thanks fore respond
sshd_confg works fine without any problem
im sure that openssh reads the /etc/ssh/ssh_config fore client configuration but I can't find out why it doesn't work
alieblice@de-10:~$ ssh -v firstname.lastname@example.org
OpenSSH_5.3p1 Debian-3ubuntu6, OpenSSL 0.9.8k 25 Mar 2009
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Applying options for *
debug1: Connecting to 172.16.49.21 [172.16.49.21] port 22.
debug1: Connection established.