net: implement DNS TCP fallback query if UDP response is truncated

Fixes #5686.

R=golang-dev, bradfitz, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/12458043

src/pkg/net/dnsclient_unix_test.go

@@ -0,0 +1,29 @@
+// Copyright 2013 The Go 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 net
+
+import (
+	"runtime"
+	"testing"
+)
+
+func TestTCPLookup(t *testing.T) {
+	if runtime.GOOS == "windows" || runtime.GOOS == "plan9" {
+		t.Skip("skipping unix dns test")
+	}
+	if testing.Short() || !*testExternal {
+		t.Skip("skipping test to avoid external network")
+	}
+	c, err := Dial("tcp", "8.8.8.8:53")
+	defer c.Close()
+	if err != nil {
+		t.Fatalf("Dial failed: %v", err)
+	}
+	cfg := &dnsConfig{timeout: 10, attempts: 3}
+	_, err = exchange(cfg, c, "com.", dnsTypeALL)
+	if err != nil {
+		t.Fatalf("exchange failed: %v", err)
+	}
+}