On 15/03/2019 12:44, Peter Lieven wrote:
> Hi Simon,
> Am 14.03.19 um 18:41 schrieb Simon Kelley:
>> Is this a use for something like the ISC dhcpd shared-network configuration.
>> In the dnsmasq case, we could have something like
>> shared-network=<interface>,<address/prefix-length>
>> or
>> shared-network=<interface-address>,<address/prefix-length>
>> In the first case dnsmasq would behave _as_if_ the specified interface
>> carried the address and netmask specified.
>> In the second case, it would behave as if the interface which carries
>> interface-address also carried the address and netmask specified.
>> If I've understood correctly, you'd just need a shared-network
>> declaration for each of your /24s.
> What you describe is exactly what I would need. However, as far
> as I understand the documentation of the shared network feature in ISC dhcpd
> they still require that the network is configured on the DHCP interface (as 
> alias, secondary etc.).
> This already works in dnsmasqd. I would need the feature that does not
> require the addresses to be actually configured on the interface as you
> describe it.

I may have miss-understood the dhcpd case - what it does or doesn't do
is probably irrelevant to what dnsmasq can  do in this case.

As far as I can see, there's no reason why it shouldn't work, with the
following caveats.

1) The DHCP server interface must have at least one address configured,
and that address needs to be reachable from configured clients. This
address gets used as the "server identifier" field in unicast
transmissions from the client to the server for things like lease
renewal. The case that the server-id is not on the same network as the
client is not new, it's the case when using a DHCP relay.

2) Dnsmasq currently guesses at the default router to send to a client,
unless it's overridden by configuration. This is either its own address
on the network where the client is given an address, or, if the DHCP
came via relay, then it's the address of the relay on the network where
the client is given an address. In the case that a client is being given
an address on a network where neither the DHCP server or the DHCP relay
have an address, there's no sensible guess for what the client's default
router should be set to, so explicit configuration will have to be



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