fbdee49965
added max_size on config, so it's likely to get noticed, since even 1M is probably too low for most sites. This should likely default to the same as databytes?
523 lines
15 KiB
Perl
523 lines
15 KiB
Perl
#!perl -w
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=head1 NAME
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helo - validate the HELO message presented by a connecting host.
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=head1 DESCRIPTION
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Validate the HELO hostname. This plugin includes a suite of optional tests,
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selectable by the I<policy> setting. The policy section details which tests
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are enforced by each policy option.
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It sets the connection notes helo_forward_match and helo_reverse_match when
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I<policy rfc> or I<policy strict> are used.
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Adds an X-HELO header with the HELO hostname to the message.
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Using I<policy rfc> will reject a very large portion of the spam from hosts
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that have yet to get blacklisted.
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=head1 WHY IT WORKS
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The reverse DNS of the zombie PCs is out of the spam operators control. Their
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only way to get past these tests is to limit themselves to hosts with matching
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forward and reverse DNS, and then use the proper HELO hostname when spamming.
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At present, this presents a very high hurdle.
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=head1 HELO VALIDATION TESTS
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=over 4
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=item is_in_badhelo
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Matches in the I<badhelo> config file, including yahoo.com and aol.com, which
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neither the real Yahoo or the real AOL use, but which spammers use a lot.
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Like qmail with the qregex patch, the B<badhelo> file can also contain perl
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regular expressions. In addition to normal regexp processing, a pattern can
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start with a ! character, and get a negated (!~) match.
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=item invalid_localhost
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Assure that if a sender uses the 'localhost' hostname, they are coming from
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the localhost IP.
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=item is_plain_ip
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Disallow plain IP addresses. They are neither a FQDN nor an address literal.
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=item is_address_literal [N.N.N.N]
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An address literal (an IP enclosed in brackets) is legal but rarely, if ever,
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encountered from legit senders.
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=item is_forged_literal
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If a literal is presented, make sure it matches the senders IP.
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=item is_not_fqdn
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Makes sure the HELO hostname contains at least one dot and has only those
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characters specifically allowed in domain names (RFC 1035).
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=item no_forward_dns
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Make sure the HELO hostname resolves.
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=item no_reverse_dns
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Make sure the senders IP address resolves to a hostname.
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=item no_matching_dns
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Make sure the HELO hostname has an A or AAAA record that matches the senders
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IP address, and make sure that the senders IP has a PTR that resolves to the
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HELO hostname.
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Per RFC 5321 section 4.1.4, it is impermissible to block a message I<soley>
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on the basis of the HELO hostname not matching the senders IP.
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Since the dawn of SMTP, having matching DNS has been a minimum standard
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expected and oft required of mail servers. While requiring matching DNS is
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prudent, requiring an exact match will reject valid email. While testing this
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plugin with rejection disabled, I noticed that mx0.slc.paypal.com sends email
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from an IP that reverses to mx1.slc.paypal.com. While that's technically an
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error, I believe it's an error to reject mail based on it. Especially since
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SLD and TLD match.
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To avoid snagging false positives, matches are extended to the first
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3 octets of the IP and the last two labels of the FQDN. The following are
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considered a match:
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192.0.1.2, 192.0.1.3
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foo.example.com, bar.example.com
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This allows I<no_matching_dns> to be used without rejecting mail from orgs with
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pools of servers where the HELO name and IP don't exactly match. This list
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includes Yahoo, Gmail, PayPal, cheaptickets.com, exchange.microsoft.com, and
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likely many more.
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=back
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=head1 CONFIGURATION
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=head2 policy [ lenient | rfc | strict ]
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Default: lenient
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=head3 lenient
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Runs the following tests: is_in_badhelo, invalid_localhost,
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is_forged_literal, and is_plain_ip.
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This setting is lenient enough not to cause problems for your Windows users.
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It is comparable to running check_spamhelo, but with the addition of regexp
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support, the prevention of forged localhost, forged IP literals, and plain
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IPs.
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=head3 rfc
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Per RFC 2821, the HELO hostname is the FQDN of the sending server or an
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address literal. When I<policy rfc> is selected, all the lenient checks and
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the following are tested: is_not_fqdn, no_forward_dns, and no_reverse_dns.
