≡ Menu

HowTo: Setup SSH Keys on a Linux / Unix System

I recently read that SSH keys provide a secure way of logging into a Linux and Unix-based server. How do I set up SSH keys on a Linux or Unix based systems? In SSH for Linux/Unix, how do I set up public key authentication?

I am assuming that you are using Linux or Unix-like server and client with the following software:

  • OpenSSH SSHD server
  • OpenSSH ssh client and friends on Linux (Ubuntu, Debian, {Free,Open,Net}BSD, RHEL, CentOS, OSX and co).

What is a public key authentication?

OpenSSH server supports various authentication schema. The two most popular are as follows:

  1. Passwords based authentication
  2. Public key based authentication. It is an alternative security method to using passwords. This method is recommended on a VPS, cloud, dedicated or even home based server.

How do I set up public key authentication?

You must generate both a public and a private key pair. For example:

     +---------------+      |       +-------------+
     | Unix/Linux    |      |       | Linux/Unix  |
     | Server with   +------+-------+ OSX/*BSD    |
     | OpenSSH SSHD  |              | Client      |
     +---------------+              +-------------+
  server1.cyberciti.biz          client1.cyberciti.biz        


  • server1.cyberciti.biz – You store your public key on the remote hosts and you have an accounts on this Linux/Unix based server.
  • client1.cyberciti.biz – Your private key stays on the desktop/laptop/ computer (or local server) you use to connect to server1.cyberciti.biz server. Do not share or give your private file to anyone.

In public key based method you can log into remote hosts and server, and transfer files to them, without using your account passwords. Feel free to replace server1.cyberciti.biz and client1.cyberciti.biz names with your actual setup. Enough talk, let’s set up public key authentication. Open the Terminal and type following commands if .ssh directory does not exists:

mkdir -p $HOME/.ssh
chmod 0700 $HOME/.ssh

#1: Create the key pair

On the computer (such as client1.cyberciti.biz), generate a key pair for the protocol.

ssh-keygen -t rsa

Sample outputs:

Generating public/private rsa key pair.
Enter file in which to save the key (/Users/vivek/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /Users/vivek/.ssh/id_rsa.
Your public key has been saved in /Users/vivek/.ssh/id_rsa.pub.
The key fingerprint is:
80:5f:25:7c:f4:90:aa:e1:f4:a0:01:43:4e:e8:bc:f5 vivek@desktop01
The key's randomart image is:
+--[ RSA 2048]----+
| oo    ...+.     |
|.oo  .  .ooo     |
|o .o. . .o  .    |
| o ...+o.        |
|  o .=.=S        |
| .  .Eo .        |
|                 |
|                 |
|                 |

You need to set the Key Pair location and name. I recommend you use the default location if you do not yet have another key there, for example: $HOME/.ssh/id_rsa. You will be prompted to supply a passphrase (password) for your private key. I suggest that you setup a passphrase when prompted. You should see two new files in $HOME/.ssh/ directory:

  1. $HOME/.ssh/id_rsa – contains your private key.
  2. $HOME/.ssh/id_rsa.pub – contain your public key.

#2: Install the public key in remote server

Use scp or ssh-copy-id command to copy your public key file (e.g., $HOME/.ssh/id_rsa.pub) to your account on the remote server/host (e.g., nixcraft@server1.cyberciti.biz). To do so, enter the following command on your client1.cyberciti.biz:

ssh-copy-id -i $HOME/.ssh/id_rsa.pub user@server1.cyberciti.biz

OR just copy the public key in remote server as authorized_keys in ~/.ssh/ directory:

scp $HOME/.ssh/id_rsa.pub user@server1.cyberciti.biz:~/.ssh/authorized_keys

A note about appending the public key in remote server

On some system ssh-copy-id command may not be installed, so use the following commands (when prompted provide the password for remote user account called vivek) to install and append the public key:

## First create .ssh directory on server ##
ssh vivek@server1.cyberciti.biz "umask 077; test -d .ssh || mkdir .ssh"
## cat local id.rsa.pub file and pipe over ssh to append the public key in remote server ##
cat $HOME/.ssh/id_rsa.pub | ssh vivek@server1.cyberciti.biz "cat >> .ssh/authorized_keys"

#3: Test it (type command on client1.cyberciti.biz)

The syntax is:

ssh user@server1.cyberciti.biz

Or copy a text file called foo.txt:

scp foo.txt user@server1.cyberciti.biz:/tmp/

You will be prompted for a passphrase. To get rid of passphrase whenever you log in the remote host, try ssh-agent and ssh-add commands.

What are ssh-agent and ssh-add, and how do I use them?

