[Dnsmasq-discuss] DHCP problem when moving from one WiFi SSID to another

[Kevin Darbyshire-Bryant]

> On 27 Dec 2018, at 08:45, Chris Green <cl at isbd.net> wrote:
> 
>>> 
>>> 
>>> My laptop seems to lose its IP address whenever I move from one
>>> Draytek's WiFi to the other but only when the IP is assigned by
>>> dnsmasq.  If I connect to my guest network (192.168.6.x) then I get a
>>> IP address assigned by the 2860n and a good connection to the outside
>>> world.  If I then reconnect to the 'local' WiFi the laptop loses its
>>> IP address.  It's as if dnsmasq doesn't see the disconnection and
>>> doesn't answer the new DHCP broadcast from my laptop.  If I leave it
>>> disconnected for a minute or two and then re-connect to the WiFi it
>>> *does* get an IP.
>>> 
>>> 
>>> Can anyone explain what might be wrong and/or a fix or workaround?
>>> 
>>> 
> 
> He didn't really.  :-)   He said:-
> 
> "but I don't have any concrete suggestions on how to fix it. I think the SSID change
> is a red-herring."
> 
> But, yes, it is basically the same issue, but now I'm not changing SSID.
> 
> I have now changed the dnsmasq configuration to set
> dhcp-authoritative, maybe that will do something.

One of the things that has always bothered me about these ‘linking lan switch ports between two wifi APs/one wifi AP/Router’ scenarios is the numerous levels of packet switching/bridging going on.  I dimly remember one of my own setups using a ‘dual AP’ that would happily roam from AP1 to AP2 but not the other way…until a bridge table entry timed out.  I wonder if that’s what is going on here?

The common scenario in terms of bridging/switching is something like (in the linux world):

AP1

Wifi Interface (or interfaces, think 2.4Ghz & 5Ghz radios) and LAN cpu interface are software bridged together.  So there’s bridge learning of which MAC is on which bridge port going on there.  The LAN cpu interface itself is often a hardware switch with built in learning of which MAC is on which physical switch port.  One of those ports is physically connected to AP2 which has:

AP2

A LAN port as part of a hardware switch with built in learning of which MAC address is on which port presented to the cpu as a LAN cpu port.  This is software bridged to a Wifi interface (or interfaces, think 2.4Ghz & 5Ghz radios) which has to learn which MAC appears on which port.


Roaming from one wireless radio on one AP to another wireless radio on another AP involves quite a lot of bridge table changes, I’m amazed it works at all.


Cheers,

Kevin D-B

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