I needed to power on and turn on a server that had been off for 8 years to check for some missing data. Of course, security standards have advanced (and that server probably wasn’t up to date when turned off), and SSH from Ubuntu 20.04 LTS would no longer connect to the old server.

Step 1: Diagnose

Turn on verbose mode.

ssh -v 192.168.0.100

That shows:

Unable to negotiate with 192.168.0.100 port 22: no matching key exchange method found. Their offer: gss-group1-sha1-toWM5Slw5Ew8Mqkay+al2g==,diffie-hellman-group-exchange-sha1,diffie-hellman-group1-sha1

Step 2: Solution

ssh -o KexAlgorithms=diffie-hellman-group-exchange-sha1 -v 192.168.0.100

I got my files, and now I’m documenting this in case it comes up again someday.