Even if LL:NG can catch logout URL trough 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.
Go in Manager, General parameters
» Advanced parameters
» Logout forward
and click on Add a key
, then fill: