[donotprint][/donotprint]To regenerate keys you need to delete old files and reconfigure openssh-server. It is also safe to run following commands over remote ssh based session. Your existing session shouldn’t be interrupted.
Why regenerate new ssh server keys?
Most Linux and Unix distribution create ssh keys for you during the installation of the OpenSSH server package. But it may be useful to be able re-generate new server keys from time to time. For example, when you duplicate VM (KVM or container) which contains an installed ssh package and you need to use different keys from cloned KVM VM guest/machine.
Steps to regenerate OpenSSH host keys on Linux
Let us see all steps
Step 1 – Delete old ssh host keys
Login as the root and type the following command to delete files on your SSHD server:
# /bin/rm -v /etc/ssh/ssh_host_*
Sample outputs:
removed '/etc/ssh/ssh_host_dsa_key' removed '/etc/ssh/ssh_host_dsa_key.pub' removed '/etc/ssh/ssh_host_ecdsa_key' removed '/etc/ssh/ssh_host_ecdsa_key.pub' removed '/etc/ssh/ssh_host_ed25519_key' removed '/etc/ssh/ssh_host_ed25519_key.pub' removed '/etc/ssh/ssh_host_rsa_key' removed '/etc/ssh/ssh_host_rsa_key.pub'
Step 2 – Debian or Ubuntu Linux Regenerate OpenSSH Host Keys
Now create a new set of keys on your SSHD server, enter:
# dpkg-reconfigure openssh-server
Sample output:
Creating SSH2 RSA key; this may take some time ... 2048 SHA256:BLUkgjGdbcFX9wCsfOoIG4gtkdSeex4K/xcnsRo0qEA root@ubuntu-box1-clone (RSA) Creating SSH2 DSA key; this may take some time ... 1024 SHA256:Ug9fJa14YMR9Fud/7bXTokffK/hM/sBVse10nSR/6Y8 root@ubuntu-box1-clone (DSA) Creating SSH2 ECDSA key; this may take some time ... 256 SHA256:Rh6izWEXkCV6HZLIpzlGQje178vhDgb77ItaZgpDsIQ root@ubuntu-box1-clone (ECDSA) Creating SSH2 ED25519 key; this may take some time ... 256 SHA256:UD4b7njwxWp1Q3wYf2R//udgPRzfGaeZ/6kE3VgZM+s root@ubuntu-box1-clone (ED25519)
You just regenerated new ssh server keys. You need to restart ssh server:
$ sudo systemctl restart ssh
OR
$ /etc/init.d/ssh restart
Step 3 – Update all ssh client(s) known_hosts files
Finally, you need to update ~/.ssh/known_hosts files on client computers, otherwise everyone will see an error message that read as follows:
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ @ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @ @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY! Someone could be eavesdropping on you right now (man-in-the-middle attack)! It is also possible that the RSA host key has just been changed. The fingerprint for the RSA key sent by the remote host is f6:67:01:41:e6:20:06:4b:4b:fa:4b:c1:f1:45:45:e0. Please contact your system administrator. Add correct host key in /home/vivek/.ssh/known_hosts to get rid of this message. Offending key in /home/vivek/.ssh/known_hosts:12 RSA host key for 202.54.xx.abc has changed and you have requested strict checking. Host key verification failed.
Either remove host fingerprint or update the file using vi text editor (command must be typed on client machine):
$ ssh-keygen -R remote-server-name-here
Now login using the ssh command:
$ ssh vivek@server1.cyberciti.biz
Conclusion
You just regenerated OpenSSH Host Keys on a Debian or Ubuntu Linux using the dpkg-reconfigure command. For more info see the man page or this wiki page here:
$ man dpkg-reconfigure
$ man sshd
11 comment
Just what I needed! Thanks.
Very helpful. Excellent article. Thanks.
Just what I was looking for, thanks!
Very helpful thanks a lot!
Great post, thanks a lot!!!!!!
thx very helpful post
At first I tried to update ~/.ssh/known_hosts on the server and this not worked.
Only later I realized that ~/.ssh/known_hosts is on my local Linux PC.
Please edit your post to make clear which files are on the server and which on the Linux PC.
Still very useful after all these years. Thanks.
Great article, right to the point, tested in Debian 8.7 and works like a charm
Can you share any links on how ssh works end to end, thanks