added local_ip option to p0f plugin

(updated patch against rspier/qpsmtpd)

The p0f plugin defaulted to binding to TCPLOCALIP, which doesn't work
when the mail server is running behind a firewall with a private IP. If
the local_ip option is set in the config file, it overrides TCPLOCALIP.

Added POD documentation for local_ip option and p0f general usage

Signed-off-by: Robert <rspier@pobox.com>

plugins/ident/p0f

@@ -1,20 +1,68 @@
 # -*- perl -*-
 
-=pod
+=head1 NAME
 
-An Identification Plugin
+p0f - A TCP Fingerprinting Identification Plugin
 
- ./p0f -u qpsmtpd -d -q -Q /tmp/.p0f_socket 'dst port 25' -o /dev/null && \
+=head1 SYNOPSIS
+
+Use TCP fingerprint info (remote computer OS, network distance, etc) to
+implement more sophisticated anti-spam policies.
+
+=head1 DESCRIPTION
+
+This p0f module inserts a 'p0f' note that other qpsmtpd plugins can inspect.
+It includes the following information about the TCP fingerprint (link,
+detail, distance, uptime, genre). Here's an example connection note:
+
+ genre    => FreeBSD
+ detail   => 6.x (1)
+ uptime   => 1390
+ link     => ethernet/modem
+ distance => 17
+
+Which was parsed from this p0f fingerprint:
+
+  24.18.227.2:39435 - FreeBSD 6.x (1) (up: 1390 hrs)
+    -> 208.75.177.101:25 (distance 17, link: ethernet/modem)
+
+=head1 MOTIVATION
+
+This p0f plugin provides a way to make sophisticated policies for email
+messages. For example, the vast majority of email connections to my server
+from Windows computers are spam (>99%). But, I have a few clients that use
+Exchange servers so I can't just block email from all Windows computers.
+
+Same goes for greylisting. Finance companies (AmEx, BoA, etc) just love to
+send notices that they won't queue and retry. Either they deliver at that
+instant or never. When I enable greylisting, I lose valid messages. Grrr.
+
+So, while I'm not willing to use greylisting, and I'm not willing to block
+connections from Windows computers, I am quite willing to greylist all email
+from Windows computers.
+
+=head1 CONFIGURATION
+
+Create a startup script for PF that creates a communication socket when your
+server starts up.
+
+ p0f -u qpsmtpd -d -q -Q /tmp/.p0f_socket 'dst port 25' -o /dev/null
  chown qpsmtpd /tmp/.p0f_socket
 
-and add 
+add an entry to config/plugins to enable p0f:
 
  ident/p0f /tmp/.p0f_socket 
 
-to config/plugins
+=head2 local_ip
 
-it puts things into the 'p0f' connection notes so other plugins can do
+Use the local_ip option to override the IP address of your mail server. This
-things based on source OS.
+is useful if your mail server has a private IP because it is running behind
+a firewall. For example, my mail server has the IP 127.0.0.6, but the world
+knows my mail server as 208.75.177.101.
+
+Example config/plugins entry with local_ip override:
+
+  ident/p0f /tmp/.p0f_socket local_ip 208.75.177.101
 
 All code heavily based upon the p0fq.pl included with the p0f distribution.
 
@@ -26,6 +74,15 @@ has been updated to provide that information when running under djb's
 tcpserver. The async, forkserver, and prefork models will likely require
 some additional changes to make sure these fields are populated.
 
+=head1 ACKNOWLEDGEMENTS
+
+Heavily based upon the p0fq.pl included with the p0f distribution.
+
+=head1 AUTHORS
+
+ Matt Simerson <msimerson@cpan.org> - 5/1/2010
+ previous unnamed author
+
 =cut
 
 use IO::Socket;
@@ -34,22 +91,24 @@ use Net::IP;
 my $QUERY_MAGIC = 0x0defaced;
 
 sub register {
-  my ($self, $qp, $p0f_socket) = @_;
+    my ($self, $qp, $p0f_socket, %args) = @_;
 
     $p0f_socket =~ /(.*)/; # untaint
     $self->{_args}->{p0f_socket} = $1;
+    foreach (keys %args) {
+        $self->{_args}->{$_} = $args{$_};
+    }
 }
 
 sub hook_connect {
   my($self, $qp) = @_;
 
   my $p0f_socket = $self->{_args}->{p0f_socket};
-  my $srcport    = 
+  my $local_ip = $self->{_args}{local_ip} || $self->qp->connection->local_ip;
-  my $destport   = $self->qp->connection->local_port;
 
   my $src = new Net::IP ($self->qp->connection->remote_ip) 
     or $self->log(LOGERROR, "p0f: ".Net::IP::Error()), return (DECLINED);
-  my $dst = new Net::IP ($self->qp->connection->local_ip) 
+  my $dst = new Net::IP($local_ip)
     or $self->log(LOGERROR, "p0f: ".NET::IP::Error()), return (DECLINED);
   my $query = pack("L L L N N S S",
                    $QUERY_MAGIC,