gross - Greylisting of suspicious sources

INTRODUCTION

Gross is a greylisting server. The features that make gross
stand out from other greylisters are:

- it's blazingly fast
- it's amazingly resource efficient
- it can be configured to query DNSBL databases, and
enforce greylisting only for hosts that are found on
those databases
- it can block hosts that match multiple DNSBL's
- it can be replicated and run parallel on two servers

It supports Sun Java System Messaging Server and Postfix,
Exim and Sendmail.

AUTHORS

Gross is written by Eino Tuominen <eino@utu.fi> and
Antti Siira <antti@utu.fi>

MOTIVATION

We decided to write a greylister of our own because
traditional greylisting was not acceptable due to it's
impact on legitimate emails. Likewise, we have not used
DNSBLs to directly refuse mail because of the possible
false positives. One of the cornerstones of our spam
fighting policy is to minimize the possibility of false
positives.

Because greylisting is found to be a very effective way
to reduce spam, and most of spam is originated from
hosts on various DNSBLs, I suggested that greylisting just 
those servers found on DNSBLs would reduce the amount
of spam received. This software has been written to test
if that hypothesis was correct.

DESIGN AND OPERATION

Gross consists of grossd, the greylisting daemon, and
a client library for SJSMS. The server also implements 
Postfix's content filtering protocol.

Upon receiving a request from a client, grossd first 
validates it. The request includes a triplet (smtp-client-ip, 
sender-address, recipient-address). A hash is then 
calculated and matched against the Bloom filter. If a
match is found, server sends an OK message.

If the triplet has not been seen before, grossd then
queries configured DNSBL databases for the smtp-client-ip.
If a positive match is found, grossd sends client a 
GREY response. If DNSBL tests are negative, an OK response
is sent. Database is then updated; currently there are 
two updating styles: either to always update or to only
update if the result was GREY.

As mentioned above, the data store is implemented with
Bloom filters. A Bloom filter is a very efficient way
to store data. It's a probabilistic data structure,
which means that there is a possibility of error when querying
the database. False positives are possible, but false
negatives are not. This means that there is a possibility
that grossd will falsely give an OK response when a
connection should be greylisted. By sizing the bloom
filters, you can control the error possibility to meet your
needs. The right bloom filter size depends on the number
of entries in the database, that is, the retention time
versus the number of handled connections.[1]

DNS queries are done asynchronously using c-ares library
(http://daniel.haxx.se/projects/c-ares/). 

DOCUMENTATION

See INSTALL for installation instructions and for more
information about configuration.

See doc/examples/grossd.conf for an example configuration
along with additional information about each feature.

There is a mailing list linked from the project's home page.
Please, don't hesitate to contact the authors directly 
if you have any questions!

PERFORMANCE AND STABILITY

Gross is currently doing greylisting for the mail gateways
of the University of Turku and the University of Wisconsin.

Gross has proven to be very stable on Solaris (9 and 10) 
on Sun Sparc.  It has also been run on a production Red Hat
server with minor stability issues.

When we enabled Gross on our servers, we saw a tremendous 
drop on our internet originated mail flow. Also,
the load on our spam filtering servers was cut in half!

DEVELOPMENT AND LICENSING

Gross is published on a BSD-like license.

See http://code.google.com/p/gross/ for more information.

If you have any suggestions regarding the software or wish
to take part on the development, feel free to contact the authors.
We'll be happy to hear if you dare to try out the software.

Should it be grey or gray?-)

  Eino Tuominen


[1] http://en.wikipedia.org/wiki/Bloom_filter
