[Dnsmasq-discuss] Always Ignore Client Identifier

Simon Kelley simon at thekelleys.org.uk
Thu Jan 30 10:07:37 GMT 2014

On 29/01/14 18:04, Linux Luser wrote:
> We have a pretty tightly-controlled private network environment which
> we've configured to have a 1-to-1-to-1 relationship between client MAC
> address, hostnames and IP addresses. Apart from "guest" IP ranges, we
> have control over when clients get added to the network. Thus, we can
> detect duplicate MAC addresses before it becomes an issue.
> In this setup, we can't need or want to use the "client identifier"
> option of DHCP. In fact, it becomes a problem when we start doing
> PXELinux installs, where a different client id gets sets during a remote
> install session, then when the install is complete and the new OS boots
> up, it gets a different IP address (because dnsmasq still knows about
> the lease it gave that same machine only 10 minutes ago!).
> To get rid of this issue, we now supply a dhcp-host option to dnsmasq
> each time we want to do a remote reinstall. The option looks something
> like this:
> dhcp-host=<MAC addr>,id:*,<hostname>,<IP addr>,set:install
> This works, since the "id:*" part tells dnsmasq to ignore the client ID
> in favor of the MAC address. But now to my question. Can this be done
> for ALL DHCP requests? Is there a global "identify-by-mac-only" option?
> If not, would you be willing to entertain the idea. I know many others
> have done this for some time now, using other DHCP server software, so I
> know it's possible and there doesn't seem to be any ill effects of this.#

There isn't a global option to do this, but there is precedent, in the 
form of --dhcp-ignore-names for adding it, and actually that's something 
more useful, since the tag system allows the setting to be applied to 
classes of clients (which could, of course, be "all clients")

> Maybe this is not a good idea? Like I said, we have a fairly controlled
> environment, so it would work for us. I could see how this would be
> unnecessary for common setups, though. Or environments that have many
> VMs running on a single host and simply bridge their network interface
> may want to use the "client identifier" option so each VM gets a unique
> IP even if they're running on the same machine or t But it would be nice to
> have a greater level of control over this.
> Thanks for your time. And GREAT piece of software, by the way. dnsmasq
> is a HUGE time saver and makes changing configurations straight-forward.
> Do you accept donations? :)

Donations by Paypal to simon at thekelleys.org.uk are always welcome, or 
you could commission me to add new features. I'm available for that on a 
consultancy basis, cheaper for stuff which goes back into the dnsmasq 
GPL codebase, more expensive for proprietary code.



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