Use crypto/rand for random id generation.

Fixes miekg#1043 and miekg#1037

msg.go

@@ -15,10 +15,9 @@ import (
 	"encoding/binary"
 	"fmt"
 	"math/big"
-	"math/rand"
+	mrand "math/rand"
 	"strconv"
 	"strings"
-	"sync"
 )
 
 const (
@@ -74,52 +73,26 @@ var (
 )
 
 // Id by default, returns a 16 bits random number to be used as a
-// message id. The random provided should be good enough. This being a
-// variable the function can be reassigned to a custom function.
-// For instance, to make it return a static value:
+// message id. This being a variable the function can be reassigned
+// to a custom function. For instance, to make it return a static value:
 //
 //	dns.Id = func() uint16 { return 3 }
+//
+// Randomness comes from crypto/rand. In the rare case that there is an error
+// getting bytes from that source, this function will silently fall back to
+// the predictable math/rand. If your program has more stringent requirements,
+// you should override Id to do something different (e.g., terminate).
 var Id = id
 
-var (
-	idLock sync.Mutex
-	idRand *rand.Rand
-)
-
 // id returns a 16 bits random number to be used as a
 // message id. The random provided should be good enough.
 func id() uint16 {
-	idLock.Lock()
-
-	if idRand == nil {
-		// This (partially) works around
-		// https://github.com/golang/go/issues/11833 by only
-		// seeding idRand upon the first call to id.
-
-		var seed int64
-		var buf [8]byte
-
-		if _, err := crand.Read(buf[:]); err == nil {
-			seed = int64(binary.LittleEndian.Uint64(buf[:]))
-		} else {
-			seed = rand.Int63()
-		}
-
-		idRand = rand.New(rand.NewSource(seed))
+	var v uint16
+	err := binary.Read(crand.Reader, binary.BigEndian, &v)
+	if err != nil {
+		v = uint16(mrand.Uint32())
 	}
-
-	// The call to idRand.Uint32 must be within the
-	// mutex lock because *rand.Rand is not safe for
-	// concurrent use.
-	//
-	// There is no added performance overhead to calling
-	// idRand.Uint32 inside a mutex lock over just
-	// calling rand.Uint32 as the global math/rand rng
-	// is internally protected by a sync.Mutex.
-	id := uint16(idRand.Uint32())
-
-	idLock.Unlock()
-	return id
+	return v
 }
 
 // MsgHdr is a a manually-unpacked version of (id, bits).