feat(headscale): add self-hosted Tailscale control server

New module: modules/headscale.nix
- Headscale service listening on localhost with Nginx reverse proxy
- SQLite database (appropriate for personal use)
- Tailscale public DERP relays for NAT traversal fallback
- MagicDNS enabled with Mullvad/Quad9 upstream resolvers
- Optional OIDC authentication (Google, GitHub, etc.)
- Default auth: pre-shared API keys (headscale apikeys create)
- Added to backup paths (SQLite DB)
- headscale CLI tool added to system packages

Configuration:
- Domain: vpn.ashisgreat.xyz
- OIDC disabled by default (documented how to enable in configuration.nix)

To register a device after deploying:
  sudo headscale apikeys create
  tailscale up --login-server=https://vpn.ashisgreat.xyz --authkey=<key>

DNS record needed: vpn.ashisgreat.xyz → VPS IP

configuration.nix

@@ -85,6 +85,7 @@
     kitty.terminfo
     htop
     tmux
+    headscale
   ];
 
   nix.settings.experimental-features = [ "nix-command" "flakes" ];
@@ -171,6 +172,22 @@
   # === CrowdSec ===
   myModules.crowdsec.enable = true;
 
+  # === Headscale (Self-hosted Tailscale) ===
+  myModules.headscale = {
+    enable = true;
+    domain = "vpn.ashisgreat.xyz";
+    # OIDC not enabled by default — use pre-shared auth keys:
+    #   headscale apikeys create
+    #   tailscale up --login-server=https://vpn.ashisgreat.xyz --authkey=<key>
+    #
+    # To enable OIDC login (Google, GitHub, etc.):
+    #   oidc.enable = true;
+    #   oidc.issuer = "https://accounts.google.com";
+    #   oidc.clientId = "your-client-id";
+    #   oidc.clientSecret = config.sops.placeholder.headscale_oidc_secret;
+    # And add headscale_oidc_secret to your secrets.yaml
+  };
+
   # === Backups (Restic + B2) ===
   myModules.backup = {
     enable = true;

modules/default.nix

@@ -11,5 +11,6 @@
     ./backup.nix
     ./adguard.nix
     ./forgejo.nix
+    ./headscale.nix
   ];
 }

modules/headscale.nix

@@ -0,0 +1,152 @@
+# Headscale Module
+# Provides: Self-hosted Tailscale control server (mesh VPN)
+#
+# Usage:
+#   myModules.headscale = {
+#     enable = true;
+#     domain = "vpn.example.com";
+#   };
+#
+# After deploying, generate an auth key to register devices:
+#   headscale apikeys create
+#
+# Then on each device:
+#   tailscale up --login-server=https://vpn.example.com --authkey=<key>
+
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+let
+  cfg = config.myModules.headscale;
+in
+{
+  options.myModules.headscale = {
+    enable = lib.mkEnableOption "Headscale (self-hosted Tailscale control server)";
+
+    domain = lib.mkOption {
+      type = lib.types.str;
+      example = "vpn.example.com";
+      description = "Public domain name for the Headscale control server";
+    };
+
+    port = lib.mkOption {
+      type = lib.types.port;
+      default = 8085;
+      description = "Internal port for Headscale to listen on";
+    };
+
+    oidc = {
+      enable = lib.mkEnableOption "OIDC authentication (e.g. Google, GitHub)";
+
+      issuer = lib.mkOption {
+        type = lib.types.str;
+        example = "https://accounts.google.com";
+        description = "OIDC issuer URL";
+      };
+
+      clientId = lib.mkOption {
+        type = lib.types.str;
+        description = "OIDC client ID (store in SOPS)";
+      };
+
+      clientSecret = lib.mkOption {
+        type = lib.types.str;
+        description = "OIDC client secret SOPS placeholder";
+      };
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    services.headscale = {
+      enable = true;
+      settings = {
+        server_url = "https://${cfg.domain}";
+        listen_addr = "127.0.0.1:${toString cfg.port}";
+        grpc_listen_addr = "127.0.0.1:50443";
+        metrics_listen_addr = "127.0.0.1:9090";
+
+        # Use SQLite for simplicity (Postgres available for larger deployments)
+        database.sqlite.path = "/var/lib/headscale/db.sqlite";
+
+        # Private key for WireGuard — auto-generated on first run
+        private_key_path = "/var/lib/headscale/private.key";
+
+        # DERP relay servers (use Tailscale's public ones)
+        derp = {
+          urls = [
+            "https://controlplane.tailscale.com/derpmap/default"
+          ];
+          auto_update = true;
+        };
+
+        # Disable built-in DERP server (use public relays only)
+        derp.server.enabled = false;
+
+        # Authentication
+        policy.mode = lib.mkIf cfg.oidc.enable "oidc";
+
+        oidc = lib.mkIf cfg.oidc.enable {
+          issuer = cfg.oidc.issuer;
+          client_id = cfg.oidc.clientId;
+          client_secret_path = config.sops.secrets.headscale_oidc_secret.path;
+          allowed_domains = [ ];
+          allowed_users = [ ];
+          scopes = [ "openid" "profile" "email" ];
+        };
+
+        # DNS configuration
+        dns = {
+          magic_dns = true;
+          base_domain = "headscale.net";
+          domains = [ ];
+          nameservers = {
+            global = [
+              "https://dns.mullvad.net/dns-query"
+              "https://dns.quad9.net/dns-query"
+            ];
+          };
+          override_local_dns = true;
+        };
+
+        # Logging
+        log.level = "info";
+
+        # Unix socket for headscale CLI
+        unix_socket = "/run/headscale/headscale.sock";
+        unix_socket_permission = "0770";
+      };
+    };
+
+    # OIDC secret from SOPS
+    sops.secrets.headscale_oidc_secret = lib.mkIf cfg.oidc.enable { };
+
+    # Nginx reverse proxy
+    myModules.nginx.domains."${cfg.domain}" = {
+      port = cfg.port;
+      extraConfig = ''
+        # Headscale needs WebSocket upgrade for client connections
+        proxy_http_version 1.1;
+        proxy_set_header Upgrade $http_upgrade;
+        proxy_set_header Connection "upgrade";
+
+        # Larger buffers for Headscale
+        proxy_buffer_size 128k;
+        proxy_buffers 4 256k;
+      '';
+      # Headscale doesn't serve HTML — skip CSP
+      contentSecurityPolicy = null;
+    };
+
+    # Add Headscale data to backups
+    myModules.backup.paths = [
+      config.services.headscale.settings.database.sqlite.path
+    ];
+
+    # Headscale needs its own group for socket permissions
+    users.groups.headscale = { };
+  };
+}