Multiple DHCP Servers with DDNS Best Practice/Workaround?

[Colin Simpson]

Hi

I'm just looking for some advice on the best practice for multiple ISC
DHCP servers with DDNS updates (each one on a different subnet). The
issue I have is that machines switching subnets are stuck with the old
names in DNS. 

Now I realise this is by design (by use of the TXT with the A records).
But if a laptop user disconnects from one network (which they invariably
do without de-registering) and reconnect to a new network, their
hostname will not get updated in DNS until the DHCP lease expires in
DHCP on the original server. This is causing issues (particularly for
Linux laptops, where people expect to reach them by SSH and NFS servers
that don't like a client that isn't properly setup in DNS (A and PTR))

I'm presuming for most situation this is acceptable so that DHCP servers
don't tread on each others DNS allocations. But can you force them to
(maybe in such a way that the original DHCP server won't delete the new
DNS record from a second server (for example by checking that it's not
in the range he manages)?

I saw a thread about this from several years ago:

https://lists.isc.org/pipermail/dhcp-users/2006-August/001335.html

Has anything changed since then (or moved forward)? The only approach in
here involved source codes changes, which from a maintainability point
of view isn't great for us.

Or has anyone got a cunning way round this? 

I thought of a bit of a hack that could remove a machines DHCP
allocation from all other DHCP servers in the environment if it appears
on a new DHCP server. Or maybe just a very short lease time (but
obviously issues with that too).

I realise there are risks to this, but we are sure (as it's possible to
be) there are no overlapping name allocations at this site (they are
assigned by us). 

And this argument against stamping on each other's toes, would be more
relevant if a malicious/stupid user couldn't already screw up say a
server, by giving their machine a server's name. I'd presume that DHCPD
would happily overwrite the static DNS entry (for the server's static
IP), esp if say a Windows DC that adds DNS entries itself to DNS (but no
TXT entries attached to them). Should I be adding my own random TXT
entries to static DNS entries if they share a zone with DHCP manage
DNS? 

Sadly the subnets in our situation don't all share a switching
infrastructure so a single server with DHCP relaying is not an option.
And does this work properly with de-registering from an existing subnet
and re-registering on a new one (out of interest)?

Any thoughts very welcome.

Thanks

Colin

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