Setup SSH Passwordless Certificate for login to Linux servers

- Generate SSH key using RSA Algorithm within 4096 bytes and compatible with PEM certificate 

$ ssh-keygen -t rsa -b 4096 -m PEM

Enter file in which to save the key (/home/user/.ssh/id_rsa): server-key

- Then you can get pem from your rsa private key.

$ openssl rsa -in server-key -outform pem > server-key.pem

, change permissions to private key in local machine

$ chmod 0400 server-key.pem

- Copy SSH Public key to your server

$ ssh-copy-id -i server-key.pub user@Sever-IP-OR-Domain

- Login using SSH Key 

$ ssh -i server-key.pem user@Sever-IP-OR-Domain

- Create SSH config file

$ nano ~/.ssh/config

# Server @datacenter1
Host web1
Hostname mydomain.com
User username
Port 1234
IdentityFile /path-to-private-key/server-key.pem

- Login using SSH config details 

$ ssh web1 

- Change SSH service settings to disable root login and prevent using passwords (remote server)

$ sudo nano /etc/ssh/sshd_config

# change default ssh port

Port 1234

# use two method authentication: publickey, password

AuthenticationMethods publickey,password

# disable root login

PermitRootLogin no

# disable login using password only

#PasswordAuthentication yes

# Allowed Users only to login

AllowUsers user1@192.168.1.0/24 user2@10.10.10.0/16

- Apply ssh new settings by restart service

$ sudo systemctl restart ssh.service

, change permissions to authorized_keys file

$ chmod 0400 ~/.ssh/authorized_keys