fix

main.cpp

@@ -5,6 +5,7 @@
 #include <netinet/in.h>
 #include <fcntl.h>
 #include <linux/if.h>
+#include <linux/ip.h>
 #include <linux/if_tun.h>
 #include <sys/ioctl.h>
 #include <unistd.h>
@@ -20,7 +21,9 @@ void inbound(int raw, int tun) {
     size_t nread;
     while ((nread = recvfrom(raw, buf, sizeof(buf), 0, (sockaddr *) &addr, &addrlen)) >= 0) {
         std::cout << "recv " << nread << " bytes from" << inet_ntoa(addr.sin_addr) << std::endl;
-        if (write(tun, buf, nread) < 0) {
+        auto *packet = (iphdr *) buf;
+        auto overhead = packet->ihl * 4;
+        if (write(tun, buf + overhead, nread - overhead) < 0) {
             perror("inbound write");
         }
     }