TCP/IP协议栈实现指南：从分层原理到代码的网络编程实战

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

#define PORT 8080
#define BACKLOG 5

int main() {
    int sockfd = socket(AF_INET, SOCK_STREAM, 0); // 创建TCP套接字
    if (sockfd == -1) perror("socket"), exit(1);

    // 地址复用（避免端口占用错误）
    int opt = 1;
    setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));

    // 绑定端口：注意字节序转换（htons=主机→网络）
    struct sockaddr_in server_addr = {
        .sin_family = AF_INET,
        .sin_port = htons(PORT),
        .sin_addr.s_addr = INADDR_ANY // 监听所有网卡
    };
    if (bind(sockfd, (struct sockaddr*)&server_addr, sizeof(server_addr)) == -1)
        perror("bind"), exit(1);

    listen(sockfd, BACKLOG); // 开始监听，BACKLOG是等待队列长度
    printf("TCP server listening on :%d
", PORT);

    while (1) {
        struct sockaddr_in client_addr;
        socklen_t addr_len = sizeof(client_addr);
        // 接受连接：返回新的socket（专门处理这个客户端）
        int new_fd = accept(sockfd, (struct sockaddr*)&client_addr, &addr_len);
        if (new_fd == -1) perror("accept"), continue;

        printf("连接来自: %s:%d
", 
            inet_ntoa(client_addr.sin_addr), // 网络IP→字符串
            ntohs(client_addr.sin_port)      // 网络端口→主机端口
        );

        // 收发数据：recv拿客户端消息，send发响应
        char buf[1024];
        int num = recv(new_fd, buf, sizeof(buf), 0);
        if (num > 0) {
            buf[num] = '';
            printf("收到: %s
", buf);
            send(new_fd, "Hello from TCP Server!", 22, 0);
        }

        close(new_fd); // 关闭客户端连接
    }
    close(sockfd);
    return 0;
}

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#define SERVER_IP "127.0.0.1"
#define SERVER_PORT 8080

int main() {
    int sockfd = socket(AF_INET, SOCK_DGRAM, 0); // UDP套接字
    if (sockfd == -1) perror("socket"), exit(1);

    // 服务器地址结构
    struct sockaddr_in server_addr = {
        .sin_family = AF_INET,
        .sin_port = htons(SERVER_PORT),
        .sin_addr.s_addr = inet_addr(SERVER_IP) // 字符串IP→网络字节序
    };

    // 发送数据：sendto需要指定服务器地址
    char msg[] = "Hello UDP!";
    sendto(sockfd, msg, strlen(msg), 0, 
           (struct sockaddr*)&server_addr, sizeof(server_addr));
    printf("发送消息到 %s:%d
", SERVER_IP, SERVER_PORT);

    // 接收响应：recvfrom会返回服务器地址
    char buf[1024];
    socklen_t addr_len = sizeof(server_addr);
    int num = recvfrom(sockfd, buf, sizeof(buf), 0, 
                       (struct sockaddr*)&server_addr, &addr_len);
    if (num > 0) {
        buf[num] = '';
        printf("收到响应: %s
", buf);
    }

    close(sockfd);
    return 0;
}

#include <stdio.h>
#include <netinet/ip.h>

// 计算IP校验和：对IP头的每16位累加，取反
unsigned short ip_checksum(unsigned short *buf, int len) {
    unsigned long sum = 0;
    while (len > 1) {
        sum += *buf++;
        len -= 2;
    }
    if (len == 1) sum += *(unsigned char*)buf; // 处理奇数长度
    sum = (sum >> 16) + (sum & 0xFFFF); // 高16位+低16位
    sum += (sum >> 16); // 再累加一次（防止溢出）
    return ~sum; // 取反
}

// 构造IP数据报：data是应用数据，src/dest是IP字符串
void build_ip_packet(char *data, int data_len, char *src_ip, char *dest_ip, 
                     char *packet, int *packet_len) {
    struct iphdr *ip = (struct iphdr*)packet;
    ip->version = 4;          // IPv4
    ip->ihl = 5;              // 头部长度：5×4=20字节（无选项）
    ip->tos = 0;              // 服务类型（默认0）
    ip->tot_len = htons(sizeof(struct iphdr) + data_len); // 总长度（网络字节序）
    ip->id = htons(12345);    // 标识（自定义）
    ip->frag_off = 0;         // 分片偏移（不分片）
    ip->ttl = 64;             // 生存时间（最多经过64个路由器）
    ip->protocol = IPPROTO_TCP;// 上层协议（TCP）
    ip->saddr = inet_addr(src_ip); // 源IP
    ip->daddr = inet_addr(dest_ip); // 目的IP
    ip->check = 0;             // 先置0，再计算校验和
    ip->check = ip_checksum((unsigned short*)ip, sizeof(struct iphdr));

    // 复制应用数据到IP头后面
    memcpy(packet + sizeof(struct iphdr), data, data_len);
    *packet_len = sizeof(struct iphdr) + data_len;
}

#include <stdio.h>
#include <net/ethernet.h> // 以太网帧结构定义

// 构造以太网帧：ip_packet是IP数据报，src/dest_mac是MAC地址（6字节数组）
void build_ethernet_frame(char *ip_packet, int ip_len, 
                          unsigned char *src_mac, unsigned char *dest_mac,
                          char *frame, int *frame_len) {
    struct ether_header *eth = (struct ether_header*)frame;
    memcpy(eth->ether_dhost, dest_mac, ETH_ALEN); // 目的MAC
    memcpy(eth->ether_shost, src_mac, ETH_ALEN);  // 源MAC
    eth->ether_type = htons(ETHERTYPE_IP);        // 上层协议：IPv4

    // 复制IP数据报到帧后面
    memcpy(frame + sizeof(struct ether_header), ip_packet, ip_len);
    *frame_len = sizeof(struct ether_header) + ip_len;
}

Transmission Control Protocol, Src Port: 54321, Dst Port: 8080, Seq: 1, Ack: 1, Len: 5
    Source Port: 54321
    Destination Port: 8080
    Sequence Number: 1    (relative sequence number)
    Acknowledgment Number: 1    (relative ack number)
    Header Length: 20 bytes
    Flags: 0x018 (PSH, ACK)
    Window Size: 65535
    Checksum: 0x1234 [validation disabled]
    Urgent Pointer: 0
    [Timestamps]
    TCP payload (5 bytes): Hello