diff --git a/msg.go b/msg.go index e04fb5d7..29381300 100644 --- a/msg.go +++ b/msg.go @@ -11,14 +11,12 @@ package dns //go:generate go run msg_generate.go import ( - crand "crypto/rand" + "crypto/rand" "encoding/binary" "fmt" "math/big" - "math/rand" "strconv" "strings" - "sync" ) const ( @@ -73,53 +71,23 @@ var ( ErrTime error = &Error{err: "bad time"} // ErrTime indicates a timing error in TSIG authentication. ) -// 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: +// Id by default returns a 16-bit random number to be used as a message id. The +// number is drawn from a cryptographically secure random number generator. +// This being a variable the function can be reassigned to a custom function. +// For instance, to make it return a static value for testing: // // dns.Id = func() uint16 { return 3 } 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 output uint16 + err := binary.Read(rand.Reader, binary.BigEndian, &output) + if err != nil { + panic("dns: reading random id failed: " + err.Error()) } - - // 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 output } // MsgHdr is a a manually-unpacked version of (id, bits).