In this post, we will learn how to configure the SSH RSA Key authentication in Linux servers. We will also setup the password less login between the servers.

Scenario :

(1) Two Servers

Server1: username : test (user must exist in server)
Server2: username : test  (user must exist in server)

(2) We want to login into Server2 from Server1 without giving passwd.
means when we do “ssh test@server2” it should not ask passwd.

How to configure ssh RSA key authentication in Linux

(a) Login into server1 by user test.

use the command: ssh-keygen -t rsa

And just hit enter enter enter , donot give any passphrase passwd.

[test@server1 ~]$ ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/test/.ssh/id_rsa):
Created directory '/home/test/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/test/.ssh/id_rsa.
Your public key has been saved in /home/test/.ssh/id_rsa.pub.
The key fingerprint is:
49:a4:1e:b8:df:13:36:fa:25:5a:d3:d2:f5:9e:77:98 test@server1
[test@server1 ~]$

Two files id_rsa and id_rsa.pub would be created in /home/test/.ssh directory

[test@server1 ~]$ ls -lrt .ssh/
total 8
-rw-r--r-- 1 test test  410 Nov 27 23:06 id_rsa.pub
-rw------- 1 test test 1671 Nov 27 23:06 id_rsa
[test@server1 ~]$

(b)  now scp the id_rsa.pub file into server2

[test@server1 ]$ cd /home/test/.ssh
[test@server1 .ssh]$ ls -lrt
total 8
-rw-r--r-- 1 test test  410 Nov 27 23:06 id_rsa.pub
-rw------- 1 test test 1671 Nov 27 23:06 id_rsa
[test@server1 .ssh]$
[test@server1 .ssh]$ scp id_rsa.pub test@server2:~

(c) Now login into Server2 by user test:

Check if /home/test/.ssh directory exist or not.

ls -ld /home/test/.ssh

if the .ssh directory does not exist make a new directory.

[test@server2]$ mkdir -p /home/test/.ssh

Now change to directory .ssh

cd /home/test/.ssh

Create one file inside .ssh called  authorized_keys

[test@server2 .ssh]$ pwd
/home/test/.ssh
[test@server1 .ssh]$ touch  authorized_keys
[test@server2 .ssh]$ ls
authorized_keys

(d) now copy the content of id_rsa.pub which u scp from server1 into the file /home/test/.ssh/authorized_keys

[test@server2 .ssh]$
[test@server2.ssh]$ cd ~
[test@server2 ~]$
[test@server2 ~]$ cat id_rsa.pub >> /home/test/.ssh/authorized_keys

(e) Check the permission and ownership of .ssh directory and authorized_keys file. It must be like below information

[test@server1 ~]$ ls -ld .ssh/
drw-r-xr-x 2 test test 4096 Nov 27 22:56 .ssh/
[test@server1 ~]$
[test@server1 ~]$ ls -lrt .ssh/
total 4
-rw-r--r-- 1 test test 412 Nov 27 22:55 authorized_keys
[test@server1 ~]$
[test@server1 ~]$

Note in Server2 : you should login with root user here

check sshd configuration:
vi   /etc/ssh/sshd_config  (on the Linux box)
RSAAuthentication   yes
PubkeyAuthentication  yes

/etc/init.d/sshd reload
or
/etc/init.d/sshd restart

Now logout from Server2 and try to ssh from Server1 to Server2,it should not ask passwd for login.

[test@server1 ~]$  ssh test@server2