9

We're running four computers connected to a hub (yes a hub not a switch) on Fedora 13. They were installed using images from a colleague that has recently left us to go back to school, they may have been set to somehow block the use of the broadcast, but we'd like to continue using this image. We've setup the static IPs for each computer using ifconfig (we've also tried ip addr add) to 10.0.1.11/24 through 10.0.1.14/24, using these settings we can't seem to ping the broadcast which is indeed set correctly to 10.0.1.255, using tcpdump we see nothing except outgoing ping requests from the computer that calls the ping request. Does anyone have any ideas or am I missing something entirely?

  • Are you using ping -b? – TomasG Feb 15 '11 at 17:13
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echo 0 >/proc/sys/net/ipv4/icmp_echo_ignore_broadcasts - as root.

You may need to also use the -b option with ping and it will most likely require root permissions.

  • Very nice, not sure how I would've found that – Jon Phenow Feb 15 '11 at 21:17
  • @jphenow - you find it like so: man ping Once inside the man program, do /broadcast. It jumps right to the relevant part. – Shawn J. Goff Feb 16 '11 at 0:58
  • Oh I knew about the -b part, it was the option that needed changing, I use manuals quite often. – Jon Phenow Feb 16 '11 at 16:38
6

Broadcast amplification was used to mount DOS (Denial of Service) attacks. As a result most IP stacks now turn off echo responses to broadcast ping. This is normal behavior now.

  • That's sort of what we were figuring, I guess I wasn't sure it was default but it would make sense... does anyone use broadcast messages anymore? – Jon Phenow Feb 15 '11 at 21:17
  • Broadcast is so 1995. Multicast is what the cool kids are doing now. – LawrenceC Feb 15 '11 at 22:33
  • Haha yea our Network professor figured he'd dig way back into the ages of Networking. – Jon Phenow Feb 16 '11 at 16:38
  • @LawrenceC Not 95, around 99 as I can remember (smurf) – peterh Feb 14 at 15:56

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