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