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:
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
or in the ~/.ssh/config file:
Host somehost.example.org KexAlgorithms +diffie-hellman-group1-sha1
The '+' before the list instructs ssh to append the algorithm to the client's default set rather than replacing the default. By appending, you will automatically upgrade to the best supported algorithm when the server starts supporting it.
Another example, this time where the client and server fail to agree on a public key algorithm for host authentication:
Unable to negotiate with legacyhost: no matching host key type found. Their offer: ssh-dss
OpenSSH 7.0 and greater similarly disable the ssh-dss (DSA) public key algorithm. It too is weak and we recommend against its use. It can be re-enabled using the HostKeyAlgorithms configuration option:
ssh -oHostKeyAlgorithms=+ssh-dss user@legacyhost
or in the ~/.ssh/config file:
Host somehost.example.org HostKeyAlgorithms +ssh-dss
Depending on the server configuration, it's possible for other connection parameters to fail to negotiate. You might find the Ciphers and/or MACs configuration options useful for enabling these. It's also possible to query which algorithms ssh supports:
ssh -Q cipher # List supported ciphers ssh -Q mac # List supported MACs ssh -Q key # List supported public key types ssh -Q kex # List supported key exchange algorithms
Finally, it's also possible to query the configuration that ssh is actually using when attempting to connect to a specific host, by using the -G option:
ssh -G firstname.lastname@example.org
which will list all the configuration options, including the chosen values for the Ciphers, MACs, HostKeyAlgorithms and KexAlgorithms parameters.