Linux Unleashed, Third Edition:Configuring Linux for Mail
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Configuration Table Locations
There are several lines in the sendmail.m4 file that define configuration tables. For the most part, these configuration files are under the directory defined by LIBDIR. This section of the sendmail.m4 file has several lines that look like
and so on for about seven configuration file definitions in total. You can change any of these values if you want, but be sure to move the files themselves to the specified location. On the whole, it is best to leave the files in their default location.
The file decnetxtable is used to translate domain names into DECnet style names. This is a holdover from earlier versions of sendmail and will probably never be necessary for Linux users (unless your Linux machine is on a DECnet system).
The domaintable file is used to force sendmail to perform specific instructions after using DNS. The file, which is almost never used on Linux systems, allows you to provide expansion of short-form names. For example, suppose you often send mail to the host reallylongname.reallybignet.com but don’t want to type that entry each time. You can place an entry in the domaintable file that looks like
reallylongname.reallybignet.com big.com
so that whenever you send mail to bill@big.com it is expanded by sendmail to bill@reallylongname.reallybignet.com. The domaintable file can also be used to correct common typographic mistakes. For example, if you find many users sending mail to abcdef.com instead of abcdfe.com, you can add a line to the domaintable file that corrects the domain name. Such a line looks like this:
abcdfe.com abcdef.com
The format of the file is always correct domain followed by the incorrect (or shortened) domain name.
The genericfrom table is used to hide local usernames and machine addresses by converting local usernames to a generic ID that has no obvious connection to the username. It is seldom used by Linux systems because the general convention is to use real names on email and similar data. The companion file, xaliases, performs the generic to real conversion when mail comes back from the outside world.
The mailertable table is used to define any special handling for hosts or domains. Most often, mailertable is used to specify how certain domains or hosts are accessed and which protocol to use for them. This file doesn’t have to be modified if your system only uses UUCP, but if you use SMTP or DNS, you should verify its contents.
The mailertable file is read from the first line down, and sendmail processes mail based on each line in the file. For this reason, place the most specific rules at the top of the file, with more general rules after. Rules are specified in a format that gives the method of connection, then the remote system or domain. The syntax is
mailer delimiter relayname remote
where mailer is the transport to use, delimiter is a special character, relayname is the name of the system to pass the mail to, and remote is the remote host or domain name. The mailer can be one of the following values:
• TCP-A TCP with Internet-style addresses
• TCP-U TCP with UUCP-style addresses
• UUCP-A UUCP with Internet-style addresses
The delimiter has a special meaning and must be one of the following characters:
• ! strips the hostname from the address before forwarding
• , doesn’t modify the address at all
• : removes the hostname only if there are intermediate hosts specified
The mailertable rules can be built quite easily when you are forwarding mail to a remote mail server. For example, to force sendmail to use UUCP through a remote mail server called wizard to connect to the remote system roy.sailing.org, add a rule like this to the mailertable file:
UUCP-A,wizard roy.sailing.org
On a more general level, you can have a rule like this one
TCP-A,wizard chatton.com
which forwards any mail destined for the remote network chatton.com to the local mail server wizard via TCP.
The pathtable table is used to define explicit routing to remote hosts and networks. The format of each line in the pathtable file uses a syntax similar to a UUCP path alias, with entries appearing alphabetically in the file. The use of the pathtable file is rare because most Linux systems can handle the routing without explicit instructions.
The uucprelays file is used to short-circuit the UUCP path to a remote site when there is a better path. For example, if your users often use the path wizard!bignet!merlin!tpci and you create a direct link to tpci, you can use uucprelays to redirect the mail. This file is seldom used by Linux system users.
The uucpxtable file is used when a UUCP-style address must be used for mail delivery. The file provides the instructions for converting a DNS format address to a UUCP format address. If you are using a mail server other than your current machine or want to use UUCP to connect to specific machines because of reliability factors, this table is necessary.
This file contains a number of entries that has a format that gives the UUCP style name followed by the domain name, such as this one:
chatton chatton.com
This tells sendmail that any mail for chatton.com should be rerouted via UUCP to chatton (UUCP-style addressing). This forces mail addresses to let’s say, yvonne@chatton.com to be rewritten as chatton!yvonne, which can be handled by UUCP.