64

I am accustomed to using Putty on a Windows box or an OSX command line terminal to SSH into a NAS, without any configuration of the client.

Ubuntu 16.04 attempts to SSH into the NAS (via LAN):

ssh root@192.168.8.109

Unable to negotiate with 192.168.8.109 port 22: no matching host key type found. Their offer: ssh-dss
  1. Is this result / response intentional?
  2. Is there a simple correction that enables SSH access to the NAS?
115

The version of OpenSSH included in 16.04 disables ssh-dss. There's a neat page with legacy information that includes this issue: http://www.openssh.com/legacy.html

In a nutshell, you should add the option -oHostKeyAlgorithms=+ssh-dss to the SSH command:

ssh -oHostKeyAlgorithms=+ssh-dss root@192.168.8.109

You can also add a host pattern in your ~/.ssh/config so you don't have to specify the key algorithm every time:

Host nas
  HostName 192.168.8.109
  HostKeyAlgorithms=+ssh-dss

This has the added benefit that you don't need to type out the IP address. Instead, ssh will recognize the host nas and know where to connect to. Of course you can use any other name in its stead.

  • I believe that these are solutions on the Ubuntu side. Is there a simple option on the NAS side? It would be nice to understand all options and seize the opportunity to harden any security weakness. Maybe this is another question for another thread? Very nice explanation \ response – gatorback Oct 13 '16 at 12:31
  • Is it possible to set this globally? LIke wildcard IP? 0.0.0.0 doesn't work – podarok Jan 19 '17 at 14:33
  • 1
    @podarok, try Host * – brownian May 31 '17 at 9:25
8

Editing the ~/.ssh/config file is the best option. If you have a number of hosts to connect to on the same subnet you can use the following method to avoid entering each host in the file:

 Host 192.168.8.*
  HostKeyAlgorithms=+ssh-dss

This works great for me as I have a number of Brocade switches to manage and they started complaining about the Host key after I moved to Ubuntu 16.04.

  • Very nice: thank you for taking the time – gatorback Jul 7 '17 at 21:45
2

If you want to use newer OpenSSH to connect to deprecated servers:

ssh -o KexAlgorithms=diffie-hellman-group14-sha1 -oHostKeyAlgorithms=+ssh-dss my.host.com

Add -v if you want to see what's happening, and -o HostKeyAlgorithms=ssh-dss if it still doesn't work:

ssh -v -o HostKeyAlgorithms=ssh-dss -o KexAlgorithms=diffie-hellman-group14-sha1 my.host.com

You can also, of course, edit /etc/ssh/ssh_config or ~/.ssh/ssh_config, and add:

Host my.host.com *.myinsecure.net 192.168.1.* 192.168.2.*
    HostKeyAlgorithms ssh-dss
    KexAlgorithms diffie-hellman-group1-sha1    

https://forum.ctwug.za.net/t/fyi-openssh-to-access-rbs-openssh-7/6069 mentions the following fix on Mikrotik Routerboards:

/ip ssh set strong-crypto=yes

(Noting this here because this answer also comes up on web searches when looking for a similar error message.)

  • it should be -o KexAlgorithms=diffie-hellman-group1-sha1 (not 14) – dalf Sep 8 '17 at 8:12
  • That depends... $ ssh -Q kex server diffie-hellman-group1-sha1 diffie-hellman-group14-sha1 diffie-hellman-group-exchange-sha1 diffie-hellman-group-exchange-sha256 ecdh-sha2-nistp256 ecdh-sha2-nistp384 ecdh-sha2-nistp521 curve25519-sha256@libssh.org – Dagelf Sep 10 '17 at 7:43

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