Perform SSH and SCP Without Entering Password on openSSH

In this article, I’ll explain how to perform ssh and scp without entering the password using the SSH Public Key authentication with SSH Agent on openSSH

There are two levels of security in the SSH key based authentication. In order for you to login, you need both the private key and the passphrase. Even if one of them is compromised, attacker still cannot login to your account, as both of them are needed to login. This is far better than typical password based authentication, where if the password is compromised, attacker can gain access to the system.

There are two ways to perform ssh and scp without entering the password:

1. No passphrase. While creating key pair, leave the passphrase empty. Use this option for the automated batch processing. for e.g. if you are running a cron job to copy files between machines this is suitable option.
2. Use passphrase and SSH Agent. If you are using ssh and scp interactively from the command-line and you don’t want to use the password everytime you perform ssh or scp, I don’t recommend the previous option (no passphrase), as you’ve eliminated one level of security in the ssh key based authentication. Instead, use the passphrase while creating the key pair and use SSH Agent to perform ssh and scp without having to enter the password everytime as explained in the steps below.

Following 8 steps explains how to perform SSH and SCP from local-host to a remote-host without entering the password on openSSH system

1. Verify that local-host and remote-host is running openSSH

[local-host]$ ssh -V
OpenSSH_4.3p2, OpenSSL 0.9.8b 04 May 2006

[remote-host]$ ssh -V
OpenSSH_4.3p2, OpenSSL 0.9.8b 04 May 2006

2. Generate key-pair on the local-host using ssh-keygen

[local-host]$ ssh-keygen
Generating public/private rsa key pair.
Enter file in which to save the key (/home/jsmith/.ssh/id_rsa):
Enter passphrase (empty for no passphrase): 
Enter same passphrase again:
Your identification has been saved in /home/jsmith/.ssh/id_rsa.
Your public key has been saved in /home/jsmith/.ssh/id_rsa.pub.
The key fingerprint is:
31:3a:5d:dc:bc:81:81:71:be:31:2b:11:b8:e8:39:a0 jsmith@local-host

The public key and private key are typically stored in .ssh folder under your home directory. In this example, it is under /home/jsmith/.sshd. 

By default the ssh-keygen on openSSH generates RSA key pair. You can also generate DSA key pair using: ssh-keygen -t dsa command.

3. Install public key on the remote-host.

Copy the content of the public key from the local-host and paste it to the /home/jsmith/.ssh/authorized_keys on the remote-host. If the /home/jsmith/.ssh/authorized_keys already has some other public key, you can append this to the end of it. If the .ssh directory under your home directory on remote-host doesn’t exist, please create it.

[remote-host]$ vi ~/.ssh/authorized_keys 
ssh-rsa ABIwAAAQEAzRPh9rWfjZ1+7Q369zsBEa7wS1RxzWR jsmith@local-host

In simple words, copy the local-host:/home/jsmith/.ssh/id_rsa.pub to remote-host:/home/jsmith/.ssh/authorized_keys

4. Give appropriate permission to the .ssh directory on the remote-host.

[remote-host]$ chmod 755 ~/.ssh
[remote-host]$ chmod 644 ~/.ssh/authorized_keys

5. Login from the local-host to remote-host using the SSH key authentication to verify whether it works properly.

[local-host]$ 

[local-host]$ ssh -l jsmith remote-host
Enter passphrase for key '/home/jsmith/.ssh/id_rsa': 
Last login: Sat Jun 07 2008 23:03:04 -0700 from 192.168.1.102
No mail.

[remote-host]$ 

6. Start the SSH Agent on local-host to perform ssh and scp without having to enter the passphrase several times.

Verify whether SSH agent is already running, if not start it as shown below.

[local-host]$ ps -ef | grep ssh-agent
 511       9789  9425  0 00:05 pts/1    00:00:00 grep ssh-agent
 
[local-host]$ ssh-agent $SHELL
 
[local-host]$ ps -ef | grep ssh-agent
 511       9791  9790  0 00:05 ?        00:00:00 ssh-agent /bin/bash
 511       9793  9790  0 00:05 pts/1    00:00:00 grep ssh-agent

7. Load the private key to the SSH agent on the local-host.

[local-host]$ ssh-add
Enter passphrase for /home/jsmith/.ssh/id_rsa: 
Identity added: /home/jsmith/.ssh/id_rsa (/home/jsmith/.ssh/id_rsa)

Following are the different options available in the ssh-add:

• ssh-add : Load a specific key file.
• ssh-add -l: List all the key loaded in the ssh agent.
• ssh-add -d : Delete a specificy key from the ssh agent
• ssh-add -D: Delete all key

8. Perform SSH or SCP to remote-home from local-host without entering the password.

[local-host]$

[local-host]$ ssh -l jsmith remote-host
Last login: Sat Jun 07 2008 23:03:04 -0700 from 192.168.1.102
 No mail.

[remote-host]$ 

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