System Administration Guide: Network Services


Use the VALIDATE option in conjunction with the COMMANDS option whenever you specify commands that are potentially dangerous to your machine's security. VALIDATE is merely an added level of security on top of the COMMANDS option, though it is a more secure way to open command access than ALL.

VALIDATE provides a certain degree of verification of the caller's identity by cross-checking the host name of a calling machine against the login name it uses. The following string ensures that if any machine other than widget or gadget tries to log in as Uwidget, the connection is refused.

LOGNAME=Uwidget VALIDATE=widget:gadget 

The VALIDATE option requires privileged computers to have a unique login and password for UUCP transactions. An important aspect of this validation is that the login and password that are associated with this entry are protected. If an outsider obtains that information, that particular VALIDATE option can no longer be considered secure.

Carefully consider which remote computers you are granting privileged logins and passwords for UUCP transactions. Giving a remote computer a special login and password with file access and remote execution capability is like giving anyone on that computer a normal login and password on your computer. Therefore, if you cannot trust someone on the remote computer, do not provide that computer with a privileged login and password.

The following LOGNAME entry specifies that if one of the remote computers that claims to be eagle, owl, or hawk logs in on your computer, it must have used the login uucpfriend:

LOGNAME=uucpfriend VALIDATE=eagle:owl:hawk 

If an outsider obtains the uucpfriend login and password, masquerading is easy.

But what does this entry have to do with the COMMANDS option, which appears only in MACHINE entries? This entry links the MACHINE entry (and COMMANDS option) with a LOGNAME entry that is associated with a privileged login. This link is needed because the execution daemon is not running while the remote computer is logged in. Actually, the link is an asynchronous process that does not know which computer sent the execution request. Therefore, the real question is: How does your computer know where the execution files came from?

Each remote computer has its own spool directory on your local machine. These spool directories have write permission that is given only to the UUCP programs. The execution files from the remote computer are put in its spool directory after being transferred to your computer. When the uuxqt daemon runs, it can use the spool directory name to find the MACHINE entry in the Permissions file and get the COMMANDS list. Or, if the computer name does not appear in the Permissions file, the default list is used.

This example shows the relationship between the MACHINE and LOGNAME entries:

MACHINE=eagle:owl:hawk REQUEST=yes \ 
COMMANDS=rmail:/usr/local/rnews \ 
LOGNAME=uucpz VALIDATE=eagle:owl:hawk \ 

The value in the COMMANDS option means that remote users can execute rmail and /usr/local/rnews.

In the first entry, you must assume that when you want to call one of the computers that is listed, you are really calling either eagle, owl, or hawk. Therefore, any files that are put into one of the eagle, owl, or hawk spool directories is put there by one of those computers. If a remote computer logs in and says that it is one of these three computers, its execution files are also put in the privileged spool directory. You therefore have to validate that the computer has the privileged login uucpz.