Multiple DHCP Servers with DDNS Best Practice/Workaround?

Shawn Routhier sar at
Wed May 11 19:49:20 UTC 2011

You may want to review the update-conflict-detection
option.  It may provide the functionality you are

On 05/11/2011 10:16, Colin Simpson wrote:
> 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:
> 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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