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28.6 The RHS

The purpose of the RHS in a rule is to rewrite the workspace. To make this rewriting more versatile, sendmail offers several special RHS operators. The complete list is shown in Table 28.2.

Table 28.2: RHS Operators
RHSDescription or Use
$digitSection 28.6.1, "Copy by Position: $digit"Copy by position
$:Section 28.6.2, "Rewrite Once Prefix: $:"Rewrite once (prefix)
$@Section 28.6.3, "Rewrite-and-Return Prefix: $@"Rewrite and return (prefix)
$>setSection 28.6.4, "Rewrite Through Another Rule Set: $>set"Rewrite through another rule set
$#Section 28.6.5, "Specify a Delivery Agent: $#"Specify a delivery agent
$[ $]Section 28.6.6, "Canonicalize Hostname: $[ and $]"Canonicalize hostname
$( $)Section 33.4, "Use Maps with $( and $) in Rules"Database lookup

28.6.1 Copy by Position: $digit

The $digit operator in the RHS is used to copy tokens from the LHS into the workspace. The digit refers to positions of LHS wildcard operators in the LHS:

R$+@$*    $2!$1
 -^  -^
 $1 $2

Here, the $1 in the RHS indicates tokens matched by the first wildcard operator in the LHS (in this case the $+), and the $2 in the RHS indicates tokens matched by the second wildcard operator in the LHS (the $*). In this example, if the workspace contains A@B.C, it will be rewritten by the RHS as follows:

$* matches    B.C    so $2 copies it to workspace
              !    explicitly placed into workspace
$+ matches    A    so $1 copies it to workspace

The $digit copies all the tokens matched by its corresponding wildcard operator. For the $+ wildcard operator, only a single token (A) is matched and copied with $1. The ! is copied as is. For the $* wildcard operator, three tokens are matched (B.C), so $2 copies all three. Thus the above rule rewrites A@B.C into B.C!A.

Not all LHS operators need to be referenced with a $digit in the RHS. Consider the following:

R$*<$*>$*   <$2>

Here, only the middle LHS operator (the second one) is required to rewrite the workspace. So only the $2 is needed in the RHS ($1 and $3 are not needed and are not present in the RHS).

Although macros appear to be operators in the LHS, they are not. Recall that macros are expanded when the configuration file is read (see Section 28.1.1, "Macros in Rules"). As a consequence, although they appear as $letter in the configuration file, they are converted to tokens when that configuration file is read. For example,

R$A @ $*   $1

Here, the macro A is defined to have the value xxx. To the unwary, the $1 appears to indicate the $A. But when the configuration file is read, the above rule is expanded into

Rxxx @ $*   $1

Clearly, the $1 refers to the $* (because $digit references only operators and $A is a macro, not an operator). The sendmail program is unable to detect errors of this sort. If the $1 were instead $2 (in a mistaken attempt to reference the $*), sendmail prints the following error and skips that rule:

ruleset replacement num out of bounds

V8 sendmail catches these errors when the configuration file is read. Earlier versions caught this error only when mail was actually sent.

The digit of the $digit must be in the range one through nine. A $0 is meaningless and causes sendmail to print the above error message and to skip that rule. Extra digits are considered tokens, rather than extensions of the $digit. That is, $11 is the RHS operator $1 and the token 1, not a reference to the eleventh LHS operator.

28.6.2 Rewrite Once Prefix: $:

Ordinarily, the RHS rewrites the workspace as long as the workspace continues to match the LHS. This looping behavior can be useful. Consider the need to strip extra trailing dots off an address in the workspace:

R$*..   $1.

Here, the $* matches any address that has two or more trailing dots. The $1. in the RHS then strips one of those two trailing dots when rewriting the workspace. For example,

xxx . . . . .     becomes -> xxx . . . .
xxx . . . .       becomes -> xxx . . .  
xxx . .           becomes -> xxx . .    
xxx . .           becomes -> xxx .      
xxx .             <- match fails

Although this looping behavior of rules can be handy, for most rules it can be dangerous. Consider the following example:

R$*	<$1>

The intention of this rule is to cause whatever is in the workspace to become surrounded with angle brackets. But after the workspace is rewritten, the LHS again checks for a match; and since the $* matches anything, the match succeeds, the RHS rewrites the workspace again, and again the LHS checks for a match:

xxx               becomes -> < xxx >
< xxx >           becomes -> < < xxx > >
< < xxx > >       becomes -> < < < xxx > > >
     and so on, until ...
sendmail prints: rewrite: expansion too long

In this case, sendmail catches the problem, because the workspace has become too large. It prints the above error message and skips that and all further rules in the rule set. If you are running sendmail in test mode, this fatal error would also be printed:

== Ruleset 0 (0) status 65

Unfortunately, not all such endless looping produces a visible error message. Consider the following example:

R$*    $1

Here is an LHS that matches anything and an RHS that rewrites the workspace in such a way that the workspace never changes. For older versions this causes sendmail to appear to hang (as it processes the same rule over and over and over). Newer versions of sendmail will catch such endless looping and print (syslog) the following error:

Infinite loop in ruleset ruleset_name, rule rule_number

In this instance the original workspace is returned.

