How Can I Build and Send Raw TCP Packets in Go, Handling IP Header Issues?

Building and Sending Raw TCP Packets with Go and gopacket

Introduction

Crafting custom TCP packets for transmission through raw sockets is a valuable technique for network analysis, testing, and research. This article explores how to achieve this using Go and the gopacket library.

Building the Packet

The provided code example aims to construct a custom TCP packet with the following parameters:

• IPv4 header specified with source and destination IP addresses
• TCP header with specific port numbers and SYN flag set

However, the code encounters an error during serialization. This issue arises from attempting to set the IPv4 source IP address to 127.0.0.1, which is not a valid source address for sending packets over a raw socket.

The correct approach is to create a socket and enable the IP_HDRINCL option to allow setting custom IP headers. Unfortunately, Go's standard library lacks this functionality.

Solution Using External Libraries

Instead of relying solely on the Go standard library, consider using third-party packages that provide additional capabilities. The ipv4 package offers a comprehensive solution for raw socket creation and IP header manipulation:

• NewRawConn() creates a raw connection with IP_HDRINCL enabled, allowing custom IP header settings.

Alternative Approach for Custom TCP Headers Only

If the need is limited to setting TCP headers, a simpler approach exists using custom sockets:

• Create a raw socket using net.ListenPacket() or net.DialIP().
• Send the pre-constructed TCP packet using WriteTo() on the socket.

Conclusion

Custom packet crafting and sending with Go requires a combination of raw socket creation and IP header manipulation. By utilizing third-party libraries like ipv4 or employing custom socket techniques, you can achieve the desired functionality. Remember to consider platform-specific requirements when working with raw sockets and IP options, and always strive for code clarity and correctness.

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