Forwarding w/o promisc on 6.4

Chris Cowart ccowart at
Mon Apr 20 20:24:17 UTC 2009

Jon Otterholm wrote:
> On 2009-04-19 11.14, "Robert Watson" <rwatson at> wrote:
>> 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.
> I tried doing this without any luck. Running GENERIC kernconf.
>>> 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.

Could you send the output from `ifconfig` and `netstat -rn`? That might
help us figure out what's going on.

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