Linux Passwordless SSH Login

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Linux Passwordless SSH Login

You can login to a remote Linux server without entering password in 3 simple steps using ssh-keygen and ssh-copy-id as explained in this article.

ssh-keygen creates the public and private keys. ssh-copy-id copies the local-host’s public key to the remote-host’s authorized_keys file. ssh-copy-id also assigns proper permission to the remote-host’s home, ~/.ssh, and ~/.ssh/authorized_keys.

This article also explains 3 minor annoyances of using ssh-copy-id and how to use ssh-copy-id along with ssh-agent.
Create public and private keys using ssh-key-gen on local-host

vagrant@local-host$ [Note: You are on local-host here]

vagrant@local-host$ ssh-keygen

Generating public/private rsa key pair.

Enter file in which to save the key (/home/vagrant/.ssh/id_rsa):[Enter key]

Enter passphrase (empty for no passphrase): [Press enter key]

Enter same passphrase again: [Pess enter key]

Your identification has been saved in /home/vagrant/.ssh/id_rsa.

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

The key fingerprint is:

33:b3:fe:af:95:95:18:11:31:d5:de:96:2f:f2:35:f9 vagrant@local-host

Step 2: Copy the public key to remote-host using ssh-copy-id

vagrant@local-host$ ssh-copy-id -i ~/.ssh/id_rsa.pub remote-host

vagrant@remote-host’s password:

Now try logging into the machine, with “ssh ‘remote-host'”, and check in:

.ssh/authorized_keys

to make sure we haven’t added extra keys that you weren’t expecting.

Note: ssh-copy-id appends the keys to the remote-host’s .ssh/authorized_key.

Step 3: Login to remote-host without entering the password

vagrant@local-host$ ssh remote-host

Last login: Sun Nov 16 17:22:33 2008 from 192.168.1.2

[Note: SSH did not ask for password.]

vagrant@remote-host$ [Note: You are on remote-host here]

Using ssh-copy-id along with the ssh-add/ssh-agent

When no value is passed for the option -i and If ~/.ssh/identity.pub is not available, ssh-copy-id will display the following error message.

vagrant@local-host$ ssh-copy-id -i remote-host

/usr/bin/ssh-copy-id: ERROR: No identities found

If you have loaded keys to the ssh-agent using the ssh-add, then ssh-copy-id will get the keys from the ssh-agent to copy to the remote-host. i.e, it copies the keys provided by ssh-add -L command to the remote-host, when you don’t pass option -i to the ssh-copy-id.

vagrant@local-host$ ssh-agent $SHELL

vagrant@local-host$ ssh-add -L

The agent has no identities.

vagrant@local-host$ ssh-add

Identity added: /home/vagrant/.ssh/id_rsa (/home/vagrant/.ssh/id_rsa)

vagrant@local-host$ ssh-add -L

ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAsJIEILxftj8aSxMa3d8t6JvM79DyBV

aHrtPhTYpq7kIEMUNzApnyxsHpH1tQ/Ow== /home/vagrant/.ssh/id_rsa

vagrant@local-host$ ssh-copy-id -i remote-host

vagrant@remote-host’s password:

Now try logging into the machine, with “ssh ‘remote-host'”, and check in:

.ssh/authorized_keys

to make sure we haven’t added extra keys that you weren’t expecting.

[Note: This has added the key displayed by ssh-add -L]

A note from one of our readers: Depending on your version of SSH you might also have to do the following changes:

  • Put the public key in .ssh/authorized_keys2
  • Change the permissions of .ssh to 700
  • Change the permissions of .ssh/authorized_keys2 to 640

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