[Dnsmasq-discuss] Safe to use static DHCP allocations within dynamic range?

Ed W lists at wildgooses.com
Fri Oct 23 17:38:11 BST 2015


Hi Simon

Thanks for the (as usual) excellent answer (and thanks Carlos!)

This behaviour is extremely desirable and as far as I can tell isn't 
available with other DHCP servers!

Cheers

Ed W


On 20/10/2015 21:51, Simon Kelley wrote:
> On 15/10/15 12:39, Ed W wrote:
>> Hi, I'm not quite clear from the manual pages, so can I please get a
>> definitive answer:
>>
>> - Am I safe to use to assign a static IP allocation using --dhcp-host
>> options, *within* an IP range allocated using --dhcp-range ?
>>
>> Specifically, whilst I realise that dnsmasq will first ping to see if an
>> IP is unused:
>> - Will there be a hard mask on the IP being handed out dynamically as a
>> result of the --dhcp-host option?
>> - ie if the static allocated device is missing from the network for some
>> long period of time, am I "safe" from having a dynamic device "pinch" my
>> static IP?
> Yes. Addresses withing the dynamic range which appear as static
> allocations are protected from being handed out to other clients.
>> Corner case:
>> - What happens if there is currently a lease allocated to IP w.x.y.z,
>> and I setup a static --dhcp-host allocation for that IP and a
>> *different* mac, ie this lease needs to be booted off.  Assuming the
>> lease is technically not expired, and I disconnect the (now errant)
>> device from the network and reconnect it, will it be able to regain it's
>> (technically still not expired) lease, or will dnsmasq observe the
>> static allocation, expire the lease and allocate a new dynamic allocation?
> For the duration of the existing lease, the existing owner will get to
> keep it. If the address is set as static for a MAC and that MAC asks for
> a lease, it will be offered a different address, and a message logged
> about why. Once the lease expires, the existing owner will not be able
> to renew the lease, and will move address. Once that's happened, the
> static-MAC will also not be able to renew its lease, and will move
> address to the originally allocated static address.
>
>>
>> Background:
>> - I do realise it's best practice to have a separate static range
>> outside of the dynamic allocation range
>> - For various technical reasons this environment would be massively
>> simpler if it were possible to safely allocate static allocations
>> *inside* the dynamic range and have them protected and always available
>> (ie even if the machine is offline for a long period)
>>
>>
> exec-summary. If you have static allocations in the dynamic range, all
> will be fine. If you add a new allocation in the dynamic range for an
> unused address, all will be fine. If you add a new allocation for an
> address that's already in use, all will eventually be fine, but clients
> will be move addresses to make it so.
>
> Cheers,
>
> Simon.
>
>
>
> Cheers,
>
> Simon.
>
>
>
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