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improve formatter example

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This commit is contained in:
Patricio Whittingslow
2026-01-26 12:12:13 -03:00
parent e8afc09a73
commit 6a6097a9ac
3 changed files with 54 additions and 37 deletions
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internet/pcap/capture.go
+8 -1
View File
@@ -1046,7 +1046,14 @@ var baseDHCPv4Fields = [...]FrameField{
Name: "Client Hardware Address",
Class: FieldClassAddress,
FrameBitOffset: 28 * octet,
BitLength: 16 * octet,
BitLength: 6 * octet, // Ethernet MAC is 6 bytes, remaining 10 in chaddr are padding
},
{
Name: "Padding",
Class: FieldClassBinaryText,
FrameBitOffset: (28 + 6) * octet, // Part of Client Hardware Address(16 bytes) but unused.
BitLength: 10 * octet,
Legacy: true,
},
{
Name: "BOOTP",
internet/pcap/capture_test.go
+42 -36
View File
@@ -395,14 +395,13 @@ func TestFieldAsUintRightAligned(t *testing.T) {
}
func ExampleFormatter_dhcp() {
// Build an Ethernet + IPv4 + UDP + DHCPv4 packet.
// Build an Ethernet + IPv4 + UDP + DHCPv4 packet using dhcpv4.Client.
const (
ethSize = 14
ipv4Size = 20
udpSize = 8
)
pktSize := ethSize + ipv4Size + udpSize + dhcpv4.OptionsOffset + 32 // room for options
pkt := make([]byte, pktSize)
pkt := make([]byte, 600) // Need 240 (DHCP header) + 255 (min options) + 42 (Eth+IP+UDP)
// Ethernet frame.
efrm, _ := ethernet.NewFrame(pkt)
@@ -410,48 +409,51 @@ func ExampleFormatter_dhcp() {
*efrm.SourceHardwareAddr() = [6]byte{0xde, 0xad, 0xbe, 0xef, 0xca, 0xfe}
efrm.SetEtherType(ethernet.TypeIPv4)
// IPv4 frame.
ifrm, _ := ipv4.NewFrame(efrm.Payload())
// IPv4 frame (will be filled by DHCP client).
ifrm, _ := ipv4.NewFrame(pkt[ethSize:])
ifrm.SetVersionAndIHL(4, 5)
ifrm.SetTotalLength(uint16(ipv4Size + udpSize + dhcpv4.OptionsOffset + 32))
ifrm.SetID(0x1234)
ifrm.SetFlags(0x4000) // Don't Fragment
ifrm.SetTTL(64)
ifrm.SetProtocol(lneto.IPProtoUDP)
*ifrm.SourceAddr() = [4]byte{0, 0, 0, 0} // 0.0.0.0 (DHCP client)
*ifrm.DestinationAddr() = [4]byte{255, 255, 255, 255} // Broadcast
ifrm.SetCRC(ifrm.CalculateHeaderCRC())
// UDP frame.
ufrm, _ := udp.NewFrame(ifrm.Payload())
// UDP frame (ports set, length will be updated).
ufrm, _ := udp.NewFrame(pkt[ethSize+ipv4Size:])
ufrm.SetSourcePort(dhcpv4.DefaultClientPort) // 68
ufrm.SetDestinationPort(dhcpv4.DefaultServerPort) // 67
ufrm.SetLength(uint16(udpSize + dhcpv4.OptionsOffset + 32))
// DHCPv4 frame (DHCP Discover).
dfrm, _ := dhcpv4.NewFrame(ufrm.Payload())
dfrm.ClearHeader()
dfrm.SetOp(dhcpv4.OpRequest)
dfrm.SetHardware(1, 6, 0) // Ethernet, 6 bytes, 0 hops
dfrm.SetXID(0xdeadbeef)
*dfrm.CHAddrAs6() = [6]byte{0xde, 0xad, 0xbe, 0xef, 0xca, 0xfe}
dfrm.SetMagicCookie(dhcpv4.MagicCookie)
// Use dhcpv4.Client to generate DHCP Discover.
var cl dhcpv4.Client
err := cl.BeginRequest(0xdeadbeef, dhcpv4.RequestConfig{
RequestedAddr: [4]byte{192, 168, 1, 100},
ClientHardwareAddr: [6]byte{0xde, 0xad, 0xbe, 0xef, 0xca, 0xfe},
Hostname: "myhost",
ClientID: "lneto-test",
})
if err != nil {
fmt.Println("begin request error:", err)
return
}
// Add DHCP options.
opts := dfrm.OptionsPayload()
n := 0
n += writeOpt(opts[n:], dhcpv4.OptMessageType, byte(dhcpv4.MsgDiscover))
n += writeOpt(opts[n:], dhcpv4.OptHostName, []byte("myhost")...)
n += writeOpt(opts[n:], dhcpv4.OptParameterRequestList,
byte(dhcpv4.OptSubnetMask),
byte(dhcpv4.OptRouter),
byte(dhcpv4.OptDNSServers),
)
opts[n] = byte(dhcpv4.OptEnd)
// Encapsulate DHCP into the packet at correct offset.
