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

[Simon Kelley]

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.