SSH returns: no matching host key type found. Their offer: ssh-dss

Question

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 [email protected]

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?

Answer 1

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 [email protected]

---

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.

Answer 2

If you came here because Bitbucket returns the following after an update to OpenSSH 8.8:

Unable to negotiate with <ip address> port 22: no matching host key type found. Their offer: ssh-rsa,ssh-dss

you should NOT enable DSS (like in the accepted answer), but rather RSA in ~/.ssh/config:

Host bitbucket.org
    HostKeyAlgorithms +ssh-rsa
    PubkeyAcceptedKeyTypes +ssh-rsa

Reference: https://community.atlassian.com/t5/Bitbucket-articles/OpenSSH-8-8-client-incompatibility-and-workaround/ba-p/1826047

Note that PubkeyAcceptedKeyTypes is a backwards compatible alias to PubkeyAcceptedAlgorithms which has been suggested in the article. If you use it, the same configuration can be used with older OpenSSH client versions, e.g. if you share the config with docker containers.

You can do the same for other hosts, or use Host * to allow RSA for any host.

Answer 3

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 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.)

Answer 4

Create new file with *.conf extension in /etc/ssh/ssh_config.d/

sudo nano /etc/ssh/ssh_config.d/my.conf

add to new empty line:

HostKeyAlgorithms ssh-rsa,ssh-dss
PubkeyAcceptedKeyTypes ssh-rsa,ssh-dss

Answer 5

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.

Answer 6

For me this added into .ssh\config worked:

Host *
HostkeyAlgorithms +ssh-dss
PubkeyAcceptedKeyTypes +ssh-dss

Answer 7

I can't change anything on the server: I created a script called ssh in my home directory which contains:

#!/bin/bash
/usr/bin/ssh -oHostKeyAlgorithms=+ssh-rsa,ssh-dss $1

I can invoke:

./ssh username@host

as before.

Answer 8

Running this one-liner on client worked to workaround the issue:

echo -e "Host *\nHostKeyAlgorithms +ssh-rsa\nPubkeyAcceptedKeyTypes +ssh-rsa\n"|sudo tee -a ~/.ssh/config

Answer 9

The 'their offer' list of algorithms are the ones that the server will accept so the client needs to offer matching ones. As mentioned one way to fix this is add the missing algorithms to your .ssh/config file. If it still doesn't work it is worth checking which algorithms your version of ssh has to offer:

ssh -Q key

If you do not see the missing algorithm listed from this command then you will need update your ssh version.

But if you do see the missing algorithm there then it's possible that either the system /etc/ssh/ssh_config file and/or /Users/USERNAME/.ssh/config contains a HostkeyAlgorithms line that limits the number of algorithms. This can happen if the algorithms are listed without + prefix which will then lead to ssh only offering those on that list e.g. (HostkeyAlgorithms ssh-dsa) so this can be fixed by just commenting that line, or explicitly adding the specific missing algorithms. The + prefix will add one or more algorithms to the list.

Answer 10

I have a different approach since I am working with many different embedded linux systems at work. First I had a script, similar to MichaelM's answer, but I created a special alias that runs ssh with the added parameters (-oHostKeyAlgorithms=+ssh-rsa)

alias sshrsa='function __sshrsa() { PARAM=$*; [ -z "$PARAM" ] && return 1; ssh -oHostKeyAlgorithms=+ssh-rsa ${PARAM}; unset -f __sshrsa; }; __sshrsa'

For this, the alias creates a private function that is then called. This makes it possible to use alias with additional input, which isn't usually supported with alias.

This way I can invoke for example

sshrsa <user>@<host>