espinudp_daemon.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <linux/udp.h>

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

    char *endptr;
    long port_number = strtol(argv[1], &endptr, 0);
    if (0 != *endptr || port_number < 0 || port_number > 65535) {
        printf("Invalid port number\n");
        return -1;
    }

    int sockfd = -1;
    if (0 > (sockfd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP))) {
        perror("socket() failed");
        return -1;
    }

    struct sockaddr_in host_addr;
    memset(&host_addr, 0, sizeof(host_addr));
    host_addr.sin_family = AF_INET;
    host_addr.sin_port = htons(port_number & 0xFFFFu);
    host_addr.sin_addr.s_addr = htonl(INADDR_ANY);

    int type = UDP_ENCAP_ESPINUDP;
    if (0 > setsockopt(sockfd, IPPROTO_UDP, UDP_ENCAP, &type, sizeof(type))) {
        perror("setsockopt() failed to set UDP_ENCAP");
        return -1;
    }

    if (0 > bind(sockfd, (struct sockaddr*) &host_addr, sizeof(host_addr))) {
        perror("bind() failed");
        return -1;
    }
    printf("Port opened\n");
    while(1) {
        sleep(1);
    }
    return 0;
}