/
xid.go
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/
xid.go
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// Copyright 2018 Google Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package dhcp4
import (
"encoding/binary"
"hash/fnv"
"math/rand"
"net"
"sync"
"time"
)
// XIDGenerator returns a function which returns DHCP transaction IDs (xid).
func XIDGenerator(hwaddr net.HardwareAddr) func() uint32 {
// https://tools.ietf.org/html/rfc2131#section-4.1 explains:
//
// A DHCP client MUST choose 'xid's in such a way as to minimize the chance
// of using an 'xid' identical to one used by another client.
//
// Hence, seed a random number generator with the current time and hardware
// address.
h := fnv.New64()
h.Write(hwaddr)
seed := int64(h.Sum64()) + time.Now().Unix()
rnd := rand.New(rand.NewSource(seed))
var rndMu sync.Mutex
buf := make([]byte, 4)
return func() uint32 {
rndMu.Lock()
defer rndMu.Unlock()
rnd.Read(buf)
return binary.BigEndian.Uint32(buf)
}
}