RBLSMTPD set, but empty for $remote_ip");
    return 1;  # don't return empty string, it evaluates to false
};

sub ip_whitelisted {
    my $self = shift;

    my $remote_ip = shift || $self->qp->connection->remote_ip;

    return
        grep { s/\.?$/./; $_ eq substr($remote_ip . '.', 0, length $_) }
            $self->qp->config('dnsbl_allow');
};

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

  my $conn = $self->connection;

  return $conn->notes('dnsbl') if $conn->notes('dnsbl');

  my %dnsbl_zones = map { (split /:/, $_, 2)[0,1] } $self->qp->config('dnsbl_zones');

  my $sel       = $conn->notes('dnsbl_sockets') or return '';
  my $dom       = $conn->notes('dnsbl_domains');
  my $remote_ip = $self->qp->connection->remote_ip;

  my $result;
  my $res = new Net::DNS::Resolver;
  $res->tcp_timeout(30);
  $res->udp_timeout(30);

  $self->log(LOGDEBUG, "waiting for dnsbl dns");

  # don't wait more than 8 seconds here
  my @ready = $sel->can_read(8);

  $self->log(LOGDEBUG, "DONE waiting for dnsbl dns, got ", scalar @ready, " answers ...");
  return '' unless @ready;

  for my $socket (@ready) {
    my $query = $res->bgread($socket);
    $sel->remove($socket);
    undef $socket;

    my $dnsbl;

    if ($query) {
      my $a_record = 0;
      foreach my $rr ($query->answer) {
        my $name = $rr->name;
        $self->log(LOGDEBUG, "name $name");
        next unless $dom->{$name};
        $self->log(LOGDEBUG, "name $name was queried");
        $a_record = 1 if $rr->type eq "A";
        ($dnsbl) = ($name =~ m/(?:\d+\.){4}(.*)/) unless $dnsbl;
        $dnsbl = $name unless $dnsbl;
        next unless $rr->type eq "TXT";
        $self->log(LOGDEBUG, "got txt record");
        $result = $rr->txtdata and last;
      }
      #$a_record and $result = "Blocked by $dnsbl";

      if ($a_record) {
        if (defined $dnsbl_zones{$dnsbl}) {
          $result = $dnsbl_zones{$dnsbl};
          #$result =~ s/%IP%/$ENV{'TCPREMOTEIP'}/g;
          $result =~ s/%IP%/$remote_ip/g;
        }
        else {
          # shouldn't get here?
          $result = "Blocked by $dnsbl";
        }
      }
    }
    else {
      $self->log(LOGERROR, "$dnsbl query failed: ", $res->errorstring)
        unless $res->errorstring eq "NXDOMAIN";
    }

    if ($result) {
      #kill any other pending I/O
      $conn->notes('dnsbl_sockets', undef);
      $result = join("\n", $self->qp->config('dnsbl_rejectmsg'), $result);
      return $conn->notes('dnsbl', $result);
    }
  }

  if ($sel->count) {
    # loop around if we have dns blacklists left to see results from
    return $self->process_sockets();
  }

  # er, the following code doesn't make much sense anymore...

  # if there was more to read; then forget it
  $conn->notes('dnsbl_sockets', undef);

  return $conn->notes('dnsbl', $result);
}

sub hook_rcpt {
  my ($self, $transaction, $rcpt, %param) = @_;

    return DECLINED if $self->is_immune();

  # RBLSMTPD being non-empty means it contains the failure message to return
  if (defined $ENV{'RBLSMTPD'} && $ENV{'RBLSMTPD'} ne '') {
    my $result = $ENV{'RBLSMTPD'};
    my $remote_ip = $self->qp->connection->remote_ip;
    $result =~ s/%IP%/$remote_ip/g;
    my $msg =  $self->qp->config('dnsbl_rejectmsg');
    $self->log(LOGINFO, "fail: $msg");
    return ( $self->get_reject_type(), join(' ', $msg, $result));
  }

    my $note = $self->process_sockets or return DECLINED;
    if ( $self->ip_whitelisted() ) {
        $self->log(LOGINFO, "skip: whitelisted");
        return DECLINED;
    };

    if ( $rcpt->user =~ /^(?:postmaster|abuse|mailer-daemon|root)$/i ) {
        $self->log(LOGWARN, "skip: don't blacklist special account: ".$rcpt->user);
        return DECLINED;
    }

    $self->log(LOGINFO, 'fail');
    return ( $self->get_reject_type(), $note);
}

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

  $self->connection->notes('dnsbl_sockets', undef);

  return DECLINED;
}

sub get_reject_type {
    my $self = shift;
    my $default = shift || DENY;
    my $deny = $self->{_args}{reject_type} or return $default;

    return   $self->{_args}{reject_type} eq 'temp'       ? DENYSOFT
           : $self->{_args}{reject_type} eq 'disconnect' ? DENY_DISCONNECT
           : $default;
};