How to Craft Custom TCP Packets in Go and Avoid the \'invalid src IP 127.0.0.1\' Panic?

Composing Raw TCP Packets in Go with gopacket

You aim to craft custom TCP packets using gopacket and send them via raw sockets. However, you encounter an issue with the code provided, resulting in the following panic: "invalid src IP 127.0.0.1."

Crafting Custom IPv4 Packets

The example code you provided also aims to craft custom IP layer 3 headers, which is a separate concern from solely handling TCP layers. Here's an in-depth explanation:

Creating Raw Sockets in Go

Contrary to some assumptions, creating raw sockets in Go is possible using net.ListenPacket, net.DialIP, or net.ListenIP:

conn, err := net.ListenIP("ip4:tcp", netaddr)

Setting IP Socket Options

To set your own IPv4 layer 3 header, you need to enable a socket option known as IP_HDRINCL. This is not supported in Core Go packages, but third-party libraries like ipv4 can provide this functionality:

conn, err := ipv4.NewRawConn(network, addr)

This function establishes a raw connection and enables IP_HDRINCL for IPv4.

Alternatives

For simpler tasks involving TCP header manipulation, the latency code may be suitable:

// Create a raw socket
fd, err := syscall.Socket(syscall.AF_INET, syscall.SOCK_RAW, syscall.IPPROTO_TCP)

However, this approach does not allow modification of IPv4 headers.

Recommendations

To resolve your issue with the provided code, you'll need to use a library like ipv4 to enable IP_HDRINCL and set custom IP headers. Alternatively, you could explore the latency package for a simplified approach to TCP header manipulation.

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