1 files changed, 104 insertions, 0 deletions
diff --git a/src/tun.c b/src/tun.c
new file mode 100644
index 0000000..8658893
--- /dev/null
+++ b/src/tun.c
@@ -0,0 +1,104 @@
+/* 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;
+}