[dhcwg] DHCP and VLANs

Bazy bazy84 at gmail.com
Wed May 17 19:26:08 UTC 2006

I'm very sorry for posting on the wrong list, I signed up at 4 AM... :-(

Thank you for your answer, I posted here because someone told me that
you cannot use DHCP on VLANd interfaces.

Thank you again David!

On 5/17/06, David W. Hankins <David_Hankins at isc.org> wrote:
> On Wed, May 17, 2006 at 02:38:16PM +0300, Bazy wrote:
> > On the Linux Server (Debian sarge) I' run dhcp3, I've worked with dhcp
> > before and I'm familiar to it. The server is connected on one of the
> > 1000Base-T ports.
> You've reached the wrong group to answer this question - I suggest
> we continue this discussion on dhcp-users at isc.org.  You can subscribe
> here:
>         http://www.isc.org/sw/dhcp/dhcp-lists.php
> The DHC WG is an IETF standards body which serves to develop and
> maintain DHCP protocol documents ("RFCs").  This isn't an appropriate
> place for this type of discussion.
> > Can I start dhcp with the -q eth1.10 eth1.11 eth1.xx? or I need to
> Generally, it's not necessary to specify the interfaces you want the
> server to use.  You merely need to configure a subnet {} statement
> for all interfaces on the system.
> But yes, you use the vlan subinterface's name.  You also have to
> set an interface option so the interface is 'baked'.  ISC DHCP doesn't
> speak 802.1q as you've noticed, so the kernel has to do this for us
> on the raw sockets (reframing our raw ethernet to 802.1q, and reframing
> 802.1q to raw ethernet for us).  I can't remember the name of the flag.
> If you ask on dhcp-users, I'm confident you'll get an answer.
> Operating DHCP over 802.1q vlans is fairly common practice.
> > So... Anyone tried this? Or can anyone tell me if it will work or not?
> I haven't, but someone has.
> As it turns out, there's a problem.  It was noticed on a college campus
> wireless access point network - where different regions of the campus
> had different broadcast domains (hence, address space), and clients might
> roam from one to another.  The VLANs were all backhauled to a single
> DHCP server similar to your setup.
> MS Windows DHCP clients that roam between vlans become confused.
> They will, according to what's been reported, first attempt to transmit
> (unsure if this is unicast or broadcast, memory if fuzzy) a DHCP
> REQUEST message, which will succeed in reaching the DHCP server either
> way (because your vlan interface mac addresses are the same on all
> interfaces).
> The server, unable to discern between unicast and broadcast, actually
> ACKS the client (presumes unicast - in which case the packet can come
> from any direction, so network attachment isn't checked).
> Windows clients do not escape this hole unless manually released and
> renewed.
> Mac OSX clients however are known to deal with this environment.
> I suspect the difference is that Mac OSX looks like INIT/REBOOT to
> the DHCP server - so network attachment detection is checked and the
> client is NAKed.  Windows' packets at these events look like RENEWING
> (which, due again to the server's inability to sense the difference
> between unicast and broadcast, looks identical to REBINDING).
> But at the moment this is mere suspicion, I haven't looked at packet
> traces of these events, only pieced together information from what's
> been reported to me.
> This isn't a very big problem if you don't expect clients to roam very
> frequently (without rebooting) if at all.  That said, there are also a
> number of ways to work around it (not the least of which is to use a
> DHCP relay instead).
> We can talk about that on dhcp-users.
> --
> David W. Hankins                "If you don't do it right the first time,
> Software Engineer                       you'll just have to do it again."
> Internet Systems Consortium, Inc.               -- Jack T. Hankins

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