Re: Multiple multicast apps

From: Christian Weisgerber (naddy_at_mips.inka.de)
Date: 10/06/04 

Date: Wed, 6 Oct 2004 19:07:47 +0000 (UTC)

James Bass <anon@somewhere.com> wrote:

> I have two applications that join two different multicast groups and read
> data off the sockets.
>
> It seems that when I start one it works fine, but when I start the second it
> stops the first receiving data, but works fine itself. When I kill the
> second application, the first then starts working fine again.

Looks like an error in the multicast filter setup for your network
card, i.e. a bug either in the network driver or more generically
in the multicast handling.

(Nearly all 100Mbit/s and better ethernet chipsets support a multicast
filter. Rather than having to take all multicast packets off the
wire and examine the header to see whether it matches any of the
groups the local host has joined, a filter can be programmed to
pass on only relevant packets and ignore others. A very common
design is a 64-entry hash filter.)

> If I run tcpdump on the interface being used to receive the multicast
> packets, then both apps work fine simultaneously - wierd huh?

No, makes perfect sense. tcpdump puts the interface into promiscuous
mode, so all packets are received.

> This is on FreeBSD 4.4 - and yes I know it is old, but we don't really want
> to upgrade.
>
> Any ideas as to cause and solution?

It's a bug and I would assume it has been fixed later in the 4.x
branch. You can probably backport a patch. If it's a driver
specific bug, switching to a different type of NIC might also serve
as a workaround. 

-- 
Christian "naddy" Weisgerber                          naddy@mips.inka.de

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