hugo-relie/content/en/posts/dns-challenge.md

---
title: "How to do HTTPS at home (when your infrastructure is private)"
date: 2024-07-02T21:00:50+02:00
draft: true
toc: true
images:
tags:
  - self-hosting
  - sysadmin
---

## The problem of having a self-hosted infrastructure

I've been maintaining a personal homelab and self-hosted infrastructure for a few years
now, but one of the most infuriating pages when starting such project is this dreaded
**Warning: Potential Security Risk Ahead** page that appears when you're using a
self-hosted certificate, or when trying to use a password on a website or app that is
served through plain HTTP.

![A screenshot of a warning from Firefox indicating that the website that is being accessed is not secure.](/images/dns_article_firefox_warning.png)

While acceptable if you're alone on your own infrastructure or dev environment, this
poses several issues if many other contexts:

- It is not acceptable to publicly expose a website presenting this issue
- It's not advisable to say "hey look, I know that your browser gives you a big red
warning, but it's okay, you can just accept" to friends/family/etc. It's just a very
bad habit to have
- After a while, it really starts to get on your nerve

Thankfully a free solution for that, which you will probably know already, has existed
for almost ten (10) years now: [Let's Encrypt and the ACME protocol](https://letsencrypt.org/)

{{< callout type="note" >}}
I promise this is not yet another Let's Encrypt tutorial, well it is, but for a more
specific use-case
{{< /callout >}}

## The Let's Encrypt solution

### What is Let's Encrypt

[Let's Encrypt](https://letsencrypt.org/) is a nonprofit certificate authority founded
in November 2014. Its main goal was to provide an easy and free way to obtain a TLS
certificate in order to make it easy to use HTTPS everywhere.

The [ACME protocol](https://letsencrypt.org/docs/client-options/) developed by Let's
Encrypt is an automated verification system aiming at doing the following:

- verifying that you own the domain for which you want a certificate
- creating and registering that certificate
- delivering the certificate to you

Most client implementation also have an automated renewal system, further reducing the
workload for sysadmins.

The current specification for the ACME protocol proposes two (2) types of challenges
to prove ownership and control over a domain: [HTTP-01](https://letsencrypt.org/docs/challenge-types/#http-01-challenge) and [DNS-01](https://letsencrypt.org/docs/challenge-types/#dns-01-challenge) challenge.

{{< callout type="note" >}}
Actually there are two (2) others: [TLS-SNI-01](https://letsencrypt.org/docs/challenge-types/#tls-sni-01) which is now disabled, and [TLS-ALPN-01](https://letsencrypt.org/docs/challenge-types/#tls-alpn-01) which is only aimed at a very
specific category of users, which we will ignore here.
{{< /callout >}}

### The common solution: HTTP challenge

The [HTTP-01](https://letsencrypt.org/docs/challenge-types/#http-01-challenge) challenge
is the most common type of ACME challenge, and will satisfy most use-cases.

![A schema describing the HTTP challenge workflow for the ACME protocol and the interactions between the application server, Let's Encrypt, and the DNS server, all of them public.](/images/dns_article_http_challenge.svg)

For this challenge, you need the following elements :

- A domain name and a record for that domain in a public DNS server (it can be a self-hosted DNS server, your providers', etc)
- Access to a server with a public IP that can be publicly reached

When performing this type of challenge, the following happens (in a very simplified way):

1. Your ACME client will ask to start a challenge to the Let's Encrypt API
2. In return, it will get a token
3. It will then either start a standalone server, or edit the configuration for your
current web server (nginx, apache, etc) to serve a file containing the token and a fingerprint of your account key.
4. Let's Encrypt will try to resolve your domain `test.example.com`.
5. If resolution works, then it will check the url `http://test.example.com/.well-known/acme-challenge/<TOKEN>`, and verify that the file from step 3 is served with the correct
content.

If everything works as expected, then the ACME client can download the certificate and key, and you can configure your reverse proxy or server to use this valid certificate,
all is well.

{{< callout type="help" >}}
Okay, but my app contains my accounts, or my proxmox management interface, and I
don't really want to make it public, so how does it work here?
{{< /callout >}}

Well it doesn't. For this type of challenge to work, the application server **must** be
public. For this challenge you need to prove that you have control over the application
that uses the target domain (even if you don't control the domain itself). But the
DNS-01 challenge bypasses this limitation.

### When it's not enough: the DNS challenge

As we saw in the previous section, sometimes, for various reasons, your application
server is in a private zone. It must be only reachable from inside a private network,
but you still want to be able to use a free Let's Encrypt certificate.

For this purpose, the [DNS-01](https://letsencrypt.org/docs/challenge-types/#dns-01-challenge) challenge is based on proving that you have control over the **DNS
server** itself, instead of the application server.

![A schema describing the DNS challenge workflow for the ACME protocol and the interaction between Let's Encrypt, the public DNS server and the private application server](/images/dns_article_dns_challenge_1.svg)

For this type of challenge, the following elements are needed :

- A public DNS server you have control over (can be a self-hosted server, or your DNS provider)
- A ACME client (usually it would be on your application server), it doesn't need to be public

Then, the challenge is done the following way :

1. Your ACME client will ask to start a challenge to the Let's Encrypt API.
2. In return, it will get a token.
3. The client then created a `TXT` record at `_acme-challenge.test.example.com` derived from the token
and your account key.
4. Let's Encrypt will try to resolve the expected `TXT` record, and verify that the content is correct.

If the verification succeeds, you can download your certificate and key, just like the other
type of challenge.

It's important to note that **at no point in time did Let's Encrypt have access to the
application server itself**, because this challenges involves proving that you control
the domain, not that you control the destination of that domain.

As someone trying to use a valid certificate for my proxmox interface, this is the way I
would want to go, because it would allow me to have a valid certificate, despite my server
not being public at all. So let's see how it works in practice.

## DNS challenge in practice

For this example, I will try to obtain a certificate for my own domain
`example.internal.faercol.me`.As this name hints, it is an internal domain and should not
be publicly reachable, so this means I'm going to use a DNS challenge. I don't really want
to use my DNS provider API for this, so I'm going to use a self-hosted [bind](https://www.isc.org/bind/)
server for that.

### Configuring the DNS server

### Enabling remote DNS zone modification

### Performing the challenge

## Bonus: completely hiding your private domains from outside

![A schema describing the DNS challenge workflow for the ACME protocol using a public and private DNS servers](/images/dns_article_dns_challenge_2.svg)

![alt text](image.png)