8.5 Complex Actions Made Simple

Beginning with V8.7 sendmail, rule-testing mode offers six simple commands that accomplish complex tasks. They are listed in Table 8-1.

A lone / character will cause the following usage message to print:

Usage: /[canon|map|mx|parse|try|tryflags]

Anything other than the commands shown in Table 8-1 (such as /foo) will produce an error:

Unknown "/" command /foo

8.5.1 Canonify a Host with /canon

The /canon rule-testing command causes sendmail to look up the canonical (official, fully qualified) name of a host and print the result. The form for this command looks like this:

/canon host

If host is missing, the following usage message is printed:

Usage: /canon address

When you correctly supply the hostname as the argument, sendmail looks up the canonical name and returns the result:

> /canon icsic
getcanonname(icsic) returns icsic.icsi.berkeley.edu
>

Here, the hostname icsic was looked up. Because its canonical name was found, that name is printed following the returns. If the hostname had not been found, sendmail would have printed that same name after the returns:

> /canon foo
getcanonname(foo) returns foo

If you wish to watch the actual process of a host being canonified, you can turn on the -d38.20 debugging switch (-d38.20) with the rule-testing -d command (Section 8.7):

> -d38.20
>

With that setting, the previous lookup of icsic produces a trace of all the steps that sendmail takes:

> /canon icsic
getcanonname(icsic), trying dns
getcanonname(icsic), trying files
text_getcanonname(icsic)
getcanonname(icsic.icsi.berkeley.edu), found
getcanonname(icsic) returns icsic.icsi.berkeley.edu

Here, sendmail first looked up icsic using DNS. That lookup failed, so sendmail fell back to looking it up in the /etc/hosts file, where it was found. The order in which these techniques are tried is defined by your service-switch (ServiceSwitchFile). If a service-switch mechanism is lacking, the order is internally defined by sendmail and varies depending on the operating system used.

Internally, the /canon rule-testing command can be watched in greater detail with the -d38.20 debugging switch (-d38.20) and with the -d8.2 debugging switch (-d8.2).

8.5.2 Look Up MX Records with /mx

The /mx rule-testing command causes sendmail to look up a specified hostname and return a list of MX records for that host. The form for this command looks like this:

/mx host

Here, host is the short or fully qualified name of a host. If host is missing, sendmail prints the following usage message:

Usage: /mx address

When host exists and has MX records associated with it, sendmail will look up and print those records. The MX records are listed in the order in which they will be tried (lowest to highest preference values). For example:

> /mx ourhost
getmxrr(ourhost) returns 2 value(s):
        mx.our.domain
        offsite.mx.domain
>

If no MX records are found (as for a.com), sendmail prints the following message:

getmxrr(a.com) returns 0 value(s):

When multiple MX records have the same preference values, sendmail randomizes the list. During a single run of sendmail the randomization will be the same each time. You can see this by looking up aol.com:

> /mx aol.com
getmxrr(aol.com) returns 4 value(s):
        mailin-02.mx.aol.com.
        mailin-01.mx.aol.com.
        mailin-04.mx.aol.com.
        mailin-03.mx.aol.com.

If you have defined the FallbackMXhost option (FallbackMXhost), the host that is specified in that option will always appear last in the list of MX hosts. As a side benefit, the fallback host will also be listed for hosts that do not exist:

% /usr/sbin/sendmail -OFallBackMXhost=mx.our.domain -bt
ADDRESS TEST MODE (ruleset 3 NOT automatically invoked)
Enter <ruleset> <address>
> /mx a.com
getmxrr(a.com) returns 1 value(s):
        mx.our.domain
>

This /mx command is available for your use only if sendmail was compiled with NAMED_BIND defined (NAMED_BIND). If NAMED_BIND was not defined, sendmail will print the following error instead of listing MX records:

No MX code compiled in

8.5.3 Look up a Database Item with /map

The /map rule-testing command causes sendmail to look up a key in a database and print the value found (if there is one). The /map command is used like this:

/map name key

Here, name is the name of a database. It is either a name you assigned using a K configuration command (Section 23.2) or a name that is internally defined by sendmail, such as aliases.files (switch). The key is the item you wish to look up in the database. If both name and key are missing, sendmail prints this usage message:

Usage: /map mapname key

If just the key is missing, sendmail prints this error:

No key specified

If the name is that of a database that does not exist, sendmail prints this error:

Map named "bad name here" not found

Otherwise, the database does exist, so sendmail looks up the key in it. If the key is not found in the database, sendmail prints this:

map_lookup: name (key) no match (error number here) 

The error number corresponds to error numbers listed in the sysexits.h file.