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If you have Windows users that send mail via your server, do not choose
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I<policy rfc> without setting I<reject> to 0 or naughty.
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Windows PCs often send unqualified HELO names and will have trouble
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sending mail. The B<naughty> plugin defers the rejection, giving the user
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the opportunity to authenticate and bypass the rejection.
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=head3 strict
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Strict includes all the RFC tests and the following: no_matching_dns, and
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is_address_literal.
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I have yet to see an address literal being used by a hammy sender. But I am
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not certain that blocking them all is prudent.
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It is recommended that I<policy strict> be used with <reject 0> and that you
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examine your logs for false positives.
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=head2 badhelo
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Add domains, hostnames, or perl regexp patterns to the F<badhelo> config
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file; one per line.
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=head2 timeout [seconds]
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Default: 5
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The number of seconds before DNS queries timeout.
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=head2 reject [ 0 | 1 | naughty ]
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Default: 1
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0: do not reject
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1: reject
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naughty: naughty plugin handles rejection
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=head2 reject_type [ temp | perm | disconnect ]
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Default: disconnect
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What type of rejection should be sent? See docs/config.pod
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=head2 loglevel
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Adjust the quantity of logging for this plugin. See docs/logging.pod
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=head1 RFC 2821
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=head2 4.1.1.1
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The HELO hostname "...contains the fully-qualified domain name of the SMTP
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client if one is available. In situations in which the SMTP client system
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does not have a meaningful domain name (e.g., when its address is dynamically
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allocated and no reverse mapping record is available), the client SHOULD send
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an address literal (see section 4.1.3), optionally followed by information
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that will help to identify the client system."
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=head2 2.3.5
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The domain name, as described in this document and in [22], is the
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entire, fully-qualified name (often referred to as an "FQDN"). A domain name
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that is not in FQDN form is no more than a local alias. Local aliases MUST
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NOT appear in any SMTP transaction.
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=head1 RFC 5321
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=head2 4.1.4
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An SMTP server MAY verify that the domain name argument in the EHLO
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command actually corresponds to the IP address of the client.
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However, if the verification fails, the server MUST NOT refuse to
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accept a message on that basis. Information captured in the
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verification attempt is for logging and tracing purposes. Note that
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this prohibition applies to the matching of the parameter to its IP
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address only; see Section 7.9 for a more extensive discussion of
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rejecting incoming connections or mail messages.
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=head1 TODO
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is_forged_literal, if the forged IP is an internal IP, it's likely one
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of our clients that should have authenticated. Perhaps when we check back
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later in data_post, if they have added relay_client, then give back the
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karma.
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=head1 AUTHOR
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2012 - Matt Simerson
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=head1 ACKNOWLEDGEMENTS
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badhelo processing from check_badhelo plugin
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badhelo regex processing idea from qregex patch
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additional check ideas from Hakura helo plugin
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=cut
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use strict;
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use warnings;
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use Qpsmtpd::Constants;
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sub register {
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my ($self, $qp) = (shift, shift);
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$self->{_args} = {@_};
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$self->{_args}{reject_type} = 'disconnect';
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$self->{_args}{policy} ||= 'lenient';
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$self->{_args}{dns_timeout} ||= $self->{_args}{timeout} || 5;
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if (!defined $self->{_args}{reject}) {
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$self->{_args}{reject} = 1;
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}
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$self->populate_tests();
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$self->init_resolver() or return;
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$self->register_hook('helo', 'helo_handler');
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$self->register_hook('ehlo', 'helo_handler');
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$self->register_hook('data_post', 'data_post_handler');
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}
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sub helo_handler {
