DHCP client moving between networks

[Bruce Hudson]

> 
> I'm keeping the 172 network on the vlan it was originally on.  I'm
> essentially moving large numbers of hosts to another interface on the
> router, and therefore the DHCP server.
> 
> Everything works after I let the leases expire on the new interface,
> as when they sent the DHCPDISCOVER, they hit the new interface.  It
> was only when the server got the DHCPREQUEST for the old address that
> they got 172s.
> 
> 
> 
> On Wed, Sep 30, 2009 at 15:40, Shawn Holland <sholland at sandara.ca> wrote:
> > On Wed, 2009-09-30 at 14:57 -0700, Tim Gavin wrote:
> >> I have a DHCP server (on Debian) that handles several VLANs.  I am
> >> also in the middle of rebuilding the physical and logical network.
> >> Occasionally, this means moving large numbers of hosts between vlans.
> >>
> >> A few weeks ago, I moved about 150 users to a new VLAN on my network
> >> in a midnight maintenance window.  When I did, something unexpected
> >> happened. . . I changed from a private NAT (172.16.x.x) to public IPs
> >> (216.x.x.x).
> >>
> >
> > So did you remove the 172.16.x.x pool completely?
> >
> >> Beforehand, I had set my main DHCP pool to one hour leases
> >
> > This main pool.. is it the new 216.x.x.x pool?
> >
> >> (there were
> >> roughly 2000 users on it originally).  When I did this, all of the
> >> clients sent DHCPREQUESTs for their 172. addresses,
> >
> > Thats expected.
> >
> >>  and I expected the
> >> server to respond with a "wrong network" or some other error
> >> condition, and issue them a new lease.
> >
> > If the server is authoritative, and it doesn't know about the lease they
> > are asking it should send a DHCPNACK
> >
> >> Instead, they sent a "DHCPACK"
> >> and the client kept their lease.
> >
> > The server would only ACK if the lease they requested was still valid.
> >
> >> Obviously, the clients couldn't get
> >> out, since their network and gateway were invalid.
> >>
> >> At the time, I just killed that VLAN interface on the server, and
> >> waited for the lease timeout, and reconnected it.  Then the users all
> >> got new leases.
> >>
> >
> > It is possible that the leases file still contained their valid lease
> > because it hadn't expired based on what was created originally with the
> > old lease time.
> >
> >> I'm going to be doing this again in the future, and will probably be
> >> doing it with larger numbers of users that will not accept the
> >> downtime waiting for everyone else's leases to expire.  Can someone
> >> tell me what I'm doing wrong, or how to work around this?  Most of my
> >> clients are using Windows clients, but a lot of them are using
> >> residential routers or computers with other OSs, so there is no client
> >> consistency.  Since I'm at an ISP, I can't mandate a change client
> >> side.
> >>
> >
> > I would make sure that the leases file was updated to expire their
> > leases.
> >
> >> I KNOW I've seen an answer to this somewhere. . . something about a
> >> guy using his laptop on multiple floors of a building or something,
> >> but I can't find it anywhere now (wish I had read it then, was looking
> >> for something else :-/ ), so a link to info would be fine :-D
> >>
> >> Thanks,
> >>
> >> Tim
> >
> > It would be very helpful to post your dhcpd.conf file before and after
> > the change. I'm just making an educated guess as to what teh problem is
> > without seeing the conf.
> >
> > --
> > Regards,
> > Shawn Holland
> > Sandara Technologies Ltd.
> >
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> >
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