The m4 macro processor program generates the sendmail.df file when it processes the macro configuration file provided by the local system administrator. Throughout the remainder of this chapter we will refer to this configuration file as the sendmail.mc file.
The configuration process is basically a matter of creating a suitable sendmail.mc file that includes macros that describe your desired configuration. The macros are expressions that the m4 macro processor understands and expands into the complex sendmail.cf syntax. The macro expressions are made up of the macro name (the text in capital letters at the start), which can be likened to a function in a programming language, and some parameters (the text within brackets) that are used in the expansion. The parameters may be passed literally into the sendmail.cf output or may be used to govern the way the macro processing occurs.
A sendmail.mc file for a minimal configuration (UUCP or SMTP with all nonlocal mail being relayed to a directly connected smart host) can be as short as 10 or 15 lines, excluding comments.
If you're an administator of a number of different mail hosts, you might not want to name your configuration file sendmail.mc. Instead, it is common practice to name it after the host—vstout.m4 in our case. The name doesn't really matter as long as the output is called sendmail.cf. Providing a unique name for the configuration file for each host allows you to keep all configuration files in the same directory and is just an administrative convenience. Let's look at two example macro configuration files so we know where we are heading.
Most sendmail configurations today use SMTP only. It is very simple to configure sendmail for SMTP. Example 18-1 expects a DNS name server to be available to resolve hosts and will attempt to accept and deliver all mail for hosts using just SMTP.
Example 18-1. Sample Configuration File vstout.smtp.m4
divert(-1) # # Sample configuration file for vstout - smtp only # divert(0) VERSIONID(`@(#)sendmail.mc 8.7 (Linux) 3/5/96') OSTYPE(`linux') # # Include support for the local and smtp mail transport protocols. MAILER(`local') MAILER(`smtp') # FEATURE(rbl) FEATURE(access_db) # end |
A sendmail.mc file for vstout at the Virtual Brewery is shown in Example 18-2. vstout uses SMTP to talk to all hosts on the Brewery's LAN, and you'll see the commonality with the generic SMTP-only configuration just presented. In addition, the vstout configuration sends all mail for other destinations to moria, its Internet relay host, via UUCP.
Example 18-2. Sample Configuration File vstout.uucpsmtp.m4
divert(-1) # # Sample configuration file for vstout # divert(0) VERSIONID(`@(#)sendmail.mc 8.7 (Linux) 3/5/96') OSTYPE(`linux') dnl # moria is our smart host, using the "uucp-new" transport. define(`SMART_HOST', `uucp-new:moria') dnl # Support the local, smtp and uucp mail transport protocols. MAILER(`local') MAILER(`smtp') MAILER(`uucp') LOCAL_NET_CONFIG # This rule ensures that all local mail is delivered using the # smtp transport, everything else will go via the smart host. R$* < @ $* .$m. > $* $#smtp $@ $2.$m. $: $1 < @ $2.$m. > $3 dnl # FEATURE(rbl) FEATURE(access_db) # end |
If you compare and contrast the two configurations, you might be able to work out what each of the configuration parameters does. We'll explain them all in detail.
A few of the items in the sendmail.mc file are required all the time; others can be ignored if you can get away with defaults. The general sequence of the definitions in the sendmail.mc is as follows:
VERSIONID
OSTYPE
DOMAIN
FEATURE
Local macro definitions
MAILER
LOCAL_* rulesets
Lines in the sendmail.mc file that begin with the # character are not parsed by m4, and will by default be output directly into the sendmail.cf file. This is useful if you want to comment on what your configuration is doing in both the input and output files.
To allow comments in your sendmail.mc that are not placed into the sendmail.cf, you can use the m4 divert and dnl tokens. divert(-1) will cause all output to cease. divert(0) will cause output to be restored to the default. Any output generated by lines between these will be discarded. In our example, we've used this mechanism to provide a comment that appears only in the sendmail.mc file. To achieve the same result for a single line, you can use the dnl token that means, literally, “starting at the beginning of the next line, delete all characters up to and including the next newline.” We've used this in our example, too.
These are standard m4 features, and you can obtain more information on them from its manual page.
VERSIONID(`@(#)sendmail.mc 8.9 (Linux) 01/10/98') |
OSTYPE(`linux') |
The OSTYPE definition should be one of the first definitions to appear in your sendmail.mc file, as many other definitions depend upon it.
The DOMAIN macro is useful when you wish to configure a large number of machines on the same network in a standard way. It you're configuring a small number of hosts, it probably isn't worth bothering with. You typically configure items, such as the name of mail relay hosts or hubs that all hosts on your network will use.
The standard installation contains a directory of m4 macro templates used to drive the configuration process. This directory is usually named /usr/share/sendmail.cf or something similar. Here you will find a subdirectory called domain that contains domain-specific configuration templates. To make use of the DOMAIN macro, you must create your own macro file containing the standard definitions you require for your site, and write it into the domain subdirectory. You'd normally include only the macro definitions that were unique to your domain here, such as smart host definitions or relay hosts, but you are not limited to these.
The sendmail source distribution comes with a number of sample domain macro files that you can use to model your own.
If you saved your domain macro file as /usr/share/sendmail.cf/domain/vbrew.m4, you'd include definitions in your sendmail.mc using:
DOMAIN(`vbrew') |
The FEATURE macro enables you to include predefined sendmail features in your configuration. These sendmail features make the supported configurations very simple to use. There are a large number, and throughout this chapter we'll talk about only a few of the more useful and important ones. You can find full details of the features available in the CF file included in the source package.
To use any of the features listed, you should include a line in your sendmail.mc that looks like:
FEATURE(name) |
FEATURE(name, param) |
The standard sendmail macro configuration files provide lots of hooks and variables with which you can customize your configuration. These are called local macro definitions. Many of them are listed in the CF file in the sendmail source package.
The local macro definitions are usually invoked by supplying the name of the macro with an argument representing the value you wish to assign to the variable the macro manages. Again, we'll explore some of the more common local macro definitions in the examples we present later in the chapter.
If you want sendmail to transport mail in any way other than by local delivery, you must tell it which transports to use. The MAILER macro makes this very easy. The current version of sendmail supports a variety of mail transport protocols; some of these are experimental, others are probably rarely used.
In our network we need the SMTP transport to send and receive mail among the hosts on our local area network, and the UUCP transport to send and receive mail from our smart host. To achieve this, we simply include both the smtp and uucp mail transports. The local mail transport is included by default, but may be defined for clarity, if you wish. If you are including both the smtp and the uucp mailers in your configuration, you must always be sure to define the smtp mailer first.
The more commonly used transports available to you using the MAILER macro are described in the following list:
This transport includes both the local delivery agent used to send mail into the mailbox of users on this machine and the prog mailer used to send messages to local programs. This transport is included by default.
This transport implements the Simple Mail Transport Protocol (SMTP), which is the most common means of transporting mail on the Internet. When you include this transport, four mailers are configured: smtp (basic SMTP), esmtp (Extended SMTP), smtp8 (8bit binary clean SMTP), and relay (specifically designed for gatewaying messages between hosts).
The uucp transport provides support for two mailers: uucp-old, which is the traditional UUCP, and uucp-new, which allows multiple recipients to be handled in one transfer.
This mailer allows you to send mail messages directly into Usenet style news networks. Any local message directed to an address of news.group.usenet will be fed into the news network for the news.group newsgroup.
If you have the HylaFAX software installed, this mailer will allow you to direct email to it so that you may build an email-fax gateway. This feature is experimental at the time of writing and more information may be obtained from http://www.vix.com/hylafax/.
There are others, such as the pop, procmail, mail11, phquery, and cyrus that are useful, but less common. If your curiosity is piqued, you can read about these in the sendmail book or the documentation supplied in the source package.
The Virtual Brewery's configuration is probably more complex than most sites require. Most sites today would use the SMTP transport only and do not have to deal with UUCP at all. In our configuration we've configured a “smart host” that is used to handle all outgoing mail. Since we are using the SMTP transport on our local network we must tell sendmail that it is not to send local mail via the smart host. The LOCAL_NET_CONFIG macro allows you to insert sendmail rules directly into the output sendmail.cf to modify the way that local mail is handled. We'll talk more about rewrite rules later on, but for the moment you should accept that the rule we've supplied in our example specifies that any mail destined for hosts in the vbrew.com domain should be delivered directly to the target hosts using the SMTP mail transport.