Reintroduce test for DHCPv4 packet marshalling and unmarshalling (#20)

* Reintroduce test for DHCPv4 packet marshalling and unmarshalling

Necessary components from `netboot/pcap` have been merged into the test.

* Fix linter rule violations

---------

Co-authored-by: Stefan Majer <[email protected]>

Author: robertvolkmann

dhcp4/packet_test.go

@@ -0,0 +1,138 @@
+// Copyright 2016 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 (
+	"bufio"
+	"bytes"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"reflect"
+	"testing"
+)
+
+const (
+	// See: https://wiki.wireshark.org/Development/LibpcapFileFormat#global-header
+	pcapHdrLen = 24
+	ethHdrLen  = 14
+	udpHdrLen  = 8
+)
+
+// See: https://wiki.wireshark.org/Development/LibpcapFileFormat#record-packet-header
+type packetHdr struct {
+	_   uint32
+	_   uint32
+	Len uint32
+	_   uint32
+}
+
+func udpFromPcap(fname string) ([][]byte, error) {
+	f, err := os.Open(fname)
+	if err != nil {
+		return nil, err
+	}
+	r := bufio.NewReader(f)
+
+	_, err = r.Discard(pcapHdrLen)
+	if err != nil {
+		return nil, fmt.Errorf("failed to discard header: %w", err)
+	}
+
+	var ret [][]byte
+	for {
+		var hdr packetHdr
+		if err = binary.Read(r, binary.LittleEndian, &hdr); err != nil {
+			break
+		}
+		bs := make([]byte, hdr.Len)
+		if _, err = io.ReadFull(r, bs); err != nil {
+			break
+		}
+		hdrLen := ethHdrLen
+		hdrLen += int(bs[hdrLen]&0xF) * 4 // IP header
+		hdrLen += udpHdrLen
+		ret = append(ret, bs[hdrLen:])
+	}
+	if err != nil && !errors.Is(err, io.EOF) {
+		return nil, err
+	}
+
+	return ret, nil
+}
+
+func TestParse(t *testing.T) {
+	rawPkts, err := udpFromPcap("testdata/dhcp.pcap")
+	if err != nil {
+		t.Fatalf("Getting test packets from pcap: %s", err)
+	}
+
+	var pkts bytes.Buffer
+	for i, rawPkt := range rawPkts {
+		pkt, err := Unmarshal(rawPkt)
+		if err != nil {
+			t.Fatalf("Parsing DHCP packet #%d: %s", i+1, err)
+		}
+		pkts.WriteString(pkt.DebugString())
+		pkts.WriteString("======\n")
+	}
+
+	expectedFile := "testdata/dhcp.parsed"
+	expected, err := os.ReadFile(expectedFile)
+	if err != nil {
+		t.Fatalf("Reading expected file: %s", err)
+	}
+
+	if pkts.String() != string(expected) {
+		if os.Getenv("UPDATE_TESTDATA") != "" {
+			_ = os.WriteFile(expectedFile, pkts.Bytes(), 0600)
+			t.Errorf("dhcp.pcap didn't decode to dhcp.parsed (updated dhcp.parsed)")
+		} else {
+			t.Fatalf("dhcp.pcap didn't decode to dhcp.parsed (rerun with UPDATE_TESTDATA=1 to get diff)")
+		}
+	}
+}
+
+func TestWriteRead(t *testing.T) {
+	rawPkts, err := udpFromPcap("testdata/dhcp.pcap")
+	if err != nil {
+		t.Fatalf("Getting test packets from pcap: %s", err)
+	}
+
+	var pkts []*Packet
+	for i, rawPkt := range rawPkts {
+		pkt, err := Unmarshal(rawPkt)
+		if err != nil {
+			t.Fatalf("Unmarshalling testdata packet #%d: %s", i+1, err)
+		}
+		pkts = append(pkts, pkt)
+	}
+
+	for _, pkt := range pkts {
+		raw, err := pkt.Marshal()
+		if err != nil {
+			t.Fatalf("Packet marshalling failed: %s\nPacket: %#v", err, pkt)
+		}
+		pkt2, err := Unmarshal(raw)
+		if err != nil {
+			t.Fatalf("Packet unmarshalling failed: %s\nPacket: %#v", err, pkt)
+		}
+		if !reflect.DeepEqual(pkt, pkt2) {
+			t.Fatalf("Packet mutated by write-then-read: %#v", pkt)
+		}
+	}
+}