/* SPDX-License-Identifier: GPL-3.0-or-later */

#include "tun.h"

#include <arpa/inet.h>
#include <errno.h>
#include <fcntl.h>
#include <linux/if.h>
#include <linux/if_tun.h>
#include <linux/sockios.h>
#include <netinet/in.h>
#include <stdio.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <unistd.h>

static int parse_cidr(const char* cidr, struct in_addr* addr, struct in_addr* netmask) {
    char ip[INET_ADDRSTRLEN];
    int prefix = 0;

    if (sscanf(cidr, "%[^/]/%d", ip, &prefix) != 2 || prefix < 0 || prefix > 32) {
        return -1;
    }

    *netmask = (struct in_addr) {
        .s_addr = htonl(0xFFFFFFFF << (32 - prefix)),
    };
    if (!inet_aton(ip, addr)) {
        return -1;
    }

    return 0;
}

int tun_alloc(const char* ifname, const char* cidr, int mtu) {
    struct in_addr addr, netmask;
    if (parse_cidr(cidr, &addr, &netmask) < 0) {
        errno = EINVAL;
        goto err;
    }

    struct ifreq ifr = {0};
    strncpy(ifr.ifr_name, ifname, IFNAMSIZ);

    // Create the tun device
    int fd = open("/dev/net/tun", O_RDWR);
    if (fd < 0) {
        goto err;
    }

    ifr.ifr_flags = (short) (IFF_TUN | IFF_NO_PI | IFF_TUN_EXCL);
    if (ioctl(fd, TUNSETIFF, &ifr) < 0) {
        goto err_close_fd;
    }

    // Configure IP address, netmask and MTU
    int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd < 0) {
        goto err_close_fd;
    }

    struct sockaddr_in* sin = (struct sockaddr_in*) &ifr.ifr_data;

    *sin = (struct sockaddr_in) {
        .sin_family = AF_INET,
        .sin_addr = addr,
    };
    if (ioctl(sockfd, SIOCSIFADDR, &ifr) < 0) {
        goto err_close_sockfd;
    }

    *sin = (struct sockaddr_in) {
        .sin_family = AF_INET,
        .sin_addr = netmask,
    };
    if (ioctl(sockfd, SIOCSIFNETMASK, &ifr) < 0) {
        goto err_close_sockfd;
    }

    ifr.ifr_mtu = mtu;
    if (ioctl(sockfd, SIOCSIFMTU, &ifr) < 0) {
        goto err_close_sockfd;
    }

    // Bring interface up
    ifr.ifr_flags = 0;
    if (ioctl(sockfd, SIOCGIFFLAGS, &ifr) < 0) {
        goto err_close_sockfd;
    }

    ifr.ifr_flags |= IFF_UP | IFF_RUNNING;
    if (ioctl(sockfd, SIOCSIFFLAGS, &ifr) < 0) {
        goto err_close_sockfd;
    }

    return fd;
err_close_sockfd:
    close(sockfd);
err_close_fd:
    close(fd);
err:
    return -1;
}