DokuWiki » Infrastructure » Operating System » CentOS 8

h1. Setup ssh-rsa-login

h2. 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

h2. 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

h2. Setup 

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

<pre><code class="shell">

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]-----+

</code></pre>

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

<pre><code class="bash">

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

b@B's password: 

</code></pre>

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

<pre><code class="bash">

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

b@B's password: 

</code></pre>

h2. Test 

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

<pre><code class="shell">

a@A:~> ssh b@B hostname

B

</code></pre>

h2. Troubleshooting 

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

<pre><code class="bash">

chmod 700 ~/.ssh

ll ~/.ssh

</code></pre>

needs permission-wise result in

<pre><code class="bash">

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_ed25519

-rw-r--r--. 1 root root  404 Aug  3 09:05 id_ed25519.pub

</code></pre>

to restore SELinux-context perform

<pre><code class="bash">

restorecon -R -v ~/.ssh

</code></pre>