--- title: SSH authentication using short-lived certificate through Cloudflare Tunnel excerpt: A quick quide to SSH certificate without using an identity provider. date: 2023-02-13 updated: 2023-02-14 tags: - cloudflare --- This article provides a quick-start guide to SSH certificate using Cloudflare Tunnel. More information can be found in the official docs. - [Public keys are not enough for SSH security](https://blog.cloudflare.com/public-keys-are-not-enough-for-ssh-security/) - [SSH with short-lived certificates](https://developers.cloudflare.com/cloudflare-one/tutorials/ssh-cert-bastion/) - [Configure short-lived certificates](https://developers.cloudflare.com/cloudflare-one/identity/users/short-lived-certificates/) - [Self-hosted applications](https://developers.cloudflare.com/cloudflare-one/applications/configure-apps/self-hosted-apps/) - [Connect with SSH through Cloudflare Tunnel](https://developers.cloudflare.com/cloudflare-one/connections/connect-apps/use_cases/ssh/) ## Introduction One unpleasant task I had previously in an enterprise with Linux servers was SSH key management, specifically checking the SSH public keys of departed staff have been removed from the Ansible config. Then I learned from [this article](https://smallstep.com/blog/use-ssh-certificates/) that it is possible to SSH using a short-lived (<1 day) certificate that is only issued to the user after successfully authenticate with the enterprise identity provider's (e.g. Azure AD) single sign-on (SSO). This means once a user is revoked from the identity provider, that user would not be issued with a new certificate to SSH again the next day. At that time, I didn't feel like configuring and integrating an identity provider, so I held off trying the feature. Recently, I wanted to try out the Cloudflare Zero Trust free tier. While reading through the [SSH configuration](https://developers.cloudflare.com/cloudflare-one/connections/connect-apps/use_cases/ssh/) guide, I found out that Cloudflare support issuing [SSH user certificate](https://developers.cloudflare.com/cloudflare-one/identity/users/short-lived-certificates/). While Cloudflare supports several [SSO integration](https://developers.cloudflare.com/cloudflare-one/identity/idp-integration/), it also supports authenticating using [one-time PIN](https://developers.cloudflare.com/cloudflare-one/identity/one-time-pin/) sent to an email address that does not have to be a Cloudflare account. Cloudflare also supports browser-based shell, just like the AWS [Session Manager](https://docs.aws.amazon.com/systems-manager/latest/userguide/session-manager.html). ## Prerequisites - A domain hosted on Cloudflare DNS - Cloudflare Zero Trust (free for 50 users) - A VM or cloud instance (optional, easier to clean up) ## Cloudflare Zero Trust Navigate to **Zero Trust** page shown on the sidebar after you login to [dash.cloudflare.com](https://dash.cloudflare.com). If this is your first time, Cloudflare will ask for billing info in which you can use an existing one or add a new credit card. You won't get charged as long as you stay within the **free tier** (50 users), I will show you how to check later in this article. The setup will then ask you to name your team domain _team-name_.cloudflareaccess.com. Just create a random name for now, you can always change it later. ## Add an application Once you're in Zero Trust console, navigate to **Access** -> **Applications**. **Add an application** and choose **Self-hosted**. **Configure app** tab, - Application name: any name - Session duration: 15 minutes. - In a corporate environment, "6 hours" is probably more user-friendly. - For sensitive server, consider "No duration". - Application domain: test.yourdomain.com - The subdomain should not have an existing website. - It may be possible to use an existing website, by specifying test.yourdomain.com/custom-path for SSH, though I haven't try it. - App Launcher visbility: No - Accept all available identity providers: No, unless you have integrated an identity provider. - Select One-time PIN - Instant Auth: Yes **Add policies** tab, - Policy name: any name - Action: Allow - Session duration: same - Configure rules: (Include) Emails = an email address - Any of your email is fine, regardless whether