How can I craft and send raw TCP packets using Golang and gopacket?

Crafting and Sending Raw TCP Packets with Golang Using gopacket

You wish to construct custom TCP packets using gopacket and transmit them via raw sockets. Allow us to guide you through the process:

Creating Custom IP and TCP Packet Headers

While your question mentions crafting custom TCP packets, your code suggests you want to modify both the IPv4 layer 3 and TCP layer 4 headers. We'll focus on IPv4 for this example:

// Example Packet Definition
import (
    "code.google.com/p/gopacket"
    "code.google.com/p/gopacket/examples/util"
    "code.google.com/p/gopacket/layers"
    "log"
    "net"
)

// Packet Data
srcIP := net.ParseIP("127.0.0.1")
dstIP := net.ParseIP("192.168.0.1")
ipLayer := layers.IPv4{
    SrcIP:    srcIP,
    DstIP:    dstIP,
    Protocol: layers.IPProtocolTCP,
}
tcpLayer := layers.TCP{
    SrcPort: layers.TCPPort(666),
    DstPort: layers.TCPPort(22),
    SYN:     true,
}

// Compute Checksums and Serialize Packet
tcpLayer.SetNetworkLayerForChecksum(&ipLayer)
buf := gopacket.NewSerializeBuffer()
err := gopacket.SerializeLayers(buf, gopacket.SerializeOptions{
    FixLengths:       true,
    ComputeChecksums: true,
}, &ipLayer, &tcpLayer)
if err != nil {
    panic(err)
}

Creating a Raw Socket in Golang

In contrast to incorrect responses on this platform, Go allows raw socket creation using net.ListenPacket, net.DialIP, or net.ListenIP. For example:

conn, err := net.ListenIP("ip4:tcp", netaddr)
if err != nil {
    log.Fatalf("ListenIP: %s\n", err)
}

Enabling IP Header Inclusion

On macOS and Linux, you can set the IP_HDRINCL socket option to allow custom IPv4 header modification.

conn.SetsockoptIPHDRINCL(true)

Sending the Packet

Finally, send the packet using the raw socket:

_, err = conn.WriteTo(buf.Bytes(), &dstIPaddr)
if err != nil {
    panic(err)
}

Note: To avoid repetition, we have focused on the core steps for assembling the packet and sending it via a raw socket. While we believe this response is technically correct and comprehensive, it is not intended to be a complete code solution.

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