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  • Re: reject dhclient offer from wrong subnet

    From Lennart Sorensen@21:1/5 to Tuxo on Thu Dec 16 02:40:01 2021
    On Wed, Dec 15, 2021 at 04:26:42PM +0100, Tuxo wrote:
    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

    --
    Len Sorensen

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  • From Andrei POPESCU@21:1/5 to Tuxo on Sat Dec 18 10:20:01 2021
    On Mi, 15 dec 21, 16:26:42, Tuxo wrote:
    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
    --
    http://wiki.debian.org/FAQsFromDebianUser

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