qpsmtpd/plugins/fcrdns

#!perl -w

=head1 NAME

Forward Confirmed RDNS - http://en.wikipedia.org/wiki/FCrDNS

=head1 DESCRIPTION

Determine if the SMTP sender has matching forward and reverse DNS. 

Sets the connection note fcrdns.

=head1 WHY IT WORKS

The reverse DNS of zombie PCs is out of the spam operators control. Their
only way to pass this test is to limit themselves to hosts with matching
forward and reverse DNS. At present, this presents a significant hurdle.

=head1 VALIDATION TESTS

=over 4

=item has_reverse_dns

Determine if the senders IP address resolves to a hostname.

=item has_forward_dns

If the remote IP has a PTR hostname(s), see if that host has an A or AAAA. If
so, see if any of the host IPs (A or AAAA records) match the remote IP.

Since the dawn of SMTP, having matching DNS has been a standard expected and
oft required of mail servers. While requiring matching DNS is prudent,
requiring an exact match will reject valid email. This often hinders the
use of FcRDNS. While testing this plugin, I noticed that mx0.slc.paypal.com
sends mail from an IP that reverses to mx1.slc.paypal.com. While that's
technically an error, so too would rejecting that connection.

To avoid false positives, matches are extended to the first 3 octets of the
IP and the last two labels of the FQDN. The following are considered a match:

  192.0.1.2, 192.0.1.3

  foo.example.com, bar.example.com

This allows FcRDNS to be used without rejecting mail from orgs with
pools of servers where the HELO name and IP don't exactly match. This list
includes Yahoo, Gmail, PayPal, cheaptickets.com, exchange.microsoft.com, etc.

=back

=head1 CONFIGURATION

=head2 timeout [seconds]

Default: 5

The number of seconds before DNS queries timeout.

=head2 reject [ 0 | 1 | naughty ]

Default: 1

0: do not reject

1: reject

naughty: naughty plugin handles rejection

=head2 reject_type [ temp | perm | disconnect ]

Default: disconnect

What type of rejection should be sent? See docs/config.pod

=head2 loglevel

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


=head1 RFC 1912, RFC 5451

From Wikipedia summary:

1. First a reverse DNS lookup (PTR query) is performed on the IP address, which returns a list of zero or more PTR records. (has_reverse_dns)

2. For each domain name returned in the PTR query results, a regular 'forward' DNS lookup (type A or AAAA query) is then performed on that domain name. (has_forward_dns)

3. Any A or AAAA record returned by the second query is then compared against the original IP address (check_ip_match), and if there is a match, then the FCrDNS check passes.


=head1 AUTHOR

2013 - Matt Simerson

=cut

use strict;
use warnings;

use Qpsmtpd::Constants;

sub register {
    my ($self, $qp) = (shift, shift);
    $self->{_args} = { @_ };
    $self->{_args}{reject_type} = 'temp';
    $self->{_args}{timeout} ||= 5;
    $self->{_args}{ptr_hosts} = {};

    if ( ! defined $self->{_args}{reject} ) {
        $self->{_args}{reject} = 0;
    };

    $self->init_resolver() or return;

    $self->register_hook('connect',   'connect_handler');
    $self->register_hook('data_post', 'data_post_handler');
};

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

    return DECLINED if $self->is_immune();

    # run a couple cheap tests before the more expensive DNS tests
    foreach my $test ( qw/ invalid_localhost is_not_fqdn / ) {
        $self->$test() or return DECLINED;
    };

    $self->has_reverse_dns() or return DECLINED;
    $self->has_forward_dns() or return DECLINED;

