socks5: add a simple test

Start up a backend service, put a SOCKS5 server in front
of it, and verify that we can get data from the backend via
SOCKS5.

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

net/socks5/socks5_test.go

@@ -0,0 +1,77 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package socks5
+
+import (
+	"fmt"
+	"io"
+	"net"
+	"testing"
+
+	"golang.org/x/net/proxy"
+)
+
+func socks5Server(listener net.Listener) {
+	var server Server
+	err := server.Serve(listener)
+	if err != nil {
+		panic(err)
+	}
+        listener.Close()
+}
+
+func backendServer(listener net.Listener) {
+	conn, err := listener.Accept()
+	if err != nil {
+		panic(err)
+	}
+	conn.Write([]byte("Test"))
+	conn.Close()
+        listener.Close()
+}
+
+func TestRead(t *testing.T) {
+	// backend server which we'll use SOCKS5 to connect to
+	listener, err := net.Listen("tcp", ":0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	backendServerPort := listener.Addr().(*net.TCPAddr).Port
+	go backendServer(listener)
+
+	// SOCKS5 server
+	socks5, err := net.Listen("tcp", ":0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	socks5Port := socks5.Addr().(*net.TCPAddr).Port
+	go socks5Server(socks5)
+
+	addr := fmt.Sprintf("localhost:%d", socks5Port)
+	socksDialer, err := proxy.SOCKS5("tcp", addr, nil, proxy.Direct)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	addr = fmt.Sprintf("localhost:%d", backendServerPort)
+	conn, err := socksDialer.Dial("tcp", addr)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	buf := make([]byte, 4)
+	_, err = io.ReadFull(conn, buf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(buf) != "Test" {
+		t.Fatalf("got: %q want: Test", buf)
+	}
+
+	err = conn.Close()
+	if err != nil {
+		t.Fatal(err)
+	}
+}