Add Cookbook S01-S22 (draft)

docs-src/pages/en/cookbook/s22-unix-socket.md

@@ -0,0 +1,64 @@
+---
+title: "S22. Talk Over a Unix Domain Socket"
+order: 41
+status: "draft"
+---
+
+When you want to talk only to other processes on the same host, a Unix domain socket is a nice fit. It avoids TCP overhead and uses filesystem permissions for access control. Local IPC and services sitting behind a reverse proxy are classic use cases.
+
+## Server side
+
+```cpp
+httplib::Server svr;
+svr.set_address_family(AF_UNIX);
+
+svr.Get("/", [](const auto &, auto &res) {
+  res.set_content("hello from unix socket", "text/plain");
+});
+
+svr.listen("/tmp/httplib.sock", 80);
+```
+
+Call `set_address_family(AF_UNIX)` first, then pass the socket file path as the first argument to `listen()`. The port number is unused but required by the signature — pass any value.
+
+## Client side
+
+```cpp
+httplib::Client cli("/tmp/httplib.sock");
+cli.set_address_family(AF_UNIX);
+
+auto res = cli.Get("/");
+if (res) {
+  std::cout << res->body << std::endl;
+}
+```
+
+Pass the socket file path to the `Client` constructor and call `set_address_family(AF_UNIX)`. Everything else works like a normal HTTP request.
+
+## When to use it
+
+- **Behind a reverse proxy**: An nginx-to-backend setup over a Unix socket is faster than TCP and sidesteps port management
+- **Local-only APIs**: IPC between tools that shouldn't be reachable from outside
+- **In-container IPC**: Process-to-process communication within the same pod or container
+- **Dev environments**: No more worrying about port conflicts
+
+## Clean up the socket file
+
+A Unix domain socket creates a real file in the filesystem. It doesn't get removed on shutdown, so delete it before starting if needed.
+
+```cpp
+std::remove("/tmp/httplib.sock");
+svr.listen("/tmp/httplib.sock", 80);
+```
+
+## Permissions
+
+You control who can connect via the socket file's permissions.
+
+```cpp
+svr.listen("/tmp/httplib.sock", 80);
+// from another process or thread
+chmod("/tmp/httplib.sock", 0660); // owner and group only
+```
+
+> **Warning:** Some Windows versions support AF_UNIX, but the implementation and behavior differ by platform. Test thoroughly before running cross-platform in production.