net: do not use reflect for DNS messages.

Fixes #3201.

R=bradfitz, bradfitz, rsc
CC=golang-dev, remy
https://golang.org/cl/5753045

src/pkg/go/build/deps_test.go

@@ -226,8 +226,8 @@ var pkgDeps = map[string][]string{
 	"os/user": {"L3", "CGO", "syscall"},
 
 	// Basic networking.
-	// TODO: Remove reflect, possibly math/rand.
-	"net": {"L0", "CGO", "math/rand", "os", "reflect", "sort", "syscall", "time"},
+	// TODO: maybe remove math/rand.
+	"net": {"L0", "CGO", "math/rand", "os", "sort", "syscall", "time"},
 
 	// NET enables use of basic network-related packages.
 	"NET": {

src/pkg/net/dnsmsg_test.go

@@ -6,6 +6,7 @@ package net
 
 import (
 	"encoding/hex"
+	"reflect"
 	"testing"
 )
 
@@ -39,6 +40,16 @@ func TestDNSParseSRVReply(t *testing.T) {
 		t.Errorf("len(addrs) = %d; want %d", g, e)
 		t.Logf("addrs = %#v", addrs)
 	}
+	// repack and unpack.
+	data2, ok := msg.Pack()
+	msg2 := new(dnsMsg)
+	msg2.Unpack(data2)
+	switch {
+	case !ok:
+		t.Errorf("failed to repack message")
+	case !reflect.DeepEqual(msg, msg2):
+		t.Errorf("repacked message differs from original")
+	}
 }
 
 func TestDNSParseCorruptSRVReply(t *testing.T) {