tcpdump -- non-local traffic not showing

Christopher Cowart ccowart at
Fri Oct 5 17:12:10 PDT 2007

On Fri, Oct 05, 2007 at 05:31:25PM -0600, freebsd at wrote:
> I'm having trouble seeing packets which are not going to or from the
> machine on which tcpdump is running.  Is there something special I
> need to do to enable this?  It's my understanding tcpdump puts the
> interface in promiscuous mode, and dmesg seems to confirm this.
> However I see the following behavior using "tcpdump -fntl -i ed1":
> If hosts .x, .y, and .z are all on the same network,
> and if tcpdump is running on host a.b.c.x
> and on host a.b.c.y I do
>  ping a.b.c.x
> I see the icmp packets.
> But if on host a.b.c.y I do
>  ping a.b.c.z
> I see nothing.
> Does the interface drop packets with a different mac address, even
> when supposedly put in promiscuous mode?
> Clues?

You're probably plugged into a switch ("learning bridge"). Switches
partition your collision domain -- they learn which MAC is available on
which port and only send on that port.

You either need a hub or a really expensive switch (the kind that you
log in to and set up port mirrors).

Chris Cowart
Lead Systems Administrator
Network & Infrastructure Services, RSSP-IT
UC Berkeley
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