用C语言实现端口全扫描程序

在C语言中实现端口扫描通常使用网络套接字编程技术，特别是TCP/IP协议栈提供的函数。这里是一个简单的例子，展示如何用C语言编写一个基础的命令行端口扫描工具，用于检测目标主机上哪些端口开放：

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

#define MAX_HOSTNAME_LEN 100
#define MAX_PORT 65535

void scan_port(int sockfd, struct sockaddr_in *target) {
    int port = 0;
    for (port = 0; port <= MAX_PORT; port++) {
        struct sockaddr_in serv_addr;
        bzero(&serv_addr, sizeof(serv_addr));
        serv_addr.sin_family = AF_INET;
        serv_addr.sin_port = htons(port);
        serv_addr.sin_addr.s_addr = target->sin_addr.s_addr;

        int result = connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr));
        if (result == -1) {
            if (errno != ECONNREFUSED) {
                perror("Error connecting to port");
                return;
            }
            printf("Port %d is open\n", port);
        } else {
            perror("Error connecting to port");
            close(sockfd);
            return;
        }
    }
}

int main(int argc, char *argv[]) {
    if (argc < 2) {
        printf("Usage: %s <hostname>\n", argv[0]);
        return 1;
    }

    char hostname[MAX_HOSTNAME_LEN];
    strncpy(hostname, argv[1], MAX_HOSTNAME_LEN);
    hostname[MAX_HOSTNAME_LEN - 1] = '\0';

    int sock = socket(AF_INET, SOCK_STREAM, 0);
    if (sock < 0) {
        perror("Failed to create socket");
        return 1;
    }

    struct sockaddr_in target;
    inet_pton(AF_INET, hostname, &target.sin_addr);
    target.sin_family = AF_INET;
    target.sin_port = htons(80); // Change this line to scan a different port

    if (connect(sock, (struct sockaddr *)&target, sizeof(target)) == 0) {
        printf("Host %s is up and accessible\n", hostname);
        scan_port(sock, &target);
    } else {
        printf("Host %s seems down or not responding on port %hu\n", hostname, ntohs(target.sin_port));
    }

    close(sock);
    return 0;
}

这个程序创建一个socket连接到指定的IP地址，并尝试打开每个端口。如果连接成功，表示该端口开放；如果连接失败并且返回`ECONNREFUSED`错误，则认为端口封闭。

注意：实际应用中，全面扫描可能会有法律风险和潜在的安全问题，因此只应在授权的情况下并遵守相应的法规进行。