If the message is for a remote machine, the message’s address is given to router drivers to determine to which host the message should be forwarded. The router drivers are specified in the routers file (usually /usr/lib/smail/routers). The file contains the names of the router drivers, each of which (in the order presented in the routers file) is given the message destination address to see whether they have information about the specific route required to send the message.

You don’t need to specify any other routers than the default configuration in most cases. The default setup uses the following router steps, in order:

•  Resolution directly by dotted quad IP address using gethostbyaddr library call

•  Resolution by symbolic name using gethostbyname library calls

•  Resolution using the pathalias database (given in the file /usr/lib/smail/paths— see the following)

•  For UUCP addresses, resolution to see whether the destination is a UUCP neighbor

•  Routing to a smart host, if one exists, when other methods to resolve the name have failed

These default routings will work for most systems, although you should comment out the UUCP router if your system is not properly configured to handle UUCP (otherwise, you will get tons of error messages). If you do not plan to use UUCP for mail, you should also comment out this router line to simplify the entire smail system.

A couple of other common situations need to be dealt with. If you are connected to the Internet, there is a problem in that smail’s router doesn’t recognize the MX record format. In order to properly support Internet mail, comment out the default router and enable the BIND router instead. (If your version of Linux doesn’t support BIND, you can obtain and link a more recent version from FTP and BBS sites.)

If you are using both SLIP/PPP and UUCP connections, you may encounter problems with smail waiting too long for a connection. To simplify this type of installation, rearrange the order the routers are checked so that the paths file is checked before the resolver router. In many cases, since UUCP is more efficient and faster than SMTP over a SLIP/PPP line, you can disable the resolver-based router entirely.

When a router identifies the best route to the destination machine, it also gives the transport required to communicate with that machine’s mail router. The actual path to the destination may be modified at this point. For example, if the remote machine chatton@bigcat.com can best be reached through a UUCP link instead of SMTP, the destination address may be modified by the router to bigcat!chatton (UUCP-style addressing). Alternatively, a destination address may become more specific. For example, the address chatton@bigcat.com may be resolved to a specific machine such as chatton@whiskers.bigcat.com if that address will get the message delivered more efficiently.

The /usr/lib/smail/paths file is used by some UUCP routers to determine a path alias. The paths file is ASCII only and contains a sorted list of entries with two columns separated by a tab: the destination site name and its UUCP bang path. No comments are allowed in the file.

Summary

This chapter looks at the configuration of both sendmail and smail mail systems for UUCP- and TCP-based mail connections. Which system you choose to use on your Linux machine is an individual choice (and a highly debated one, too). Either system works well and should provide you with trouble-free mail service. From here, there are a number of chapters you may want to read for more information. To learn about:

Setting up your Linux system to access and display Usenet newsgroups, see Chapter 41, “Configuring Linux for News.”

Buttoning up your network and preventing access from the outside, read Chapter 42, “Network Security.”

Backups and how to perform them (so you don’t have to reinstall your mail server again!), read Chapter 45, “Backups.”

Scheduling tasks to run automatically without your intervention, read Chapter 46, “cron and at.”