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.

Implement a C program that constructs a real UDP datagram from stdin parameters and computes its RFC 768 checksum.

Input (stdin, whitespace-separated):

source IP, destination IP (dotted quad)
source port, destination port, payload length, payload fill byte (decimal)

Requirements:

Build the 8-byte UDP header: source port, destination port, length (= 8 + payload), checksum placeholder, all in network byte order
Fill payload-length payload bytes with the fill byte
Compute the checksum over the IPv4 pseudo-header (source IP, destination IP, zero byte, protocol 17, UDP length) followed by the UDP header and payload, using RFC 1071 one's-complement arithmetic with an odd-byte zero pad
If the computed checksum is 0x0000, transmit 0xFFFF instead (a transmitted all-zero checksum means "no checksum" in IPv4)
Print the field summary line and the full datagram hex dump exactly as specified

Output format:

Line 1: src=IP dst=IP sport=N dport=N len=N checksum=0xNNNN
Line 2: the complete datagram bytes as two-digit lowercase hex, space-separated, prefixed with "hex="

Success Criteria:

Length field equals header plus payload bytes
Checksum includes the pseudo-header (verify against a known vector if unsure)
Odd-length payloads are handled with the zero-octet pad

Craft a Custom UDP Packet from Scratch

Topic Overview

UDP Header Fields

Pseudo-Header Checksum

Raw Socket Modes

Why This Exercise

Implementation Task

Progressive Hints

Resources

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