Hey there,
I managed to forget the passphrase for a server (it is very rarely
rebooted). 
It it still running, the LUKS device is opened, and the filesystem
laying on top of it is mounted. 
So, I do have a fresh backup, and can keep on trying to remember the
passphrase with `cryptsetup luksAddKey /dev/mapper/foobar`
However, it would be a lot of hasle to re-install the system entirely,
as the services cannot suffer a long downtime. 
Is there a way to recover the passphrase or add a new keyslot while the
system's running, the crypto device mounted, the fs opened? 
Or do I necessarily have to jump towards the "How do I recover the
master key from a mapped LUKS container?"  FAQ entry? 
Thanks in advance for your answer,
--
mike dentifrice <fluor at poivron.org>