Macros by Function

The m4 technique uses a huge number of macros to accomplish the complex task of creating configuration files for all possible circumstances. Most are detailed in the reference section at the end of this chapter. Others are documented in chapters dedicated to particular subjects. Here, we summarize all m4 macros by classification or function.

Options

Options may be set, unset, and changed in your mc file with simple define statements. For example, the following line sets the location of the alias file and thus the AliasFile (A) option:

define(`ALIAS_FILE', `nis:-N mail.aliases')

Configuring options with the m4 technique is described in "Configuring with V8 m4 Options" (with the individual m4 option names listed in Table 34.3 of that section). Options are described in general in Options.

Define Macros

Defined macros may be declared in your mc file. Those that are useful are listed in Table 31.5 of "Macros with the m4 Technique". That section also describes the general technique of defining macros via m4. To illustrate, for example,

define(`BITNET_RELAY', `host.domain')

causes the value host.domain to be assigned to the $B macro. Non-m4 specific defined macros can be declared with the LOCAL_CONFIG technique (see ).

Rules and rule sets

The m4 technique allows custom rules and rule sets to be inserted in a variety of convenient ways. The complete list of m4 keywords that affect rules is shown in Table 19.5.

To illustrate, consider the following technique for adding a rule to rule set 0:

LOCAL_RULE_0 R$* <@ $=w . $=m> $* $#local $: $1 @here.ourdomain

Here, we add a rule to rule set 0 that accepts any address with a host part in the class $=w (see $=w) that is also in one of the local domains as listed in the class $=m (see $=m) as local address.

Masquerading

Masquerading is the process of transforming the local hostname in addresses into that of another host. This results in the mail message appearing to come from that other host rather than from the local host. Masquerading is most often used in domains where email is addressed to the domain rather than to individual hosts inside the domain.

Masquerading usually rewrites header-sender addresses. Some m4 features allow you also to rewrite envelope addresses. The complete list of all definitions and features that affect masquerading is shown in Table 19.6.

Relays

A relay is a rule that sends all of one type of mail to a specific destination. One example would be email FAX transmissions. Clearly, even though local mail should be delivered locally, mail to the pseudo-user fax should always be sent to a special FAX-handling machine.

The complete list of relays supported by the V8 sendmail m4 technique is shown in Table 19.7.

All relays are declared in the same fashion. For example,

define(`LOCAL_RELAY',`agent:host')

Here, agent is the name of a delivery agent to use, and host is the name of the machine to which all such mail will be relayed. If agent: is missing, it defaults to a literal relay:.

If the host is listed under a domain that uses wild-card MX records (see "Wildcard MX Records"), you should specify it with a trailing dot; for example,

define(`LOCAL_RELAY', `smtp:relay.sub.domain.')  trailing dot

Relays fit into the flow of rules through rule set 0 like this:

1. Basic canonicalization (list syntax, delete local host, etc.)
2. Basic canonicalization (list syntax, delete local host, etc.)
3. Basic canonicalization (list syntax, delete local host, etc.)
4. LOCAL_RULE_0 (see )
5. FEATURE(mailertable) (see )
6. UUCP_RELAY, BITNET_RELAY, FAX_RELAY, DECNET_RELAY
7. LOCAL_NET_CONFIG (see )
8. SMART_HOST
9. SMTP, local, etc. delivery agents

UUCP

The m4 configuration technique includes four UUCP options to choose from. They are listed in Table 19.8.

Note that two items in the table are marked as obsolete. This is because all of their functions have been moved into the mailertable feature (see ). They are included for backward compatibility with early configuration file versions.

Support for UUCP can be included in your mc file with the MAILER command (see ):

MAILER(uucp)

This declares six [3] delivery agents and the rules to support them. They are listed in Table 19.9.

[3] Actually, there are only four; uucp and uucp-old are synonyms for the same agents, as are suucp and uucp-new.

If support for SMTP delivery agents is also included prior to UUCP, then the last two additional delivery agents are included (uucp-dom and uucp-uudom). Note that smtp must be first for this to happen:

MAILER(smtp) MAILER(uucp)

If uucp is first, uucp-dom and uucp-uudom are excluded.

When processing UUCP mail (addresses that contain a ! and those that end in a .UUCP suffix), sendmail routes to those hosts on the basis of the class in which they were found. Hosts that are found in $=U deliver via uucp-old, hosts in $=Y deliver via uucp-new, and hosts in $=Z deliver via uucp-uudom.

The choice of which delivery agent to use for UUCP delivery is under the control of the SITECONFIG m4 macro described above (see ). Which you choose depends on what version of UUCP you are running locally and what version is being run at the other end of the connection. There are far too many variations on UUCP to allow specific recommendations here. In general, you need to choose between a domain form of address (gw@wash.dc.gov) and a UUCP form (wash!gw) and then go with the delivery agent that makes the most sense for you. We recommend that you start with the most domain-correct agent, uucp-dom, and see if it works for you. If not, scale back to uucp-uudom, then to uucp-new and finally to uucp-old as a last resort.

uucp-old (aka uucp)

If you are running an old version of UUCP, you may have to use this delivery agent. All addresses are turned into the ! form even if they were in domain form:

user becomes  yourhost!user user@host.domain becomes  yourhost!host.domain!user

This delivery agent can deliver only to one recipient at a time, so it can spend a lot of time transmitting duplicate messages. If at all possible, avoid using this delivery agent.

uucp-new (aka suucp)

Newer releases of UUCP can send to multiple recipients at once. If yours is such a release, you may use the uucp-new delivery agent. It is just like uucp-old except that it can perform multiple deliveries.

uucp-uudom

More modern implementations of UUCP can understand and correctly handle domain style addresses in headers (although they still require the ! form in the envelope). If yours is such an implementation, you may use the uucp-uudom delivery agent.

At the receiving end, the message mail arrives with the five character "From " line showing the sender address in the ! form. The "From " line reflects the envelope address.

uucp-dom

The uucp-dom is the most domain-correct form of the available UUCP delivery agents. All addresses, envelope and header, no matter whether or not they began in the ! form, are sent out in domain form. Essentially, this uses UUCP as a transport mechanism, but in all other respects it adheres to the Internet standards.