Golalta

There’s no denying that SSH is the de facto tool for *nix server administration. It’s far from perfect, but it was designed with security in mind and there’s been a huge amount of tooling written over the years to make it easier to use. In addition, many popular products and just about every server deployment system integrates with SSH somehow. It is universally supported across pretty much all architectures and distributions, from Raspberry Pis all the way up to massive supercomputer clusters.

1. Generate Ssh Public Private Key Pair Linux
2. Generate Public Ssh Key Windows
3. Ssh Generate Public Key Pair Linux
4. Public Key Example
5. Ssh Generate Public Key Pair 2017
6. Generate Ssh Public Key Pair

SSH is a powerful tool which often grants a lot of access to anyone using it to log into a server. In this post, I’m going to talk about a few different ways that you can easily improve the security of your SSH model without needing to deploy a new application or make any huge changes to user experience.

SSH certificates

Most people can agree that using public key authentication for SSH is generally better than using passwords. Nobody ever types in a private key, so it can’t be keylogged or observed over your shoulder. SSH keys have their own issues, however, some of which we’ve covered in a previous post about SSH key management.

Apr 15, 2020  To see whether you have SSH keys on the system, run the command: ls -al /.ssh/id.pub. If the output tells you there are no such files, move on to the next step, which shows you how to generate SSH keys. In case you do have them, you can use the existing keys, back them up and create a new pair. To generate the public/private key pair, enter this in the Command Prompt: ssh-keygen At the first prompt, “Enter file in which to save the key,” press Enter to save it in the default location.

The next level up from SSH keys is SSH certificates. OpenSSH has supported the use of certificates since OpenSSH 5.4 which was released back in 2010.

With SSH certificates, you generate a certificate authority (CA) and then use this to issue and cryptographically sign certificates which can authenticate users to hosts, or hosts to users. You can generate a keypair using the ssh-keygen command, like this:

The host_ca file is the host CA’s private key and should be protected. Don’t give it out to anyone, don’t copy it anywhere, and make sure that as few people have access to it as possible. Ideally, it should live on a machine which doesn’t allow direct access and all certificates should be issued by an automated process.

In addition, it’s best practice to generate and use two separate CAs - one for signing host certificates, one for signing user certificates. This is because you don’t want the same processes that add hosts to your fleet to also be able to add users (and vice versa). Using separate CAs also means that in the event of a private key being compromised, you only need to reissue the certificates for either your hosts or your users, not both at once.

As such, we’ll also generate a user_ca with this command:

The user_ca file is the user CA’s private key and should also be protected in the same way as the host CA’s private key.

Issuing host certificates (to authenticate hosts to users)

In this example, we’ll generate a new host key (with no passphrase), then sign it with our CA. You can also sign the existing SSH host public key if you’d prefer not to regenerate a new key by copying the file (/etc/ssh/ssh_host_rsa_key.pub) from the server, signing it on your CA machine, and then copying it back.

ssh_host_rsa_key-cert.pub contains the signed host certificate.

Here’s an explanation of the flags used:

• -s host_ca: specifies the filename of the CA private key that should be used for signing.
• -I host.example.com: the certificate’s identity - an alphanumeric string that will identify the server. I recommend using the server’s hostname. This value can also be used to revoke a certificate in future if needed.
• -h: specifies that this certificate will be a host certificate rather than a user certificate.
• -n host.example.com: specifies a comma-separated list of principals that the certificate will be valid for authenticating - for host certificates, this is the hostname used to connect to the server. If you have DNS set up, you should use the server’s FQDN (for example host.example.com) here. If not, use the hostname that you will be using in an ~/.ssh/config file to connect to the server.
• -V +52w: specifies the validity period of the certificate, in this case 52 weeks (one year). Certificates are valid forever by default - expiry periods for host certificates are highly recommended to encourage the adoption of a process for rotating and replacing certificates when needed.

Configuring SSH to use host certificates

Generate Ssh Public Private Key Pair Linux

You also need to tell the server to use this new host certificate. Copy the three files you just generated to the server, store them under the /etc/ssh directory, set the permissions to match the other files there, then add this line to your/etc/ssh/sshd_config file:

Once this is done, restart sshd with systemctl restart sshd.

Your server is now configured to present a certificate to anyone who connects. For your local ssh client to make use of this (and automatically trust the host based on the certificate’s identity), you will also need to add the CA’s public key to your known_hosts file.

