Linux Network Administrator Guide, Second Edition

director to use such data is as follows:

lists:
 driver = forwardfile
 file = /etc/exim/lists/${local_part}
 no_check_local_user
 errors_to = ${local_part}-request

When this director runs, the values of the file and errors_to options are expanded. Expansion causes certain portions of the strings beginning with dollar characters to be replaced every time the string is used. The simplest kind of expansion is the insertion of the value of one of Exim's variables, and this is what is happening here. The substring ${local_part} substitutes the value of the $local_part, which is the local part of the address that is being processed.

For each mailing list, a user (or alias or mailing list) named listname-request should exist; any errors occurring when resolving an address or delivering to a list member are reported to this address.

Protecting Against Mail Spam

Mail spam, or unsolicited email advertising, is an annoying problem for many users. A project has been formed to address this problem called the Mail Abuse Protection System (MAPS), and a mechanism has been built that reduces the problem, called the Real Time Blackhole List (RBL). Information on how the MAPS RBL works can be obtained from its online documentation at http://maps.vix.com/rbl/. The idea is simple. Sites that are caught generating mail spam are added into the database and mail transfer agents like Exim are able to query the database to confirm that a source is not a spammer before accepting mail from it.

Since the advent of the RBL, several other similar lists have been created. One of the most useful is the Dial-Up List (DUL), which lists the IP addresses of dial-up hosts. These should normally send outgoing mail only to their ISP's mail servers. Many sites block mail from external dial-ups because when such a host avoids its own ISP's server, it is usually up to no good.

Exim provides support for the real-time and other blacklists. It is very easily configured. To enable it, add the following lines to your /etc/exim.conf file:

# Vixie / MAPS RBL (http://maps.vix.com/rbl)
rbl_domains = rbl.maps.vix.com: dul.maps.vix.com

This example checks both the RBL and the DUL, rejecting any messages from hosts that are on either list. The rbl_hosts option allows you to specify groups of hosts to which RBL checking does (or does not) apply. The default setting is:

rbl_hosts = *

which means that all hosts are subject to RBL checking. If you wanted to override blacklisting and accept mail from a specific host without performing the RBL checking you could, for example, use:

rbl_hosts =! nocheck.example.com: *

The exclamation mark before the first item in this list indicates a negated item: if the calling host is nocheck.example.com, it will match this item. But because of the negation, RBL checking is not performed. Any other host matches the second item in the list.

UUCP Setup

Exim does not have any specific code for transporting mail via UUCP, nor does it support UUCP bang path addresses. However, if domain addressing is being used, Exim can be interfaced to UUCP fairly simply. Here is a configuration fragment for sending certain domains to UUCP, taken from a real installation:

# Transport
uucp:
 driver = pipe
 user = nobody
 command = '/usr/local/bin/uux -r -
  ${substr_-5:$host}!rmail ${local_part}'
 return_fail_output = true
# Router
uucphost:
 transport = uucp
 driver = domainlist
 route_file = /usr/exim/uucphosts search_type = lsearch

In a complete configuration file, the transport would be inserted among the other transports, and the router probably defined as the first router. The file /usr/exim/uucphosts contains entries like this:

darksite.example.com: darksite.UUCP

which is interpreted to mean, 'Send mail addressed to the domain darksite.example.com to the UUCP host darksite.' This configuration could be set up more simply without the router adding the suffix.UUCP to darksite only to have the transport take it off again, but this way is useful because it makes clear the distinction between the domain name darksite.example.com and the UUCP host name darksite.

Whenever the router comes across a domain that is in the route file, it will send the address to the UUCP transport, which subsequently pipes it to the uux command (described in Chapter 16, Managing Taylor UUCP). If there is a problem, uux will generate some output and terminate with a non-zero error code. The setting of return_fail_output makes sure that the output is returned to the sender.

If incoming UUCP messages are grouped into files in batched SMTP format, they can be passed directly to Exim using a command like this:

exim -bS ‹/var/uucp/incoming/001

However, there is one catch. When Exim receives a message locally, it insists that the sender is the logged-in user that calls it, but for a UUCP batch we want the senders to be taken from the incoming messages. Exim will do this if the process that calls it is running as a trusted user. If you arrange for incoming UUCP to be handled by a user called uucp, for example, you need to specify:

trusted_users = uucp

in the Exim configuration file to ensure that sender addresses are correctly handled.

Chapter 20. Netnews

Netnews, or Usenet news, remains one of the most important and highly valued services on computer networks today. Dismissed by some as a mire of unsolicited commercial email and pornography, Netnews still maintains several cases of the high-quality discussion groups that made it a critical resource in pre-web days. Even in these times of a billion web pages, Netnews is still a source for online help and community on many topics.