#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/socket.h>
#define BUF_SIZE 1024
struct NetworkConnection {
    uint16_t magic;
    struct in6_addr clientAddr;
    struct in6_addr proxyAddr;
    uint16_t clientPort;
    uint16_t proxyPort;
};
void handleConn(int sockfd) {
    struct sockaddr_in clientAddr;
    socklen_t addrLen = sizeof(clientAddr);
    unsigned char buf[BUF_SIZE];
    ssize_t n = recvfrom(sockfd, buf, BUF_SIZE, 0, (struct sockaddr *)&clientAddr, &addrLen);
    if (n < 0) {
        perror("Error reading from UDP connection");
        return;
    }
    // Convert the received bytes to a NetworkConnection struct.
    struct NetworkConnection nc;
    nc.magic = ntohs(*(uint16_t *)buf);
    if (nc.magic == 0x56EC) { // Magic with the value 0x56EC indicates an SPP header.
        memcpy(&nc.clientAddr, buf + 2, 16);
        memcpy(&nc.proxyAddr, buf + 18, 16);
        nc.clientPort = ntohs(*(uint16_t *)(buf + 34));
        nc.proxyPort = ntohs(*(uint16_t *)(buf + 36));
        printf("Received packet:\\n");
        printf("\\tmagic: %x\\n", nc.magic);
        char clientIp[INET6_ADDRSTRLEN];
        char proxyIp[INET6_ADDRSTRLEN];
        inet_ntop(AF_INET6, &nc.clientAddr, clientIp, INET6_ADDRSTRLEN);
        inet_ntop(AF_INET6, &nc.proxyAddr, proxyIp, INET6_ADDRSTRLEN);
      
        // Print the SPP header information, including magic, real client IP and port, as well as proxy IP and port.
        printf("\\tclient address: %s\\n", clientIp);
        printf("\\tproxy address: %s\\n", proxyIp);
        printf("\\tclient port: %d\\n", nc.clientPort);
        printf("\\tproxy port: %d\\n", nc.proxyPort);
        // Print the actual and effective payload.
        printf("\\tdata: %.*s\\n\\tcount: %zd\\n", (int)(n - 38), buf + 38, n);
    } else {
        printf("\\tdata: %.*s\\n\\tcount: %zd\\n", (int)n, buf, n);
    }
    // Respond with a packet. Note: The 38-byte SPP header must be returned completely.
    sendto(sockfd, buf, n, 0, (struct sockaddr *)&clientAddr, addrLen);
}
int main() {
    int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd < 0) {
        perror("Failed to create socket");
        exit(EXIT_FAILURE);
    }
    // Create a UDP address using the local address and port.
    struct sockaddr_in serverAddr;
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_addr.s_addr = INADDR_ANY;
    serverAddr.sin_port = htons(6666);
    if (bind(sockfd, (struct sockaddr *)&serverAddr, sizeof(serverAddr)) < 0) {
        perror("Failed to bind");
        exit(EXIT_FAILURE);
    }
    while (1) {
        handleConn(sockfd);
    }
}