If you get the following error while trying to connect with SSH...
"Unable to negotiate with xx.xxx.xx.xx port 22: no matching key exchange method
found. Their offer: diffie-hellman-group-exchange-sha1,diffie-hellman-group1-sha"
That error is due to the fact that diffie-helllman is an older less secure connection method.
Apple has removed it from the MacOS starting with MacOS Mojave.
If the server you are trying to connect to cannot upgrade their key exchange method for some reason, You can add diffie-hellman to the MacOS.
Here's how to do that:
Launch the terminal app and enter the following command.
sudo nano /etc/ssh/ssh_config
Locate the line ‘ # MACs hmac-md5,hmac-sha1,hmac-sha2-256,umac-64@openssh.com,hmac-ripemd160′ and remove the Hash/Pound sign from the beginning, and add the extra hashing algorithm that I’ve shown above in red.
Locate the line ‘ # Ciphers aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc’ and remove the Hash/Pound sign from the beginning.
Then paste the following on the end;
HostkeyAlgorithms ssh-dss,ssh-rsa
KexAlgorithms +diffie-hellman-group1-sha1
The Modified text should look like this:
Ciphers aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc
HostkeyAlgorithms ssh-dss,ssh-rsa
KexAlgorithms +diffie-hellman-group1-sha1
MACs hmac-md5,hmac-sha1,hmac-sha2-256,umac-64@openssh.com
Now you should be able to connect with SSH
