Editing X display manager (section)

==X Display Manager Control Protocol==
{{anchor|XDMCP}}
The X Display Manager Control Protocol (XDMCP)  uses [[User Datagram Protocol|UDP]] [[TCP and UDP port|port]] 177. An X server requests that a display manager start a session by sending a <code>Query</code> packet. If the display manager allows access for that X server, it responds by sending a <code>Willing</code> packet back to the X server. (The X server can also send <code>BroadcastQuery</code> or <code>IndirectQuery</code> packets to start a session - this mechanism for requesting a session resembles using [[DHCP]] to request an IP address.)

The display manager must authenticate itself to the server. To do this the X server sends a <code>Request</code> packet to the display manager, which returns an <code>Accept</code> packet. If the <code>Accept</code> packet contains the response the X server expects, the display manager is authenticated. Producing the correct response might require the display manager to have access to a [[secret key]], for example. If authentication succeeds, the X server sends a <code>Manage</code> packet to inform the display manager. Then the display manager displays its login screen by connecting to the X server as a regular X client.

During the session, the server can send <code>KeepAlive</code> packets to the display manager at intervals. If the display manager fails to respond with an <code>Alive</code> packet within a certain time, the X server presumes that the display manager has ceased running, and can terminate the connection.
<!-- missing: 1. when is the connection terminated 2. more on indirect and broadcast -->
=== Security ===
One problem with XDMCP is that, similarly to [[telnet]], the authentication takes place unencrypted. If snooping is possible, this leaves the system vulnerable to attack. It is more secure to use an [[Secure Shell|ssh]] [[tunneling protocol|tunnel]] for X traffic.<ref>{{Cite web|url=http://www.gnome.org/projects/gdm/docs/2.14/security.html|title=Apps - GNOME Wiki!}}</ref>