Forwarding w/o promisc on 6.4

Robert Watson rwatson at
Sun Apr 19 09:14:20 UTC 2009

On Sun, 19 Apr 2009, Jon Otterholm wrote:

> I have a router running 6.4R that does not forward packets if I disable 
> PROMISC on the interface. Hardware is a Dell PE with two Intel 82541EI 
> chipsets (if_em). I have a number (~100) of vlan-interfaces on em0. 
> Everything works as aexpected if I turn on PROMISC on em0 but forwarding 
> stops if I disable it, I can still communicate with the router directly on 
> the same logical network (for example pinging interface adress on a vlan_if 
> from a client on that vlan) but all forwarding stops.

Try disabling hardware VLAN taggging/processing?  I believe you should be able 
to do this with "ifconfig em0 -vlanhwtag" (substituting appropriate interface 
names).  It could be there's a bug in how hardware-optimized tag handling is 
being managed, as when promiscuous mode is used we re-insert vlan headers in 
software for the benefits of BPF.

Robert N M Watson
Computer Laboratory
University of Cambridge

> Some info:
> net.inet.ip.forwarding: 1
> net.inet.ip.fastforwarding: 0 (enableing this does not help)
> net.inet.tcp.recvspace=1048576
> net.inet.tcp.sendspace=1048576
> kern.ipc.maxsockbuf=16777216
> I use PF for filtering and disableing this does not help either.
> Anyone with a clue?
> //JO
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