Saturday, August 30, 2014

Hm, kex protocol error: type 30 seq 1 [preauth]

When tried to SSH Fedora server 23 soon after the installation, discovered that it's not working & clue less about the problem, decided to look into the "sshd" service status, without much breakthrough on the error messages decided to perform a packet capture.

The server was throwing the error message "Hm, kex protocol error: type 30 seq 1 [preauth]"

Packet capture of a failed SSH connection

First 3 packets show that handshake is successfully completed. Packet 4, 5, 6 & 7 show the Server & Client has agreed on a protocol version. It explains from packet 9 to 13 the client & server has initiated the key exchange.

On 14 packet the client starts the key exchange with "Diffie-Hellman Group Exchange Request" the server sends a "ack" and the connection gets stuck.

To fix the issue open PuTTY. In PuTTY, go to “Connection → SSH → Kex”. There, select “Diffie-Hellman group exchange” and move it to the bottom of the list, so it is not used. 

The connection should start working & Below is the packet capture for a successful connection.

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