Implement a UDP Client and Server Using Only Syscalls

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:

c
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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:

c
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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.

Implement a UDP client and server using only raw syscalls.

Requirements:

Create UDP socket with socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)
Manually fill sockaddr_in with sin_family, sin_port (htons), sin_addr
Bind server socket to a port using bind()
Use sendto() to send datagrams
Use recvfrom() to receive datagrams
Show sockaddr_in fields after initialization
Print the syscall flow: socket -> bind -> sendto -> recvfrom
Demonstrate both client and server concepts in one program
Handle errors for each syscall

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:

c
Loading…

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:

c
Loading…

Key fields:

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

Why This Exercise

Implement a UDP client and server using only raw syscalls.

Requirements:

Create UDP socket with socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)
Manually fill sockaddr_in with sin_family, sin_port (htons), sin_addr
Bind server socket to a port using bind()
Use sendto() to send datagrams
Use recvfrom() to receive datagrams
Show sockaddr_in fields after initialization
Print the syscall flow: socket -> bind -> sendto -> recvfrom
Demonstrate both client and server concepts in one program
Handle errors for each syscall

Topic Overview

Creating a UDP Socket

Filling sockaddr_in by Hand

Why This Exercise

Implementation Task

Progressive Hints

Resources

Open Source

Implement a UDP Client and Server Using Only Syscalls

Topic Overview

Creating a UDP Socket

Filling sockaddr_in by Hand

Why This Exercise

Implementation Task

Progressive Hints

Resources

Open Source