All of our users use Filezilla to connect to our SFTP server but one cannot seem to connect. Every time this person tries to connect the connection gets terminated right after the key exchange. I did notice something strange about the key exchange though. When he tries to connect is see that the DH Key sizes are much lower than all the other clients. Has anyone else experienced a similar issue?
In the attached picture the failed attempts are above the highlighted line and a successful from another client is below it.
It looks like the one user experiencing issues is using an almost 7 year old version of OpenSSH to connect. There is no error from Cerberus FTP Server’s perspective. The client is simply disconnecting after the key exchange. The client-side error would reveal the reason for the disconnect. I’m guessing the client doesn’t like something about the key exchange.
Upgrading to a newer OpenSSH version will almost certainly fix the problem (we haven’t had any compatibility problems reported by modern OpenSSH builds) and will result in a larger and more secure key being used during key exhange.