The /map rule-testing command is very useful for testing databases of your own design. If a rule that uses the database fails to work as predicted, use /map to test that database by hand. To illustrate, consider the sampling of maps in the following sections.

8.5.3.1 The aliases database map

The aliases map is used to convert a local address into one or more new addresses. Using the rule-testing /map command, you can see how sendmail looks up an alias:

> /map aliases root
map_lookup: aliases (root) returns you, hans@other.site (0)

8.5.3.2 The host map

The host database behaves the same as the /canon command shown earlier. It looks up a hostname by using sendmail's internal host map (Section 23.4.3), which returns the canonical name of the looked-up host:

> /map host localhost
map_lookup: host (localhost) returns localhost.our.domain. (0)
> /map host bogus.no.domain
map_lookup: host (bogus.no.domain) no match (68)

8.5.3.3 The dequote map

The dequote map (dequote) is not really a database at all, but a hook into a routine that removes quotation marks from addresses:

> /map dequote "a"@"@b"
map_lookup: dequote ("a"@"@b") returns a@@b (0)
> /map dequote "a
map_lookup: dequote ("a) no match (0)
> /map dequote "<a"
map_lookup: dequote ("<a") no match (0)
> /map dequote "(a"
map_lookup: dequote ("(a") no match (0)

Note in the second example that it removes only balanced quotation marks. Note in the last two examples that it will remove quotation marks only if the enclosed expression is a valid address expression. In neither of the last two examples were the enclosing angle braces or parentheses balanced.

8.5.4 Select Whom to /parse or /try with /tryflags

Before we cover the /parse and /try commands, we need to mention the /tryflags rule-testing command because it is used to select the sender, recipient, headers, and envelope for the /parse and /try commands. The /tryflags command is used like this:

/tryflags h         set headers
/tryflags e         set envelope
/tryflags s         set sender
/tryflags r         set recipient
/tryflags er        set envelope recipient

The arguments are single letters that can appear in uppercase or lowercase and in any order. Any letter other than those shown is silently ignored.

The default setting when sendmail first starts to run in rule-testing mode is er for envelope-recipient. Omitting the argument causes sendmail to print the following usage statement:

Usage: /tryflags [Hh|Ee][Ss|Rr]

8.5.5 Parse an Address with /parse

The /parse rule-testing command instructs sendmail to pass an address through a predetermined sequence of rules to select a delivery agent and to put the $u macro ($u) into its final form. The /parse command is used like this:

/parse address

If the address is missing, sendmail prints the following usage message:

Usage: /parse address

The following example shows a local address being fed into /parse. Note that the numbers on the left are for later reference and are not part of sendmail's output:

   > /parse you@localhost (Your Name)
1    Cracked address = $g (Your Name)
2    Parsing envelope-recipient address
3    canonify           input: you @ localhost
4    Canonify2          input: you < @ localhost  >
5    Canonify2        returns: you < @ here . our. domain .  >
6    canonify         returns: you < @ here . our. domain .  >
7    parse              input: you < @ here . our. domain .  >
8    Parse0             input: you < @ here . our. domain .  >
9    Parse0           returns: you < @ here . our. domain .  >
10   ParseLocal         input: you < @ here . our. domain .  >
11   ParseLocal       returns: you < @ here . our. domain .  >
12   Parse1             input: you < @ here . our. domain .  >
13   Parse1           returns: $# local $: you
14   parse            returns: $# local $: you
15   2                  input: you
16   2                returns: you
17   EnvToL             input: you
18   EnvToL           returns: you
19   final              input: you
20   final            returns: you
21   mailer local, user you

The address you@localhost is first fed into crackaddr (line 1) to separate it from any surrounding RFC822 comments such as "(Your Name)." If mail were actually to be sent, the address would be stored in the $g macro before being passed to rules. This is illustrated by line 1, which uses $g as a place holder to show where the address was found.

The next line (line 2) shows that the address will be treated as that of an envelope recipient. The /tryflags command (Section 8.5.4) sets whether it is treated as a header or envelope or as a sender or recipient address.

