#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>

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

#include <net/if.h>
#include <sys/ioctl.h>
#include <linux/if.h>

#define MAX_PACKET_LEN 65507  // Max UDP payload size (practical)

void print_socket_options(int fd) {
    int val;
    socklen_t len = sizeof(val);

    // SO_SNDBUF
    if (getsockopt(fd, SOL_SOCKET, SO_SNDBUF, &val, &len) == 0) {
        printf("Socket send buffer size (SO_SNDBUF): %d bytes\n", val);
    } else {
        perror("getsockopt SO_SNDBUF");
    }

    // IPV6_DONTFRAG
    if (getsockopt(fd, IPPROTO_IPV6, IPV6_DONTFRAG, &val, &len) == 0) {
        printf("IPV6_DONTFRAG: %s\n", val ? "ENABLED (no fragmentation)" : "DISABLED (fragmentation allowed)");
    } else {
        perror("getsockopt IPV6_DONTFRAG");
    }
}

void print_interface_mtu(const char *ifname) {
    int sock;
    struct ifreq ifr;

    sock = socket(AF_INET6, SOCK_DGRAM, 0);
    if (sock < 0) {
        perror("socket (for ioctl)");
        return;
    }

    memset(&ifr, 0, sizeof(ifr));
    strncpy(ifr.ifr_name, ifname, IFNAMSIZ - 1);

    if (ioctl(sock, SIOCGIFMTU, &ifr) == 0) {
        printf("Interface MTU for %s: %d bytes\n", ifname, ifr.ifr_mtu);
    } else {
        perror("ioctl SIOCGIFMTU");
    }

    close(sock);
}

int main(int argc, char *argv[]) {
    int fd;
    struct sockaddr_in6 sa6;
    socklen_t sa_len;
    char *buf;
    ssize_t n;
    size_t len = MAX_PACKET_LEN;
    const char *iface = "lo";  // Default interface (loopback)
    const char *target_ip = "::1";  // Default target

    if (argc > 1) {
        len = atoi(argv[1]);  // Allow custom payload length
    }
    if (argc > 2) {
        iface = argv[2];      // Allow custom interface
    }

    printf("Attempting to send %zu bytes via IPv6 UDP\n", len);
    printf("Target: [%s]:9999\n", target_ip);
    printf("Interface: %s\n\n", iface);

    print_interface_mtu(iface);

    // Create IPv6 UDP socket
    fd = socket(AF_INET6, SOCK_DGRAM, 0);
    if (fd < 0) {
        perror("socket");
        exit(EXIT_FAILURE);
    }

    print_socket_options(fd);

    // Set up destination address
    memset(&sa6, 0, sizeof(sa6));
    sa6.sin6_family = AF_INET6;
    sa6.sin6_port = htons(9999);
    if (inet_pton(AF_INET6, target_ip, &sa6.sin6_addr) <= 0) {
        perror("inet_pton");
        close(fd);
        exit(EXIT_FAILURE);
    }
    sa_len = sizeof(sa6);

    // Allocate and fill buffer
    buf = malloc(len);
    if (!buf) {
        perror("malloc");
        close(fd);
        exit(EXIT_FAILURE);
    }
    memset(buf, 'X', len);

    // Send the packet
    n = sendto(fd, buf, len, 0, (struct sockaddr *)&sa6, sa_len);
    if (n < 0) {
        perror("sendto");
        fprintf(stderr, "errno = %d (%s)\n", errno, strerror(errno));
    } else {
        printf("\n✅ Sent %zd bytes successfully over IPv6 UDP\n", n);
    }

    free(buf);
    close(fd);
    return 0;
}