Building a Web Server in Assembly

What You Will Learn

How socket communication works at the system call level
The sequence of C socket API calls to create a server
How to map C socket functions to Linux syscalls
Why this knowledge is valuable for exploit development

What Is It?

Building a web server from scratch — using sockets in C or directly in assembly — teaches you exactly how network communication works at the lowest level. This knowledge is fundamental for exploit development, writing shellcode that connects back, and understanding network programs.

Why It Matters

Shellcode for reverse shells uses these exact syscalls
Understanding socket code helps you read and modify exploits
Binary analysis of network programs requires knowing the socket API
CTF challenges often involve writing custom network clients

How Sockets Communicate

For two computers to communicate over a network, both nodes need to specify how they will send and receive data. One node listens (the server) while the other connects (the client).

The server-side socket creation process:

socket()
   |
bind()
   |
listen()
   |
accept()
   |
read()/write()

Socket API in C

Creating a Socket

int socket(int domain, int type, int protocol);

Example:

int sfd = socket(AF_INET, SOCK_STREAM, 0);
// AF_INET = IPv4, SOCK_STREAM = TCP, 0 = default protocol

Binding the Socket

struct sockaddr_in my_addr;
my_addr.sin_family = AF_INET;
my_addr.sin_port   = htons(8080);
my_addr.sin_addr.s_addr = INADDR_ANY;

bind(sfd, (struct sockaddr *)&my_addr, sizeof(my_addr));

The sockaddr_in structure:

struct sockaddr_in {
    sa_family_t    sin_family;   // address family: AF_INET
    in_port_t      sin_port;     // port in network byte order
    struct in_addr sin_addr;     // internet address
};

Listening for Connections

int listen(int sockfd, int backlog);
// backlog = max number of pending connections in the queue

Accepting a Connection

struct sockaddr_in client_addr;
socklen_t client_len = sizeof(client_addr);

int conn_fd = accept(sfd, (struct sockaddr *)&client_addr, &client_len);

Reading and Writing

char buf[1024];
int n = read(conn_fd, buf, sizeof(buf));
write(conn_fd, "HTTP/1.1 200 OK\r\n\r\nHello!", 25);
close(conn_fd);

Complete Minimal HTTP Server

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <netinet/in.h>

int main() {
    int sfd = socket(AF_INET, SOCK_STREAM, 0);

    struct sockaddr_in addr = {0};
    addr.sin_family      = AF_INET;
    addr.sin_port        = htons(8080);
    addr.sin_addr.s_addr = INADDR_ANY;

    bind(sfd, (struct sockaddr *)&addr, sizeof(addr));
    listen(sfd, 5);

    while (1) {
        int conn = accept(sfd, NULL, NULL);
        char buf[4096];
        read(conn, buf, sizeof(buf));

        char response[] = "HTTP/1.1 200 OK\r\nContent-Length: 13\r\n\r\nHello, World!";
        write(conn, response, sizeof(response));
        close(conn);
    }
    return 0;
}

Linux Syscall Numbers for Sockets

On x86-64 Linux, socket operations map to syscall number 41:

socket   = 41
bind     = 49
listen   = 50
accept   = 43
connect  = 42
send     = 44
recv     = 45
close    = 3

Reference: Linux x86-64 System Call Table

Socket Syscall in Assembly

; socket(AF_INET=2, SOCK_STREAM=1, 0) = sockfd
mov rax, 41         ; sys_socket
mov rdi, 2          ; AF_INET
mov rsi, 1          ; SOCK_STREAM
mov rdx, 0          ; protocol
syscall