bridged networking does not work starting with 2.6.35 on TAP-devices

[Michael Kromer]

As you can read on

​http://amailbox.org/mailarchive/linux-netdev/2010/10/23/6288128/thread

VBox stops working with bridged Networking and TAP devices since 2.6.35.

I don't think VBox.log will help you much, the only thing which might help is that transmits are ok, but recv is completely dead.

A short test with

#include <fcntl.h> #include <stdbool.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <unistd.h> #include <sys/ioctl.h> #include <sys/stat.h> #include <sys/types.h> #include <net/if.h> #include <linux/if_tun.h>

int main(int argc, const char argv) {

struct ifreq ifr; char *p; int fd;

if (argc < 2) {

fprintf(stderr, "Need tun name\n"); return EXIT_FAILURE;

}

fd = open("/dev/net/tun", O_RDWR); if (fd < 0) {

perror("open"); return EXIT_FAILURE;

} memset(&ifr, 0, sizeof(ifr)); ifr.ifr_flags = IFF_TAP | IFF_NO_PI; p = strncpy(ifr.ifr_name, argv[1], sizeof(ifr.ifr_name) - 1); if (ioctl(fd, TUNSETIFF, &ifr) < 0) {

perror("ioctl"); return EXIT_FAILURE;

} while (true)

sleep(3600);

return EXIT_SUCCESS;

}

results in a positive fix.

I think the problem is, that vboxnetflt simply transmits via skb->dev=tapX and doesn't open the tap/tun fd. The upstream (kernel) patch is suboptimal for vbox in a way that it actually needs to open the file descriptor - otherwise the interface has status NO-CARRIER and packets will not be accepted by core kernel.