How to setup passwordless login to the SSH server

Renesh Bedre    3 minute read

Background

  • When you work on the remote host (SSH server) regularly (e.g. for big data analysis), it is convenient to set up passwordless login to the SSH server from your local host (SSH client).
  • SSH (Secure Shell) is a software package that enables SSH client to securely connect to an SSH server over a insecure network.
  • SSH is secure and encrypts the connection between the SSH client and SSH server. By using it, you can protect your data, remote server authorizations, file transfers, and remote server work (e.g. commands) from network hacking attacks.
  • SSH protocol uses the client-server model to make a secure connection between the SSH client and SSH server. In this model, the client and server interact with each other using SSH keys to make a secure connection.

SSH client-server model

How to setup passwordless login

SSH is a default software package on Linux, Mac OS, and Windows, and hence you do not have to install it. In addition, you can also use other SSH clients such as PuTTY and MobaXterm on Windows platform.

This tutorial is intended for Linux and Mac OS platforms only.

1. Check if public and private SSH key pair exists

You can use ls commands to see the files ending with either .rsa or .dsa in ~/.ssh folder

ls ~/.ssh

If you see a file either of id_rsa or id_dsa and id_rsa.pub or id_dsa.pub, it means the SSH key pair already exists. In this case, you can either replace the existing key pair by creating new one or skip to next step.

If you do not see these files, you can create SSH key pairs as described in following step.

2. Generate public and private SSH key pairs

SSH keys can be generated using ssh-keygen. You can use RSA or DSA algorithm for the generation of SSH key pair using ssh-keygen. The RSA is preferred over the DSA algorithm for SSH key pair generation. You can see the difference between these two algorithms here

ssh-keygen -t rsa
# follow the instructions from this command until you create a SSH keys

SSH key generation

You may have to enter a passphrase for generating SSH keys. The passphrase is similar to a password and is used for encrypting the private key.

This will create id_rsa (private key) and id_rsa.pub (public key) under the ~/.ssh folder. The key size is 2048 bits which is recommended for RSA. You can also increase the key size to 4096 bits by using ssh-keygen -t rsa -b 4096

3. Copy public key to SSH server

The public key (id_rsa.pub) need to be copied into authorized_keys file into SSH server using ssh-copy-id

ssh-copy-id username@remote_host

At this step, you may have to enter the password for the SSH server

You should never copy private key (id_rsa) to any remote host

4. Login to SSH server

Once the public key is added to the SSH server authorized file, you should be able to login to a SSH server without a password. But at this step, it may ask you to enter a passphrase that you have created during the ssh-keygen step.

ssh username@remote_host

If it asks for a passphrase for connection to a SSH server, follow the below steps to add the private key to SSH Agent (ssh-agent).

Start SSH agent,

eval `ssh-agent`

Add SSH private key (listed in ~/.ssh) to the SSH agent

ssh-add

Now, you should be able to login to a SSH server without a password and passphrase,

ssh username@remote_host

Summary

In this article, the background of the SSH client-server model is explained, along with how it works. SSH provides a secure connection between the local host and remote host. This article taught you how to log into a SSH server without a password and passphrase.

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References

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