Be Paranoid-- Call Sequence Checks

Another way to fend off and detect impostors is to use call sequence checks. Call sequence checks help you protect against intruders that somehow managed to find out the password you log into your UUCP system with.

When using call sequence checks, both machines keep track of the number of connections established so far. It is incremented with each connection. After logging in, the caller sends its call sequence number, and the callee checks it against its own number. If they don't match, the connection attempt will be rejected. If the initial number is chosen at random, attackers will have a hard time guessing the correct call sequence number.

But call sequence checks do more for you than this: even if some very clever person should detect your call sequence number as well as your password, you will find this out. When the attacker call your UUCP feed and steals your mail, this will increase the feeds call sequence number by one. The next time you call your feed and try to log in, the remote uucico will refuse you, because the numbers don't match anymore!

If you have enabled call sequence checks, you should check your log files regularly for error messages that hint at possible attacks. If your system rejects the call sequence number the calling system offers it, uucico will put a message into the log file saying something like ``Out of sequence call rejected''. If your system is rejected by its feed because the sequence numbers are out of sync, it will put a message in the log file saying ``Handshake failed (RBADSEQ)''.

To enable call sequence checks, you have to add following command to the system entry:

Beside this, you have to create the file containing the sequence number itself. Taylor UUCP keeps the sequence number is in a file called .Sequence in the remote site's spool directory. It must be owned by uucp, and must be mode 600 (i.e. readable and writable only by uucp). It is best to initialize this file with an arbitrary, agreed-upon start value. Otherwise, an attacker might manage to guess the number by trying out all values smaller than, say, 60.

Of course, the remote site has to enable call sequence checks as well, and start by using exactly the same sequence number as you.