SSH Legacy Key Exchange

SSH Legacy Key Exchange Algorithm1 #

When an SSH client connects to a server, each side offers sets of connection parameters to the other. For a successful connection, there must be at least one mutually compatible set for each parameter.
If the client and the server cannot agree on a mutual set, in this case, the key exchange algorithm, the connection will fail and OpenSSH will return an error message like this:

Unable to negotiate with port 22: no matching key exchange method found. Their offer: diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1

The server offered diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1. OpenSSH supports these methods but does not enable it by default because is weak and within the theoretical range of the so-called Logjam attack.

More about the Logjam attack in Imperfect Forward Secrecy:How Diffie-Hellman Fails in Practice.

Solution #

If upgrading is not immediately possible you can re-enable the algorithms either on the command-line:

ssh -oKexAlgorithms=+diffie-hellman-group1-sha1 root@

or in the ~/.ssh/config file:

    KexAlgorithms +diffie-hellman-group1-sha1

  1. “OpenSSH: Legacy Options.” OpenSSH,↩︎