reject dhclient offer from wrong subnet

[Lennart Sorensen]

Hi list
My router and my docsys modem power on at the same time. the modem is
handing out dhcp offers of 192.168.100/24 . I assume they are meant for
internal set up purposes and dhclient on my router should not catch or
respond to them. Only the final offer after modem bootup has completed does
contain the WAN subnet and a lease time of 4 hours.
Can I configure dhclient on my router to discard lease offers from a certain
subnet? I could also try to match the lease time, the 192.168.100/24 lease
time is only several seconds (!!) short, the real one will be 4 hours or
more and come with a valid WAN subnet mask.
I never had this problem before, so I don't know how to change dhclient's
behaviour to a bogus lease offer.
Maybe iptables could assist and block multicast traffic from/to the wrong
subnet? I'm not sure I would still get the actual WAN lease then.

Which dhcp client?

if dhclient, the config allows specifying a reject subnet range, so you
could do something like:
reject 192.168.100.0/24

[Andrei POPESCU]

Hi list
My router and my docsys modem power on at the same time. the modem is
handing out dhcp offers of 192.168.100/24 . I assume they are meant for
internal set up purposes and dhclient on my router should not catch or
respond to them. Only the final offer after modem bootup has completed does
contain the WAN subnet and a lease time of 4 hours.

Hello,

This appears to be a general networking question, that happens to
involve one or more ARM systems.

If one of your systems is running Debian (the router?) the question is
better suited on the debian-user list

https://lists.debian.org/debian-user

I would suggest you add more information on the router (hardware and
software configuration, which port is connected to the modem, etc.).

This list is meant for issues with porting packages to the arm*
architectures in Debian or issues with getting Debian to run at all on
specific ARM hardware.

Kind regards,
Andrei