OpenSSH Legacy Options

Owned by Jermaine Alls
Last updated: Oct 23, 2017
2 min read

Source: https://www.openssh.com/legacy.html

OpenSSH implements all of the cryptographic algorithms needed for compatibility with standards-compliant SSH implementations, but since some of the older algorithms have been found to be weak, not all of them are enabled by default. This page describes what to do when OpenSSH refuses to connect with an implementation that only supports legacy algorithms.

When a SSH client connects to a server, each side offers lists of connection parameters to the other. These are, with the corresponding ssh_config keyword:

  • KexAlgorithms: the key exchange methods that are used to generate per-connection keys
  • Ciphers: the ciphers to encrypt the connection
  • MACs: the message authentication codes used to detect traffic modification
  • PubkeyAcceptedKeyTypes: the public key algorithms that the server can use to authenticate itself to the client

For a successful connection, there must be at least one mutually-supported choice for each parameter.

If the client and server are unable to agree on a mutual set of parameters then the connection will fail. OpenSSH (7.0 and greater) will produce an error message like this:

Unable to negotiate with legacyhost: no matching key exchange method found. 
Their offer: diffie-hellman-group1-sha1


In this case, the client and server were unable to agree on the key exchange algorithm. The server offered only a single method diffie-hellman-group1-sha1. OpenSSH supports this method, but does not enable it by default because is weak and within theoretical range of the so-called Logjam attack.

The best resolution for these failures is to upgrade the software at the other end. OpenSSH only disables algorithms that we actively recommend against using because they are known to be weak. In some cases, this might not be immediately possible so you may need to temporarily re-enable the weak algorithms to retain access.

For the case of the above error message, OpenSSH can be configured to enable the diffie-hellman-group1-sha1 key exchange algorithm (or any other that is disabled by default) using the KexAlgorithms option - either on the command-line:

ssh -oKexAlgorithms=+diffie-hellman-group1-sha1 user@legacyhost