Kali Linux remote SSH – How to configure openSSH server

Step 3: Change Kali default ssh keys to avoid MITM attack

At this point you will have openssh-server installed on Kali Linux and enabled at runlevel 2,3,4 and 5. But now we got a problem. Every Linux system that you install via a CD or DVD or similar uses a default SSH key. This is same for all first installation that means, anyone with a similar version can perform a Man in the Middle Attack (MITM) and listen to your encrypted traffic. To fix that we will do the followings:

Note: At least one reader confirms that this step is not necessary in new Kali Linux installation as the keys are different for each installation. I will leave the decision of changing default SSH keys to the readers.

Step 3.1: Move the default Kali ssh keys to a new folder:

Issue the following commands one line at a time:

root@kali:~#  cd /etc/ssh/
root@kali:/etc/ssh#  mkdir default_kali_keys
root@kali:/etc/ssh# 
root@kali:/etc/ssh#  mv ssh_host_* default_kali_keys/
root@kali:/etc/ssh#

This will move your default keys to the new folder.

Step 3.2: Regenerate the keys

Use the following command to regenerate SSH keys

root@kali:/etc/ssh#  dpkg-reconfigure openssh-server
Creating SSH2 RSA key; this may take some time ...
Creating SSH2 DSA key; this may take some time ...
Creating SSH2 ECDSA key; this may take some time ...
[ ok ] Restarting OpenBSD Secure Shell server: sshd.
root@kali:/etc/ssh#

Step 3.3: Verify ssh key hashes are different

Use the following commands to verify SSH key hashes are different

root@kali:/etc/ssh#  md5sum ssh_host_*
d5dff2404dd43ee0d9ed967f917fb697  ssh_host_dsa_key
2ec88dc08f24c39077c47106aab1e7f4  ssh_host_dsa_key.pub
ab96da6ffc39267f06e7f9497c4f5755  ssh_host_ecdsa_key
614e36d18dc2c46178d19661db4dbd7b  ssh_host_ecdsa_key.pub
abcc037705e48b3da91a2300d42e6a2b  ssh_host_rsa_key
e26eaa1c5cff38457daef839937fcedd  ssh_host_rsa_key.pub
root@kali:/etc/ssh#

Compare new key hashes to the hashes below)

root@kali:/etc/ssh#  cd default_kali_keys/
root@kali:/etc/ssh#
root@kali:/etc/ssh/default_kali_keys#  md5sum *
9a09f49be320e561dc6cf95463d4378c  ssh_host_dsa_key
1a52709d596569224822e870239c9298  ssh_host_dsa_key.pub
65d0af7fdc5c50f67f90cb953460ba61  ssh_host_ecdsa_key
606d1ac71100c8b38e0f87951bb94855  ssh_host_ecdsa_key.pub
c871ecf961924389f2cddbd5888b5037  ssh_host_rsa_key
99d4c4c68224900d0430f0bee9baf28e  ssh_host_rsa_key.pub
root@kali:/etc/ssh/default_kali_keys#

Restart SSH.

root@kali:/etc/ssh/default_kali_keys# service ssh restart

Thanks to pentest09 for providing this excellent and easy to follow info on Kali Linux forum.

Step 4: Set MOTD with a nice ASCII

So far, we have installed and configured Kali Linux remote SSH – openssh-server, enabled openssh-server to run on boot, changed Kali default SSH keys to avoid MITM attacks.

Now the usual SSH MOTD (Message of the Day – Banner) is boring. I want my name on that and add some useful info. Following is what a usual MOTD looks like:

Well, that’s just plain and boring for me.

Go to http://patorjk.com/software/taag/

Type something in “Type Something” Box! Play around with the settings and you get a nice ASCII art.

Edit the following file and add your text.

root@kali:~# vi /etc/motd 
root@kali:~# service ssh restart

Save the file and restart/reload SSH … both should just work. I’ve added blackMORE Ops as ASCII and http://www.blackmoreops.com/ as a second line…

So next time I try to login I get this nice screen with some more info

Pretty cool!

Step 5: Troubleshooting

Because I changed SSH keys in the middle of change, (I was logged in before), I had this BIG warning message coming up

 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
 @ 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
 26:65:52:75:81:71:a8:c5:4c:ad:b6:81:78:58:18:af.
 Please contact your system administrator.
 Add correct host key in /root/.ssh/known_hosts to get rid of this message.
 Offending key in /root/.ssh/known_hosts:1
 RSA host key for localhost has changed and you have requested strict checking.
 Host key verification failed.

It kicked me right out.

Usually this is the sign of something bad. As you can see MITM attacks does this:

 IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
 Someone could be eavesdropping on you right now (man-in-the-middle attack)!

This was the reason I changed Kali Linux default key (You wouldn’t even notice MITM if you’re using the default key).

This is a rather easy fix. You just need to delete the offending line in known_hosts file.

 Add correct host key in /root/.ssh/known_hosts to get rid of this message.
 Offending key in /root/.ssh/known_hosts:1

Use the following command:

root@kali:~# vi /root/.ssh/known_hosts

Following was the key I had

Just delete the line, save the file and try to SSH again.

and it worked.