string.
Ruleset Semantics
Each of the sendmail rulesets is called upon to perform a different task in mail processing. When you are writing rules, it is important to understand what each of the rulesets are expected to do. We'll look at each of the rulesets that the m4 configuration scripts allow us to modify:
LOCAL_RULE_3
Ruleset 3 is responsible for converting an address in an arbitrary format into a common format that sendmail will then process. The output format expected is the familiar looking
Ruleset 3 should place the hostname part of the converted address inside the ‹ and › characters to make parsing by later rulesets easier. Ruleset 3 is applied before sendmail does any other processing of an email address, so if you want sendmail to gateway mail from some system that uses some unusual address format, you should add a rule using the LOCAL_RULE_3 macro to convert addresses into the common format.
LOCAL_RULE_0 and LOCAL_NET_CONFIG
Ruleset 0 is applied to recipient addresses by sendmail after Ruleset 3. The LOCAL_NET_CONFIG macro causes rules to be inserted into the
Ruleset 0 is expected to perform the delivery of the message to the recipient, so it must resolve to a triple that specifies each of the mailer, host, and user. The rules will be placed before any smart host definition you may include, so if you add rules that resolve addresses appropriately, any address that matches a rule will not be handled by the smart host. This is how we handle the direct smtp for the users on our local LAN in our example.
LOCAL_RULE_1 and LOCAL_RULE_2
Ruleset 1 is applied to all sender addresses and Ruleset 2 is applied to all recipient addresses. They are both usually empty.
Interpreting the rule in our example
Our sample in Example 18.3 uses the LOCAL_NET_CONFIG macro to declare a local rule that ensures that any mail within our domain is delivered directly using the smtp mailer. Now that we've looked at how rewrite rules are constructed, we will be able to understand how this rule works. Let's take another look at it.
Example 18.3: Rewrite Rule from vstout.uucpsmtp.m4
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
We know that the LOCAL_NET_CONFIG macro will cause the rule to be inserted somewhere near the end of ruleset 0, but before any smart host definition. We also know that ruleset 0 is the last ruleset to be executed and that it should resolve to a three-tuple specifying the mailer, user, and host.
We can ignore the two comment lines; they don't do anything useful. The rule itself is the line beginning with R. We know that the R is a sendmail command and that it adds this rule to the current ruleset, in this case ruleset 0. Let's look at the lefthand side and the righthand side in turn.
The lefthand side looks like: $* ‹ @ $*.$m. › $*.
Ruleset 0 expects ‹ and › characters because it is fed by ruleset 3. Ruleset 3 converts addresses into a common form and to make parsing easier, it also places the host part of the mail address inside ‹›s.
This rule matches any mail address that looks like: 'DestUser ‹ @ somehost.ourdomain. › Some Text'. That is, it matches mail for any user at any host within our domain.
You will remember that the text matched by metasymbols on the lefthand side of a rewrite rule is assigned to macro definitions for use on the righthand side. In our example, the first $* matches all text from the start of the address until the ‹ character. All of this text is assigned to $1 for use on the righthand side. Similarly the second $* in our rewrite rule is assigned to $2, and the last is assigned to $3.
We now have enough to understand the lefthand side. This rule matches mail for any user at any host within our domain. It assigns the username to $1, the hostname to $2, and any trailing text to $3. The righthand side is then invoked to process these.
Let's now look at what we're expecting to see outputed. The righthand side of our example rewrite rule looks like: $#smtp $@ $2.$m. $: $1 ‹ @ $2.$m. › $3.
When the righthand side of our ruleset is processed, each of the metasymbols are interpreted and relevant substitutions are made.
The $# metasymbol causes this rule to resolve to a specific mailer,
The $@ resolves the target host. In our example, the target host is specified as $2.$m., which is the fully qualified domain name of the host on in our domain. The FQDN is constructed of the hostname component assigned to $2 from our lefthand side with our domain name (.$m.) appended.
The $: metasymbol specifies the target user, which we again captured from the lefthand side and had stored in $1.
We preserve the contents of the ‹› section, and any trailing text, using the data we collected from the lefthand side of the rule.
Since this rule resolves to a mailer, the message is forwarded to the mailer for delivery. In our example, the message would be forwarded to the destination host using the SMTP protocol.
Configuring sendmail Options
sendmail has a number of options that allow you to customize the way it performs certain tasks. There are a large number of these, so we've listed only a few of the more commonly used ones in the upcoming list.
To configure any of these options, you may either define them in the m4 configuration file, which is the preferable method, or you may insert them directly into the
define(`confSEPARATE_PROC',`true')
The corresponding
O ForkEachJob=true
The following list describes common
confMIN_FREE_BLOCKS (MinFreeBlocks)
There are occasions when a problem might prevent the immediate delivery of mail messages, causing messages to be queued in the mail spool. If your mail host processes large volumes of mail, it is possible for the mail spool to grow to such a size that it fills the filesystem supporting the spool. To prevent this, sendmail provides this option to specify the minimum number of free disk blocks that must exist before a mail message will be accepted. This allows you to ensure that sendmail never causes your spool filesystem to be filled (Default:
