Ssh Generate Public Key For User

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am a new Ubuntu 18.04 LTS user and I would like to setup ssh public key authentication. How do I set up ssh keys based authentication on Ubuntu Linux 18.04 LTS server? How do I set up SSH keys on an Ubuntu Linux 18.04 LTS server? In Ubuntu 18.04 LTS, how do I set up public key authentication?

Adding your SSH key to the ssh-agent. Before adding a new SSH key to the ssh-agent to manage your keys, you should have checked for existing SSH keys and generated a new SSH key. When adding your SSH key to the agent, use the default macOS ssh-add command, and not an application installed by macports, homebrew, or some other external source. Sep 06, 2019 Generating your key pair and propagating your public key is simpler than it sounds. Let’s walk through it. Generating the key. The minimum effort to generate a key pair involves running the ssh-keygen command, and choosing the defaults at all the prompts: $ ssh-keygen Generating public/private rsa key pair.

Introduction: OpenSSH is a free and open source client/server technology for secure remote login. It is an implementation of the SSH protocol. OpenSSH divided into sshd (server) and various client tools such as sftp, scp, ssh and more. One can do remote login with OpenSSH either using password or combination of private and public keys named as public key based authentication. It is an alternative security method for user passwords. This method is recommended on a VPS, cloud, dedicated or even home-based server or laptop. This page shows how to set up SSH keys on Ubuntu 18.04 LTS server.
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Ubuntu 18.04 Setup SSH Public Key Authentication

The procedure to set up secure ssh keys on Ubuntu 18.04:

  1. Create the key pair using ssh-keygen command.
  2. Copy and install the public key using ssh-copy-id command.
  3. Add yourself to sudo admin account on Ubuntu 18.04 server.
  4. Disable the password login for root account on Ubuntu 18.04.

Sample set up for SSH Keys on Ubuntu 18.04


Where,

  • 202.54.1.55 – You store your public key on the remote hosts and you have an accounts on this Ubuntu Linux 18.04 LTS server.
  • Linux/macbook laptop – Your private key stays on the desktop/laptop/computer (or local server) you use to connect to 202.54.1.55 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 202.54.1.55 and client names with your actual setup. Enough talk, let’s set up public key authentication on Ubuntu Linux 18.04 LTS.

How to create the RSA/ed25519 key pair on your local desktop/laptop

Open the Terminal and type following commands if .ssh directory does not exists:
$ mkdir -p $HOME/.ssh
$ chmod 0700 $HOME/.ssh
Next generate a key pair for the protocol, run:
$ ssh-keygen
OR
$ ssh-keygen -t rsa 4096 -C 'My key for Linode server'
These days ED25519 keys are favored over RSA keys when backward compatibility is not needed:
$ ssh-keygen -t ed25519 -C 'My key for Linux server # 42'

How to install the public key in Ubuntu 18.04 remote server

The syntax is as follows:
ssh-copy-id your-user-name@your-ubuntu-server-name
ssh-copy-id -i ~/.ssh/file.pub your-user-name@your-ubuntu-server-name
For example:
## for RSA KEY ##
ssh-copy-id -i $HOME/.ssh/id_rsa.pub user@202.54.1.55
## for ED25519 KEY ##
ssh-copy-id -i $HOME/.ssh/id_ed25519.pub user@202.54.1.55
## install SSH KEY for root user ##
ssh-copy-id -i $HOME/.ssh/id_ed25519.pub root@202.54.1.55

I am going to install ssh key for a user named vivek (type command on your laptop/desktop where you generated RSA/ed25519 keys):
$ ssh-copy-id -i ~/.ssh/id_ed25519.pub vivek@202.54.1.55

Test it

Now try logging into the Ubuntu 18.04 LTS server, with ssh command from your client computer/laptop using ssh keys:
$ ssh your-user@your-server-name-here
$ ssh vivek@202.54.1.55

What are ssh-agent and ssh-add, and how do I use them on Ubuntu 18.04?

To get rid of a passphrase for the current session, add a passphrase to ssh-agent (see ssh-agent command for more info) 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:
$ ssh-add
Enter your private key passphrase. Now try again to log into vivek@202.54.1.55 and you will NOT be prompted for a password:
$ ssh vivek@202.54.1.55

How to disable the password based login on a Ubuntu 18.04 server

Login to your server, type:
## client commands ##
$ eval $(ssh-agent)
$ ssh-add
$ ssh vivek@202.54.1.55
Now login as root user:
$ sudo -i
OR
$ su -i
Edit sshd_config file:
# vim /etc/ssh/sshd_config
OR
# nano /etc/ssh/sshd_config
Find PermitRootLogin and set it as follows:
PermitRootLogin no
Save and close the file. I am going to add a user named vivek to sudoers group on Ubuntu 18.04 server so that we can run sysadmin tasks:
# adduser vivek sudo
Restart/reload the sshd service:
# systemctl reload ssh
You can exit from all session and test it as follows:
$ ssh vivek@202.54.1.55
## become root on server for sysadmin task ##
$ sudo -i

How do I add or replace a passphrase for an existing private key?

To to change your SSH passphrase type the following command:
$ ssh-keygen -p

How do I backup my existing private/public SSH keys

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

How do I protect my ssh keys?

  1. Always use a strong passphrase.
  2. Do not share your private keys anywhere online or store in insecure cloud storage or gitlab/github servers.
  3. Restrict privileges of the account.

Tip: Create and setup an OpenSSH config file to create shortcuts for servers

See how to create and use an OpenSSH ssh_config file for more info.

How do I secure my OpenSSH server?

See “OpenSSH Server Best Security Practices” for more info.

Conclusion

You learned how to create and install ssh keys for SSH key-based authentication for Ubuntu Linux 18.04 LTS server. See OpenSSH server documents here and here for more info.

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Password authentication is the default method most SSH (Secure Shell) clients use to authenticate with remote servers, but it suffers from potential security vulnerabilities, like brute-force login attempts. An alternative to password authentication is public key authentication, in which you generate and store on your computer a pair of cryptographic keys and then configure your server to recognize and accept your keys. Using key-based authentication offers a range of benefits:

  • Key-based login is not a major target for brute-force hacking attacks.

  • If a server that uses SSH keys is compromised by a hacker, no authorization credentials are at risk of being exposed.

  • Because a password isn’t required at login, you are able to able to log in to servers from within scripts or automation tools that you need to run unattended. For example, you can set up periodic updates for your servers with a configuration management tool like Ansible, and you can run those updates without having to be physically present.

This guide will explain how the SSH key login scheme works, how to generate an SSH key, and how to use those keys with your Linode.

Note
If you’re unfamiliar with SSH connections, review the Getting Started with Linode guide.

How SSH Keys Work

SSH keys are generated in pairs and stored in plain-text files. The key pair (or keypair) consists of two parts:

  • A private key, usually named id_rsa. The private key is stored on your local computer and should be kept secure, with permissions set so that no other users on your computer can read the file.

    Caution

  • A public key, usually named id_rsa.pub. The public key is placed on the server you intend to log in to. You can freely share your public key with others. If someone else adds your public key to their server, you will be able to log in to that server.

When a site or service asks for your SSH key, they are referring to your SSH public key (id_rsa.pub). For instance, services like GitHub and Gitlab allow you to place your SSH public key on their servers to streamline the process of pushing code changes to remote repositories.

The authorized_keys File

In order for your Linode to recognize and accept your key pair, you will need to upload your public key to your server. More specifically, you will need to upload your public key to the home directory of the user you would like to log in as. If you would like to log in to more than one user on the server using your key pair, you will need to add your public key to each of those users.

To set up SSH key authentication for one of your server’s users, add your public key to a new line inside the user’s authorized_keys file. This file is stored inside a directory named .ssh/ under the user’s home folder. A user’s authorized_keys file can store more than one public key, and each public key is listed on its own line. If your file contains more than one public key, then the owner of each key listed will be able to log in as that user.

Granting Someone Else Access to your Server

To give someone else access to your server’s user, simply add their public key on a new line in your authorized_keys file, just as you would add your own. To revoke access for that person, remove that same line and save the changes.

Challenge-Response

When logging in to a server using SSH, if there is a public key on file on that server, the server will create a challenge. This challenge will be crafted in such a way that only the holder of the private SSH key will be able to decipher it.

This challenge-response action happens without any user interaction. If the person attempting to log in has the corresponding private key, then they will be safely logged in. If not, the login will either fail or fall back to a password-based authentication scheme.

