Nebula-Cert-Maker

A script which allows you to create new host certificates and keys

Originally written for an offline Nebula CA for my home network and VPS, this script uses features in the Nebula Certificate Authority certificate to pre-fill certain fields that help with my Nebula overlay network.

Create the CA

I'm using a Raspberry Pi that I can boot offline from the rest of my network. I generated my root certificate like this:

nebula-cert ca -ips 192.0.2.0/24 -name nebula.example.org

By generating the CA certificate like this, I can derive the DNS search domain (somehost.nebula.example.org) and the base CIDR for the nodes on the network.

This certificate defaults to 1 year (or 8760 hours), so after 6 months, a second CA would need to be generated in order to ensure that the certificates you have created don't expire. (You can put longer durations in, but I'm electing to keep it as 1 year. Ideally, I'd be doing ACME style certificate management with a 3 month span... but... not yet!)

Using the tool

certmaker.sh has help, which can be exposed using ./certmaker.sh --help or running it with no parameters.

With the most simple action, running ./certmaker.sh --name somehost (which can also be run as ./certmaker.sh -n somehost), you'll get a random IP address in the first defined subnet in the CA cert. If you've not defined a subnet in the CA Cert, then you'll get an IP address in the subnet 10.44.88.0/24 (which was picked at random by the author). The hostname will be defined as somehost.nebula.example.org, or whatever suffix the CA cert has specified as the CA Name.

If you want to specify one or more specific FQDN suffixes, you can also use the -f or --fqdn_root flag, like this ./certmaker.sh -n somehost -f nebula.example.net. (Remember that the -f and and -n flags expand to --fqdn_root and --name respectively.)

If you want to add a specific IP to the subnet, perhaps if you want your lighthouses to be 192.0.2.1, for example, you can use the -i or --ip flag, like this ./certmaker.sh -n lighthouse -i 1. (Again, -i expands to --ip.)

If your CA has multiple permitted subnets, perhaps ["192.0.2.0/24","198.51.100.0/24"], you can use the -d or --index flag to tell it which subnet base to use, where 0 would be 192.0.2.0/24 and 1 would be 198.51.100.0/24 in this context. By default, it will pick the first subnet.

If the CA does not have a subnet defined, certmaker.sh will default to using 10.44.88.0/24. You can manually specify a subnet to use, with the -b or --subnet flag, like this ./certmaker.sh -n somehost -b "203.0.113.0/24". (As before, -b expands to --subnet.)

This script assumes that you will be generating certificates in /root/nebula. If this is not the case, you can specify the path to your CA key and certificate files with the -c or --cert_path flags, like this ./certmaker.sh -n somehost -c /tmp/certmaker. Note that this flag will check for both ca.crt and ca.key in this path.

Lastly, a key function of Nebula is the use of groups to define a firewall policy. Three basic and common groups have been defined with a collection of simple flags, which are:

• -l/--lighthouse
• -s/--server
• -w/--workstation

These flags will assign the groups, respectively "Lighthouse", "Server" and "Workstation". If you want to add additional groups, these can be assigned with the flag -g or --group and then the group string, like this ./certmaker.sh -n somehost -g "ADevice" -l -s which would add the groups Lighthouse, Server, and ADevice.

Once you've put your command together, the script will generate a certificate and key, in the current working directory, named for the name of the device.

If you've created the public key to use (perhaps on a mobile device) already, you may use the -k or --key option to specify the path for that public key, like this ./certmaker.sh -n mobile -k mobile.key.

Putting it together

Let's build some certificates.

mkdir -p /tmp/nebula_ca
# Create the CA
nebula-cert ca -out-crt /tmp/nebula_ca/ca.crt -out-key /tmp/nebula_ca/ca.key -ips 192.0.2.0/24,198.51.100.0/24 -name nebula.example.org
# First lighthouse, lighthouse1.nebula.example.org - 192.0.2.1, group "Lighthouse"
./certmaker.sh --cert_path /tmp/nebula_ca --name lighthouse1 --ip 1 --lighthouse
# Second lighthouse, lighthouse2.nebula.example.org - 192.0.2.2, group "Lighthouse"
./certmaker.sh -c /tmp/nebula_ca -n lighthouse2 -i 2 -l
# First webserver, webserver1.nebula.example.org - 192.0.2.168, groups "Server" and "web"
./certmaker.sh --cert_path /tmp/nebula_ca --name webserver1 --server --group web
# Second webserver, webserver2.nebula.example.org - 192.0.2.191, groups "Server" and "web"
./certmaker.sh -c /tmp/nebula_ca -n webserver2 -s -g web
# Database Server, db.nebula.example.org - 192.0.2.182, groups "Server" and "db"
./certmaker.sh --cert_path /tmp/nebula_ca --name db --server --group db
# First workstation, admin1.nebula.example.org - 198.51.100.205, group "Workstation"
./certmaker.sh --cert_path /tmp/nebula_ca --index 1 --name admin1 --workstation
# Second workstation, admin2.nebula.example.org - 198.51.100.77, group "Workstation"
./certmaker.sh -c /tmp/nebula_ca -d 1 -n admin2 -w
# First Mobile device - Create the private/public key pairing first
nebula-cert keygen -out-key mobile1.key -out-pub mobile1.pub
# Then sign it, mobile1.nebula.example.org - 198.51.100.217, group "mobile"
./certmaker.sh --cert_path /tmp/nebula_ca --index 1 --name mobile1 --group mobile --public mobile1.pub
# Second Mobile device - Create the private/public key pairing first
nebula-cert keygen -out-key mobile2.key -out-pub mobile2.pub
# Then sign it, mobile2.nebula.example.org - 198.51.100.22, group "mobile"
./certmaker.sh -c /tmp/nebula_ca -d 1 -n mobile2 -g mobile -p mobile2.pub

Contributors ✨

Thanks goes to these wonderful people (emoji key):

Jon "The Nice Guy" Spriggs 💻 📖 🤔

Samuel Walker 🐛

This project follows the all-contributors specification. Contributions of any kind welcome!