sendmail, 4th Edition

F=S

F=SAssume specified user-id and group-id V8.9 and later

There are three major ways in which sendmail can be run:^[298] as a set-user-id root process (that is, with the permissions of root regardless of who runs it), as a root process because it was run by root, or as an ordinary process run by an ordinary (nonprivileged) user. When sendmail is running with root privilege and when the F=S delivery agent flag is specified for a delivery agent, sendmail always invokes that delivery agent as the effective user and effective group specified by the U= delivery agent equate.^[299] If the U= delivery agent equate is unspecified or is specified as zero, it runs as the effective user root. In both instances, the real user and real group IDs remain those of the recipient.

If the F=S flag is omitted from the delivery agent, the following scenarios occur:

• If delivery is to a file, and if the set-user-id bit is set in the file’s permission bits, and if the execute-bit is not set, sendmail sets its user and group identities to those of the owner and group of the file.

• Otherwise, if the set-user-id bit is not set, or if delivery is not to a file, and if there is a controlling user (C line on page 447) for the address, sendmail sets its identity to that of the controlling user for delivery.

• Otherwise, if the user or group part of the U= delivery agent equate was missing or was 0, sendmail assumes the identity of the DefaultUser option (DefaultUser on page 1000).

• Otherwise, sendmail assumes the identity ...