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

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

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

{{< 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 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 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 and DNS-01 challenge.

{{< callout type="note" >}} Actually there are two (2) others: TLS-SNI-01 which is now disabled, and 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 challenge is the most common type of ACME challenge, and will satisfy most use-cases.

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 challenge is based on proving that you have control over the DNS server itself, instead of the application server.

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 server for that.

Configuring the DNS server

The first step is configuring the DNS server. For this, I'll just use a bind server installed from my usual package manager.

# example on Debian 12
sudo apt install bind9

Most of the configuration happens in the /etc/bind directory, mostly in /etc/bind/named.conf.local

root@dns-server: ls /etc/bind/
bind.keys  db.127  db.empty  named.conf         named.conf.local    rndc.key
db.0    db.255  db.local  named.conf.default-zones  named.conf.options  zones.rfc1918

Let's declare a first zone, for internal.example.com. Add the following config to /etc/bind/named.conf.local

zone "internal.example.com." IN {
  type master;
  file "/var/lib/bind/internal.example.com.zone";

This simply declares a new zone which is described in the file /var/lib/bind/internal.example.com.zone

Let's now create the zone itself. A DNS zone has a base structure that you must follow

$ORIGIN .
$TTL 7200 ; 2 hours
internal.example.com IN SOA ns.internal.example.com. admin.example.com. (
                            2024070301 ; serial
                            3600       ; refresh (1 hour)
                            600        ; retry (10 minutes)
                            86400      ; expire (1 day)
                            600        ; minimum (10 minutes)
                            )
                    NS ns.internal.example.com.

$ORIGIN internal.example.com.
ns      A     1.2.3.4
test    A     192.168.1.2

This file declares a zone internal.example.com which master is ns.internal.example.com. It also sets the parameters (time to live for the records, and the current serial for the zone config).

Finally, two (2) A records are created, associating the name ns.internal.example.com to the IP address 1.2.3.4, and test.internal.example.com (the domain for which we want a certificate) to a local IP address 192.168.1.2.

A simple systemctl restart bind9 would be enough to apply the modification, but we still have one thing to do, which is allowing remote modifications to the zone.

Enabling remote DNS zone modification

To allow remote modification of our DNS zone, we are going to use TSIG which stands for Transaction signature. It's a way to secure server to server operations to edit a DNS zone, and is preferred to access control based on IP addresses.

Let's start with creating a key using the command tsig-keygen <keyname>

➜ tsig-keygen letsencrypt
key "letsencrypt" {
 algorithm hmac-sha256;
 secret "oK6SqKRvGNXHyNyIEy3hijQ1pclreZw4Vn5v+Q4rTLs=";
};

This creates a key with the given name using the default algorithm (which is hmac-sha256). The entire output of this command is actually a code block that you can add to your bind9 configuration.

Finally, using update-policy, allow this key to be used to update the zone.

update-policy {
  grant letsencrypt. zonesub txt;
};

{{< callout type="note" >}} Doing so allows users to update everything in your zone using this key. In fact you would only need to update _acme-challenge.test.internal.example.com as seen in the DNS challenge description.

If you want a better restriction, then you can use the following configuration instead

update-policy {
  grant letsencrypt. name _acme-challenge.test.internal.example.com. txt;
};

{{< /callout >}}

This means your entire named.conf.local would become something like this

key "letsencrypt" {
 algorithm hmac-sha256;
 secret "oK6SqKRvGNXHyNyIEy3hijQ1pclreZw4Vn5v+Q4rTLs=";
};

zone "internal.example.com." IN {
  type master;
  file "/var/lib/bind/internal.example.com.zone";
  update-policy {
    grant letsencrypt. zonesub txt;
  };
};

{{< callout type="warning" >}} Be very cautious about the . at the end of the zone name and the key name, they are easy to miss, and forgetting them will cause issues that would be hard to detect. {{< /callout >}}

With that being done, you can restart the DNS server and everything is ready server side, the only remaining thing to do would be the DNS challenge itself.

Performing the challenge

Start by installing the certbot with the RFC2136 plugin (to perform the DNS challenge).

apt install python3-certbot-dns-rfc2136

It's handled using a .ini configuration file, let's put it in /etc/certbot/credentials.ini

dns_rfc2136_server = <you_dns_ip>
dns_rfc2136_port = 53
dns_rfc2136_name = letsencrypt.
dns_rfc2136_secret = oK6SqKRvGNXHyNyIEy3hijQ1pclreZw4Vn5v+Q4rTLs=
dns_rfc2136_algorithm = HMAC-SHA512

Finally, run the challenge using certbot (if it's the first time you're using certbot on that machine, it might ask for an email to handle admin stuff).

root@toolbox:~# certbot certonly --dns-rfc2136 --dns-rfc2136-credentials /etc/certbot/credentials.ini -d 'test.internal.example.com'

Saving debug log to /var/log/letsencrypt/letsencrypt.log
Requesting a certificate for test.internal.example.com
Waiting 60 seconds for DNS changes to propagate

Successfully received certificate.
Certificate is saved at: /etc/letsencrypt/live/test.internal.example.com/fullchain.pem
Key is saved at:         /etc/letsencrypt/live/test.internal.example.com/privkey.pem
This certificate expires on 2024-09-30.
These files will be updated when the certificate renews.
Certbot has set up a scheduled task to automatically renew this certificate in the background.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
If you like Certbot, please consider supporting our work by:
 * Donating to ISRG / Let's Encrypt:   https://letsencrypt.org/donate
 * Donating to EFF:                    https://eff.org/donate-le
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

And that's done, you have a certificate, and a no point in time did you need to actually expose your application to the outside world.

