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Server & Website Managed Operations

Logging into Linux Servers via SSH Key Authentication

Published 2015-08-15By adminCategory Server & Website Managed Operations11 min read

In addition to password authentication, SSH offers another commonly used method: key-based authentication. Its advantages include enhanced security and the ability to enable passwordless login between Linux systems, facilitating management or batch maintenance.

This article explains how to configure key-based passwordless login for Linux servers on both Windows and Linux.

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[root@A ~]# ssh-keygen -t rsa

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Generating public/private rsa key pair.

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Enter file in which to save the key (/root/.ssh/id_rsa):

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// Path and filename where the key will be saved

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Enter passphrase (empty for no passphrase):

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// Passphrase for the key; leave empty for passwordless login

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Enter same passphrase again:

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Your identification has been saved in /root/.ssh/id_rsa

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// Private key generated at /root/.ssh/id_rsa

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Your public key has been saved in /root/.ssh/id_rsa.pub

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// Public key generated at /root/.ssh/id_rsa.pub

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The key fingerprint is:

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82:8c:2c:52:06:c6:f3:c4:1c:1c:35:b7:cd:5b:42:a2 root@A

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[root@A ~]# ls /root/.ssh/ // Check the directory; these two files should now exist

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authorized_keys id_rsa id_rsa.pub known_hosts

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[root@A ~]#

Check the path where the server’s SSH service stores public keys:

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[root@A ~]# cat /etc/ssh/sshd_config |grep AuthorizedKeysFile

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#AuthorizedKeysFile .ssh/authorized_keys

The default location for storing public keys on the server is .ssh/authorized_keys.

Therefore, you need to copy the generated public key file to this file on the target server using scp or any other method.

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[root@B ~]# cd .ssh/

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[root@B .ssh]# cat id_rsa.pub >authorized_keys

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// The default private key path read by the local SSH client includes /.ssh/id_rsa

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[root@A .ssh]# cat /etc/ssh/ssh_config |grep IdentityFile

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# IdentityFile ~/.ssh/identity

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# IdentityFile ~/.ssh/id_rsa

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# IdentityFile ~/.ssh/id_dsa

At this point, you can directly execute ssh ipB from Server A to log in to Server B. It’s that simple.

A simpler method is to use the ssh-copy-id command to directly add your local key to the verification file on the remote server:

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[root@A .ssh]# ssh-copy-id

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Usage: /usr/bin/ssh-copy-id [-i [identity_file]] [user@]machine

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[root@A .ssh]# ssh-copy-id -i /root/.ssh/id_rsa root@ipB

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The authenticity of host 'ipB (ipB)' can't be established.

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RSA key fingerprint is 17:21:32:02:da:3b:a1:d1:a2:69:12:36:a2:d7:59:ad.

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Are you sure you want to continue connecting (yes/no)? yes

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root@ipB's password:

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Now try logging into the machine, with "ssh 'root@ipB'", and check in:

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.ssh/authorized_keys

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[root@A ~]# ssh ipB

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Last login: Sun Dec 15 19:33:07 2013 from ipA

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[root@B ~]#

After executing this, you can log in to the server directly.

The above methods involve connecting via the Linux SSH client. If you want to use key-based login from a local machine, the process is similar.

Download the private key (id_rsa) to your local machine and import it into the key authentication configuration of your client.

For example, in SecureCRT, select the downloaded private key file as shown in the configuration below.

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