13.2.1 The Location of the Queue Directory

We have already described queue directories (in Section 3.1.2, "The Queue Directory"). Queued mail always looks like authentic mail to sendmail. That is, the sendmail program trusts the mail it finds there, believing that only root placed it there. If the queue directory were world-writable (as /tmp is), anyone could create queued mail and thereby create forged mail messages. To help prevent forged mail, the queue directory should be writable only by root. Unfortunately, for the purpose of our exercises, this would prevent you from sending mail using the client.cf file. You would need to be root, which isn't desirable and may not be possible.

Instead, we will temporarily declare the location of the queue directory to be /tmp. To help you to remember to change the client.cf file later, add a comment now showing the need to make the change and the correct path to use:

O QueueDirectory=/tmp           # BEWARE: use /var/spool/mqueue upon release

Note that on some systems, /var needs to be replaced with /usr.

13.2.2 Limit the Life of Queued Messages

Mail is usually placed into the queue because it could not be transmitted immediately. Periodically, sendmail attempts to retransmit each queued message. If the message has not been delivered after a reasonable interval, sendmail sends a warning to the sender, informing the sender that the message has not yet been delivered but that sendmail will continue to try. After a longer interval in the queue, messages that have not been successfully transmitted are bounced. The Timeout (r) option is used to specify both intervals:

O Timeout.queuewarn=4h
O Timeout.queuereturn=5d

The Timeout (r) option is special in that it takes a dot and a keyword to tell it what to time out. In the first case the keyword queuewarn sets the warning interval. In the second, the keyword queuereturn sets the return (bounce) interval. (The complete list of keywords that apply to the Timeout option is in Section 34.8.70.)

The Timeout option is one that takes a time interval as its argument. Here, 4h represents four hours (if the message is undelivered after four hours, warn the sender and continue to try) and 5d represents five days (return the message to the sender as bounced mail). The letter following the number in each specifies the units. The queuereturn, for example, could have been represented like this:

O Timeout.queuereturn=120h

This tells sendmail to bounce queued mail after 120 hours, which is the same as five days.

Five days may seem like a long time. After all, the mail hub should always be up and always be accepting mail. But suppose the hub crashed on Friday evening and replacement parts weren't available until Thursday morning. In this situation, queued mail on all the clients would start bouncing before the hub was repaired.

In choosing a value for the Timeout (r) option's queuereturn keyword, take into account the worst-case scenario you can imagine. If the hub has same-day service, a value of 1d might be enough. If the hub has to be shipped out for repair, you may want to consider a value such as 14d (two weeks). [3]

[3] You should also consider including MX records for the hub (see Section 21.3, "Set Up MX Records") so that mail will be sent to another server if the hub is down.

13.2.3 The Default Delivery Mode

There are several modes in which the sendmail program can run. Each determines how sendmail interacts with the program that invoked it. For the client.cf file, you want the user's MUA to execute sendmail for message transmission but to give the appearance that the message was sent almost instantaneously. This prevents users from waiting for their prompt to return whenever the mail hub is busy.

The delivery mode to use is called background because it causes sendmail to accept a message and then run in the background (thus allowing the MUA to continue on). The delivery mode is set with the DeliveryMode (d) option:

O DeliveryMode=background

Note that sendmail recognizes only the b of background. So you will sometimes find this same declaration in other configuration files more succinctly expressed like this:

O DeliveryMode=b

Other possible values for the DeliveryMode (d) option are documented in Section 34.8.16.

13.2.4 The Default File Permissions

The sendmail program frequently needs to create new files (such as files in its queue). The file permissions that are given to each created file are determined by the value of the TempFileMode (F) option. That value can range from 0600 (readable and writable only by sendmail) to 0666 (readable and writable by anyone in the world). For security, we'll select the first value - the most restrictive:

O TempFileMode=0600

Note that the value must be expressed in octal notation. (See chmod(1) for details.) The TempFileMode (F) option is further documented in Section 34.8.68.