To get rid of a passphrase for the current session, add a passphrase to ssh-agent and you will not be prompted for it when using ssh or scp/sftp/rsync to connect to hosts with your public key. The syntax is as follows:

eval $(ssh-agent)

Type the ssh-add command to prompt the user for a private key passphrase and adds it to the list maintained by ssh-agent command:


Enter your private key passphrase. Now try again to log into user@server1.cyberciti.biz and you will not be prompted for a password:

ssh user@server1.cyberciti.biz

#4: Disable the password based login on a server

Login to your server, type:

## client commands ##
eval $(ssh-agent)
ssh user@server1.cyberciti.biz<

Edit /etc/ssh/sshd_config on server1.cyberciti.biz using a text editor such as nano or vim:
$ sudo vim /etc/ssh/sshd_config
OR directly jump to PermitRootLogin line using a vim text editor:
$ sudo vim +/PermitRootLogin /etc/ssh/sshd_config
Find PermitRootLogin and set it as follows:

PermitRootLogin no

Save and close the file. Reload/restart sshd server, type command as per your Linux/Unix version:

## CentOS/RHEL/Fedora Linux server reload sshd ##
sudo service sshd reload
## Debian/Ubuntu Linux server reload sshd ##
/etc/init.d/ssh reload
## Generic Unix method to reload sshd ##
kill -HUP `cat /var/run/sshd.pid`

#5: How to add or replace a passphrase for an existing private key?

To to change your passphrase type the following command:

ssh-keygen -p

#6: How to backup an existing private/public key?

Just copy files to your backup server or external USB pen/hard drive:

## Copy files to  home based nas server ##
rsync -avr $HOME/.ssh user@home.nas-server:/path/to/encrpted/nas/partition/
## Copy files to  usb pen drive mounted at /mnt/usb ##
cp -avr $HOME/.ssh/ /mnt/usb/backups/
See also

And, there you have it, ssh set up with public key based authentication for Linux or Unix-like systems.

Share this tutorial on:
{ 14 comments… add one }
  • Casper March 9, 2014, 9:26 pm

    Things might have changed, but did you not forget to setup “authorized_keys” ?

    • Nix Craft March 10, 2014, 4:26 am

      Thanks for the heads up! The faq has been updated.

  • Topper March 10, 2014, 6:28 am

    Some typo error:
    cp -avr $HONE/.ssh/ /mnt/usb/backups/
    has to be
    cp -avr $HOME/.ssh/ /mnt/usb/backups/

  • Terry March 13, 2014, 11:34 am

    To add to the story, I do this often with keys setup from my office desktop.

    for i in server1 server2 server3 server4; do ssh mylogin@$i "hostname" ; done

    This one line will login to the four servers and run the command hostname .

    Replace hostname with your hearts desire.

  • Louise August 20, 2014, 12:30 pm

    I have my ssh keys setup with up on one server and connecting with two other servers successfully. I now have to rename and reip all three servers, i noticed with the pub key the server name is listed. When i rename and re-ip my servers is there any way to update the keys or do i have re-create again?

    Thank you

  • rajesh September 23, 2014, 4:55 pm

    i have generate ssh key on A server for communicate server B. then i can able to do ssh with out passwd to server B.

    but when i try to login in Server B . ..then trying to do ssh to server A ..it is asking password..why lit happens ?

    it should be vise versa ..rt ? server A – server B , Server B- server A


    • rajesh December 2, 2014, 3:34 am

      A to B password less
      server A:
      ssh-keygen -t rsa
      press enter,enter
      two keys is create the path /root/.ssh
      next go to the cd .ssh
      ls -lrt
      scp id_pas.pub root@serverB:/home
      server B:
      check the cd /home
      cd /root/.ssh
      in server B is not .ssh directory
      create the directory
      mkdir /root/.ssh
      chmod 700 /root/.ssh

      cd /root/.ssh
      cp /home/id_pas.pub authorized_keys

      already authorized_keys in another keys
      so append data because over read the data
      cat>>authorized_keys</home/id_pas.pub -this command is append the data not over read the old data
      next go to server A
      ssh root@serverB
      login the passwd lesss

  • milosz October 23, 2014, 12:33 pm

    I believe you forgot the pipe character:

    cat $HOME/.ssh/id_rsa.pub ssh vivek@server1.cyberciti.biz cat >> .ssh/authorized_keys

    should be:

    cat $HOME/.ssh/id_rsa.pub | ssh vivek@server1.cyberciti.biz cat >> .ssh/authorized_keys

  • amrx December 12, 2014, 6:30 am

    How can I access this remote server from different network ?

  • Jason September 17, 2015, 11:32 am

    Having an issue.
    When I am prompted for a key in Ubuntu 14.04, ther terminal will not let me enter one.
    I press buttons but nothing appears, and I don’t want to set one up without passphrase.

  • Dan May 3, 2016, 3:57 pm

    Great article, I have found many use full command and scripts to help with setting up a kickstart configuration that loads and configures without interaction using PXEboot.

    Do you have an article about a one line to configure ssh public/private key that will run silent and without interaction.

    Thank you

  • volcano May 17, 2016, 12:41 pm

    I did everything as said in article, but there’s an error everytime I connect to server: “Server refused our key”. I generated key on windows machine with puttygen. May be there are some settings I should change in sshd_config?

Leave a Comment

   Tagged with: , , , ,