Dealing with old Cisco gear and SSH

I've spent enough time dealing with old Cisco get to know that the old outdated Ciphers and Key exchanges can be tricky to deal with. Unfortunately, we can't just run the latest and greatest in a LAB and it generally considered isolated, so we have to live with this to a certain degree even if insecure.

I'm documenting the process of how to use an SSH client (Linux) to force it to use the right KEX and cypher etc. so people (including myself) don't have to piece the solution together from different sources every single time.

First off, the answers to the command-line parameters required lie in debugging in the client application itself.

ssh -vvv $host

This spits out a lot of information, which I could not seem to be able to filter through egrep, nonetheless, key items are listed here for reference

debug2: ciphers ctos: chacha20-poly1305@openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.com
debug2: ciphers stoc: chacha20-poly1305@openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.com
debug2: KEX algorithms: diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1
debug2: host key algorithms: ssh-rsa

With that information gleaned, I was able to construct the parameters required to successfully connect to an SSH session in a lab.

ssh -oStrictHostKeyChecking=no -oKexAlgorithms=+diffie-hellman-group1-sha1,diffie-hellman-group14-sha1 -oCiphers=aes128-ctr -oHostkeyAlgorithms=+ssh-rsa $host