Sep 142015

If you run ancient operating system with an old version of SSH client then you are going to hit this “No Kex Alg” problem soon.

have u tried

For example Solaris 9

$ ssh -Vxx
Sun_SSH_1.1, SSH protocols 1.5/2.0, OpenSSL 0x0090700f

So what a hell is it? What’s causing it? Well, modern operating system like Debian Jessie are packaged with OpenSSH 6.7 or newer  – and Openssh 6.7 disables a number of ciphers, as per changelog  As Russel rightly pointed out in comments section below ‘”kex” is “key exchange”.x

So it’s time to upgrade your client! However, if for some bizarre reasons those pesky sysadmins are refusing to upgrade client software then that leaves you with two options:

  • if you have physical access to client simply spill coffee or some other beverage on it (alright, just joking)
  • or edit /etc/ssh/sshd_config on the server, append the following line and restart sshd daemon
KexAlgorithms diffie-hellman-group1-sha1,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp512,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1

Now your old client should be able to connect to server plus you have successfully created security vulnerability on your machine. How exciting!

If you’re still dying to know what mechanisms your system supports run:

ssh -Q cipher
ssh -Q mac
ssh -Q kex

I know more about ssh ciphers, macs, kex now that I ever wanted to know.

  7 Responses to “no kex alg”

  1. Ha ha… yet I laugh with real joy.. since I have *finally* jailbroken my iPad and have just logged into it from one of my Linux boxes …! I got the Cydia stuff running on the iPad, and first install was OpenSSH package, but coming from an old Windows-XP box, with “ssh”, all I could get was this goofy “no kex alg” message.. – hmmm now to decode the linux-crypto-msg…? “no” is easy, probably means “no, you dumbguy!”… “kex” sounded like an ancient breakfast cereal, but “alg” – now that’s gotta be short for “algorithm” (finance hackers use “algo”, so pretty close, eh? .. ) but “kex” had me stumped. Now what could that be??
    Of course, my SSH client on the winbox is ancient (and the iPad is Gen-0, from the days when Steve Jobs was still on earth), so probably it was an unsupported algo which my ancient winbox ssh used, but now is compromised and easy to hack? Am I getting a bit warm here?
    Anyway, your webpage was really helpful. Thanx for taking the time to publish it.
    I suppose a modern version of PuTTY might work? What I find really impressive, is that I can login to my old iPad version 1, and from a terminal session window on a CentOS Linux box, enter “ls -l –col”, and get a directory list with colour codings…
    … Ok, I just tried it.
    My Winbox “ssh” returns “no kex alg”, but old PuTTY with Blowfish and triple DES seems to work.
    THere. I can login to the *unix O/S running on my iPad, from my Windows-XP session, using an old version of PuTTY.
    Just one dumb question: What the heck does “Kex” mean?
    (I suppose I will have to break down, and actually look at the OpenSSH release notes, eh? )
    – Rus

  2. Of course.. ‘”kex” is “key exchange”.

  3. Thank you!

  4. Thank you, I started to get this message after upgrading from debian jessie to debian stretch. De client being an old solaris machine. This article saved me from a lot of trouble…

  5. Very useful article, thank you. I’ve a really ancient server – as in hardware dates from the 1990s and OS is a version of SuSE Linux from about 2005 – in the basement that holds my mp3 music collection. Internal network only and one of those things that “just works” so had never upgraded or replaced it. But now am replacing it with an all-new Ubuntu 20.04 / ZFS rig, just need to rsync the files across – and got the no kex alg error. Added the KexAlgorithms line and all now happily transferring. Line will be removed from new system, and old server decommissioned permanently, once all needed data is across. Appreciated the info being available!

  6. Did you mean …nistp512 instead of nistp521? At any rate, thanks for the post, it helped to me to connect from an old client to a new Ubuntu 20.04

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