Linux Network Administrator Guide, Second Edition

That would take longer, especially on a busy system where the log files can get quite big. The exigrep utility comes into its own when looking for information about more than one message. Its first argument is a regular expression, and it picks out all the log lines concerned with any messages that have at least one log line that matches the expression. Thus it can be used to pick out all messages for one specific address, or all those to or from a specific host.

You can keep a general watch on what a running Exim is doing by running tail on its main log file. Another way of doing this is to run the eximon utility that comes with Exim. This is an X11 application that puts up a scrolling display of the main log, and also shows a list of messages that are awaiting delivery, as well as some stripcharts about delivery activity.

Miscellaneous config Options

Here are a few of the more useful options you can set in the configuration file:

message_size_limit

Setting this option limits the size of message that Exim will accept.

return_size_limit

Setting this option limits the amount of an incoming message that Exim will return as part of a bounce message.

deliver_load_max

If the system load exceeds the value given for this option, all mail delivery is suspended, though messages are still accepted.

smtp_accept_max

This is the maximum number of simultaneous incoming SMTP calls Exim is prepared to accept.

log_level

This option controls the amount of material that is written to the log. There are also some options with names beginning with log_ that control the logging of specific information.

Message Routing and Delivery

Exim splits up mail delivery into three different tasks: routing, directing, and transporting. There are a number of code modules of each type, and each is separately configurable. Usually a number of different routers, directors, and transports are set up in the configuration file.

Routers resolve remote addresses, determining which host the message should be sent to and which transport should be used. In Internet-connected hosts there is often just one router, which does the resolution by looking up the domain in the DNS. Alternatively, there may be one router that handles addresses destined for hosts on a local LAN, and a second to send any other addresses to a single smart host; for example, an ISP's mail server.

Local addresses are given to the directors, of which there are normally several, to handle aliasing and forwarding as well as identifying local mailboxes. Mailing lists can be handled by aliasing or forwarding directors. If an address gets aliased or forwarded, any generated addresses are handled independently by the routers or directors, as necessary. By far the most common case will be delivery to a mailbox, but messages may also be piped into a command or appended to a file other than the default mailbox.

A transport is responsible for implementing a method of delivery; for example, sending the message over an SMTP connection or adding it to a specific mailbox. Routers and directors select which transport to use for each recipient address. If a transport fails, Exim either generates a bounce message or defers the address for a later retry.

With Exim, you have a lot of freedom in configuring these tasks. For each of them, a number of drivers are available, from which you can choose those you need. You describe them to Exim in different sections of its configuration file. The transports are defined first, followed by the directors, and then the routers. There are no built-in defaults, though Exim is distributed with a default configuration file that covers simple cases. If you want to change Exim's routing policy or modify a transport, it is easiest to start from the default configuration and make changes rather than attempt to set up a complete configuration from scratch.

Routing Messages

When given an address to deliver, Exim first checks whether the domain is one that is handled on the local host by matching it against a list in the local_domains configuration variable. If this option is not set, the local host name is used as the only local domain. If the domain is local, the address is handed to the directors. Otherwise, it is handed to the routers to find out which host to forward a message to.[118]

Delivering Messages to Local Addresses

Most commonly, a local address is just a user's login name, in which case the message is delivered to the user's mailbox, /var/spool/mail/user-name. Other cases include aliases, mailing list names, and mail forwarding by the user. In these cases, the local address expands to a new list of addresses, which may be either local or remote.

Apart from these 'normal' addresses, Exim can handle other types of local message destinations, like filenames and pipe commands. When delivering to a file, Exim appends the message, creating the file if necessary. File and pipe destinations are not addresses in their own right, so you can't send mail to, say, /etc/[email protected] and expect to overwrite the password file; deliveries to a specific file are valid only if they come from forwarding or alias files. Note, however, that /etc/[email protected] is a syntactically valid email address, but if Exim received it, it would (typically) search for a user whose login name was /etc/passwd, fail to find one, and bounce the message.

In an alias list or forwarding file, a filename is anything that begins with a slash (/) that does not parse as a fully qualified email address. For example, /tmp/junk in a forwarding or alias file is interpreted as a file name, but /tmp/[email protected] is an email address, though it is not likely to be a very useful one. However, valid addresses of this type are seen when sending mail through X.400 gateways, because X.400 addresses start with a slash.

Similarly, a pipe command may be any Unix command preceded by the pipe symbol (|), unless the string parses as a valid email address complete with domain. Unless you have changed the configuration, Exim does not use a shell to run the command; instead, it splits it up into a command name, arguments itself, and runs it directly. The message is fed to the command on its standard input.

For example, to gate a mailing list into a local newsgroup, you might use a shell script named gateit, and set up a local alias that delivers all messages from this mailing list to the script using |gateit. If the command line contains a comma, it and the preceding pipe symbol must be enclosed in double quotes.