SSH Key Passphrases

You can optionally provide an additional level of security for your SSH keys by encrypting them with a passphrase at the time of creation. When you attempt to log in using an encrypted SSH key, you will be prompted to enter its passphrase. This is not to be confused with a password, as this passphrase only decrypts the key file locally and is not transferred over the Internet as a password might be.

If you’d like to set up your logins so that they require no user input, then creating a passphrase might not be desirable, but it is strongly recommended nevertheless.

Linux and macOS

Generate a Key Pair

Perform the steps in this section on your local machine.

  1. Create a new key pair.

    Caution

    This command will overwrite an existing RSA key pair, potentially locking you out of other systems.

    If you’ve already created a key pair, skip this step. To check for existing keys, run ls ~/.ssh/id_rsa*.

    If you accidentally lock yourself out of the SSH service on your Linode, you can still use the Lish console to login to your server. After you’ve logged in via Lish, update your authorized_keys file to use your new public key. This should re-establish normal SSH access.

    The -b flag instructs ssh-keygen to increase the number of bits used to generate the key pair, and is suggested for additional security.

  2. Press Enter to use the default names id_rsa and id_rsa.pub in the /home/your_username/.ssh directory before entering your passphrase.

  3. While creating the key pair, you will be given the option to encrypt the private key with a passphrase. This means that the key pair cannot be used without entering the passphrase (unless you save that passphrase to your local machine’s keychain manager). We suggest that you use the key pair with a passphrase, but you can leave this field blank if you don’t want to use one.

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Upload your Public Key

There are a few different ways to upload your public key to your Linode from Linux and macOS client systems:

Using ssh-copy-id

ssh-copy-id is a utility available on some operating systems that can copy a SSH public key to a remote server over SSH.

  1. To use ssh-copy-id, pass your username and the IP address of the server you would like to access:

  2. You’ll see output like the following, and a prompt to enter your user’s password:

  3. Verify that you can log in to the server with your key.

Using Secure Copy (scp)

Secure Copy (scp) is a tool that copies files from a local computer to a remote server over SSH:

Caution
These instructions will overwrite any existing contents of the authorized_keys file on your server. If you have already set up other public keys on your server, use the ssh-copy-id command or enter your key manually.

  1. Connect to your server via SSH with the user you would like to add your key to:

  2. Create the ~/.ssh directory and authorized_keys file if they don’t already exist:

  3. Give the ~/.ssh directory and authorized_keys files appropriate file permissions:

  4. In another terminal on your local machine, use scp to copy the contents of your SSH public key (id_rsa.pub) into the authorized_keys file on your server. Substitute in your own username and your server’s IP address:

  5. Verify that you can log in to the server with your key.

Manually Copy an SSH Key

You can also manually add an SSH key to a server:

  1. Begin by copying the contents of your public SSH key on your local computer. You can use the following command to output the contents of the file:

    You should see output similar to the following:

    Note that the public key begins with ssh-rsa and ends with [email protected].

  2. Once you have copied that text, connect to your server via SSH with the user you would like to add your key to:

  3. Create the ~/.ssh directory and authorized_keys file if they don’t already exist:

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  4. Give the ~/.ssh directory and authorized_keys files appropriate file permissions:

  5. Open the authorized_keys file with the text editor of your choice (nano, for example). Then, paste the contents of your public key that you copied in step one on a new line at the end of the file.

  6. Save and close the file.

    Note

    If you initially logged into the server as root but edited the authorized_keys file of another user, then the .ssh/ folder and authorized_keys file of that user may be owned by root. Set that other user as the files’ owner:

  7. Verify that you can log in to the server with your key.

Connect to the Remote Server

  1. SSH into the server from your local machine:

  2. If you chose to use a passphrase when creating your SSH key, you will be prompted to enter it when you attempt to log in. Depending on your desktop environment, a window may appear:

    Caution
    Do not allow the local machine to remember the passphrase in its keychain unless you are on a private computer which you trust.

    You may also see the passphrase prompt at your command line:

  3. Enter your password. You should see the connection establish in the local terminal.

Windows

The following instructions use the PuTTY software to connect over SSH, but other options are available on Windows too.

Generate a Key Pair with PuTTY

  1. Download PuTTYgen (puttygen.exe) and PuTTY (putty.exe) from the official site.

  2. Launch puttygen.exe. The RSA key type at the bottom of the window is selected by default for an RSA key pair but ED25519 (EdDSA using Curve25519) is a comparable option if your remote machine’s SSH server supports DSA signatures. Do not use the SSH-1(RSA) key type unless you know what you’re doing.

