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## Connecting to legacy servers with OpenSSH

Phasing out legacy cryptographic algorithms can always be an interesting endeavor as terminating to early breaks stuff and to late it can lead to a compromise. OpenSSH disabled DSA with version 7.0 in March 2015 as 5 years earlier it was discovered that DSA was compromised and labelled as insecure. Normally this shouldn’t be a problem with a normal software life cycle, but sometimes you will encounter a legacy box that will not be upgraded as it will break things. Now it will stop new connections being setup from upgraded to machines as with SSH.

$ssh user@server.example.org Unable to negotiate with server.example.org port 22: no matching host key type found. Their offer: ssh-dss  For an incidental connection from the command line the algorithm can be enabled again to connect with a legacy machine. $ ssh -o HostKeyAlgorithms=+ssh-dss user@server.example.org


For automated processed or when scripts can’t be modified a setting for OpenSSH can also be set in $HOME/.ssh/config for the account depending on this option to be set. Host server.example.org HostKeyAlgorithms=+ssh-dss  Re-enabling broken algorithms like DSA should only be done for a limited time and scope. In a lot of commercial environments these algorithms aren’t allowed to be enabled again. Also in most cases the code to run these obsolete algorithms can be removed in a later version as already is the case with SSL 3.0 and earlier for example. Categories ## 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 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


Just before the SSH-client prompts for the user’s password two interesting debug lines are shown. The first line is about the authentication methods we can use and the next line shows 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 permission denied as our client doesn’t have 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 then ask the user to type in the password instead of getting it from a password file as was allowed with the password authentication method. Allot of SSH-clients start to ignore this convention, but some enterprise environments still depend on this convention. If we add a password to the list of preferred authentication methods we see the password prompt is offered again. $ ssh -o PreferredAuthentications=keyboard-interactive,password -o PubkeyAuthentication=no user@host.example.org

 \$ ssh -L 3306:localhost:3306 remote.server 
 Host remote.server LocalForward 3306 localhost:3306