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

debian ssh openssh key-authentication ssh-agent

804,248

Solution 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.

Solution 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,[email protected],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

Solution 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 [email protected]
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 [email protected]

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 [email protected]                    
protocol identification string lack carriage return
Warning: Permanently added '123.123.123.123' (RSA) to the list of known hosts.
Password:


This computer system is restricted to authorized users.
 Unauthorized access attempts will be prosecuted.
 If unauthorized, disconnect now.

bookmiday#

804,248

j1088099.mvrht.com.

Updated on September 18, 2022

Comments

j1088099.mvrht.com. over 1 year
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,[email protected],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.
- Admin over 3 years
  
  In 2020 and Ubuntu 20 I have solved this problem by installing putty, simply: apt install putty then used putty to connect to old server...
- Admin over 3 years
  
  I got Unable to negotiate with 192.168.1.1 port 22: no matching key exchange method found. Their offer: curve25519-sha256,[email protected],ecdh-sha2-nis‌tp521,ecdh-sha2-nist‌p384,ecdh-sha2-nistp‌256,[email protected]‌cc.asn.au so I added the first offer into config and it worked
- Admin almost 3 years
  
  Why would you restart your OpenSSH server after tinkering with the client-side config? Admittedly it's the system-wide client-side config, but that still doesn't make it relevant for the server.
Luv33preet almost 7 years

I also faced this problem today, but that was due to the network. I changed the network and the issue was gone
knb over 6 years

This works on the server side (unlike the very similar, accepted answer which focused on the client-side.)
eigenfield almost 6 years

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
Stephen Kitt almost 6 years

@ifelsemonkey that’s a different problem, note that the offer you get isn’t the same as the one in the question.
eigenfield almost 6 years

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.
DYoung about 5 years

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
raven about 4 years

-oKexAlgorithms resulted in the same error for me(alghough I'm using git not ssh). but the ~/.ssh/config way worked.
gaoithe over 3 years

Add -oCiphers=aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-c‌bc also on the ssh command-line will make it work if you get the no matching cipher message.
Hashman over 3 years

For many "internal" servers (where the whole network is locked down to trusted users) diffie-hellman is perfectly sufficient. Disabling it breaks a lot of legacy tools for no good reason, requiring expensive development effort to modify the legacy code that no one has looked at in ages. So having a way to enable it server-side is still valuable.
basZero over 3 years

I have the issue that I can't connect to my new Ubuntu 20 server anymore from my iTerminal App on my iPhone. How do I solve this?
Stephen Kitt over 3 years

@basZero you’re trying to connect to a server requiring new algorithms with a client which doesn’t support them, which is the opposite scenario to that addressed in my answer; see arod’s answer which explains how to configure your server to allow such connections.
Aaron Franke over 3 years

Is there a way to enable this algorithm globally and permanently, so that I don't need to write this in the command every time?
Stephen Kitt over 3 years

@Aaron see the middle of the answer, “or more permanently, adding ... to ~/.ssh/config.” You can specify Host *.
ali about 3 years

Thank you - your second solution (with the addition of the Ciphers) fixed my problem - I was trying unsuccessfully to ssh to an ancient Mac OS X 10.4 (Tiger) G5 system and this actually worked.
Bob Stein about 3 years

rsync -e 'ssh -oKexAlgorithms=+diffie-hellman-group1-sha1' ... for anyone who gets this "Unable to negotiate" error with rsync.
Samir almost 3 years

Adding KexAlgorithms +diffie-hellman-group1-sha1 and the host/server IP on the client side resolved the issue for me.
Arrow_Raider about 2 years

Just to put some justification out there: my Cisco devices with SFTP clients use SHA1 and I can't change that. So I needed a server that allows it. I am getting this error: /etc/ssh/sshd_config line 25: Bad SSH2 cipher spec '3des-cbc,blowfish-cbc,aes128-cbc,aes128-ctr,aes256-ctr'