From e6308f6e28fe40312d9a99b8c3ce97a37c496b70 Mon Sep 17 00:00:00 2001
From: PancakeTAS <pancake@mgnet.work>
Date: Sat, 13 Jun 2026 22:33:14 +0200
Subject: feat: Implement TUN module

---
 CMakeLists.txt |   1 +
 src/tun.c      | 104 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 src/tun.h      |  14 ++++++++
 3 files changed, 119 insertions(+)
 create mode 100644 src/tun.c
 create mode 100644 src/tun.h

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0820f7e..ad8e726 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -11,6 +11,7 @@ add_executable(mpu
   "src/heap.c"
   "src/mpu.c"
   "src/p2p.c"
+  "src/tun.c"
   "src/main.c")
 
 set_target_properties(mpu PROPERTIES
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;
+}
diff --git a/src/tun.h b/src/tun.h
new file mode 100644
index 0000000..98a307d
--- /dev/null
+++ b/src/tun.h
@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: GPL-3.0-or-later */
+
+#pragma once
+
+//
+// This module implements a Linux tun device. The tun_alloc() functions creates a tun device
+// with a given name, IP address, subnet mask and MTU.
+//
+
+#include <linux/if.h>
+
+/// Allocate a tun device with the given CIDR (e.g. "172.16.0.1/24") and MTU.
+/// Returns fd on success, -1 on failure (errno is set).
+int tun_alloc(const char* ifname, const char* cidr, int mtu);
-- 
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