    $self->log(LOGINFO, "pass");
    return DECLINED;
}

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

    my $match = $self->connection->notes('fcrdns_match') || 0;
    $transaction->header->add('X-Fcrdns', $match ? 'Yes' : 'No', 0 );
    return (DECLINED);
};

sub invalid_localhost {
    my ( $self ) = @_;
    return 1 if lc $self->qp->connection->remote_host ne 'localhost';
    if (   $self->qp->connection->remote_ip ne '127.0.0.1'
        && $self->qp->connection->remote_ip ne '::1' ) {
        $self->adjust_karma( -1 );
        $self->log( LOGINFO, "fail, not localhost" );
        return;
    };
    $self->adjust_karma( 1 );
    $self->log( LOGDEBUG, "pass, is localhost" );
    return 1;
};

sub is_not_fqdn {
    my ($self) = @_;
    my $host = $self->qp->connection->remote_host or return 1;
    return 1 if $host eq 'Unknown'; # QP assigns this to a "no DNS result"

    # Since QP looked it up, perform some quick validation
    if ( $host !~ /\./ ) { # has no dots
        $self->adjust_karma( -1 );
        $self->log(LOGINFO, "fail, not FQDN");
        return;
    };
    if ( $host =~ /[^a-zA-Z0-9\-\.]/ ) {
        $self->adjust_karma( -1 );
        $self->log(LOGINFO, "fail, invalid FQDN chars");
        return;
    };
    return 1;
};

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

    my $res = $self->init_resolver();
    my $ip  = $self->qp->connection->remote_ip;

    my $query = $res->query( $ip ) or do {
        if ( $res->errorstring eq 'NXDOMAIN' ) {
            $self->adjust_karma( -1 );
            $self->log( LOGINFO, "fail, no rDNS: ".$res->errorstring );
            return;
        };
        $self->log( LOGINFO, "fail, error getting rDNS: ".$res->errorstring );
        return;
    };

    my $hits = 0;
    $self->{_args}{ptr_hosts} = {};  # reset hash
    for my $rr ($query->answer) {
        next if $rr->type ne 'PTR';
        $hits++;
        $self->{_args}{ptr_hosts}{ $rr->ptrdname } = 1;
        $self->log(LOGDEBUG, "PTR: " . $rr->ptrdname );
    };
    if ( ! $hits ) {
        $self->adjust_karma( -1 );
        $self->log( LOGINFO, "fail, no PTR records");
        return;
    };

    $self->log(LOGDEBUG, "has rDNS");
    return 1;
};

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

    my $res = $self->init_resolver();

    foreach my $host ( keys %{ $self->{_args}{ptr_hosts} } ) {

        $host .= '.' if '.' ne substr( $host, -1, 1); # fully qualify name
        my $query = $res->search($host) or do {
            if ( $res->errorstring eq 'NXDOMAIN' ) {
                $self->log(LOGDEBUG, "host $host does not exist" );
                next;
            }
            $self->log(LOGDEBUG, "query for $host failed (", $res->errorstring, ")" );
            next;
        };

        my $hits = 0;
        foreach my $rr ($query->answer) {
            next unless $rr->type =~ /^(?:A|AAAA)$/;
            $hits++;
            $self->check_ip_match( $rr->address ) and return 1;
        }
        if ( $hits ) {
            $self->log(LOGDEBUG, "PTR host has forward DNS") if $hits;
            return 1;
        };
    };
    $self->adjust_karma( -1 );
    $self->log(LOGINFO, "fail, no PTR hosts have forward DNS");
    return;
};

sub check_ip_match {
    my $self = shift;
    my $ip = shift or return;

    if ( $ip eq $self->qp->connection->remote_ip ) {
        $self->log( LOGDEBUG, "forward ip match" );
        $self->connection->notes('fcrdns_match', 1);
        $self->adjust_karma( 1 );
        return 1;
    };

# TODO: make this IPv6 compatible
    my $dns_net = join('.', (split(/\./, $ip))[0,1,2] );
    my $rem_net = join('.', (split(/\./, $self->qp->connection->remote_ip))[0,1,2] );

    if ( $dns_net eq $rem_net ) {
        $self->log( LOGNOTICE, "forward network match" );
        $self->connection->notes('fcrdns_match', 1);
        return 1;
    };
    return;
};