The address is passed to the canonify rule set 3 (Section 19.3) because all addresses are rewritten by the canonify rule set 3 first. The job of the canonify rule set 3 is to focus on (surround in angle brackets) the host part of the address, which it does (line 4). The canonify rule set 3, in this example, then passes the address to the Canonify2 rule set to see whether localhost is a synonym for the local machine's name. It is, so the Canonify2 rule set makes that translation (line 5).

The output of the canonify rule set 3 is passed to the parse rule set 0, whose job is to select a delivery agent (line 7). Because here.our.domain is the local machine, the parse rule set 0 (by way of other rule sets) selects the local delivery agent (line 5).

Line 14 shows that the $: part of the delivery agent "triple" (Section 19.5) will eventually be tucked into $u ($u) for use by the delivery agent's A= equate (A=). But before that happens, that address needs to be passed through its own set of specific rules. It is given to rule set 2 because all recipient addresses are given to rule set 2 (line 15). It is then given to rule set EnvToL because the R= equate (R=) for the local delivery agent specifies rule set EnvToL for the envelope recipient (line 17). Finally, it is given to the final rule set 4 (Section 19.4) because all addresses are lastly rewritten by the final rule set 4 (line 19).

The last line of output shows that the local delivery agent was selected and that the value that would be put into $u (were mail really being sent) would be you.

When you /parse an address that is not local, the parse rule set 3 will also select a host ($@) part for delivery:

parse            returns: $# esmtp $@ uofa . edu . $: friend < @ uofa . edu .  >

In this instance, the last line of /parse output will also include the host information that will be placed into $h:

mailer esmtp, host uofa.edu., user friend@uofa.edu

When you /parse an address that is illegal (from the point of view of rules), sendmail selects the #error delivery agent:

> /parse @host
Cracked address = $g
Parsing envelope-recipient address
canonify           input: @ host
Canonify2          input: < @ host >
Canonify2        returns: < @ host >
canonify         returns: < @ host >
parse              input: < @ host >
Parse0             input: < @ host >
Parse0           returns: $# error $@ 5 . 1 . 3 $: "553 User address required"
parse            returns: $# error $@ 5 . 1 . 3 $: "553 User address required"
@host... User address required
mailer *error*, host 5.1.3, user "553 User address required"

The error here was that the address lacked a user part. The meanings of all the parts of the #error delivery agent are described in error. The second from the last line in this example shows the message that would be printed or returned if such an address appeared in actual mail. The delivery agent *error* is internal to sendmail and cannot be directly used.

8.5.6 Try a Delivery Agent with /try

In the SMTP RCPT command, sendmail is required to express the recipient's address relative to the local host. For domain addresses, this simply means that the address should be RFC2822-compliant (such as you@here.our.domain). For UUCP addresses, this can mean reversing the path (such as you@there reverses to there!you). The /try rule-testing command causes an address to be rewritten so that it appears to be correct relative to the local host.

The /try command is used like this:

/try agent address

Here, agent is the delivery agent, and address is the address to rewrite. The following usage message is produced if both agent and address are missing or if just the address is missing:

Usage: /try mailer address

The delivery agent (mailer) is used to select only the R= or S= rule set for the address. The /tryflags command (Section 8.5.4) determines which is selected (by selecting recipient or sender).

In the following example the numbers to the left are for reference only and are not part of sendmail's output:

   > /try smtp you
   Trying envelope-recipient address you for mailer esmtp
1    canonify           input: you
2    Canonify2          input: you
3    Canonify2        returns: you
4    canonify         returns: you
5    2                  input: you
6    2                returns: you
7    EnvToSMTP          input: you
8    PseudoToReal       input: you
9    PseudoToReal     returns: you
10   MasqSMTP           input: you
11   MasqSMTP         returns: you < @ *LOCAL*  >
12   EnvToSMTP        returns: you < @ here . our . domain .  >
13   final              input: you < @ here . our . domain .  >
14   final            returns: you @ here . our . domain
15   Rcode = 0, addr = you@here.our.domain

Here, the envelope-recipient address you is rewritten on the basis of the smtp delivery agent. Rule set canonify is called first (line 1) because all addresses are rewritten by it first. Rule set 2 (line 5) is called because all recipient addresses get rewritten by it. Rule set EnvToSMTP (line 13) is called because that rule set was indicated by the esmtp delivery agent's R= equate. That rule set detects that the envelope-recipient address (you) is local (line 7). Rule set final (always the last to rewrite) sees the special tag *LOCAL* and converts that tag to the canonical name of your local machine (line 11).