It is not always desirable (or even possible) to write "loop-proof" rules. To prevent looping, sendmail offers the $: RHS prefix. By starting the RHS of a rule with the $: operator, you are telling sendmail to rewrite the workspace exactly once.

R$*   $: <$1>

Again the rule causes the contents of the workspace to be surrounded by a pair of angle brackets. But here the $: prefix prevents the LHS from checking for another match after the rewrite.

Note that the $: prefix must begin the RHS to have any effect. If it instead appears inside the RHS, its special meaning is lost:

foo  rewritten by  $:$1   becomes ->   foo
foo  rewritten by  $1$:   becomes ->   foo $:

28.6.3 Rewrite-and-Return Prefix: $@

The flow of rules is such that each and every rule in a series of rules (a rule set) is given a chance to match the workspace:

Rxxx   yyy
Ryyy   zzz

The first rule matches xxx in the workspace and rewrites the workspace to contain yyy. The first rule then tries to match the workspace again but, of course, fails. The second rule then tries to match the workspace. Since the workspace contains yyy, a match is found, and the RHS rewrites the workspace to be zzz.

There will often be times when one rule in a series performs the appropriate rewrite and no subsequent rules need to be called. In the above example, suppose xxx should only become yyy and that the second rule should not be called. To solve problems like this, sendmail offers the $@ prefix for use in the RHS.

The $@ prefix tells sendmail that the current rule is the last one that should be used in the current rule set. If the LHS of the current rule matches, any rules that follow (in the current rule set) are ignored:

Rxxx   $@yyy
Ryyy   zzz

If the workspace contains anything other than xxx, the first rule does not match, and the second rule is called. But if the workspace contains xxx, the first rule matches and rewrites the workspace. The $@ prefix for the RHS of that rule prevents the second rule (and any subsequent rules) from being called.

Note that the $@ also prevents looping. The $@ tells sendmail to skip further rules and to rewrite only once. The difference between $@ and $: is that both rewrite only once, but $@ doesn't proceed to the next rule, whereas $: does.

The $@ operator must be used as a prefix because it has special meaning only when it begins the RHS of a rule. If it appears anywhere else inside the RHS it loses its special meaning:

foo  rewritten by  $@$1   becomes ->   foo
foo  rewritten by  $1$@   becomes ->   foo $@

28.6.4 Rewrite Through Another Rule Set: $>set

Rules are organized in sets that can be thought of as subroutines. Occasionally, a rule or series of rules can be common to two or more rule sets. To make the configuration file more compact and somewhat clearer, such common series of rules can be made into separate subroutines.

The RHS $>set operator tells sendmail to perform additional rewriting using a secondary set of rules. The set is the rule-set name or number of that secondary set. If set is the name or number of a nonexistent rule set, the effect is the same as if the subroutine rules were never called (the workspace is unchanged).

If the set is numeric and is greater than the maximum number of allowable rule sets, sendmail prints the following error and skips that rule:

bad ruleset bad_number (maximum max)

If the set is a name and the rule-set name is unknown, sendmail prints the following error and skips that rule:

Unknown ruleset bad_name

Neither of these errors is caught when the configuration file is read. They are caught only when mail is sent, because a rule set name may be a macro:

$> $&{SET}

The $& prefix prevents the macro named {SET} from being expanded when the configuration file is read. Therefore the name or number of the rule set cannot be known until mail is sent.

The process of calling another set of rules proceeds in five stages: First As usual, if the LHS matches the workspace, the RHS gets to rewrite the workspace. Second The RHS ignores the $>set part and rewrites the rest as usual. Third The rewritten workspace is then given to the set of rules specified by set. They either rewrite the workspace or do not. Fourth The original RHS (the one with the $>set) leaves the possibly rewritten workspace as is, as though it had performed the subroutine's rewriting itself. Fifth The LHS gets a crack at the new workspace as usual unless it is prevented by a $: or $@ prefix in the RHS.

For example, consider the following two sets of rules:

# first set
R$*..   $:$>22 $1.     strip extra trailing dots

# second set
R$*..    $1.           strip trailing dots

Here, the first set of rules contains, among other things, a single rule that removes extra dots from the end of an address. But because other rule sets may also need extra dots stripped, a subroutine (the second set of rules) is created to perform that task.

