GTPv1U SerializeTo: GTPExtensionHeaders out of place #67
Description
When encoding a packet that contains a GTPv1U layer that is not the last layer, the GTPExtensionHeaders are incorrectly placed at the end of the packet, instead of right after the GTPv1U minimum header before the next layer.
Environment
Go v1.22.4
GoPacket revision 8c73654
This issue is copied from google/gopacket#1175 but adjusted for this fork.
Snippet to Reproduce
https://go.dev/play/p/3hnQC2spi_k
package main
import (
"encoding/hex"
"fmt"
"net"
"github.com/gopacket/gopacket"
"github.com/gopacket/gopacket/layers"
)
func main() {
payload := gopacket.Payload{0xA0, 0xA1, 0xA2, 0xA3}
udpI := &layers.UDP{SrcPort: 6363, DstPort: 6363}
ip4I := &layers.IPv4{Version: 4, TTL: 64, Protocol: layers.IPProtocolUDP, SrcIP: net.ParseIP("192.168.60.4"), DstIP: net.ParseIP("192.168.60.3")}
gtpExt := []layers.GTPExtensionHeader{{Type: 0x85, Content: []byte{10, 01}}}
gtp := &layers.GTPv1U{Version: 1, ProtocolType: 1, MessageType: 0xFF, MessageLength: uint16(8 + 20 + 8 + len(payload)), TEID: 0x10000008, GTPExtensionHeaders: gtpExt}
udp := &layers.UDP{SrcPort: 2152, DstPort: 2152}
ip4 := &layers.IPv4{Version: 4, TTL: 64, Protocol: layers.IPProtocolUDP, SrcIP: net.ParseIP("192.168.37.2"), DstIP: net.ParseIP("192.168.37.1")}
eth := &layers.Ethernet{SrcMAC: net.HardwareAddr{0x02, 0x00, 0x00, 0x00, 0x00, 0x02}, DstMAC: net.HardwareAddr{0x02, 0x00, 0x00, 0x00, 0x00, 0x01}, EthernetType: layers.EthernetTypeIPv4}
udp.SetNetworkLayerForChecksum(ip4)
udpI.SetNetworkLayerForChecksum(ip4I)
buf := gopacket.NewSerializeBuffer()
if e := gopacket.SerializeLayers(buf, gopacket.SerializeOptions{
FixLengths: true,
ComputeChecksums: true,
}, eth, ip4, udp, gtp, ip4I, udpI, payload); e != nil {
panic(e)
}
fmt.Println(hex.EncodeToString(buf.Bytes()))
}Expected Output
0200000000010200000000020800 Ethernet
4500004c000000004011af4dc0a82502c0a82501 outer-IPv4
086808680038d511 outer-UDP
34ff00281000000800000085010a0100 GTPv1U
450000200000000040118175c0a83c04c0a83c03 inner-IPv4
18db18db000c9182 inner-UDP
a0a1a2a3 payload
Actual Output
0200000000010200000000020800 Ethernet
4500004c000000004011af4dc0a82502c0a82501 outer-IPv4
086808680038d511 outer-UDP
34ff002810000008 GTPv1U:header
450000200000000040118175c0a83c04c0a83c03 inner-IPv4
18db18db000c9182 inner-UDP
a0a1a2a3 payload
00000085010a0100 GTPv1U:extension
Root Cause
Line 132 in 8c73654
SerializeBuffer.AppendBytes is called on a non-empty buffer, so that extension headers are placed after the inner layers.