Implement a UDP Client and Server Using Only Syscalls

Topic Overview

Creating a UDP Socket

UDP is connectionless with no handshake or reliability guarantees. For example, a DNS client on port 49152 talks to 8.8.8.8:53 with a single sendto() call. The syscall sequence starts with:

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Arguments: AF_INET for IPv4, SOCK_DGRAM for datagrams, IPPROTO_UDP (or 0).

Filling sockaddr_in by Hand

Without getaddrinfo(), you construct the address structure yourself:

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Key fields:

• sin_family: always AF_INET
• sin_port: use htons() for network byte order
• sin_addr: 32-bit IPv4 address

Then bind() for servers, sendto()/recvfrom() for each datagram. Unlike TCP's connect()/send() pattern, UDP is stateless: every sendto() must carry the destination address, and every recvfrom() tells you who sent the packet. This makes UDP ideal for DNS queries to 8.8.8.8:53 where one request expects one reply.

Why This Exercise

This task requires you to build a UDP client and server using only raw syscalls. Forgetting htons() on the port field is an extremely common bug on little-endian machines: the value silently becomes garbage instead of erroring. Every DNS resolver and DHCP client on Linux ultimately calls socket(AF_INET, SOCK_DGRAM, 0) and fills sockaddr_in exactly as you do here, since getaddrinfo() is just a convenience wrapper around this sequence.

Given the endpoints and payload of a UDP datagram, compute its on-the-wire framing without touching a real socket - the byte-level work sendto() and the kernel do for you.

Input (stdin): src_ip dst_ip src_port dst_port payload_len, then (only if payload_len > 0) the payload as a hex string. Example: 192.168.1.10 8.8.8.8 49152 53 8 aabbccddeeff0011

Output (stdout):

• udp_length=<8 + payload_len> - the value stored in the UDP length field
• checksum=0xHHHH - the UDP checksum per RFC 768: sum 16-bit big-endian words over the pseudo-header (source IP, destination IP, one zero byte plus protocol 17, UDP length), the UDP header (source port, destination port, length, checksum field 0), and the payload, padding an odd-length payload's last word with a zero byte. Fold carries into the one's complement sum, then complement; a computed 0 is transmitted as 0xFFFF.

Every DNS resolver builds exactly this header when it queries a server like 8.8.8.8:53, and htons() is why the ports and lengths live on the wire as big-endian words.