You can do this by taking the contents of the host_ca.pub file, adding @cert-authority *.example.com to the beginning, then appending the contents to ~/.ssh/known_hosts:

The value *.example.com is a pattern match, indicating that this certificate should be trusted for identifying any host which you connect to that has a domain of *.example.com - such as host.example.com above. This is a comma-separated list of applicable hostnames for the certificate, so if you’re using IP addresses or SSH config entries here, you can change this to something like host1,host2,host3 or 1.2.3.4,1.2.3.5 as appropriate.

Once this is configured, remove any old host key entries for host.example.com in your ~/.ssh/known_hosts file, and start an ssh connection. You should be connected straight to the host without needing to trust the host key. You can check that the certificate is being presented correctly with a command like this:

At this point, you could continue by issuing host certificates for all hosts in your estate using your host CA. The benefit of doing this is twofold: you no longer need to rely on the insecure trust on first use (TOFU) model for new hosts, and if you ever redeploy a server and therefore change the host key for a certain hostname, your new host could automatically present a signed host certificate and avoid the dreaded WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! message.

Issuing user certificates (to authenticate users to hosts)

In this example, we’ll generate a new user key and sign it with our user CA. It’s up to you whether you use a passphrase or not.

user-key-cert.pub contains the signed user certificate. You’ll need both this and the private key (user-key) for logging in.

Here’s an explanation of the flags used:

• -s user_ca: specifies the CA private key that should be used for signing
• -I [email protected]: the certificate’s identity, an alphanumeric string that will be visible in SSH logs when the user certificate is presented. I recommend using the email address or internal username of the user that the certificate is for - something which will allow you to uniquely identify a user. This value can also be used to revoke a certificate in future if needed.
• -n ec2-user,gus: specifies a comma-separated list of principals that the certificate will be valid for authenticating, i.e. the *nix users which this certificate should be allowed to log in as. In our example, we’re giving this certificate access to both ec2-user and gus.
• -V +1d: specifies the validity period of the certificate, in this case +1d means 1 day. Certificates are valid forever by default, so using an expiry period is a good way to limit access appropriately and ensure that certificates can’t be used for access perpetually.

If you need to see the options that a given certificate was signed with, you can use ssh-keygen -L:

Configuring SSH for user certificate authentication

Once you’ve signed a certificate, you also need to tell the server that it should trust certificates signed by the user CA. To do this, copy the user_ca.pub file to the server and store it under /etc/ssh, fix the permissions to match the other public key files in the directory, then add this line to /etc/ssh/sshd_config:

Once this is done, restart sshd with systemctl restart sshd.

Your server is now configured to trust anyone who presents a certificate issued by your user CA when they connect. If you have a certificate in the same directory as your private key (specified with the -i flag, for example ssh -i /home/gus/user-key [email protected]), it will automatically be used when connecting to servers.

Checking logs

If you look in your server’s sshd log (for example, by running journalctl -u sshd), you will see the name of the certificate being used for authentication, along with the fingerprint of the signing CA:

If the user tries to log in as a principal (user) which they do not have permission to use (for example, their certificate grants ec2-user but they try to use root), you’ll see this error in the logs:

If the certificate being presented has expired, you’ll see this error in the logs:

One way that you could make further use of user certificates is to set up a script which will use your CA to issue a certificate to log into production servers, valid only for the next two hours. Every use of this script or process could add logs as to who requested a certificate and embed their email address into the certificate. After this is done, every time the user uses that certificate to access a server (regardless of which principal they log in as), their email address will be logged. This can provide a useful part of an audit trail.

There are many other useful things you can do with SSH certificates, such as forcing a specific command to be run when presenting a certain certificate, or denying the ability to forward ports, X11 traffic or SSH agents. Take a look at man ssh-keygen for some ideas.

Enforce the use of a bastion host

Another way to improve your SSH security is to enforce the use of a bastion host - a server which is specifically designed to be the only gateway for access to your infrastructure. Lessening the size of any potential attack surface through the use of firewalls enables you to keep a better eye on who is accessing what.

Switching to the use of a bastion host doesn’t have to be an arduous task, especially if you’re using SSH certificates. By setting up your local ~/.ssh/config file, you can automatically configure all connections to hosts within a certain domain to go through the bastion.

Here’s a very quick example of how to force SSH access to any host in the example.com domain to be routed through a bastion host, bastion.example.com:

To make this even simpler, if you add user-key to your local ssh-agent with ssh-add user-key, you can also remove the IdentityFile entries from the SSH config file.

Once you’re using the bastion host for your connections, you can use iptables (or another *nix firewall configuration tool of your choosing) on servers behind the bastion to block all other incoming SSH connections. Here’s a rough example using iptables:

It’s a good idea to leave a second SSH session connected to the bastion while running these commands so that if you inadvertently input the wrong IP address or command, you should still have working access to the bastion to fix it via the already-established connection.