it's a Cloudflare account. - Cloudflare _will not_ create an account using that email, it will only be used to receive one-time PIN. **Setup** tab: - CORS settings: leave it as is - Cookies settings: - SameSite Attribute: blank or Lax - Either setting is practically the same, browsers default to Lax when SameSite is not set. - "Strict" value cannot be used because Cloudflare will authenticate the user on _team-name_.cloudflareaccess.com and issue a cookie on test.yourdomain.com. - HTTP Only: Yes - Additional settings: - Enable automatic cloudflared authentication: Yes - Browser rendering: SSH ## Generate a CA certificate Navigate to **Access** -> **Service Auth** -> **SSH** tab. Select the application you just created and **Generate certificate**. Copy the generated public key and save it to `/etc/ssh/ca.pub` in your host (the host you're going to SSH into). ``` sudo -e /etc/ssh/ca.pub ``` ## Create a tunnel Navigate to **Access** -> **Tunnels** - Name: any name **Install connector** tab, choose the relevant OS and run the installation command. Once installed, you should see "connected" status. **Route tunnel** tab, - Public hostname: test.example.com - This is the application domain in the [Add an application](#Add-an-application) step. - Service - SSH type: URL = localhost:22 - Replace 22 with the custom SSH port you are going to use. After finishing creating a tunnel, you should have a new CNAME DNS record that points to _tunnel-id_.cfargotunnel.com. If there is no CNAME entry, grab the tunnel ID and create a new DNS record. ## Start SSH server Install `openssh-server`. `sudo -e /etc/ssh/sshd_config.d/cf.conf` ``` TrustedUserCAKeys /etc/ssh/ca.pub ListenAddress 127.0.0.1 ListenAddress ::1 PasswordAuthentication no # Uncomment below line for custom port # Port 1234 ``` `systemctl restart ssh` or `systemctl restart sshd` ## Create a test user The easiest setup is one where a Unix username matches the email that you configured to receive one-time PIN in previous steps. For example, if you set **loremipsum**@youremail.com, then create a new user **loremipsum**. `sudo adduser loremipsum` Set a random password and leave everything else blank. ## Initiate SSH connection Install `cloudflared` on the host that you're going to SSH from. `cloudflared access ssh-config --hostname test.yourdomain.com --short-lived-cert` Example output: ``` Match host test.example.com exec "/usr/local/bin/cloudflared access ssh-gen --hostname %h" ProxyCommand /usr/local/bin/cloudflared access ssh --hostname %h IdentityFile ~/.cloudflared/%h-cf_key CertificateFile ~/.cloudflared/%h-cf_key-cert.pub ``` or ``` Host test.example.com ProxyCommand bash -c '/usr/local/bin/cloudflared access ssh-gen --hostname %h; ssh -tt %r@cfpipe-test.example.com >&2 <&1' Host cfpipe-test.example.com HostName test.example.com ProxyCommand /usr/local/bin/cloudflared access ssh --hostname %h IdentityFile ~/.cloudflared/test.example.com-cf_key CertificateFile ~/.cloudflared/test.example.com-cf_key-cert.pup ``` Save the output to `$HOME/.ssh/config`. Now, the moment of truth. `ssh loremipsum@test.example.com` (replace the username with the one you created in [Create a test user](#Create-a-test-user) step.) The terminal should launch a website to _team-name_.cloudflareaccess.com. Enter the email you configured in [Add an application](#Add-an-application) step and then enter the received 6-digit PIN. Back to the terminal, wait for at least 5 seconds and you should see the usual SSH authentication. > You may wondering why you still see fingerprint warning, I find this article [SSH Best Practices using Certificates, 2FA and Bastions](https://goteleport.com/blog/how-to-ssh-properly/) explains it well. ## Browser-based shell As a bonus, head to test.yourdomain.com (see [Add an application](#Add-an-application) step) which will redirect you to a login page just the previous step. After login with a 6-digit PIN, you shall see a browser-based shell. ## Usage monitoring Head to **Settings** -> **Account** to monitor how many users you have, each email address you configured to receive one-time PIN is counted as one user. To delete user(s), head to **Users**, tick the relevant users, **Update status** and then **Remove**. The seat usage column should show _Inactive_.