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Setup Caddy with a CA and ACME server on Alpine Linux

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Now that we have a WireGuard VPN with an awesome internal DNS server, let's get a web server with HTTPS!

Caddy

Install

You will need to enable the community repo first.

doas apk add caddy

Configuration

Create a folder to serve stuff from, I placed it in

/srv/www

Create the config in

/etc/caddy/Caddyfile

Here's the config, it's very simple to get started:

intra.philt3r:80
root * /srv/www
file_server browse

This config will only launch an HTTP server, the HTTPS will come later.

It should work only from the WireGuard peers, since they can resolve the DNS name intra.philt3r.

If there is no index.html in the folder, it will serve static files directly.

Script to launch

Caddy already has a service!

Generate keys and certificates

We will generate the Root CA, the Intermediate CA.

Generate these with openssl installed on a computer, preferabbly offline.

Make sure the keys are stored in a safe place, I will store mine inside of a KeePassXC keystore.

OpenSSL Configuration

inside a folder, create a file

config.conf

In [CA_root], make sure to put your folder dir

# OpenSSL root CA configuration file.

[ ca ]
# `man ca`
default_ca = CA_root

[ CA_root ]
# Directory and file locations.
dir               = /home/phil/ca
certs             = $dir/certs
crl_dir           = $dir/crl
new_certs_dir     = $dir/newcerts
database          = $dir/index.txt
serial            = $dir/serial
RANDFILE          = $dir/private/.rand

# The root key and root certificate.
# Match names with Smallstep naming convention
private_key       = $dir/root_ca_key
certificate       = $dir/root_ca.crt

# For certificate revocation lists.
crlnumber         = $dir/crlnumber
crl               = $dir/crl/ca.crl.pem
crl_extensions    = crl_ext
default_crl_days  = 30

# SHA-1 is deprecated, so use SHA-2 instead.
default_md        = sha256

name_opt          = ca_default
cert_opt          = ca_default
default_days      = 25202
preserve          = no
policy            = policy_strict

[ policy_strict ]
# The root CA should only sign intermediate certificates that match.
# See the POLICY FORMAT section of `man ca`.
countryName             = match
stateOrProvinceName 	= supplied
localityName	    	= supplied
organizationName        = match
commonName              = supplied

[ req ]
# Options for the `req` tool (`man req`).
default_bits        = 4096
distinguished_name  = req_distinguished_name
string_mask         = utf8only

# SHA-1 is deprecated, so use SHA-2 instead.
default_md          = sha256

# Extension to add when the -x509 option is used.
x509_extensions     = v3_ca

[ req_distinguished_name ]
# See <https://en.wikipedia.org/wiki/Certificate_signing_request>.
countryName          = Country (2 letter code)
stateOrProvinceName  = State or Region
localityName         = City
commonName           = Common Name
0.organizationName   = Organization Name

[ v3_ca ]
# Extensions for a typical CA (`man x509v3_config`).
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid:always,issuer
basicConstraints = critical, CA:true
keyUsage = critical, digitalSignature, cRLSign, keyCertSign

[ v3_intermediate_ca ]
# Extensions for a typical intermediate CA (`man x509v3_config`).
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid:always,issuer
basicConstraints = critical, CA:true, pathlen:0
keyUsage = critical, digitalSignature, cRLSign, keyCertSign

After, run

mkdir newcerts
touch index.txt
echo 1420 > serial

We are now ready to generate keys and certificates.

Root key and certificate

Generate key:

openssl genrsa -aes256 -out root_ca_key 4096

It will ask for a passphrase, I generated mine with my KeePassXC.

Generate root certificate:

openssl req -config config.conf -key root_ca_key -days 3650 -new -x509 -sha256 -extensions v3_ca -out root_ca.crt

My root CA will last for 3650 days (10 years).

Here's the info I provided:

Country (2 letter code) []:FR
State or Region []:Bretagne
City []:Rennes
Common Name []:philt3r CA
Organization Name []:philt3r

I saved the root_ca_key and root_ca.crt inside my KeePassXC.

