[bdNOG] DHCP Lease and Network Problem Help (Mohammad Shahjahan)
bunty.ctg at hotmail.com
Thu Dec 10 08:42:51 BDT 2015
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 (192.168.1.1) 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
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:
Example: Suppose your gateway IP is 192.168.1.1, then you can
implement Standard ACL on the switchport (from where your users
are connected to)
Switch(config)#access-list 1 deny host 192.168.1.1
1 permit any
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 192.168.1.1 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 192.168.1.100 to the
Internet, say IP address 220.127.116.11.
Neither of those packets has 192.168.1.1 as a source or destination
address, so it won't be affected by the filter. However, the PC
knows that its default gateway is 192.168.1.1, so will send an ARP
request for 192.168.1.1 to learn the MAC address.
Your router will respond, but so will the hijacker who has manually
configured 192.168.1.1 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 192.168.1.1 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:
ip helper-address 192.0.0.1
ip helper-address 192.0.2.2
The simplest way to make this work is to give the two DHCP servers
non-overlapping pools of addresses (e.g. 192.168.1.10-149 and
192.168.1.150-249). 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
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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nog at bdnog.org
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