Ubuntu Manpage: greylistd - simple greylisting system for mail transport agents

NAME

       greylistd - simple greylisting system for mail transport agents

SYNOPSIS

       greylistd

DESCRIPTION

Greylisting
This daemon provides a simple greylisting implementation for use with Exim and other mail
transport agents (MTAs). For a more elaborate introduction to greylisting, please refer
to Evan Harris' whitepaper at: http://projects.puremagic.com/greylisting/

Greylisting is a simple but highly effective means to weed out messages that are being
delivered via spamware/ratware tools. The idea is to establish whether a prior
relationship exists between the sender and the receiver of a message. Most of the time it
does, and the delivery proceeds normally.

On the other hand, if no prior relationship exists, the delivery is temporarily rejected,
using a 451 SMTP response. Legitimate MTAs will treat this response accordingly, and
retry the delivery in a while. In contrast, ratware will usually fail to retry the
delivery in a normal fashion.

As a result, greylisting is currently more than 90% effective in blocking incoming junk
mail, while nearly all legitimate mail goes through.

Three pieces of information (herafter called a triplet) from the delivery attempt are
cached for future reference:

- The address of the host attempting the delivery
- The envelope sender address (MAIL FROM:)
- The envelope recipient address (RCPT TO:)

If a delivery attempt was temporarily rejected, then after an initial timeout (60 minutes
by default), but before a retry expiration time (8 hours by default), new delivery
attempts with the same triplet are accepted, and the triplet is added to a whitelist.
This allows for delivery retries, presumably from legitimate MTAs, and ensures that future
mail from the same contact is not subject to greylisting.

If a whitelisted triplet has not been seen for an extended duration (by default 60 days),
it is expired. This prevents unlimited growth of the list.

The downside to greylisting is that legitimate mail from people who have never sent you
mail in the past (or, at least, within the last 60 days) are subject to a one-hour delay.

The upside is that the current generation of ratware tools will not be able to deliver
spam or virii to you. Even if, as a result of lots of sites incorporating the greylisting
concept, ratware tools are modified such that temporarily rejected deliveries are retried,
you stand an increased chance of blocking such mail. That is because within the mandatory
1-hour initial delay, chances are that the sending host's IP address has been listed in
one or more DNS block lists (such as bl.spamcop.net, cbl.abuseat.org, etc..), and can be
rejected by your MTA by consulting these lists directly, or via anti-spam software like
SpamAssassin.

greylistd
greylistd is meant to be installed on a server that accepts incoming mail. The MTA on
this server connects to the greylistd daemon over a UNIX domain socket (by default
/var/run/greylistd/socket), or alternatively via the command greylist(1), and submits a
string (triplet) that identifies a particular host/sender/recipient relationship.
greylistd responds "white", "grey" or "black", depending on the current listing status of
the provided triplet. Alternatively, if either of the "--white", "--grey", or "--black"
options precede the data, greylistd responds "true" or "false", indicating whether the
triplet is currently in the corresponding state.

EXAMPLES

   Exim 4
       A sample greylistd statement for Exim 4 is provided with this package, and can normally be
       found in "/usr/share/doc/greylistd/examples/exim4-acl-example.txt".

   Others
       What others?  :-)

       A prerequisite to greylisting in general  is  the  ability  to  perform  custom  filtering
       throughout  the  various  stages  in the SMTP transaction, most notably after the RCPT TO:
       SMTP command.  In particular, greylistd(8) can be invoked either over a UNIX domain socket
       or via the supplied greylist(1) utility.

       Although greylistd(8) is written mainly with Exim in mind, it should be possible to use it
       with any MTA that:

         -    Allows  arbitrary  strings  to  be   passed   on   via   a   UNIX   domain   socket
              (/var/run/greylistd/socket) or supplied to external programs (greylist(1)).

         -    Can defer the incoming delivery, based on the response.

       Some  MTAs  either  have  limited  or  no  support  for  such external filters in the SMTP
       transaction (e.g. Sendmail), or define a very custom  interface  for  such  filters  (e.g.
       Postifx "Policy Servers").

       That  said,  solutions  exist  for  these  other  MTAs  as  well.  For Postfix, check into
       "postgrey", and for Sendmail there is "relaydelay".  For other MTAs, check  the  links  on
       Evan Harris' greylisting project page:

           http://projects.puremagic.com/greylisting/links.html

FILES

/etc/greylistd/config
Configuration settings. Currently, this file consists of three sections:

[timeout]
Lists various timeouts used to determine how long to keep a new triplet greylisted,
and when to expire previosly known triplets.

[socket]
Specifies path and permissions of the UNIX domain socket on which greylistd will
listen.

[data]
Specifies the paths to the data files, containing the data items and statistics, as
well as an update interval specifying how often data will be written to these files.

/var/lib/greylistd/states
(default path, can be modified in the configuration file)

Runtime data. Theare are four sections: [white], [grey], [black] and [statistics]. The
first three sections consist of lines of the form:

hash = lastseen firstseen count

where:

- hash is a 32-bit value representing a given triplet,

- lastseen is a 32-bit value representing the timestamp of last delivery attempt for
this triplet,

- firstseen is a 32-bit value representing the timestamp of first known delivery attempt
for this triplet,

- count is a 32-bit value representing the number of delivery attempts that have been
made for this triplet in this time period.

The [statistics] section contains a counter for each of the three lists, indicating how
many items that has ever made its way into these lists by way of the update protocol.

/var/lib/greylistd/triplets
(default path, can be modified in the configuration file)

Unhashed data - i.e. the original triplets passed to greylistd. Internally, greylistd(8)
hashes the provided data into a single 32-bit value for efficiency. Prior to version 0.6,
the original data was not retained; as of version 0.6, data is optionally saved into this
file.

Data items are saved in the form:
hash = data ...

/var/run/greylistd/socket
(default path, can be modified in the configuration file)

The UNIX domain socket providing the main interface to "greylistd". The MTA can either
connect to this socket directly, or use the supplied "greylist" utility to do so.

BUGS

       Because triplets and timestamps are hashed into simple 32-bit values, there is a very slim
       chance that deliveries that should have been greylisted are allowed through.  More so  for
       very busy sites.

       Commands  are actually executed in the daemon, not the "greylist" client.  If the user who
       invokes "greylist" interactively has a different time zone than the daemon  process,  time
       and date representations in the output will reflect those of the daemon.

AUTHOR

       This  python  script  and  manual  page  is written by Tor Slettnes, originally for Debian
       GNU/Linux.

COPYRIGHT

       Copyright © 2004-2005 Tor Slettnes.

       This program is free software; you can redistribute it and/or modify it under the terms of
       the  GNU  General  Public  License  as  published  by the Free Software Foundation; either
       version 2 of the License, or (at your option) any later version.

       This program is distributed in the hope that it will be useful, but WITHOUT ANY  WARRANTY;
       without  even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
       See the GNU General Public License for more details.

       On a Debian GNU/Linux system, the full text of the GPL is available in  /usr/share/common-
       licenses/GPL.  It is also available at:

           http://www.gnu.org/licenses/gpl.html