sendmail, 4th Edition

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 such as 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, such as 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.