We normally log in to a remote SSH server using password authentication method. Username and password combination is the most common authentication method for SSH and is a good enough method for most people. Sometimes, you might want to automatically log in to an SSH server without entering your username and password.

Passwordless SSH login could be convenient for some, but it is vitally important for those doing automation work. A script could be configured to automatically run commands on a remote server or transfer files using scp command.

You can automatically log in to an SSH server without entering your username and password using the public-key authentication method.

Steps to use passwordless login in SSH:

  1. Launch terminal application.
  2. Generate an SSH key pair on the local host if you don't already have one.
    $ ssh-keygen
    Generating public/private rsa key pair.
    Enter file in which to save the key (/home/user/.ssh/id_rsa):
    Enter passphrase (empty for no passphrase):
    Enter same passphrase again:
    Your identification has been saved in /home/user/.ssh/id_rsa.
    Your public key has been saved in /home/user/.ssh/id_rsa.pub.
    ##### snipped

    Make sure to not set any passphrase for the key pair

  3. Enable public key authentication in the remote SSH server.
    $ sudo grep PubkeyAuthentication /etc/ssh/sshd_config
    [sudo] password for user:
    PubkeyAuthentication yes

    Public key authentication is normally enabled by default.

  4. Copy your SSH public key from local host to the SSH server.
    $ ssh-copy-id user@remote-host
    /usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
    /usr/bin/ssh-copy-id: INFO: 2 key(s) remain to be installed -- if you are prompted now it is to install the new keys
    user@remote-host's password:
    
    Number of key(s) added: 2
    
    Now try logging into the machine, with:   "ssh 'user@remote-host'"
    and check to make sure that only the key(s) you wanted were added.
  5. Log in to the remote server to test.
    $ ssh user@remote-host
    Last login: Fri Jun 28 00:12:15 2019 from 192.168.111.135
    [user@remote-host ~]$

    You will no longer be prompted for a password when logging in to the server.

Guide compatibility:

Operating System
Ubuntu Linux
Debian Linux
Red Hat Enterprise Linux
Fedora Linux
CentOS Linux
openSUSE Linux
SUSE Linux Enterprise Server
FreeBSD
OpenBSD
NetBSD
macOS
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