Ssh-rsa key algorithm not accepted in Rocky 9.4

Rocky Linux Tech Help
1

Hi Team,

I have recently upgraded one of my server from Centos7.9 to Rocky 9.4.

After upgrading the server, I get the below error if I try to ssh to the server from another CentOS 7.9 server.

Unable to negotiate with xxx.xxx.xxx.xx port 22: no matching host key type found. Their offer: ssh-rsa,ssh-dss

Tried below solution, but nothing found to be working.

  1. PubkeyAcceptedAlgorithms=+ssh-rsa (add in sshd_config)
  2. Tried SSH using below command;

ssh -vvv -o HostKeyAlgorithms=+ssh-rsa -o PubkeyAcceptedKeyTypes=+ssh-rsa root@xxx.xxx.xxx.xx

Any suggestion how to make Rocky9 accept ssh-rsa key algorithm?

Thanks,
Scott

2

If you look at sshd.service in Rocky 9 – with:

cat /usr/lib/systemd/system/sshd.service

you should see that the service wants sshd-keygen.target and that in turn:

Wants=sshd-keygen@rsa.service
Wants=sshd-keygen@ecdsa.service
Wants=sshd-keygen@ed25519.service

In other words, the Rocky 9 system should have RSA, ECDSA, and ED25519 host keys, or else the sshd would refuse to start.
Furthermore, running sudo sshd -T | grep hostkey on the Rocky 9 should show plenty – but not ssh-dsa nor ssh-rsa.

If you have default ‘sshd_config’ in el9, then you have only three (actual) lines in it:

$ sudo grep "^[^#]" /etc/ssh/sshd_config
Include /etc/ssh/sshd_config.d/*.conf
AuthorizedKeysFile	.ssh/authorized_keys
Subsystem	sftp	/usr/libexec/openssh/sftp-server

The first reads additional files (in alphabetical order). One of them does:
Include /etc/crypto-policies/back-ends/opensshserver.config
that file sets the ‘PubkeyAcceptedAlgorithms’.
Does that include “ssh-rsa”? If not, does sshd -T show that your addition
takes effect? (SSH uses/prefers first occurrence.)

The algorithms can be adjusted with Chapter 3. Using system-wide cryptographic policies | Red Hat Product Documentation
i.e. update-crypto-policies --set DEFAULT:SHA1

However, I can right now ssh from el7 to el9, where the el9 has policy DEFAULT. The ssh client on the el7 did store ECDSA hostkey of el9 to knownhosts.

3

One more thing:
I do hope that by “upgrade” you do mean fresh install of Rocky 9, and not any dark magick.

4

One more thing to consider would be the key length of the RSA pubkey. I have seen servers reject if it wasn’t 2048 or higher. Perhaps it’s an old generated ssh-key that is too short?

5

I missed conectiva for its distro upgrade - and downgrade - versatility, just with apt4rpm (another best-of-breed over DidNotFinish). Wow, did that work a treat.

6

That is quite true. Alas, the error message would be different:

[me@el9 ~]$ ssh old-server-with-short-rsa-key
Bad server host key: Invalid key length

[me@el9 ~]$ ssh -o RSAMinSize=1024 old-server-with-short-rsa-key
[me@old-server-with-short-rsa-key ~]$
7

Are you sure you have an RSA key and that’s what you’re actually sending? From the server message, it clearly says it accepts RSA. Why don’t you just generate a new RSA key with ssh-keygen -t rsa and try that? On some systems, it defaults to Ed25519 so you need to explicitly set it to RSA.

What is the output of ssh -v when you connect?

Just in case you’re actually using Ed25519, you should know it won’t work if the OS is running in FIPS mode.