xauth

xauth is a command-line utility used with the X Window System to view and modify the X authority file, which stores credentials that govern access to an X server. The authorization data is typically in the form of cookies, most commonly MIT-MAGIC-COOKIE-1, which a client must present to connect to a running X server. By managing these cookies, xauth controls which clients can create or display X sessions.

The default authority file is ~/.Xauthority, though xauth can operate on other files with the -f option. Typical operations include listing cookies (xauth list), adding or merging cookies (xauth add, xauth merge), removing entries (xauth remove), and generating new cookies for a host (xauth generate). Cookies are stored as a pairing of a display name and a hexadecimal cookie value; outputs can be displayed or used to feed other xauth commands (xauth nlist).

X11 forwarding in SSH relies on xauth to transfer the remote host's authority cookie to the local X server, enabling secure remote graphical sessions. Because the authority data is sensitive, proper file permissions and careful handling are important. Xauth is part of the X.Org/XFree86 utilities and is typically provided by the xauth package in Unix-like systems.