13.2.5 The Default User Identities

Again, for security, sendmail tries to avoid running as root whenever possible. When delivering failed mail to your ~/dead.letter file, for example, it runs as you. If it finds itself in a situation in which it must not be root but cannot otherwise decide on a real user's identity, sendmail assumes the identity of the user defined by the DefaultUser (u) option:

O DefaultUser=1:1

The uid under which to run (the number to the left of the colon) is here defined to be 1 (for the user daemon). The gid under which to run (the number to the right of the colon) is here defined as 1 (for the group daemon).

The values given to these options may also be names:

O DefaultUser=daemon:daemon

At security-conscious sites these are often set to the user nobody and the group nogroup. The DefaultUser (u) option is further documented in Section 34.8.15.

13.2.6 The Default Logging Level

Recall that the DeliveryMode (d) option told sendmail to run in the background. Because it is running in the background, sendmail should not print information about its activities to your screen. On the other hand, you do want to record information about its activities to help solve future problems.

The method used by sendmail to record its activities is called logging. [4] The setting of the LogLevel (L) option allows you to turn logging completely off or to specify a logging level. The higher the logging level, the more detailed the information that is logged. That is, low levels log only serious problems, middle levels also log statistical information, and high levels include debugging information.

[4] The actual mechanism used is called syslog(3) and is described in Chapter 26, Logging and Statistics.

O LogLevel=9

Here, we've chosen a level of 9. This is a middle level, which, in addition to causing serious problems to be logged, also causes statistics such as message size to be logged.

Typically, logged information is written by the system into a file called syslog by a means documented in Section 26.1, "Logging with syslog". The LogLevel (L) option is further documented in Section 34.8.33.

13.2.7 Accept Old-Style Lists of Addresses

The current standard for specifying multiple recipients is to separate each address from the others with commas. Unfortunately, this has not always been the standard; old software may still exist that separates addresses with spaces:

abe,george,andrew       new style
abe george andrew       old style

To prevent old software from breaking, you need to tell sendmail that the use of spaces is acceptable and that if it finds such old-style lists, it should replace the spaces with commas. You tell sendmail this by specifying the OldStyleHeaders (o) option:

O OldStyleHeaders=True

The value is either true (accept and convert) or false (don't accept). The True makes it true. In actual practice, only the first letter is recognized, and either T or t will work. To turn it off, use F or f (for false) or omit the entire declaration. If you omit the true or false but include the option, it defaults to true. The OldStyleHeaders (o) option is further documented in Section 34.8.44.

13.2.8 The Unquoted Space Replacement Character

Recall from Chapter 8, Addresses and Rules, that any address can be split up into tokens in the workspace. The address is then rewritten according to rules specified in rule sets. After all the tokens have been (possibly) rewritten, they are rejoined to form an address again.

The BlankSub (B) option exists for those times when two adjoining tokens are just words (rather than a word and a separating character). For example, suppose the workspace began by containing the following tokens:

a @ b . c

Then suppose some rule always changed the last two tokens into the single word LOCAL. The result of rewriting would look like this:

a @ b LOCAL

Here we have four tokens, the last two of which are text. The question becomes: What do we insert between them? Unless you tell sendmail to do otherwise, it always sticks a space between them. Therefore the default is to join these tokens together into this:

a@b LOCAL

Because we set the OldStyleHeaders (o) option above to true, this single (but odd) address wrongly becomes two:

a@b, LOCAL

To prevent this kind of mishap, we use the BlankSub (B) option to change the replacement character from a space to a dot:

O BlankSub=.                    # Replace unquoted spaces with a dot.

With this declaration in the configuration file the previous tokens are joined together like this:

a@b.LOCAL

This forms a single address, which is what is wanted. But what does the "unquoted" in the comment mean?

When parts of an address are surrounded in full quotation marks, those parts are viewed by sendmail as a single token. Therefore an address like this:

"abe lincoln"@wash.dc.gov

is tokenized like this:

"abe lincoln"   @   wash   .   dc   .   gov

When these tokens are joined back together, the quoted words abe and lincoln are viewed by sendmail as one token (with a space included), rather than two (with need of a space replacement character). [5] The BlankSub (B) option is further documented in Section 34.8.5.

[5] Actually, the address is sent as is to another site. The BlankSub (B) option at the other site causes the confusion, because the address arrives there unquoted.