Note that the first rule strips one trailing dot from the workspace and then calls rule set 22 (the $>22), which then strips any additional dots. The workspace as rewritten by rule set 22 becomes the workspace yielded by the RHS in the first rule. The $: prevents the LHS of the first rule from looking for a match a second time.

Prior to V8.8 sendmail the subroutine call must begin the RHS (immediately follow any $@ or $: prefix, if any) and only a single subroutine may be called. That is, the following causes rule set 22 to be called but does not call 23:

$>22 xxx $>23 yyy

Instead of calling rule set 23, the $> operator and the 23 are copied as is into the workspace, and that workspace is passed to rule set 22:

xxx $> 23 yyy      <- passed to rule set 22

Beginning with V8.8 [5] sendmail, subroutine calls may appear anywhere inside the RHS, and there may be multiple subroutine calls. Consider the same RHS as above:

[5] Using code derived from IDA sendmail.

$>22 xxx $>23 yyy

Beginning with V8.8 sendmail, rule set 23 is called first and is given the workspace yyy to rewrite. The workspace, as rewritten by rule set 23, is added to the end of the xxx, and the combined result is passed to rule set 22.

Under V8.8 sendmail, subroutine rule-set calls are performed from right to left. The result (rewritten workspace) of each call is appended to the RHS text to the left.

You should beware of one problem with all versions of sendmail. When ordinary text immediately follows the number of the rule set, that text is likely to be ignored. This can be witnessed by using the -d21.3 debugging switch.

Consider the following RHS:


Because sendmail parses the 3 and the uucp as a single token, the subroutine call succeeds, but the uucp is lost. The -d21.3 switch illustrates this problem:

-----callsubr 3uucp (3)       <-  sees this
-----callsubr 3 (3)           <- but should have seen this

The 3uucp is interpreted as the number 3, so it is accepted as a valid number despite the fact that uucp was attached. Since the uucp is a part of the number, it is not available for comparison to the workspace and so is lost. The correct way to write the above RHS is

$>3 uucp.$1

Note that the space between the 3 and the uucp causes them to be viewed as two separate tokens.

This problem can also arise with macros. Consider the following:


Here, the $M is expanded when the configuration file is parsed. If the expanded value lacks a leading space, that value (or the first token in it) is lost.

Note that operators that follow a rule-set number are correctly recognized:


Here, the 3 is immediately followed by the $[ operator. Because operators are token separators, the call to rule set 3 will be correctly interpreted as

-----callsubr 3 (3)           <- good

But as a general rule, and just to be safe, the number of a subroutine call should always be followed by a space. [6]

[6] As a stylistic point, it is easier to read rules that have spaces between all patterns that are expected to match separate tokens. For example, use $+ @ $* $=m instead of $+@$*$=m. This style handles subroutine calls automatically.

28.6.5 Specify a Delivery Agent: $#

The $# operator in the RHS is copied as is into the workspace and functions as a flag advising sendmail that a delivery agent has been selected. The $# must be the first token copied into the rewritten workspace for it to have this special meaning: If it occupies any other position in the workspace, it loses its special meaning.

$# local           <- selects delivery agent
xxx $# local       <- no special meaning

When it occurs first in the rewritten workspace, the $# operator tells sendmail that the second token in the workspace is the name of a delivery agent. The $# operator is useful only in rule sets 0 and 5.

Note that the $# operator may be prefixed with a $@ or a $: without losing its special meaning, because those prefix operators are not copied to the workspace:

$@ $# local     rewritten as -> $# local

However, those prefix operators are not necessary, because the $# acts just like a $@ prefix. It prevents the LHS from attempting to match again after the RHS rewrite, and it causes any following rules to be skipped. When used in nonprefix roles in rule sets 0 and 5, $@ and $: also act like flags, conveying host and user information to sendmail (see Section 29.6, "Rule Set 0").

28.6.6 Canonicalize Hostname: $[ and $]

Tokens that appear between a $[ and $] pair of operators in the RHS are considered to be the name of a host. That hostname is looked up by using DNS [7] and replaced with the full canonical form of that name. If found, it is then copied to the workspace, and the $[ and $] are discarded.

[7] Or other means, depending on the setting of service switch file, if you have one, or the state of the ServiceSwitchFile option (see Section 34.8.61, ServiceSwitchFile).

For example, consider a rule that looks for a hostname in angle brackets and (if found) rewrites it in canonical form:

R<$*>     $@ <$[ $1 $]>     canonicalize host name

Such canonicalization is useful at sites where users frequently send mail to machines using the short version of a machine's name. The $[ tells sendmail to view all the tokens that follow (up to the $]) as a single hostname.