// offsetToIP=14 (after Ethernet), offsetToFrame=42 (after Ethernet+IPv4+UDP)
dhcpLen, err := cl.Encapsulate(pkt, ethSize, ethSize+ipv4Size+udpSize)
if err != nil {
fmt.Println("encapsulate error:", err)
return
}
// Update lengths now that we know DHCP size.
totalLen := ipv4Size + udpSize + dhcpLen
ifrm.SetTotalLength(uint16(totalLen))
ufrm.SetLength(uint16(udpSize + dhcpLen))
ifrm.SetCRC(ifrm.CalculateHeaderCRC())
pkt = pkt[:ethSize+totalLen]
// Capture and format the packet.
var cap PacketBreakdown
cap.SubfieldLimit = 10 // Capture up to 10 DHCP options
cap.SubfieldLimit = 10
frames, err := cap.CaptureEthernet(nil, pkt, 0)
if err != nil {
fmt.Println("capture error:", err)
@@ -471,11 +473,15 @@ func ExampleFormatter_dhcp() {
fmt.Println(string(out))
// Output:
// Ethernet len=14; destination=ff:ff:ff:ff:ff:ff; source=de:ad:be:ef:ca:fe; protocol=0x0800
// IPv4 len=20; version=0x04; (Header Length)=5; (Type of Service)=0x00; (Total Length)=300; identification=0x1234; identification=0x1234; flags=0x4000; (Time to live)=0x40; protocol=0x11; checksum=0x278e; source=0.0.0.0; destination=255.255.255.255
// UDP [RFC768] len=8; (Source port)=68; (Destination port)=67; size=280; checksum=0x0000
// DHCPv4 len=240; op=1; (Hardware Address Type)=0x01; (Hardware Address Length)=6; Hops=0x00; (Transaction ID)=0xdeadbeef; (Start Time)=0x0000; Flags=0x0000; (Client Address)=0.0.0.0; (Offered Address)=0.0.0.0; (Server Next Address)=0.0.0.0; (Relay Agent Address)=0.0.0.0; (Client Hardware Address)=dead:beef:cafe::; options((DHCP message type.)=1
// IPv4 len=20; version=0x04; (Header Length)=5; (Type of Service)=0x00; (Total Length)=312; identification=0x6043; identification=0x6043; flags=0x4000; (Time to live)=0x40; protocol=0x11; checksum=0xd972; source=0.0.0.0; destination=255.255.255.255
// UDP [RFC768] len=8; (Source port)=68; (Destination port)=67; size=292; checksum=0x0000
// DHCPv4 len=240; op=1; (Hardware Address Type)=0x01; (Hardware Address Length)=6; Hops=0x00; (Transaction ID)=0xdeadbeef; (Start Time)=0x0001; Flags=0x0000; (Client Address)=0.0.0.0; (Offered Address)=0.0.0.0; (Server Next Address)=255.255.255.255; (Relay Agent Address)=0.0.0.0; (Client Hardware Address)=de:ad:be:ef:ca:fe; options
// (DHCP message type.)=1
// (Parameter request list)=0x0102031a1c060f2a
// (DHCP maximum message size)=558
// (Requested IP address)=192.168.1.100
// (Client identifier)="lneto-test"
// (Hostname string)="myhost"
// (Parameter request list)=0x010306)
}
func writeOpt(dst []byte, opt dhcpv4.OptNum, data ...byte) int {
dst[0] = byte(opt)
internet/pcap/format.go
+4
View File
@@ -9,6 +9,7 @@ import (
"slices"
"strconv"
"strings"
"sync"
_ "time"
"github.com/soypat/lneto"
@@ -28,6 +29,7 @@ type Formatter struct {
// may be very large in size but meaningless to the actual network functioning.
// Enabling DisableLegacyFilter means these fields will be printed as is.
DisableLegacyFilter bool
mubuf sync.Mutex
buf []byte
}
@@ -119,6 +121,8 @@ func (f *Formatter) formatField(dst []byte, pktStartOff int, field FrameField, p
dst = append(dst, ')')
}
dst = append(dst, '=')
f.mubuf.Lock()
defer f.mubuf.Unlock()
f.buf, err = appendField(f.buf[:0], pkt, field.FrameBitOffset+pktStartOff, field.BitLength, field.RightAligned)
if err != nil {
return dst, err