[bdNOG] DHCP Lease and Network Problem Help (Mohammad Shahjahan)

Mohammad Shahjahan bunty.ctg at hotmail.com
Thu Dec 10 08:42:51 BDT 2015

Dear Brian,

Thank you so much for your cordial help. Your are right the access list wont work because our network provide dhcp lease through a layer 3 switch with several vlan's with ip helper address ( as said.

But there are no options to change helper ip address that because there are lots of routing issue with other country's.

So, that's why i think there are no hope to stop using GATEWAY IP ADDRESS in android mobile user. We have to think about another internet policy.

I really appreciate about all of support and thank you so much.

Engr. Mohammad Shahjahan
Member of Institute of Engineer Bangladesh
Membership Number: M/31195
Chittagong, Bangladesh
Contact Information: +8801752789798

To: nog at bdnog.org
From: brian at nsrc.org
Date: Mon, 7 Dec 2015 08:36:50 +0000
Subject: Re: [bdNOG] DHCP Lease and Network Problem Help (Mohammad Shahjahan)

    On 07/12/2015 06:00,
      nog-request at bdnog.org wrote:

      For an
        Example: Suppose your gateway IP is, then you can
        implement Standard ACL on the switchport (from where your users
        are connected to)

      Switch(config)#access-list 1 deny host
        1 permit any 

        fastEthernet 0/1
      Switch(config-if)#ip access-group 1 in

    I'm not sure that's going to help, unless it filters ARP responses
    as well as IP datagrams. That is: it will stop the person with
    address from using the Internet, but it probably won't
    stop the breakage of the other users.


    Consider a packet which is going out from PC to the
    Internet, say IP address


    Neither of those packets has as a source or destination
    address, so it won't be affected by the filter. However, the PC
    knows that its default gateway is, so will send an ARP
    request for to learn the MAC address.


    Your router will respond, but so will the hijacker who has manually
    configured as their IP address. Whichever response
    arrives first will be the one which is used, so it's just a race.


    If the hijacker's response arrives first, the PC will encapsulate
    their packet with the hijacker's MAC address and deliver it there
    instead of to the Internet. If the hijacker has IP forwarding
    enabled they may re-send it to the router (note that they will be
    able to sniff all outbound traffic!) But most likely the packet will
    be dropped on the floor and connectivity will be broken.


    So, to fix this problem properly you would have to do filtering of
    ARP responses; enforce that ARP responses for which
    enter through a port other than the router's port are discarded. I
    believe the Cisco "IP address tracking" feature might be able to do


    However a simpler approach may be to *detect* the problem: for
    example by running a tool like arpwatch on that network segment.
    Then you have an Acceptable Usage Policy which says that the manual
    configuration of IP addresses is *forbidden* and will result in
    serious consequences (make use of your company's or university's
    existing disciplinary procedure)


    In my experience, the reason that people actually configure manual
    IP addresses is because the DHCP service is unreliable, and they are
    forced to do this to get work done. If that's the case, then the
    real solution is to make your DHCP service reliable.


    It's fine to have two DHCP servers for redundancy:


    interface Foo

      ip helper-address

      ip helper-address


    The simplest way to make this work is to give the two DHCP servers
    non-overlapping pools of addresses (e.g. and This avoids then need to run DHCP clustering
    where the two servers are both aware of each other's allocations. 
    The user will receive two DHCP offers, will pick the first one and
    use that.


    Also it's better if your lease times are sufficiently large (say 12
    hours) so that once they have an address at the start of the day, it
    works all day. For that to work well you might need bigger subnets,
    say a /22 instead of a /24, especially with wireless, but that's not
    a problem if you are using private IP addresses.







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