2003-05-19 17:27:14

by Denis Zaitsev

Subject: Impossible to turn BROADCAST mode off on ethernet device?

Kernel 2.4.20. I do:

ifconfig eth0 -broadcast

And BROADCAST isn't turned off... The ethernet modules are: eepro100,
tulip, 3c59x. So, what does it mean?

Thanks in advance.

2003-05-19 18:05:14

by Richard B. Johnson

Subject: Re: Impossible to turn BROADCAST mode off on ethernet device?

On Mon, 19 May 2003, Denis Zaitsev wrote:

> Kernel 2.4.20. I do:
>
> ifconfig eth0 -broadcast
>
> And BROADCAST isn't turned off... The ethernet modules are: eepro100,
> tulip, 3c59x. So, what does it mean?
>
> Thanks in advance.


Your `ifconfig` might not allow broadcast to be turned off as
long a eth0 is `up`. So I just made a little program to get the flags,
and reset the flags. This shows that your observation is, indeed,
correct.

If you mess around with this, I think you will find that you can't
set broadcast OFF as long at the IFF_UP flag is set. This may be
the required behavior because an interface without broadcast
capability will not work on ethernet because ARP requires it.

#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <string.h>
#include <net/if.h>

int main()
{
struct ifreq ifr;
int s, status;
if((s = socket(PF_INET, SOCK_DGRAM, IPPROTO_IP)) == -1)
{
fprintf(stderr, "socket() failed (%s)\n", strerror(errno));
return s;
}
memset(&ifr, 0x00, sizeof(ifr));
ifr.ifr_netmask.sa_family = AF_INET;
strcpy(ifr.ifr_name, "eth0");
status = ioctl(s, SIOCGIFFLAGS, &ifr);
ifr.ifr_flags &= ~IFF_BROADCAST;
status = ioctl(s, SIOCGIFFLAGS, &ifr);

(void)close(s);
return status;
}



Cheers,
Dick Johnson
Penguin : Linux version 2.4.20 on an i686 machine (797.90 BogoMips).
Why is the government concerned about the lunatic fringe? Think about it.