From owner-freebsd-net@FreeBSD.ORG  Thu Jan 27 22:41:16 2005
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Date: Thu, 27 Jan 2005 14:41:10 -0800
From: Charlie Schluting <charlie@schluting.com>
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Subject: vlan + promisc + em(4)
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On 1/20/2005 2:33 AM, Robert Watson wrote:
> Try "ifconfig em0 -vlanhwtag" and see if that helps.  If not, take a look
> in if_em.c:em_setup_interface(), and you'll see two lines like this: 
> 
> #if __FreeBSD_version >= 500000
>         ifp->if_capabilities |= IFCAP_VLAN_HWTAGGING | IFCAP_VLAN_MTU;
>         ifp->if_capenable |= IFCAP_VLAN_HWTAGGING | IFCAP_VLAN_MTU;
> #endif
> 
> Delete the contents "|FCAP_VLAN_HWTAGGING |" from each line, and that
> should disable support for hardware vlan tagging and stripping in the
> driver.  There are several bugs relating to the handling of hardware vlan
> tagging and promiscuous mode in both if_re and if_em.  I had hoped to have
> a chance to resolve them over the past couple of months but have not as
> yet been able to do so. 

I'm sad to report that neither worked. After doing the ifconfig -vlanhwtag, 
the interface stopped recieving packets in about an hour.
After deleting IFCAP_VLAN_HWTAGGING and recompiling/rebooting, it worked for 
about 4 hours, then stopped.

tcpdump sees nothing when it happens.. bringing the interface down; then back 
up seems to fix it. We've got a cron on the job now :)

-Charlie