Setup ssh-rsa-login¶

Requirements¶

To install ssh you will need the following:

• a installed and supported operating system (e.g. CentOS 8.x)
• root-access
• a fast internet connection

Preliminary Note¶

You want to use Linux and OpenSSH to automize your tasks. Therefore you need an automatic login from host A / user a to Host B / user b. You don't want to enter any passwords, because you want to call ssh from a within a shell script.
This information was taken from Mathias Kettner @ http://linuxproblem.org/art_9.html

Setup¶

First log in on A as user a and generate a pair of authentication keys. Do not enter a passphrase:

a@A:~> ssh-keygen -t ed25519
Generating public/private ed25519 key pair.
Enter file in which to save the key (/root/.ssh/id_ed25519): 
Created directory '/root/.ssh'.
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /root/.ssh/id_ed25519.
Your public key has been saved in /root/.ssh/id_ed25519.pub.
The key fingerprint is:
SHA256:cWzFIp9zUKCHz8tijMdZn7Nx07gS1nJKeSNnBP1vrFX a@A
The key's randomart image is:
+--[ED25519 256]--+
|          o=o    |
|        .+..o    |
| E      +o =     |
|         B+      |
|        S O  + .E|
| e     + . .B X.+|
|      . B +o Xo==|
|       + o .-. ==|
|            o++oo|
+----[SHA256]-----+

Now use ssh to create a directory ~/.ssh as user b on B. (The directory may already exist, which is fine):

a@A:~> ssh b@B mkdir -p .ssh
b@B's password: 

Finally append a's new public key to b@B:.ssh/authorized_keys and enter b's password one last time:

a@A:~> cat .ssh/id_ed25519.pub | ssh b@B 'cat >> .ssh/authorized_keys'
b@B's password: 

Test¶

From now on you can log into B as b from A as a without password:

a@A:~> ssh b@B hostname
B

Troubleshooting¶

If for any reason this is not working it might be because of wrong permissions or SELinux-context

chmod 700 ~/.ssh
ll ~/.ssh

needs permission-wise result in

drwx------. 2 root root 4096 Aug  3 11:04 .
dr-xr-x---. 6 root root 4096 Aug  3 11:23 ..
-rw-r--r--. 1 root root  410 Aug  3 11:04 authorized_keys
-rw-------. 1 root root 1671 Aug  3 09:05 id_rsa
-rw-r--r--. 1 root root  404 Aug  3 09:05 id_rsa.pub

to restore SELinux-context perform

restorecon -R -v ~/.ssh