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.

[Michael Kromer]

Correction: Problem exists since 2.6.36, not 2.6.35 with commit bee31369ce16fc3898ec9a54161248c9eddb06bc (follow the link posted, it provides exact information)