  3. Increase the RSA key size from 2048 bits 4096 and click Generate:

  4. PuTTY uses the random input from your mouse to generate a unique key. Once key generation begins, keep moving your mouse until the progress bar is filled:

  5. When finished, PuTTY will display the new public key. Right-click on it and select Select All, then copy the public key into a Notepad file.

  6. Save the public key as a .txt file or some other plaintext format. This is important–a rich text format such as .rtf or .doc can add extra formatting characters and then your private key won’t work:

  7. Enter a passphrase for the private key in the Key passphrase and Confirm passphrase text fields. Important: Make a note of your passphrase, you’ll need it later:

  8. Click Save private key. Choose a file name and location in Explorer while keeping the ppk file extension. If you plan to create multiple key pairs for different servers, be sure to give them different names so that you don’t overwrite old keys with new:

Manually Copy the SSH Key with PuTTY

  1. Launch putty.exe. Find the Connection tree in the Category window, expand SSH and select Auth. Click Browse and navigate to the private key you created above:

  2. Scroll back to the top of the Category window and click Session. Enter the hostname or IP address of your Linode. PuTTY’s default TCP port is 22, the IANA assigned port for for SSH traffic. Change it if your server is listening on a different port. Name the session in the Saved Sessions text bar and click Save:

  3. Click the Open button to establish a connection. You will be prompted to enter a login name and password for the remote server.

  4. Once you’re logged in to the remote server, configure it to authenticate with your SSH key pair instead of a user’s password. Create an .ssh directory in your home directory on your Linode, create a blank authorized_keys file inside, and set their access permissions:

  5. Open the authorized_keys file with the text editor of your choice (nano, for example). Then, paste the contents of your public key that you copied in step one on a new line at the end of the file.

  6. Save, close the file, and exit PuTTY.

  7. Verify that you can log in to the server with your key.

Using WinSCP

Uploading a public key from Windows can also be done using WinSCP:

Caution
These instructions will overwrite any existing contents of the authorized_keys file on your server. If you have already set up other public keys on your server, use the PuTTY instructions instead.

  1. In the login window, enter your Linode’s public IP address as the hostname, the user you would like to add your key to, and your user’s password. Click Login to connect.

  2. Once connected, WinSCP will show two file tree sections. The left shows files on your local computer and the right shows files on your Linode. Using the file explorer on the left, navigate to the file where you saved your public key in Windows. Select the public key file and click Upload in the toolbar above.

  3. You’ll be prompted to enter a path on your Linode where you want to upload the file. Upload the file to /home/your_username/.ssh/authorized_keys.

  4. Verify that you can log in to the server with your key.

Connect to the Remote Server with PuTTY

Start PuTTY and Load your saved session. You’ll be prompted to enter your server user’s login name as before. However, this time you will be prompted for your private SSH key’s passphrase rather than the password for your server’s user. Enter the passphrase and press Enter.

Troubleshooting

If your SSH connections are not working as expected, or if you have locked yourself out of your system, review the Troubleshooting SSH guide for troubleshooting help.

Upload your SSH Key to the Cloud Manager

It is possible to provision each new Linode you create with an SSH public key automatically through the Cloud Manager.

  1. Log in to the Cloud Manager.

  2. Click on your username at the top right hand side of the page. Then click on My Profile in the dropdown menu that appears:

    Note
    If you are viewing the Cloud Manager in a smaller browser window or on a smaller device, then the My Profile link will appear in the sidebar links. To view the sidebar links, click on the disclosure button to the left of the blue Create button at the top of the page.

  3. From the My Profile page, select the SSH Keys tab, and then click Add a SSH Key:

  4. Create a label for your key, then paste in the contents of your public SSH key (id_rsa.pub):

  5. Click Add Key.

  6. When you next create a Linode you’ll be given the opportunity to include your SSH key in the Linode’s creation. This key will be added to the root user of the new Linode.

    In the Create Linode form, select the SSH key you’d like to include. This field will appear below the Root Password field:

Ssh Generate Public Key For User Guide

Next Steps

After you set up your SSH keys and confirm they are working as expected, review the How to Secure Your Server guide for instructions on disabling password authentication for your server.

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Ssh Generate Public Key For User Portal

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