Intermediate key and certificate

Generate key:

openssl genrsa -aes256 -out intermediate_ca_key 4096

It will ask for a passphrase, I generated mine with my KeePassXC.

Generate certificate request:

openssl req -config config.conf -new -sha256 -key intermediate_ca_key -out intermediate_ca.csr.pem

Here's the info I provided:

Country (2 letter code) []:FR
State or Region []:Bretagne
City []:Rennes
Common Name []:philt3r Intermediate CA 
Organization Name []:philt3r

Sign certificate request with Root key:

openssl ca -config config.conf -keyfile root_ca_key -cert root_ca.crt -extensions v3_intermediate_ca -days 1825 -notext -md sha256 -in intermediate_ca.csr.pem -out intermediate_ca.crt

My Intermediate certificate will last for 1825 days (5 years).

Save these files, I saved them in my KeePassXC:

Once everything is saved and backed up, delete everything from your computer securely.

CA and ACME server

I discovered Smallstep, which allows to become your own ACME server.

Install

They provide packages for Alpine, but one of them is only in the testing repos.

Edit

/etc/apk/repositories

And add:

@testing http://mirrors.ircam.fr/pub/alpine/edge/testing

Afterwards, run

apk update

to refresh the packages.

Install the packages with

apk add step-cli step-certificates@testing

The @testing is to tell apk to pull the package from the testing repo.

Configuration

Start by creating the folder where step will save all the configs:

mkdir /etc/step-ca -p

Let's configure step-ca!

STEPPATH=/etc/step-ca step ca init --name="philt3r" --acme --address="10.131.111.1:444" --provisioner="philt3r" --deployment-type standalone

I ask it to run on the address 10.131.111.1 (the WireGuard ip) and on the port 444. The port 443 will be used for a https server, so I picked 443 + 1.

Since I want an ACME server, I asked to get one.

Step will ask what IP address the clients will use to reach your ca, reply with 10.131.111.1, because only WireGuard peers and the server should be allowed.

This will prompt a password, put one.

Step will generate a root and intermediate key, as well as an intermediate certificate. We don't want that, since we already generated our own.

Copy these files in /etc/step-ca/certs:

Copy intermediate_ca_key in /etc/step-ca/secrets folder. I use the key directly, but in a safe environment use a Yubikey, but I don't have one.

Start the CA/ACME server

Run

step-ca /etc/step-ca/config/ca.json

to start the server. It will ask your password to decrypt the intermediate_ca_key. Provide the password.

The server should start, stop it.

We will now create a file containing the password of the intermediate_ca_key, since we want to have the ACME server starting when Alpine will boot.

Why put the password inside a file? Well, simply because we can't type the password at boot. Again, in an ideal environment, use a Yubikey.

Create a file at

/etc/step-ca/password.txt

and place the password inside that file.

step should run as the user step-ca, so update the permissions on the config folder:

chown step-ca:step-ca -Rv /etc/step-ca/

To verify that everything worked, run:

step-ca /etc/step-ca/config/ca.json --password-file=/etc/step-ca/password.txt

Stop the server again.

Script to launch

Step already has a service!

Use ACME with Caddy

Now let's tell Caddy to get TLS certificates with our ACME server.

Edit the /etc/caddy/Caddyfile:

# global
{
        # step-ca ACME server
        acme_ca https://10.131.111.1:444/acme/acme/directory
}

intra.philt3r intra.philt3r:80 {
        root * /srv/www
        file_server browse
}

Make sure step-ca is started, and restart Caddy to make sure everything is good:

rc-service caddy restart

Now we need to tell our system to trust the certificates.

Download the file containing the certificates. It is available at this URL:

https://10.131.111.1:444/roots.pem

On every device you want to trust your certificates, you will need to download the file on the device, then you will need to tell your operating system to trust it.

Start on boot

Start caddy and step-ca on startup with:

rc-update add step-ca
rc-update add caddy

Reboot to make sure everything works.

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