[Dnsmasq-discuss] Behavior on DHCP denied

[Simon Kelley]

On 18/04/2023 16:35, 0zl wrote:
> Hello,
> 
> This is an issue I've experienced with ESP8266 and proxy ARP on my WiFi 
> network. I was able to work around it by assigning the devices an 
> infinite lease, however I think dnsmasq's behavior is undesirable.
> 
> In short, ESP8266 is on a network with proxy ARP setup; getting the 
> initial lease works fine, however once proxy ARP kicks in it fails to 
> renew. This is the chain of events leading up to the issue:
> 
>   * MCU tries to renew the address
>   * dnsmasq properly renews its address
>   * MCU sends an ARP request to check if the address is in use and
>     receives an ARP reply from the router because of proxy ARP
>   * MCU mistakenly believes that the address was assigned already even
>     though it was not, sends a DENIED message back to dnsmasq and tries
>     again
>   * dnsmasq then allocates the exact same address that the MCU just rejected
> 
> I think in this scenario, dnsmasq should try to allocate a different 
> address because MCU has rejected it already.
> 
> Not sure what people in this mailing list think, but it feels like 
> dnsmasq shouldn't be doing this.

This situation was considered and there should be sensible behaviour.

Dnsmasq uses a hash of the MAC address to determine which the address to 
offer to the client, which would cause the same address always to be 
offered to the client. But if a client returns a DHCPDECLINE reply then 
a global variable is incremented. That variable is also used as an input 
to the hash function, so when the client asks again for an address it 
should get offered a different one.

So, this situation has been considered, but something is going wrong in 
your setup. Please could you post more details of the configuration you 
are using and logs of what happens so we can try and work out what is 
going wrong?


Cheers,

Simon.


> 
> All the best,
> 0zl.
> 
> 
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