Using explicit SSH authentication methods

For many SSH is a magic sauce to get access to a server and to transfer files between servers. But when things go wrong this magic sauce becomes a problem. Let start with one an example when things go wrong and how to debug it. First we start to add to option -v to our command to connect to another server to get some basic debug information about the SSH handshake and getting to the point the user has to authenticate.

$ ssh -v user@host.example.org
...
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,password
debug1: Next authentication method: password
user@host.example.org's password:

Just before the SSH-client prompts for the users password two interesting debug lines are shown. The first line is about the authentication methods we can use and next line shows the our client selected method password as we don’t have any methods configured in our SSH-client like publickey. So we manually disable publickey authentication and set the preferred authentication methods to keyboard-interactive.

$ ssh -v -o PreferredAuthentications=keyboard-interactive -o PubkeyAuthentication=no user@host.example.org
...
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,password
debug1: No more authentication methods to try.
Permission denied (publickey,gssapi-keyex,gssapi-with-mic,password).

We now get a permission denied as our client doesn’t has a matching set of authentication methods. Over a decade ago some commercial SSH-servers would require keyboard-interactive as authentication method as the client must than ask the user to type in the password instead of getting it from a password file as was allowed with the password authentication method. Al lot of SSH-clients start to ignore this convention, but some enterprise environments still depend on this convention. If we add password to the list of preferred authentication method we see the password prompt is offered again.

$ ssh -o PreferredAuthentications=keyboard-interactive,password -o PubkeyAuthentication=no user@host.example.org
user@host.example.org's password:

This method can also be used to temporarily disable public key authentication without changing any SSH configuration to test of the account is still working correctly or the password of the target account is still working.

Port Forwarding met SSH

Uit veiligheid wil je soms niet elk protocol zomaar over Internet transporteren of verkeer door firewalls te krijgen. Zo ook met MySQL bijvoorbeeld, want packetfilters en SSL-verbindingen zijn ook niet echt ideaal om protocollen te beperken of te beveiligen. Gelukkig levert SSH een mooie tijdelijke oplossing in de vorm van portforwarding. Met het volgende commando wordt al het verkeer naar de lokale poort 3306 aan de andere kant van de tunnel verzonden naar localhost op port 3306.

$ ssh -L 3306:localhost:3306 remote.server

Als dit vaker moet plaats vinden dan is het onderstaande op te nemen in ~/.ssh/config, waarna de tunnel wordt opgezet op het moment dat je verbinding maakt met de remote.server.

Host remote.server
LocalForward 3306 localhost:3306

Het is belangrijk om te beseffen dat die tunnel niet exclusief voor jouw is. Als er andere gebruikers zijn op het systeem van waar je verbinding maakt dan kan dit een security probleem zijn.