{ config, ... }:

# Headscale — self-hosted control server for the tailnet. Every host's
# services/vpn/tailscale.nix points --login-server at https://vpn.mgaction.town
# (this host). MagicDNS base_domain "orbit.sol" matches the
# "jupiter.orbit.sol" names used in this repo's Caddy vhosts
# (hosts/neptun/configuration.nix) — changing base_domain means changing
# those too, and re-pointing neptun's dnsmasq stub at the new suffix.
#
# TLS terminates at Caddy (see the host's configuration.nix); headscale
# itself only listens on localhost.
{
  services.headscale = {
    enable = true;
    port = 8082;   # off the default 8080 to stay clear of other web apps

    settings = {
      server_url = "https://vpn.mgaction.town";

      dns = {
        # Deliberately OUTSIDE mgaction.town. That zone has a wildcard A+AAAA
        # pointing at neptun, and DNS wildcards match multi-label names — so
        # with base_domain = hosts.mgaction.town, `jupiter.hosts.mgaction.town`
        # resolved publicly to NEPTUN and Caddy proxied to itself: a silent
        # loop rather than a lookup failure.
        #
        # `.sol` is the LAN domain pihole serves, so this nests the tailnet
        # inside it: planets sit on the LAN as jupiter.sol, and reach each
        # other in orbit as jupiter.orbit.sol. Resolution is unambiguous
        # because tailscale matches routes by LONGEST suffix, so orbit.sol
        # goes to MagicDNS even when everything else funnels to pihole.
        #
        # Never give a LAN host the name `orbit`: pihole's
        # `address=/<host>.sol/<ip>` lines match a name AND everything under
        # it, so an `orbit` host would swallow this entire zone.
        base_domain = "orbit.sol";
        # pihole on mercury, over the tailnet — so every roaming device gets
        # ad blocking and .sol names wherever it is, not just on the LAN.
        # Deliberately NO public fallback: tailscale treats the list as a set,
        # so adding 9.9.9.9 here would let queries slip past the filter
        # whenever mercury is briefly slow. Strict blocking, at the cost of
        # mercury being a single point of failure for tailnet DNS.
        #
        # ⚠️  A hardcoded tailnet address, so it changes if mercury re-enrols
        #     — check `headscale nodes list` if DNS dies tailnet-wide.
        nameservers.global = [ "100.64.0.7" ];

        # Must be set, and must be HERE rather than via the module's
        # `dns.split` option. nixpkgs renders that option one level too high
        # (a sibling of `nameservers:`), but headscale reads
        # dns.nameservers.split (hscontrol/types/config.go:722) and so does
        # headplane. So the module's option is dead, and the missing key makes
        # headplane's DNS page die with
        #   TypeError: Cannot convert undefined or null to object
        # from Object.keys(config.dns.nameservers.split).
        nameservers.split = { };

        # Point every node's resolver at MagicDNS, which forwards on to the
        # global nameserver above. That is the only way to get pihole onto a
        # roaming device: with this false, globalResolvers land in the
        # netmap's FallbackResolvers (hscontrol/types/config.go:826-830) and a
        # phone with carrier DNS never consults them.
        #
        # The cost is that every node's DNS now depends on mercury and on the
        # home connection, so mercury going down costs name resolution
        # everywhere, not just `.sol`. neptun and mercury opt out of this
        # individually with --accept-dns=false — see their configuration.nix.
        override_local_dns = true;
      };

      # Authentik as the login provider, so `tailscale up --login-server ...`
      # sends you to a browser instead of needing a pre-auth key. This is a
      # SEPARATE Authentik application from headplane's — its own provider,
      # slug `headscale`, redirect https://vpn.mgaction.town/oidc/callback
      # (headscale's own callback; headplane's is under /admin).
      #
      # ⚠️  headscale performs OIDC discovery at STARTUP and a failure is
      # FATAL ("creating OIDC provider from issuer config: 404 Not Found") —
      # it will not boot, taking the whole tailnet's control plane with it.
      # Never point `issuer` at an application that doesn't exist yet; verify
      # with:
      #   curl -s <issuer>.well-known/openid-configuration
      #
      # Headless hosts still enrol with pre-auth keys. Note also that users
      # created here are distinct from `headscale users create` ones: matching
      # is by the OIDC `sub` claim against the user's providerId, and 0.28
      # dropped map_legacy_users, so CLI-made users never gain one.
      oidc = {
        issuer = "https://auth.mgaction.town/application/o/headscale/";
        client_id = "14vhRYaLiONHmI2YFIxbQEveJDLu5cCvzSkTb9oq";
        client_secret_path = config.sops.secrets.headscale_oidc_client_secret.path;
      };

      # Run our own DERP relay instead of pulling Tailscale's map.
      #
      # With the default (urls = [controlplane.tailscale.com/derpmap/default],
      # auto_update_enabled = true) headscale fetches that map at startup and
      # treats failure as FATAL — so a DNS blip or a Tailscale outage stops the
      # control server from booting at all. A self-hosted control plane that
      # can't start without Tailscale's infrastructure rather misses the point.
      #
      # The relay itself rides Caddy on :443 (hence the flush_interval -1 on
      # that vhost); only STUN needs its own UDP port.
      derp = {
        urls = [ ];
        auto_update_enabled = false;
        server = {
          enabled = true;
          region_id = 999;          # 900-999 is the custom range
          region_code = "neptun";
          region_name = "neptun";
          stun_listen_addr = "0.0.0.0:3478";
          automatically_add_embedded_derp_region = true;
        };
      };
    };
  };

  # STUN for the embedded DERP server above. Also needs a matching inbound-UDP
  # rule in netcup's edge firewall — it is stateless and defaults to denying
  # inbound UDP outright, which silently kills every DNS/NTP reply too.
  networking.firewall.allowedUDPPorts = [ 3478 ];
}