Cannot connect: unexpected message type 30 (expected one of [34])

Some clients cannot connect to your Syncplify Server! (v6 and later), and when you look at the log of the virtual site's worker process you find this message:

ssh: unexpected message type 30 (expected one of [34])

Why it happens

Message type 30 is the group exchange request that predates RFC 4419, and support for it was removed from OpenSSH 6.1 and later. To prevent potential vulnerabilities, and in the ongoing effort to keep our customers and users as safe as possible, we removed it from Syncplify Server! too. When a client still sends that message during the Diffie-Hellman group exchange, the server logs the error above and aborts the key exchange.

The fix

The fix is on the client: update or reconfigure the client software so that it sends the current message type 34 to request a diffie-hellman-group-exchange key exchange algorithm. Do not try to work around it on the server; the old message has no place in a modern key exchange.

If the client has been updated and still cannot connect, look at the other causes in Connection reset by peer.