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my ($self, $transaction, $host) = @_;
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if (!$host) {
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$self->log(LOGINFO, "fail, tolerated, no helo host");
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return DECLINED;
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}
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return DECLINED if $self->is_immune();
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foreach my $test (@{$self->{_helo_tests}}) {
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my @err = $self->$test($host);
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if (scalar @err) {
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$self->adjust_karma(-1);
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return $self->get_reject(@err);
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}
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}
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$self->log(LOGINFO, "pass");
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return DECLINED;
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}
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sub data_post_handler {
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my ($self, $transaction) = @_;
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$transaction->header->delete('X-HELO');
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$transaction->header->add('X-HELO', $self->qp->connection->hello_host, 0);
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return (DECLINED);
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}
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sub populate_tests {
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my $self = shift;
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my $policy = $self->{_args}{policy};
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@{$self->{_helo_tests}} =
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qw/ is_in_badhelo invalid_localhost is_forged_literal is_plain_ip /;
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if ($policy eq 'rfc' || $policy eq 'strict') {
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push @{$self->{_helo_tests}},
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qw/ is_not_fqdn no_forward_dns no_reverse_dns /;
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}
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if ($policy eq 'strict') {
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push @{$self->{_helo_tests}}, qw/ is_address_literal no_matching_dns /;
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}
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}
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sub is_in_badhelo {
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my ($self, $host) = @_;
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my $error = "I do not believe you are $host.";
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$host = lc $host;
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foreach my $bad ($self->qp->config('badhelo')) {
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if ($bad =~ /[\{\}\[\]\(\)\^\$\|\*\+\?\\\!]/) { # it's a regexp
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return $self->is_regex_match($host, $bad);
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}
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if ($host eq lc $bad) {
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return ($error, "in badhelo");
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}
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}
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return;
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}
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sub is_regex_match {
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my ($self, $host, $pattern) = @_;
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my $error = "Your HELO hostname is not allowed";
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#$self->log( LOGDEBUG, "is regex ($pattern)");
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if (substr($pattern, 0, 1) eq '!') {
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$pattern = substr $pattern, 1;
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if ($host !~ /$pattern/) {
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#$self->log( LOGDEBUG, "matched ($pattern)");
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return ($error, "badhelo pattern match ($pattern)");
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}
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return;
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}
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if ($host =~ /$pattern/) {
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#$self->log( LOGDEBUG, "matched ($pattern)");
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return ($error, "badhelo pattern match ($pattern)");
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}
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return;
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}
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sub invalid_localhost {
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my ($self, $host) = @_;
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return if lc $host ne 'localhost';
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if ($self->qp->connection->remote_ip ne '127.0.0.1') {
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#$self->log( LOGINFO, "fail, not localhost" );
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return ("You are not localhost", "invalid localhost");
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}
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$self->log(LOGDEBUG, "pass, is localhost");
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return;
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}
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sub is_plain_ip {
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my ($self, $host) = @_;
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return if $host =~ /[^\d\.]+/; # has chars other than digits and a dot
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return if $host !~ m/^(\d{1,3}\.){3}\d{1,3}$/;
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$self->log(LOGDEBUG, "fail, plain IP");
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return ("Plain IP is invalid HELO hostname (RFC 2821)", "plain IP");
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}
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sub is_address_literal {
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my ($self, $host) = @_;
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return if $host !~ m/^\[(\d{1,3}\.){3}\d{1,3}\]$/;
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$self->log(LOGDEBUG, "fail, bracketed IP");
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return ("RFC 2821 allows an address literal, but we do not",
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"bracketed IP");
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}
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sub is_forged_literal {
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my ($self, $host) = @_;
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return if $host !~ m/^\[(\d{1,3}\.){3}\d{1,3}\]$/;
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# should we add exceptions for reserved internal IP space? (192.168,10., etc?)
