mirror of https://gitlab.com/curben/blog
post: SSH authentication using short-lived certificate through Cloudflare Tunnel
This commit is contained in:
Ming Di Leom
parent
da79ea8455
commit
392170d9ca
|
|
@ -0,0 +1,169 @@
|
||||||
|
---
|
||||||
|
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
|
||||||
|
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.
|
||||||
Loading…
Reference in New Issue