Re: vr0 only working if tcpdump -n -i vr0 is running

[Chuck Swiger <cswiger%mac.com@localhost>]

On Oct 12, 2011, at 2:26 PM, Jonathan Schleifer wrote:
> I'm having a strange problem with the vr0 in my barebone. It is only working 
> when running tcpdump -n -i vr0 in the background. As soon as I stop it, it 
> stops working again.

As you discovered, tcpdump is putting the NIC into promiscuous mode, so it 
processes all packets it sees, rather than ignoring packets which it doesn't 
believe it should listen to.

This tends to indicate layer-2 problems such as it is not generating 
ARPOP_REPLYs for its IP with the right MAC, or the ARP tables on other machines 
in the network don't match.

Running "tcpdump -exxn -s 0 arp" and "arp -a" ought to be informative.

> There are several things strange with this network anyway:
> 
> • I have to use a specific MAC, so I do ifconfig vr0 link $some_mac active
>  first.

OK.  Maybe there is a bug where the vr0 NIC isn't getting the receive filter 
reprogrammed to pickup the changed MAC.  You might be able to work around the 
issue by convincing the thing which wants a specific MAC to look for the MAC 
which your vr0 NIC originally has.

You aren't trying to do proxy arp'ing or layer-2 bridging, by any chance...?

> • Many NICs did not even get a signal, my WiFi access point only gets a signal
>  sometimes, but if it got one, it keeps it. But until it gets one, I have to
>  restart it several times. (I guess the cable is too long or something).

What does "signal" mean here?

Ethernet link light?  Seeing traffic from the rest of the network?

Regards,
-- 
-Chuck