[Dnsmasq-discuss] DHCP errors with vlans and multiple subnets

Simon Kelley simon at thekelleys.org.uk
Wed Mar 5 08:48:25 UTC 2014

Is there anything (probably new, quite possibly not the access point or 
the server running dnsmasq) which could be bridging the vlans? That 
would give the symptoms you are seeing.

Hmm, except that the log indicates that the requests are arriving 
sequentially, a few seconds apart on each VLAN. That points to the 
access points doing individual requests on each VLAN, and must therefore 
be an acesss point configuration thing.

Good luck!



On 04/03/14 21:17, David Joslin wrote:
> Thanks, Simon.
> The access points are all on vlan 9 (the management vlan). They serve
> wireless clients on vlans 3, 4 and 5 but they don't have interfaces on
> these vlans.
> What's really baffling me is that nothing has changed with the network
> configuration. I've been through the configuration of the router (with
> it's dnsmasq dhcp server) and the switches on the network over and over
> again and I can't find anything that's changed.
> Each subnet is defined on the router (with dhcp enabled for each subnet)
> and each vlan is associated with a particular subnet. The way it used to
> work was this: I simply reserved the IP addresses for the access points
> on the router (and I can see that these reservations have gone into
> dnsmasq's dhcp-hosts file) and the access points were given these
> addresses (on vlan9) when they asked for them - simple! Now, when they
> request addresses, they are being offered addresses from every vlan
> apart from vlan 9! And when they request the offered address, dnsmasq
> pumps out the warning messages that the requested address conflicts with
> the address in the hosts file. I even disabled the dhcp server on all
> the subnets apart from that assigned to vlan9 to see if that would force
> the dhcp server to assign the correct 10.10.99... address but when I did
> this dnsmasq logged the following messages over and over again:
> no address range available for DHCP request via br0
> no address range available for DHCP request via br1
> no address range available for DHCP request via br2
> It wouldn't offer a vlan9 address.
> There appeared to be a simple solution to this which was to assign
> static IPs to all vlan9 devices (switches, access points). But when I
> did, DHCP requests from the access points still appeared at the router!
> This would seem to be a fairly major bug (in the Ubiquti UniFi Wi-Fi
> system) but I'm wondering if this is in some way related to this problem
> (I can't think how, though).
> The access points have only one network interface and don't seem to
> offer any way to configure client-IDs so I still can't see a solution
> (apart from reverting everything to factory settings and building it all
> from scratch again - not something I want to do).
> If you've got any advice I'd be grateful.
> Cheers
> David
> On 4 March 2014 17:59, Simon Kelley <simon at thekelleys.org.uk
> <mailto:simon at thekelleys.org.uk>> wrote:
>     So, the same machine, with the same MAC address, seems to be talking to
>     the dnsmasq DHCP server from (at least) three different subnets
>     more-or-less simultaneously. This is not good, as the DHCP protocol (for
>     IPv4, at least) assumes each interface will get _one_ address. Dnsmasq
>     is chasing its tail, giving the machine one address, then abandoning
>     that and giving it another, and so on.
>     The APs have interfaces on multiple VLANS? If so you need to do one of
>     two things
>     1) Get them to use different MAC addresses on each distinct VLAN,
>     2) Get them to use DHCP client-IDs and ensure that _those_ are distinct.
>     As a unique identifier, client-ids override MAC addresses, so you should
>     be OK leaving the interfaces with a single MAC address if you take the
>     client-id route.
>     Most DHCP clients have a way to configure which client-id they
>     should use.
>     Cheers,
>     Simon.
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