Even if LL:NG can catch logout URL through virtual host rules, you can have the need to forward a logout to other applications, to close their local sessions.
LL::NG has a logout forward mechanism, that will add a step in logout process, to send logout requests (indeed, GET requests on application logout URL) inside hidden iframes.
The logout request will be sent even if the user did not use the application.
Go in Manager,
General parameters »
Advanced parameters »
Logout forward and click on
Add a key, then fill:
Key: application name
Value: application logout URL
The request on logout URL will be sent after user is disconnected, so you should unprotect this URL if it is protected by an LL::NG Handler.