Add xray server with xtls-based proxy

Problem: Wireguard and OpenVPN protocols are blocked by ISPs
in _some countries_.

Solution: Add xray server with xtls-based proxy.

servers/mebsuta/default.nix

@@ -7,6 +7,7 @@ in
   imports = [
     inputs.serokell-nix.nixosModules.hetzner-cloud
     inputs.subspace.nixosModule
+    ./xray.nix
   ];
 
   networking.hostName = "mebsuta";

servers/mebsuta/xray.nix

@@ -0,0 +1,115 @@
+{ config, lib, pkgs, ... }:
+
+let
+  vs = config.vault-secrets.secrets;
+  dataDir = "/var/lib/xray";
+  xrayPort = 9433;
+in
+{
+  networking.firewall.allowedTCPPorts = [
+    xrayPort
+  ];
+  users.users.xray = {
+    isSystemUser = true;
+    group = "xray";
+    home = dataDir;
+    createHome = true;
+  };
+  users.groups.xray = {};
+  vault-secrets.secrets.xray = {
+    user = "xray";
+    group = "xray";
+  };
+  services.xray = {
+    enable = true;
+    settingsFile = "${dataDir}/config.json";
+  };
+  systemd.services.xray = let
+    privateKeyPlaceholder = "<private-key>";
+    clientIdPlaceholder = "<client-id>";
+    shortIdPlaceholder = "<short-id>";
+    # Settings file without secrets that is safe to store in /nix/store
+    settings = pkgs.writeText "settings-without-secrets" (builtins.toJSON {
+      log = {
+        loglevel = "warning";
+      };
+      inbounds = [
+        { port = xrayPort;
+          protocol = "vless";
+          tag = "vless";
+          sniffing = {
+            enable = true;
+            destOverrides = ["http" "tls"];
+          };
+          settings = {
+            clients = [
+              {
+                id = clientIdPlaceholder;
+                flow = "xtls-rprx-vision";
+              }
+            ];
+            decryption = "none";
+          };
+          streamSettings = {
+            network = "tcp";
+            security = "reality";
+            realitySettings = {
+              show = false;
+              dest = "serokell.io:443";
+              serverNames = [
+                "serokell.io"
+              ];
+              privateKey = privateKeyPlaceholder;
+              shortIds = [ shortIdPlaceholder ];
+            };
+          };
+        }
+      ];
+      outbounds = [
+        { protocol = "freedom";
+          tag = "direct";
+        }
+        { protocol = "blackhole";
+          tag = "block";
+        }
+        # IPv4-only outbound for Google
+        { tag = "IPv4";
+          protocol = "freedom";
+          settings = {
+            domainStrategy = "UseIPv4";
+          };
+        }
+      ];
+      routing = {
+        domainStrategy = "IPIfNonMatch";
+        domainMatcher = "hybrid";
+        # Force IPv4 for Google, otherwise it keeps giving 403
+        rules = [
+          { type = "field";
+            outboundTag = "IPv4";
+            domain = [ "geosite:google" ];
+          }
+        ];
+        balancers = [];
+      };
+    });
+  in {
+    # Add secrets from Vault to the config file
+    preStart = ''
+      cp --no-preserve=mode "${settings}" "${config.services.xray.settingsFile}"
+      private_key="$(cat "${vs.xray}/private-key")"
+      client_id="$(cat "${vs.xray}/client-id")"
+      short_id="$(cat "${vs.xray}/short-id")"
+      ${pkgs.gnused}/bin/sed -i -e "s/${privateKeyPlaceholder}/$private_key/" -e "s/${clientIdPlaceholder}/$client_id/" -e "s/${shortIdPlaceholder}/$short_id/" \
+        "${config.services.xray.settingsFile}"
+    '';
+    serviceConfig = {
+      # In nixpkgs this service uses DynamicUser=true. However, this doesn't work with vault-secrets because the user that needs
+      # to read secrets is not know in prior.
+      DynamicUser = lib.mkForce "false";
+      User = "xray";
+      Group = "xray";
+      ReadWritePaths = [ "/var/lib/xray" ];
+    };
+  };
+}