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$host = substr $host, 1, -1;
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return if $host eq $self->qp->connection->remote_ip;
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return ("Forged IPs not accepted here", "forged IP literal");
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}
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sub is_not_fqdn {
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my ($self, $host) = @_;
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return if $host =~ m/^\[(\d{1,3}\.){3}\d{1,3}\]$/; # address literal, skip
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if ($host !~ /\./) { # has no dots
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return ("HELO name is not fully qualified. Read RFC 2821", "not FQDN");
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}
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if ($host =~ /[^a-zA-Z0-9\-\.]/) {
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return ("HELO name contains invalid FQDN characters. Read RFC 1035",
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"invalid FQDN chars");
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}
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return;
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}
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sub no_forward_dns {
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my ($self, $host) = @_;
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return if $self->is_address_literal($host);
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my $res = $self->init_resolver();
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$host = "$host." if $host !~ /\.$/; # fully qualify name
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my $query = $res->search($host);
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if (!$query) {
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if ($res->errorstring eq 'NXDOMAIN') {
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return ("HELO hostname does not exist", "no such host");
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}
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$self->log(LOGERROR, "skip, query failed (", $res->errorstring, ")");
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return;
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}
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my $hits = 0;
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foreach my $rr ($query->answer) {
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next unless $rr->type =~ /^(?:A|AAAA)$/;
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$self->check_ip_match($rr->address);
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$hits++;
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last if $self->connection->notes('helo_forward_match');
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}
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if ($hits) {
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$self->log(LOGDEBUG, "pass, forward DNS") if $hits;
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return;
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}
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return ("HELO hostname did not resolve", "no forward DNS");
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}
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sub no_reverse_dns {
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my ($self, $host, $ip) = @_;
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my $res = $self->init_resolver();
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$ip ||= $self->qp->connection->remote_ip;
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my $query = $res->query($ip) or do {
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if ($res->errorstring eq 'NXDOMAIN') {
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return ("no rDNS for $ip", "no rDNS");
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}
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$self->log(LOGINFO, $res->errorstring);
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return ("error getting reverse DNS for $ip",
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"rDNS " . $res->errorstring);
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};
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my $hits = 0;
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for my $rr ($query->answer) {
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next if $rr->type ne 'PTR';
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$self->log(LOGDEBUG, "PTR: " . $rr->ptrdname);
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$self->check_name_match(lc $rr->ptrdname, lc $host);
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$hits++;
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}
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if ($hits) {
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$self->log(LOGDEBUG, "has rDNS");
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return;
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}
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return ("no reverse DNS for $ip", "no rDNS");
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}
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sub no_matching_dns {
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my ($self, $host) = @_;
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# this is called iprev, or "Forward-confirmed reverse DNS" and is discussed
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# in RFC 5451. FCrDNS is done for the remote IP in the fcrdns plugin. Here
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# we do it on the HELO hostname.
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# consider adding status to Authentication-Results header
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if ( $self->connection->notes('helo_forward_match')
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&& $self->connection->notes('helo_reverse_match'))
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{
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$self->log(LOGDEBUG, "foward and reverse match");
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$self->adjust_karma(1); # a perfect match
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return;
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}
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if ($self->connection->notes('helo_forward_match')) {
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$self->log(LOGDEBUG, "name matches IP");
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return;
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}
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if ($self->connection->notes('helo_reverse_match')) {
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$self->log(LOGDEBUG, "reverse matches name");
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return;
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}
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$self->log(LOGINFO, "fail, no forward or reverse DNS match");
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return ("That HELO hostname fails FCrDNS", "no matching DNS");
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}
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sub check_ip_match {
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my $self = shift;
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my $ip = shift or return;
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if ($ip eq $self->qp->connection->remote_ip) {
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$self->log(LOGDEBUG, "forward ip match");
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$self->connection->notes('helo_forward_match', 1);
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return;
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}
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my $dns_net = join('.', (split(/\./, $ip))[0, 1, 2]);
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my $rem_net =
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join('.', (split(/\./, $self->qp->connection->remote_ip))[0, 1, 2]);
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if ($dns_net eq $rem_net) {
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$self->log(LOGNOTICE, "forward network match");
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$self->connection->notes('helo_forward_match', 1);
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}
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}
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sub check_name_match {
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my $self = shift;
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my ($dns_name, $helo_name) = @_;
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return if !$dns_name;
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return if split(/\./, $dns_name) < 2; # not a FQDN
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if ($dns_name eq $helo_name) {
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$self->log(LOGDEBUG, "reverse name match");
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$self->connection->notes('helo_reverse_match', 1);
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return;
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}
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my $dns_dom = join('.', (split(/\./, $dns_name))[-2, -1]);
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my $helo_dom = join('.', (split(/\./, $helo_name))[-2, -1]);
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if ($dns_dom eq $helo_dom) {
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$self->log(LOGNOTICE, "reverse domain match");
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$self->connection->notes('helo_reverse_match', 1);
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}
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}
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