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

Question

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.

Answer 1

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.

Answer 2

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

Answer 3

I was also facing the same issue but resolved it by executing below command. You can do it without restarting SSH server-

Problem:

ssh test@123.123.123.123
protocol identification string lack carriage return
Unable to negotiate with 123.123.123.123 port 22: no matching key exchange method found. Their offer: diffie-hellman-group1-sha1
root@shoesdekho#

Solution:

ssh -o KexAlgorithms=diffie-hellman-group1-sha1 test@123.123.123.123

On other system I have observed that using above command also it was not working. Try adding Ciphers with the command. See full command as below-

root@shoesdekho#ssh -o KexAlgorithms=diffie-hellman-group1-sha1 -o Ciphers=aes256-cbc test@123.123.123.123                    
protocol identification string lack carriage return
Warning: Permanently added '123.123.123.123' (RSA) to the list of known hosts.
Password:


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