Recently I encountered the following issue when trying to login to a Linux server running Redhat Enterprise Linux 9.

Issue description

I cannot log in via SSH to a Linux server running Redhat Enterprise Linux 9 (Rhel9).

pgulian@Petrus-MacBook-Pro ~ % ssh petru@172.16.10.102     
petru@172.16.10.102's password: 
Permission denied, please try again.
petru@172.16.10.102's password: 
Permission denied, please try again.
petru@172.16.10.102's password: 
petru@172.16.10.102: Permission denied (publickey,gssapi-keyex,gssapi-with-mic,password).
pgulian@Petrus-MacBook-Pro ~ %

Trobleshooting

• I checked the SSH configuration. I compared it with another server which is running Rocky Linux 9. Both servers have the same SSH configuration. I am able to log in via the SSH on Rocky Linux, but not on Rhel9. I used post servers in the past and was able to login to both of them via SSH.

• I checked the logs on the Redhat Enterprise Linux 9 (Rhel9) and I found some clues there.

Aug  2 14:05:52 rhel92 sshd[925]: main: sshd: ssh-rsa algorithm is disabled

I did some research and found this KB from Redhat: https://access.redhat.com/solutions/6966079

Root Cause

I encountered the issue because SHA1 is disabled in Rhel 9.

Solution

1. To fix the issue you will need to enable the SHA1.

[sudo] password for petru: 
Setting system policy to DEFAULT:SHA1
Note: System-wide crypto policies are applied on application start-up.
It is recommended to restart the system for the change of policies
to fully take place.
[petru@rhel92 ~]$

2. Reboot your system.

[petru@rhel92 ~]$ shutdown -r now

3. Confirm that SHA1 was enabled.

[petru@rhel92 ~]$ sudo update-crypto-policies --show
[sudo] password for petru: 
DEFAULT:SHA1
[petru@rhel92 ~]$ 

4. Check if you are able to connect to the system via SSH.

[petru@rocky9 ~]$ ssh petru@172.16.10.102
petru@172.16.10.102's password: 
Activate the web console with: systemctl enable --now cockpit.socket

Register this system with Red Hat Insights: insights-client --register
Create an account or view all your systems at https://red.ht/insights-dashboard
Last login: Fri Aug  2 14:31:40 2024 from 172.16.10.121
[petru@rhel92 ~]$