4 Steps to login Linux server without password

In this article, we will learn how to enable password-less login on Linux using ssh key.

Using Password-less login with SSH key will increase the trust between two Linux servers.

Here are 4 steps to log in to Linux without the password.

• create SSH Key on Client
• Copy public key to remote server
• Disable password login on the server – optional
• login server with the private key

Create SSH Key on Client

We can use ssh-keygen command to generate SSH keys in Linux.

1. Generate a key pair with the following command. The default SSH key type is RSA. Check this post to know which SSH Key type is more secure in Linux.
2. Press Enter to confirm the default location (that is, ~/.ssh/id_rsa) for the newly created key.
3. Enter a passphrase, and confirm it by entering it again when prompted to do so.
4. Check the SSH public key file and private key file with ls command under ~/.ssh/ directory
5. Change the permissions of the ~/.ssh/ directory and key files

Here are the commands we use for generating SSH keys.

# ssh-keygen -t rsa -b 4096
Generating public/private rsa key pair.
Enter file in which to save the key (/home/john/.ssh/id_rsa):
The key fingerprint is:
SHA256:6ezTTbbipomsipqJrsmqHFkGeM0VMPDv24PhimnbD+Y john@TOCAO-M-F13P
The key’s randomart image is:
+—[RSA 4096]—-+
| ..o.o. |
|. + o |
|… + |
| .. . . |
| o . S |
| + ..o o |
| o o..oo. + . |
|+=o* oo=o.+ o |
|^==oE+= =*.. |
+—-[SHA256]—–+

$ chmod 700 ~/.ssh

$ chmod 600 ~/.ssh/id_rsa.pub
$ chmod 600 ~/.ssh/id_rsa

Copy public key to remote server

There are two parts for this.

1. Append the content of ~/.ssh/id_rsa.pub into the ~/.ssh/authorized_keys file on the server-side.
2. Change the permissions of the ~/.ssh/authorized_keys file and ~/.ssh using the following command on the server-side.

We can use these three commands to copy the public key to the remote server and change permission.

cat ~/.ssh/id_rsa.pub | ssh user@ssh-server.example.com “cat >> ~/.ssh/authorized_keys”

$ chmod 600 ~/.ssh/authorized_keys

$ chmod 700 ~/.ssh

We can also use command ssh-copy-id for this.

$ ssh-copy-id -i keyfile user@hostname

Disable Password login on the server – optional

We need to change the sshd configuration to disable password login. This part is optional.

1. Ensure this option “PasswordAuthentication no” in /etc/ssh/sshd_config in server
2. Add this configuration “PubkeyAuthentication yes ” in /etc/ssh/sshd_config
3. To enable the change, restart the SSH daemon with this command “systemctl restart sshd “

Now we can log in to the remote server without a password.

Login server with the private key

Use the key to log in to the SSH server as shown in the following example, which loads the key in file ~/.ssh/id_rsa and logs in as user user@ssh-server.example.com

ssh -i ~/.ssh/id_rsa user@ssh-server.example.com

Troubleshooting Guide for SSH login without Password

• most time the root user is not allowed to log in with ssh. This can be verified with the configuration in /etc/ssh/sshd_config file.
• check the log /var/log/messages. and /var/log/secure for the login issue
• check the permission of the ssh key directory and even the .ssh directory
• use ssh -vvvv to print debug info about access process

Related:

• Which SSH Key Is More Secure in Linux?
• Filtering SSH Packets with Tcpdump
• Setup SSH Keys to Login Linux Without Password