net/socks5: add password auth support

Conforms to RFC 1929.

To support Java HTTP clients via libtailscale, who offer no other
reliable hooks into their sockets.

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

net/socks5/socks5_test.go

@@ -4,6 +4,7 @@
 package socks5
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"net"
@@ -74,3 +75,80 @@ func TestRead(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestReadPassword(t *testing.T) {
+	// backend server which we'll use SOCKS5 to connect to
+	ln, err := net.Listen("tcp", ":0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	backendServerPort := ln.Addr().(*net.TCPAddr).Port
+	go backendServer(ln)
+
+	socks5ln, err := net.Listen("tcp", ":0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() {
+		socks5ln.Close()
+	})
+	auth := &proxy.Auth{User: "foo", Password: "bar"}
+	go func() {
+		s := Server{Username: auth.User, Password: auth.Password}
+		err := s.Serve(socks5ln)
+		if err != nil && !errors.Is(err, net.ErrClosed) {
+			panic(err)
+		}
+	}()
+
+	addr := fmt.Sprintf("localhost:%d", socks5ln.Addr().(*net.TCPAddr).Port)
+
+	if d, err := proxy.SOCKS5("tcp", addr, nil, proxy.Direct); err != nil {
+		t.Fatal(err)
+	} else {
+		if _, err := d.Dial("tcp", addr); err == nil {
+			t.Fatal("expected no-auth dial error")
+		}
+	}
+
+	badPwd := &proxy.Auth{User: "foo", Password: "not right"}
+	if d, err := proxy.SOCKS5("tcp", addr, badPwd, proxy.Direct); err != nil {
+		t.Fatal(err)
+	} else {
+		if _, err := d.Dial("tcp", addr); err == nil {
+			t.Fatal("expected bad password dial error")
+		}
+	}
+
+	badUsr := &proxy.Auth{User: "not right", Password: "bar"}
+	if d, err := proxy.SOCKS5("tcp", addr, badUsr, proxy.Direct); err != nil {
+		t.Fatal(err)
+	} else {
+		if _, err := d.Dial("tcp", addr); err == nil {
+			t.Fatal("expected bad username dial error")
+		}
+	}
+
+	socksDialer, err := proxy.SOCKS5("tcp", addr, auth, 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)
+	if _, err := io.ReadFull(conn, buf); err != nil {
+		t.Fatal(err)
+	}
+	if string(buf) != "Test" {
+		t.Fatalf("got: %q want: Test", buf)
+	}
+
+	if err := conn.Close(); err != nil {
+		t.Fatal(err)
+	}
+}