qpsmtpd/plugins/bogus_bounce

#!perl -w

=head1 NAME

bogus_bounce - Check that a bounce message isn't bogus

=head1 DESCRIPTION

This plugin is designed to reject bogus bounce messages.

In our case a bogus bounce message is defined as a bounce message
which has more than a single recipient.

=head1 CONFIGURATION

Only a single argument is recognized and is assumed to be the default
action.  Valid settings are:

=over 8

=item log

Merely log the receipt of the bogus bounce (the default behaviour).

=item deny

Deny with a hard error code.

=item denysoft

Deny with a soft error code.

=back

=head1 AUTHOR

2010 - Steve Kemp - http://steve.org.uk/Software/qpsmtpd/

2013 - Matt  Simerson - added Return Path check

=cut

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

    if (@_ % 2) {
        $self->{_args}{action} = shift;
    }
    else {
        $self->{_args} = {@_};
    }

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

    # we only need to check for deferral, default is DENY
    if ($self->{_args}{action} && $self->{_args}{action} =~ /soft/i) {
        $self->{_args}{reject_type} = 'temp';
    }
}

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

    #
    # Find the sender, quit processing if this isn't a bounce.
    #
    my $sender = $transaction->sender->address || undef;
    if ($sender && $sender ne '<>') {
        $self->log(LOGINFO, "pass, not a null sender");
        return DECLINED;
    }

    #  at this point we know it is a bounce, via the null-envelope.
    #
    #  Count the recipients. Valid bounces have a single recipient
    #
    my @to = $transaction->recipients || ();
    if (scalar @to != 1) {
        $self->log(LOGINFO, "fail, bogus bounce to: " . join(',', @to));
        return $self->get_reject(
                         "fail, this bounce message does not have 1 recipient");
    }

    # validate that Return-Path is empty, RFC 3834

    my $rp = $transaction->header->get('Return-Path');
    if ($rp && $rp ne '<>') {
        $self->log(LOGINFO,
                   "fail, bounce messages must not have a Return-Path");
        return $self->get_reject(
                               "a bounce return path must be empty (RFC 3834)");
    }

    $self->log(LOGINFO, "pass, single recipient, empty Return-Path");
    return DECLINED;
}