Now because I like to go way too far, I can propose two (2) improvements to this setup:

• Using ACL in addition to the TSIG key to secure operations on the DNS server
• Using a second DNS server only locally accessible for your private records, and using the public server to only perform challenges

Bonus 1: adding a second layer of authentication to connect to the DNS

In our setup, we used TSIG to secure our access to the DNS server, meaning that having the key is necessary to perform the operations. If you are paranoid, or if you want to do a little bit more, then you could add a second layer of authentication based on Access Control List (ACL).

ACL allow to filter allowed operations based on several characteristics, such as IP address, TSIG key, subnet. In our case, we will use an IPV4 subnet from inside a Wireguard tunnel between the application servers (DNS clients) and the DNS server. It could be any form of tunnel, but Wireguard is easy to configure and perfect for point-to-point tunnels such as what we are doing here.

Wireguard configuration

First, let's create the Wireguard tunnel.

We start by creating two wireguard key pairs, which can be done this way

# Install wireguard tools
apt install wireguard-tools

# Create the keypair
wg genkey | tee privatekey | wg pubkey > publickey

Private key is in the privatekey file, and public key in the publickey file.

Then we can create the server configuration, create a file /etc/wg/wg0.conf on the DNS server.

[Interface]
PrivateKey = <server_private_key>
Address = 192.168.42.1/24
ListenPort = 51820

[Peer]
PublicKey = <client_public_key>
AllowedIPs = 192.168.42.0/24

Then on the client side you can do the same

[Interface]
PrivateKey = <client_private_key>
Address = 192.168.42.2/24

[Peer]
PublicKey = <server_public_key>
Endpoint = <dns_public_ip>:51820
AllowedIPs = 192.168.42.1/32

Then you can start the tunnel on both sides using wg-quick up wg0, check that ip works by pinging the server from the client

root@toolbox:~ ping 192.168.42.1
PING 192.168.42.1 (192.168.42.1) 56(84) bytes of data.
64 bytes from 192.168.42.1: icmp_seq=1 ttl=64 time=19.2 ms
64 bytes from 192.168.42.1: icmp_seq=2 ttl=64 time=8.25 ms

Basically, we created a new network 192.168.42.0/24 which links the DNS server and our client, and we can restrict modification to the DNS zone to force them to be from inside the virtual network, instead of allowing them from anywhere.

{{< callout type="note" >}} The ACL that we are going to use here can have many other purposes, such as hiding some domains, or serving different versions of a zone depending on the origin of the client. This is not our topic of concern here though. {{< /callout >}}

DNS configuration

Using ACLs, we are going to split the DNS zone into several views based on the source IP. Basically our goal is to say that

• Users coming from inside our wireguard network 192.168.42.0/24 can modify DNS records in our zone using the TSIG key defined earlier.
• Users coming from any other IP can read the DNS zone, but nothing else, so they can't update it, even using the correct key.

ACL can be defined inside named.conf.local using the following syntax.

acl local {
  127.0.0.0/8;
  192.168.42.0/24;
};

This means that local addresses, and addresses coming from our wireguard network will be considered as local and can be referenced as such in the rest of the configuration.

Then, a view can be created like this:

view "internal" {
  match-clients { local; };
  zone "internal.example.com." IN {
    type master;
    file "/var/lib/bind/internal.example.com.zone";
    update-policy {
      grant letsencrypt. zonesub txt;
    };
  };
};

Basically this means that the view internal is only used for clients that match the local ACL defined above. In this view we define the zone internal.example.com, which is the zone we defined ealier.

We also need to declare the zone for non-local users who wouldn't match the local ACL. It's important to note that you cannot use the same zone file twice in different zones, so we cannot define the public view exactly the same way. Our public view will be defined the following way:

view "public" {
  zone "internal.example.com." IN {
    in-view internal;
  };
};

This way, in the public view, we define the internal.example.com zone, and we define this zone as being inside the internal view. This way, we will serve the exact same DNS zone whatever the origin, but the update policy only applies to user from local addresses, and they will be the only ones able to edit the zone.

In summary, our named.conf.local file should now look like this.

acl local {
  127.0.0.0/8;
  192.168.42.0/24;
};

key "letsencrypt." {
  algorithm hmac-sha512;
  secret "oK6SqKRvGNXHyNyIEy3hijQ1pclreZw4Vn5v+Q4rTLs=";
};

view "internal" {
  match-clients { local; };
  zone "internal.example.com." IN {
    type master;
    file "/var/lib/bind/internal.example.com.zone";
    update-policy {
      grant letsencrypt. zonesub txt;
    };
  };
};

view "public" {
  zone "internal.example.com." IN {
    in-view internal;
  };
};

And now

Bonus 2: completely hiding your private domains from outside