Working with a Cisco router

Simon Hobson dhcp1 at
Fri Feb 5 19:31:38 UTC 2010

Tim Gavin wrote:
>I have just identified a problem where a small group of people are
>'stealing' their IPs.  It appears that they're grabbing their DHCP
>assigned address and setting it in their router as a static.  This has
>the obvious consequences.
>What I'm wondering is if anyone knows of a script or app that will
>work with ISC DHCP that can set a static ARP in my Cisco router based
>on the DHCP lease.  Maybe using SNMP or something. . . I know it's a
>strange request, but am hoping someone has dealt with this before.

If your leases are long enough, then you could periodically scan the 
leases file to get a list of IP-MAC pairs and script an update to a 
switch or router. Or possibly more useful, compare this with the 
router ARP table and build a blacklist of devices to redirect into a 
walled garden.

Actually, this could be a reasonably safe way to do it - you could 
include logic that would include expired leases less than a certain 
age which would avoid false blockings just after a lease expires. 
However, a lease would disappear if the address got reallocated to 
another customer, but in that case, the faster you deal with the 
offender the better.

Simon Hobson

WANTED: "Software CD ROM Kit" for Canon CLBP 360-PS printer (Canon 
part no RH6-3612, or possibly RH6-3810, or RH6-3610 might do). I've a 
dead HD and need this CD so I can replace the disk and re-install the 
printer OS on it. If anyone knows where I might get hold of one I'd 
be grateful - requests to Canon drew a blank, it's been out of 
support for years.
Alternatively, if anyone has one of these and would let me image 
their hard disk ...

Visit for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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