25.7.1 Unscrambling Forwards

The traditional use of the ~/.forward file, as its name implies, is to forward mail to another site. Unfortunately, as users move from machine to machine, they can leave behind a series of ~/.forward files, each of which points to the next machine in a chain. As machine names change and as old machines are retired, the links in this chain can be broken. One common consequence is a bounced mail message ("host unknown") with a dozen or so Received: (see Section 35.10.25, Received: ) header lines.

As the mail administrator, you should beware of the ~/.forward files of users at your site. If any contain offsite addresses, you should periodically use the SMTP expn command [8] to examine them. For example, consider a local user whose ~/.forward contains the following line:

[8] Under old versions of sendmail the vrfy and expn commands are interchangeable. Under V8 sendmail and other, modern SMTP servers, the two commands differ.

[email protected]

This causes all local mail for the user to be forwarded to the host remote.domain for delivery there. The validity of that address can be checked with telnet (1) at port 25 [9] and the SMTP expn command:

[9] In place of specifying port 25, you can use either mail or smtp . These are more mnemonic and easier to remember (although we "oldtimers" tend to still use 25).

% 

telnet remote.domain 25

 Trying 123.45.123.45 ... Connected to remote.domain. Escape character is '^]'. 220 remote.domain Sendmail 8.6.13/8.6.12 ready at Tue, 7 May 1996 13:39:21 -0700 220 ESMTP spoken here 

expn user

 250 <[email protected]> 

quit

 221 remote.domain closing connection Connection closed by foreign host. %

This shows that the user is known at remote.site but also shows that mail will be forwarded (yet again) from there to another.site . By repeating this process, you will eventually find the site at which the user's mail will be delivered. Depending on your site's policies, you can either correct the user's ~/.forward file or have the user correct it. It should contain the address of the host where that user's mail will ultimately be delivered.

25.7.2 Forwarding Loops

Because ~/.forward files are under user control, the administrator occasionally needs to break loops caused by improper use of those files. To illustrate, consider a user who wishes to have mail delivered on two different machines (call them machines A and B). On machine A the user creates a ~/.forward file like this:

\user, user@B

Then, on machine B the user creates this ~/.forward file:

\user, user@A

The intention is that the backslashed name ( \user ) will cause local delivery and the second address in each will forward a copy of the message to the other machine. Unfortunately, this causes mail to go back and forth between the two machines (delivering and forwarding at each) until the mail is finally bounced with the error message "too many hops."

On the machine that the administrator controls, a fix to this looping is to temporarily edit the aliases database and insert an alias for the offending user like this:

user:  \user

This causes mail for user to be delivered locally and that user's ~/.forward file to be ignored. After the user has corrected the offending ~/.forward files, this alias can be removed.

25.7.3 Appending to Files

The ~/.forward file can contain the names of files onto which mail is to be appended. Such filenames must begin with a slash character that cannot be quoted. For example, if a user wishes to keep a backup copy of incoming mail:

\user /home/user/mail/in.backup

The first line ( \user ) tells sendmail to deliver directly to the user's mail spool file using the local delivery agent. The second line tells sendmail to append a copy of the mail message to the file specified ( in.backup ).

Note that, prior to V8, sendmail did no file locking, so writing files by way of the ~/.forward file was not recommended. Beginning with V8, however, sendmail locks those files during writing, so such use of the ~/.forward file is now okay.

If the SafeFileEnvironment option (see Section 34.8.58, SafeFileEnvironment ) is set, the user should be advised to specify the path of that safe directory:

\user /arch/bob.backup          
 here /arch was specified by the SafeFileEnvironment option

When the SafeFileEnvironment option is used, the cooperation of the system administration may be needed if users are to have the capability of saving mail to files via the ~/.forward file.

25.7.4 Piping Through Programs

The ~/.forward file can contain the names of programs to run. A program name is indicated by a leading pipe ( | ) character, which may or may not be quoted (see Section 24.2.3, "Delivery via Programs" ). For example, a user may be away on a trip and want mail to be handled by the vacation (1) program:

\user, "|/usr/ucb/vacation user"

Recall that prefixing a local address with a backslash tells sendmail to skip additional alias transformations. For \user this causes sendmail to deliver the message (via the local delivery agent) directly to the user's spool mail box.

The quotes around the vacation program are necessary to prevent the program and its single argument ( user ) from being viewed as two separate addresses. The vacation program is run with the command-line argument user , and the mail message is given to it via its standard input.

Beginning with V8 sendmail , a user must have a valid shell to run programs from the ~/.forward file. See Section 18.8.34, PATH... for a description of this process and for methods to circumvent it at the system level.

Because sendmail sorts all addresses and deletes duplicates before delivering to any of them, it is important that programs in ~/.forward files be unique. Consider a program that doesn't take an argument and suppose that two users both specified that program in their ~/.forward files:

user 1   \user1, "|/bin/notify"
 
user 2   \user2, "|/bin/notify"

Prior to V8 sendmail , when mail was sent to both user1 and user2 , the address /bin/notify appeared twice in the list of addresses. The sendmail program eliminated what seems to be a duplicate, [10] and one of the two users did not have the program run.

[10] V8 sendmail uses the owner of the ~/.forward file in addition to the program name when comparing.

If a program requires no arguments (as opposed to ignoring them), the ~/.forward program specifications can be made unique by including a shell comment:

user 1   \user1, "|/bin/notify #user1"
 
user 2   \user2, "|/bin/notify #user2"

25.7.5 Specialty Programs for Use with ~/.forward

Rather than expecting users to write home-grown programs for use in ~/.forward files, offer them any or all of the publicly available alternatives. The most common are listed below.

25.7.6 Force Requeue on Error

Normally, a program in the user's ~/.forward file is executed with the Bourne shell. The precise means that is used is defined by the prog delivery agent.

Mprog, P=/bin/sh,   F=lsDFMeuP,  S=10, R=20, A=sh -c $u                                                  

                                               
The Bourne shell

One drawback to using the Bourne shell to run programs is that it exits with a value of 1 when the program cannot be executed. When sendmail sees the exit value 1, it bounces the mail message.

There will be times when bouncing a mail message because the program could not execute is not desirable. For example, consider the following ~/.forward file:

 "| /usr/local/lib/slocal -user george"

If the directory /usr/local/lib is unavailable (perhaps because a file server is down or because an automounter failed), the mail message should be queued, rather than bounced. To arrange for requeueing of the message on failure, users should be encouraged to construct their ~/.forward files like this:

 "| /usr/local/lib/slocal -user george || exit 75"

Here, the || tells the Bourne shell to perform what follows (the exit 75 ) if the preceding program could not be executed or if the program exited because of an error. The exit value 75 is special, in that it tells sendmail to queue the message for later delivery, rather than to bounce it.