SMM:08-22
Sendmail Installation and Operation Guide
listed in the .forward file. Note that aliases are fully expanded before forward files are referenced.
For example, if the home directory for user "mckusick" has a .forward file with contents:
mckusick@ernie
kirk@calder
then any mail arriving for "mckusick" will be redirected to the specified accounts.
Actually, the configuration file defines a sequence of filenames to check. By default, this is
the user's .forward file, but can be defined to be more generally using the ForwardPath option. If
you change this, you will have to inform your user base of the change; .forward is pretty well incor-
porated into the collective subconscious.
2.9. Special Header Lines
Several header lines have special interpretations defined by the configuration file. Others
have interpretations built into sendmail that cannot be changed without changing the code. These
built-ins are described here.
2.9.1. Errors-To:
If errors occur anywhere during processing, this header will cause error messages to go to
the listed addresses. This is intended for mailing lists.
The Errors-To: header was created in the bad old days when UUCP didn't understand the
distinction between an envelope and a header; this was a hack to provide what should now be
passed as the envelope sender address. It should go away. It is only used if the UseErrorsTo
option is set.
The Errors-To: header is officially deprecated and will go away in a future release.
2.9.2. Apparently-To:
RFC 822 requires at least one recipient field (To:, Cc:, or Bcc: line) in every message. If a
message comes in with no recipients listed in the message then sendmail will adjust the header
based on the "NoRecipientAction" option. One of the possible actions is to add an "Apparently-
To:" header line for any recipients it is aware of.
The Apparently-To: header is non-standard and is both deprecated and strongly discour-
aged.
2.9.3. Precedence
The Precedence: header can be used as a crude control of message priority. It tweaks the
sort order in the queue and can be configured to change the message timeout values. The prece-
dence of a message also controls how delivery status notifications (DSNs) are processed for that
message.
2.10. IDENT Protocol Support
Sendmail supports the IDENT protocol as defined in RFC 1413. Note that the RFC states a
client should wait at least 30 seconds for a response. The default Timeout.ident is 5 seconds as
many sites have adopted the practice of dropping IDENT queries. This has lead to delays process-
ing mail. Although this enhances identification of the author of an email message by doing a ``call
back'' to the originating system to include the owner of a particular TCP connection in the audit
trail it is in no sense perfect; a determined forger can easily spoof the IDENT protocol. The follow-
ing description is excerpted from RFC 1413:
6. Security Considerations