Add 2-factor authentication to your SSH logins

2-factor authentication makes it more difficult for bad actors to log into your systems by enforcing the need for two different “factors” or methods to be able to successfully authenticate.

This usually comes down to needing both “something you know” (like a password, or SSH certificate in our example) and “something you have” (like a token from an app installed on your phone, or an SMS with a unique code). One other possibility is requiring the use of “something you are” - for example a fingerprint, or your voice.

In this example, we’ll install the google-authenticatorpluggable authentication module, which will require users to input a code from the Google Authenticator app on their phone in order to log in successfully. You can download the app for iOS here and Android here.

As a general note, it’s always important to consider the user experience when enforcing security measures. If your measures are too draconian then users may attempt to find ways to defeat and work around them, which will eventually reduce the overall security of your systems and lead to the creation of back doors. To give our users a reasonable experience in this example, we are only going to require 2-factor authentication to be able to log into our bastion host. Once authenticated there, users should be able to log into other hosts simply by using their valid SSH certificate. This combination should give an acceptable level of security without interfering too much with user workflows. With this in mind, however, it is always prudent and appropriate to enforce extra security measures in specific environments which contain critical production data or sensitive information.

Install google-authenticator

On RHEL/CentOS based systems, you can install the google-authenticator module from the EPEL repository:

For Debian/Ubuntu-based systems, this is available as the libpam-google-authenticator package:

The google-authenticator module has many options you can set which are documented here. In the interest of saving time, we are going to use some sane defaults in this example: disallow reuse of the same token twice, issue time-based rather than counter-based codes, and limit the user to a maximum of three logins every 30 seconds. To set up Google 2-factor authentication with these settings, a user should run this command:

You can also run google-authenticator with no flags and answer some prompts to set up interactively if you prefer.

This will output a QR code that the user can scan with the app on their phone, plus some backup codes which they can use if they lose access to the app. These codes should be stored offline in a secure location.

Scan the generated QR code for your user now with the Google Authenticator app and make sure that you have a 6-digit code displayed. If you need to edit or change any settings in future, or remove the functionality completely, the configuration will be stored under ~/.google_authenticator.

Configure PAM for 2-factor authentication

To make the system enforce the use of these OTP (one-time password) codes, we’ll first need to edit the PAM configuration for the sshd service (/etc/pam.d/sshd) and add this line to the end of the file:

The nullok at the end of this line means that users who don’t have a second factor configured yet will still be allowed to log in so that they can set one up. Once you have 2-factor set up for all your users, you should remove nullok from this line to properly enforce the use of a second factor.

We also need to change the default authentication methods so that SSH won’t prompt users for a password if they don’t present a 2-factor token. These changes are also made in the /etc/pam.d/sshd file:

• On RHEL/CentOS-based systems, comment out auth substack password-auth by adding a # to the beginning of the line: #auth substack password-auth
• On Debian/Ubuntu-based systems, comment out @include common-auth by adding a # to the beginning of the line: #@include common-auth

Save the /etc/pam.d/sshd file once you’re done.

Configure SSH for 2-factor authentication

We also need to tell SSH to require the use of 2-factor authentication. To do this, we make a couple of changes to the /etc/ssh/sshd_config file.

Firstly, we need to change ChallengeResponseAuthentication no to ChallengeResponseAuthentication yes to allow the use of PAM for credentials.

We also need to set the list of acceptable methods for authentication by adding this line to the end of the file (or editing the line if it already exists):

This tells SSH that it should require both a public key (which we are going to be satisfying using an SSH certificate) and a keyboard-interactive authentication (which will be provided and satisfied by the sshd PAM stack). Save the /etc/ssh/sshd_config file once you’re done.

At this point, you should restart sshd with systemctl restart sshd. Make sure to leave an SSH connection open so that you can fix any errors if you need to. Restarting SSH will leave existing connections active, but new connections may not be allowed if there is a configuration problem.

Test it out

Connect to your bastion host directly and you should see a prompt asking you for your 2-factor code:

Type the code presented by your Google Authenticator app and your login should proceed normally.

If you check the sshd log with journalctl -u sshd, you should see a line indicating that your login succeeded:

Conclusion

The methods above give practical examples of several ways in which you can improve the security of your SSH infrastructure, all while giving users the flexibility to keep using the tools they’re familiar with.

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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.

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.

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:

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:

Generate Public Ssh Key Windows

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:

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.

Ssh Generate Public Key Pair Linux

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

Public Key Example

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

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

Ssh Generate Public Key Pair 2017

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:

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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This guide is published under a CC BY-ND 4.0 license.