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From: Eugene Grosbein <egrosbein@rdtc.ru>
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To: David Duchscher <daved@tamu.edu>
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Cc: hackers@freebsd.org
Subject: Re: Disabling ethernet link on an Intel nic?
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11.08.2012 20:35, David Duchscher пишет:
> Greetings, 
> 
> I have a need to turn off the link of an ethernet port on a Intel nic.  The issue is not a big deal but one we would like to solve.  we have no way of signaling an upstream router that a path is down but via turning off the link of the ethernet port.  This would only be used when something goes wrong. The hope is that we could automate some problems and handle other situations remotely avoiding having to visit the system in person.
> 
> Any thoughts?  We can modify the system so if it means modifying the driver or some other code, I am willing to try.  Just need a little direction.
> 
> Thanks for your time,

I made such patch a couple of years ago and run it in production with copper-based
em(4) and igb(4) NICs. The patch introduces new sysctls like dev.em.X.down_disables_link
and dev.igb.X.down.disables_link with default zero values. 
Setting sysctl dev.em.0.down_disables_link=1 makes "ifconfig em0 down" bring link down.

http://www.grosbein.net/freebsd/patches/em_sysctl-8.3.diff.gz
http://www.grosbein.net/freebsd/patches/igb_sysctl-8.3.diff.gz