Craft a Custom UDP Packet from Scratch

UDP Header Fields

UDP's 8-byte header is defined by RFC 768. For example, a DNS query from ephemeral port 49152 to 53 on 8.8.8.8 packs source port, destination port, total length, and checksum into exactly eight bytes.

The four fields:

• Source port (2 bytes): e.g. 49152
• Destination port (2 bytes): e.g. 53 for DNS
• Length (2 bytes): header plus payload
• Checksum (2 bytes): optional in IPv4, mandatory in IPv6

Every multi-byte field must be in network byte order via htons().

Pseudo-Header Checksum

When computing the UDP checksum, you prepend a pseudo-header that never goes on the wire:

• Source IP (192.168.1.10)
• Destination IP (8.8.8.8)
• Zero-padded protocol field (17)
• UDP length

This protects against misrouted packets. Sum pseudo-header + UDP header + payload using one's-complement arithmetic.

Raw Socket Modes

• IPPROTO_UDP: you supply UDP header and payload, kernel builds IP
• IPPROTO_RAW + IP_HDRINCL: you build the entire IP datagram too

Why This Exercise

This task requires you to construct a UDP packet byte by byte and print each field in hex. Packet crafters like hping3 and Scapy build datagrams exactly this way, including the pseudo-header trick for checksums. The same field-by-field construction underlies UDP-based DNS amplification attacks where a small forged query triggers a large reflected response.