SSH won't Work - expecting SSH2_MSG_KEX_DH_GEX_GROUP

Short Post for a little-big Problem.

If your firewall is open for ssh, telnet on ssh port works but ssh won't work.

Try to debug the ssh connection by using verbose debug

ssh -v user@yourip

if ssh hangs on

"expecting SSH2_MSG_KEX_DH_GEX_GROUP"

and then ssh returns with the following error: "Read from socket failed: Operation timed out"

Probably you have an MTU/fragmentation problem and you will solve the problem by set correctly a new mtu value for network interface.

So launch by terminal the following command:

sudo ifconfig en1 mtu 576

Where en1 is your active network interface.