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Add DNS support to pcap (#100)

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* begin adding dns pcap functionality

* improve question formatting

* work on capturing answers as well

* finalize DNS printing

* increase subfield limits

* add FlagContainer type

* prettier DNS printing
This commit is contained in:
Pat Whittingslow
2026-04-24 19:41:10 -03:00
committed by GitHub
parent b2c3d11c4c
commit 198db3789b
8 changed files with 401 additions and 23 deletions
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internet/pcap/capture_test.go
+88 -5
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@@ -10,6 +10,7 @@ import (
"github.com/soypat/lneto"
"github.com/soypat/lneto/dhcpv4"
"github.com/soypat/lneto/dns"
"github.com/soypat/lneto/ethernet"
"github.com/soypat/lneto/http/httpraw"
"github.com/soypat/lneto/internal/ltesto"
@@ -486,9 +487,91 @@ func ExampleFormatter_dhcp() {
// (Hostname string)="myhost"
}
func writeOpt(dst []byte, opt dhcpv4.OptNum, data ...byte) int {
dst[0] = byte(opt)
dst[1] = byte(len(data))
copy(dst[2:], data)
return 2 + len(data)
func ExampleFormatter_dns() {
const (
ethSize = 14
ipv4Size = 20
udpSize = 8
)
// Build a DNS query for "example.com" A record.
var msg dns.Message
msg.Questions = []dns.Question{
{Name: dns.MustNewName("example.com"), Type: dns.TypeA, Class: dns.ClassINET},
{Name: dns.MustNewName("temu.com"), Type: dns.TypeAAAA, Class: dns.ClassANY},
}
msg.Answers = []dns.Resource{
dns.NewResource(dns.MustNewName("abc.com"), dns.TypeALL, dns.ClassANY, 64, []byte{10, 0, 11, 1}),
dns.NewResource(dns.MustNewName("123.com"), dns.TypeA, dns.ClassINET, 64, []byte{20, 0, 22, 2}),
}
dnsPayload, err := msg.AppendTo(nil, 0x1234, dns.NewClientHeaderFlags(dns.OpCodeQuery, true))
if err != nil {
fmt.Println("dns encode error:", err)
return
}
pkt := make([]byte, ethSize+ipv4Size+udpSize+len(dnsPayload))
efrm, _ := ethernet.NewFrame(pkt)
*efrm.DestinationHardwareAddr() = [6]byte{0x00, 0x00, 0x00, 0x00, 0x00, 0x01}
*efrm.SourceHardwareAddr() = [6]byte{0x00, 0x00, 0x00, 0x00, 0x00, 0x02}
efrm.SetEtherType(ethernet.TypeIPv4)
ifrm, _ := ipv4.NewFrame(pkt[ethSize:])
ifrm.SetVersionAndIHL(4, 5)
ifrm.SetTTL(64)
ifrm.SetProtocol(lneto.IPProtoUDP)
ifrm.SetTotalLength(uint16(ipv4Size + udpSize + len(dnsPayload)))
ifrm.SetCRC(ifrm.CalculateHeaderCRC())
ufrm, _ := udp.NewFrame(pkt[ethSize+ipv4Size:])
ufrm.SetSourcePort(58200)
ufrm.SetDestinationPort(dns.ServerPort)
ufrm.SetLength(uint16(udpSize + len(dnsPayload)))
copy(pkt[ethSize+ipv4Size+udpSize:], dnsPayload)
var cap PacketBreakdown
cap.SubfieldLimit = 20
frames, err := cap.CaptureEthernet(nil, pkt, 0)
if err != nil {
fmt.Println("capture error:", err)
return
}
var fmtr Formatter
fmtr.SubfieldLimit = cap.SubfieldLimit
fmtr.FrameSep = "\n"
fmtr.FieldSep = "; "
fmtr.SubfieldSep = "\n\t"
out, err := fmtr.FormatFrames(nil, frames, pkt)
if err != nil {
fmt.Println("format error:", err)
return
}
fmt.Println(string(out))
// Output:
// Ethernet len=14; destination=00:00:00:00:00:01; source=00:00:00:00:00:02; protocol=0x0800
// IPv4 len=20; version=0x04; (Header Length)=5; (Type of Service)=0x00; (Total Length)=117; identification=0x0000; flags=0x0000; (Time to live)=0x40; protocol=0x11; checksum=0x7a79; source=0.0.0.0; destination=0.0.0.0
// UDP len=8; (Source port)=58200; (Destination port)=53; size=97; checksum=0x0000
// DNS len=89; identification=0x1234; flags=0x0100; Questions=2; Answers=2; Authorities=0; Additionals=0; Questions
// Name=example.com
// Type=1
// Class=1
// Name=temu.com
// Type=28
// Class=255; Answers
// Name=abc.com
// Type=255
// Class=255
// TTL=0x00000040
// Length=4
// Data=10.0.11.1
// Name=123.com
// Type=1
// Class=1
// TTL=0x00000040
// Length=4
// Data=20.0.22.2
}