sendmail, 4th Edition

C line

Set controlling user V5.62 and later

To ensure secure handling of delivery, recipient addresses that are either a file or a program require that sendmail perform delivery as the owner of the file or program rather than as the user defined by the DefaultUser option (DefaultUser on page 1000). A file address is one that begins with a / character. A program address is one that begins with a | character. Both characters are detected after quotation marks have been stripped from the address.

To prevent potential security violations, sendmail must take special precautions when addresses in the qf file result from reading a ~/.forward or :include: file. When such an address is to be placed into the qf file (whether as a recipient’s address in an R line or as an error recipient’s address in an E line), sendmail first places a C line (for Controlling user) into the file and then the recipient’s address. The C line specifies the owner of the ~/.forward or :include: file:

Cgeorge
RPF:/u/users/george/mail/archive
Cben
RPF:|/u/users/ben/bin/mailfilter

Here, when sendmail later delivers to the recipients in this qf file, it first converts its user identity to that of the user george, and then resets itself back to being root again. The same process repeats with the next recipient, except that sendmail changes from root to ben and back again. If there is no C line preceding an R line, the previous C line’s value is carried down:

Cgeorge RPF:/u/users/george/mail/archive RPF:|/u/users/ben/bin/mailfilter ...