Putty on client - error couldn't agree a key exchange algorithm

Question

My issue is that I have a putty client on a windows machine, trying to ssh to a Linux server (CentOS). The putty version is very old and it is NOT possible to take a newer version.

I would like to allow to use diffie-hellman-group1-sha1, but the explanations I found here => https://unix.stackexchange.com/questions/340844/how-to-enable-diffie-hellman-group1-sha1-key-exchange-on-debian-8-0 and here => http://www.openssh.com/legacy.html don't seem to be OK for my case. Because in my case the client is old, not the server. And this is putty on windows, not ssh from a Linux.

Do you have an idea how I could allow sha1 on my Linux server please ?

Thank you,

Answer 1

For future readers coming to this question via a search engine (as I did):

This recently happened to me on a Windows machine.

I was using PuTTY 0.63 and the current version was 0.81.

I downloaded PuTTY 0.81 and unzipped it into a different folder.

I then ran my scripts, which control PuTTY and automate collecting information from a large number of switches, and started seeing this error:

"Couldn't agree a key exchange algorithm"

SOLUTION (for my case):

At one point the script was calling PuTTY from the old location - that is, it was using the older version of PuTTY. As soon as I modified that link to call the 0.81 version of PuTTY, problem solved.

POSTSCRIPT:

To be clear, I can switch back and forth between the versions with no problem - both versions continue to work independently. But there was something odd when connecting to a switch with the newer version, and then trying to re-connect to it again with the older version. Something is being cached somewhere... but sticking to the newer version resolved the problem.