ssh error: unable to negotiate with IP: no matching cipher found

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Mattias Geniar, September 24, 2018

Follow me on Twitter as @mattiasgeniar

I have a pretty old Synology NAS at home that I use for some basic file storage. I also abuse it as a cron-server, doing some simple rsync’s from remote systems. When I last tried to SSH into it, I was greeted with this error.

$ ssh admin@nas.home
Unable to negotiate with 192.168.126.100 port 22: no matching cipher found.
  Their offer: aes128-cbc,3des-cbc,aes192-cbc,aes256-cbc

Turns out my clients’ SSH was updated and was blocking several insecure ciphers by default. And this Synology runs an ancient SSH daemon, that only supports those ancient outdated ciphers.

To check which ciphers your client supports, run this:

$ ssh -Q cipher
3des-cbc
aes128-cbc
aes192-cbc
aes256-cbc
rijndael-cbc@lysator.liu.se
aes128-ctr
aes192-ctr
aes256-ctr
aes128-gcm@openssh.com
aes256-gcm@openssh.com
chacha20-poly1305@openssh.com

In this list are several ciphers that are supported by my ancient SSH server as well as the client, they’re just blocked by default on the client. Things like 3des-cbc, aes128-cbc, aes256-cbc, … etc.

To enable those ciphers anyway, you can force their use with the -c parameter.

$ ssh -c aes256-cbc admin@nas.home

This will re-allow access to that ancient SSH server.

Now to find a way to upgrade the SSH daemon on that Synology without breaking it …

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