ACCESS CONTROL

An X server can use several types of access control. Mechanisms provided in Release 5 are:

• Host Access Simple host-based access control.
• MIT-MAGIC-COOKIE-1 Shared plain-text "cookies".
• XDM-AUTHORIZATION-1 Secure DES based private-keys.
• SUN-DES-1 Based on Sun's secure rpc system.

Xdm initializes access control for the server, and also places authorization information in a file accessible to the X Version 11.

Normally, the list of hosts from which connections are always accepted should be empty, so that only clients with are explicitly authorized can connect to the display. When you add entries to the host list (with xhost), the server no longer performs any authorization on connections from those machines. Be careful with this. The file from which Xlib extracts authorization data can be specified with the environment variable XAUTHORITY, and defaults to the file .Xauthority in the home directory.

Xdm uses $HOME/.Xauthority and will create it or merge in authorization records if it already exists when a user logs in. If you use several machines, and share a common home direc- tory across all of the machines by means of a network file system, then you never really have to worry about authoriza- tion files, the system should work correctly by default. Otherwise, as the authorization files are machine-independent, you can simply copy the files to share them.

To manage authorization files, use xauth. This program allows you to extract records and insert them into other files. Using this, you can send authorization to remote machines when you login, if the remote machine does not share a common home directory with your local machine. Note that authorization information transmitted ``in the clear'' through a network file system or using ftp or rcp can be ``stolen'' by a network eavesdropper, and as such may enable unauthorized access. In many environments this level of security is not a concern, but if it is, you need to know the exact semantics of the particular authorization data to know if this is actually a problem. For more information on access control, see the Xsecurity manual page.