57

I am unable to ssh to a server that asks for a diffie-hellman-group1-sha1 key exchange method:

ssh 123.123.123.123
Unable to negotiate with 123.123.123.123 port 22: no matching key exchange method found. Their offer: diffie-hellman-group1-sha1

How to enable the diffie-hellman-group1-sha1 key exchange method on Debian 8.0?

I have tried (as proposed here) to

  1. add the following lines to my /etc/ssh/ssh_config

    KexAlgorithms diffie-hellman-group1-sha1,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1
Ciphers 3des-cbc,blowfish-cbc,aes128-cbc,aes128-ctr,aes256-ctr

  2. regenerate keys with 

    ssh-keygen -A

  3. restart ssh with 

    service ssh restart

    but still get the error.

|improve this question
  • I have the same happening to me with Debian 9. – Rui F Ribeiro Apr 9 '17 at 14:37
  • Try this diffie-hellman-group-exchange-sha256 – Miguel Mar 22 '18 at 16:46
81

The OpenSSH website has a page dedicated to legacy issues such as this one. It suggests the following approach, on the client:

ssh -oKexAlgorithms=+diffie-hellman-group1-sha1 123.123.123.123

or more permanently, adding

Host 123.123.123.123
    KexAlgorithms +diffie-hellman-group1-sha1

to ~/.ssh/config.

This will enable the old algorithms on the client, allowing it to connect to the server.

|improve this answer
  • I also faced this problem today, but that was due to the network. I changed the network and the issue was gone – Luv33preet Jun 14 '17 at 9:59
  • Tried above, but got Unable to negotiate with 192.168.1.123 port 22222: no matching cipher found. Their offer: aes128-cbc,3des-cbc,aes256-cbc,twofish256-cbc,twofish-cbc,twofish128-cbc,blowfish-cbc – typelogic Jul 31 '18 at 22:01
  • @ifelsemonkey that’s a different problem, note that the offer you get isn’t the same as the one in the question. – Stephen Kitt Aug 1 '18 at 6:20
  • 1
    Confirmed it was a different problem. I was able to resolve it by adding the following entry into my ~/.ssh/config file. Host 192.168.1.123 and under it Ciphers aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc. – typelogic Aug 1 '18 at 17:18
16

I tried this solution, but my problem was that I had many (legacy) clients connecting to my recently upgraded server (ubuntu 14 -> ubuntu 16).

The change from openssh6 -> openssh7 disabled by default the diffie-hellman-group1-sha1 key exchange method.

After reading this and this I came up with the changes I needed to do to the /etc/ssh/sshd_config file:

#Legacy changes
KexAlgorithms +diffie-hellman-group1-sha1
Ciphers +aes128-cbc

But a more wide legacy set of changes is (taken from here)

#Legacy changes
KexAlgorithms diffie-hellman-group1-sha1,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1
Ciphers 3des-cbc,blowfish-cbc,aes128-cbc,aes128-ctr,aes256-ctr
|improve this answer
  • 4
    Hopefully you’ll be able to upgrade your clients at some point, the legacy algorithms were disabled for very good reasons and shouldn’t be re-enabled lightly (you probably realise that, I just thought it worth pointing out for other readers). – Stephen Kitt Jun 14 '17 at 11:10
  • 1
    This works on the server side (unlike the very similar, accepted answer which focused on the client-side.) – knb Sep 15 '17 at 12:50
  • i am trying to use the same to enable the old keys. but since i am a newbie i don't really know what IP to place when i write ssh -oKexAlgorithms=+diffie-hellman-group1-sha1 123.123.123.123 – Yousi Nov 27 '18 at 8:14
  • To add for future people to find, I was connecting via SSH from a Mac running OpenSSH_7.9p1 to a Cisco 3750 switch running: Cisco IOS Software, C3750 Software (C3750-IPSERVICESK9-M), Version 12.2(55)SE12, RELEASE SOFTWARE (fc2). I added the following to the client config and was able to ssh in: KexAlgorithms +diffie-hellman-group1-sha1 Ciphers +aes128-cbc – DYoung Mar 12 at 4:36

protected by Community♦ Nov 25 '18 at 20:54

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.