dhcp not matching subnet if not first address on interface

Steve Clark sclark at netwolves.com
Tue Sep 6 14:14:01 UTC 2011

On 09/06/2011 09:04 AM, Simon Hobson wrote:
> Steve Clark wrote:
>> Should I include subnet declaration for all interfaces? Or just the
>> subnets on the
>> interface where the subnet is that I will actually be handing out ips for.
> You **MUST** include subnet declarations for all interfaces where :
> 1) You will be offering a DHCP service (whether it's directly
> connected or remote)
> 2) It is on an interface you will be configuring the server to use.
> So lets say you have :
> eth0 with subnets A and B directly attached
> eth1 with subnet C directly attached
> Subnet D as a remote network (serviced via a Relay Agent (aka DHCP Helper))
> Now if you configure the server to listen on eth0 but not eth1, then
> you'd need to define A and B (as a shared network), but not C as you
> won't be listening on eth1.
> You will also need to define subnet D.
> But, you also need to consider how packets from subnet D are going to
> get to your server - ie the network topology. If they will arrive on
> eth0 then it'll work. But if they will arrive on eth1 then you will
> need to have the server listen on eth1 AND define subnet C.
> Just for completeness, it is possible to compile the server without
> the (IIRC) use_sockets option. Then it doesn't listen on raw
> interfaces, only via the host OS network stack - meaning it doesn't
> handle broadcast packets. You would need to ensure that the relay
> agent servicing subnet D uses unicast packets to the server AND you
> cannot service subnets A, B, or C.
Thanks for the excellent description and examples.


Stephen Clark
Sr. Software Engineer III
Phone: 813-579-3200
Fax: 813-882-0209
Email: steve.clark at netwolves.com
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