Homelab: Secrets management

Since the introduction of the certificates, each script was ending with some old-fashioned (i.e. ssh/scp-based commands) as follows:

TARGET_DIR=/etc/nixos/certificates/servers/$SERVICE
TARGET_LOGIN=black@192.168.0.4
echo "Copy '$CRT_DIR/server.key' and '$CRT_DIR/server.crt' to barracuda@$TARGET_DIR"
ssh $TARGET_LOGIN sudo mkdir -p $TARGET_DIR
scp $CRT_DIR/server.{key,crt} $TARGET_LOGIN:~
ssh $TARGET_LOGIN sudo chown root:root server.key server.crt
ssh $TARGET_LOGIN sudo mv server.key server.crt $TARGET_DIR
ssh $TARGET_LOGIN sudo systemctl restart nginx.service

I won't say that old things are always bad, but there are many shortcomings:

• My server's IP is hard-coded
• My server file hierarchy is implicitly coupled
• My script handles (poorly) nginx
• My script is stateful and does not handle well failures (no retry/restart/rollback)

All of these making it hard to maintain/reuse.

If I really wanted to make this clean from here, I would use something like ansible (I mean JetPorch), or Terraform, which I'm more use to.

I could simply reference the files in my configuration.nix file, but it would end-up in my git history, which is not a big deal as it is a local git repository, the real concern come from my nix store, which will store the certificate and make them available to everyone on my machine, in clear-text, without permission requirements.

Hopefully, there's a solution called agenix for that.

It relies on age, which is a pgp successor, which aims to provide a simple way to cipher/decipher files.

Lastly, there's also agebox which helps to manage multiple ciphered files/directories.

Let's start by creating a key:

age-keygen -o secrets/age00.txt

Then I save the public key:

echo age... > keys/age00.pub.txt

I could generate it from an SSH key, but I'm used to having my SSH keys dedicated to SSH usage.

Then, I use agebox to convert all my secrets (certificates and keys):

agebox init
agebox encrypt server/generated/services/*/server.{crt,key} client/generated/*/ca/{ca.pem,crl/crl.pem}

Which suffixes all the files with .agebox.

Then I upload keys to Barracuda.

Finally, the nix part.

First, we have to declare all files:

age = {
  identityPaths = [ "/etc/nixos/secrets/age00.txt" ];
  secrets =
    let
      nginx = localPath: { file = localPath; mode = "400"; owner = "nginx"; group = "nginx"; };
    in
    {
      resticClientCA = nginx ./certificates/client/generated/restic/ca/ca.pem.agebox;
      resticClientCRL = nginx ./certificates/client/generated/restic/ca/crl/crl.pem.agebox;
      withingsClientCA = nginx ./certificates/client/generated/withings/ca/ca.pem.agebox;
      withingsClientCRL = nginx ./certificates/client/generated/withings/ca/crl/crl.pem.agebox;
    };
};

Quick note here: by default everything is done to use SSH keys, I have to explicitly set the private part of the age key.

Lastly, we can reference the declared secrets:

virtualHosts = {
  "restic.barracuda.local" = {
    # ...
    extraConfig = ''
      ssl_client_certificate ${config.age.secrets.withingsClientCA.path};
      ssl_crl ${config.age.secrets.withingsClientCRL.path};
      ssl_verify_client on;
    '';
  };
};

Here we are, now, our scripts aren't tied to Barracuda, IP, file hierarchy or deployment failure mishandling. There's more, aside of /etc/nixos/secrets, we don't even need a specific file organisation, which means less headaches for me.