If the name cannot be canonicalized (perhaps because there is no such host), the name is copied as is into the workspace. For configuration files lower than 2, no indication is given that it could not be canonicalized (more about this soon).

Note that if the $[ is omitted and the $] is included, the $] loses its special meaning and is copied as is into the workspace.

The hostname between the $[ and $] can also be an IP address. By surrounding the hostname with square brackets ([ and ]), you are telling sendmail that it is really an IP address:            <- a host name
[]          <- an IP address

When the IP address between the square brackets corresponds to a known host, the address and the square brackets are replaced with that host's canonical name.

If the version of the configuration file is 2 or greater (as set with the V configuration command; see Section 27.5, "The V Configuration Command"), a successful canonicalization has a dot appended to the result:

myhost       becomes ->  myhost . domain .   <- success
nohost       becomes ->  nohost              <- failure

Note that a trailing dot is not legal [8] in an address specification, so subsequent rules (such as rule set 4) must remove these added trailing dots.

[8] Under DNS the trailing dot signifies the root (topmost) domain. Therefore under DNS a trailing dot is legal. For mail, however, RFC1123 specifically states that no address is to be propagated that contains a trailing dot.

Also, the K configuration command (see Section 33.3, "The K Configuration Command") can be used to redefine (or eliminate) the dot as the added character. For example,

Khost host -a.found

This causes sendmail to add the text .found to a successfully canonicalized hostname instead of the dot.

One difference between V8 sendmail and other versions is in the way it looks up names from between the $[ and $] operators. The rules for V8 sendmail are as follows: First If the name contains at least one dot (.) anywhere within it, it is looked up as is; for example, host.CS. Second If that fails, it appends the default domain to the name (as defined in /etc/resolv.conf) and tries to look up the result; for example, host.CS.our.Sub.Domain. Third If that fails, the leftmost part of the subdomain (if any) is discarded and the result is appended to the original host; for example, host.our.Sub.Domain. Fourth If the original name did not have a dot in it, it is looked up as is; for example, host.

This approach allows names such as host.CS to first match a site in the Czech Republic, such as vscht.CS (if that was intended), rather than to wrongly match a host in your local Computer Science (CS) department. This is particularly important if you have wildcard MX records for your site. An example of canonicalization

The following two-line configuration file can be used to observe how sendmail canonicalizes hostnames:

R$*        $@ $[ $1 $]

If this file were called, sendmail could be run in rule-testing mode with a command like the following:

% /usr/lib/sendmail -oQ. -bt

Thereafter, hostname canonicalization can be observed by specifying rule set 0 and a hostname. One such run of tests is as follows:

ADDRESS TEST MODE (ruleset 3 NOT automatically invoked)
Enter <ruleset> <address>
> 0 wash
rewrite: ruleset  0   input: wash
rewrite: ruleset  0 returns: wash . dc . gov .
> 0 nohost
rewrite: ruleset  0   input: nohost
rewrite: ruleset  0 returns: nohost

Note that the known host named wash is rewritten in canonicalized form (with a dot appended because of the V2). The unknown host named nohost is unchanged and has no dot appended. Default in canonicalization: $:

IDA and V8 sendmail both offer an alternative to leaving the hostname unchanged when canonicalization fails with $[ and $]. A default can be used instead of the failed hostname by prefixing that default with a $::

$[ host $: default $]

The $:default must follow the host and precede the $]. To illustrate its use, consider the following rule:

R$*    $:$[ $1 $: $1.notfound $]

If the hostname $1 can be canonicalized, the workspace becomes that canonicalized name. If it cannot, the workspace becomes the original hostname with a .notfound appended to it. If the default part of the $:default is omitted, a failed canonicalization is rewritten as zero tokens.

28.6.7 Other Operators

Many other operators (depending on your version of sendmail) may also be used in rules. Because of their individual complexity, all of the following are detailed in other chapters. We outline them here, however, for completeness.

Class macros

Class macros are described in Section 32.2.1, "Matching Any in a Class: $=" and Section 32.2.2, "Matching Any Not in a Class: $~" of Chapter 32, Class Macros. Class macros may appear only in the LHS. They begin with the prefix $= to match a token in the workspace to one of many items in a class. The alternative prefix $~ causes a token in the workspace to match if it does not appear in the list of items that are the class.


The conditional macro operator $? is rarely used in rules (see Section 31.6, "Macro Conditionals: $?, $|, and $."). When it is used in rules, the result is often not what was intended. Its else part, the $| conditional operator is used by the check_compat rule set (see Section 29.10.4, "The check_compat Rule Set") to separate the sender from the recipient address.

Database Operators

The database operators, $( and $), are used to look up tokens in various types of database files and network database services. They also provide access to internal services, such as dequoting and looking up MX records (see Chapter 33, Database Macros).

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