[unisog] DHCP lease times, reserved IP addresses, etc

George C. Kaplan gckaplan at ack.berkeley.edu
Tue May 11 20:57:37 GMT 2004

In message <s0a0bc62.063 at wpo.cso.niu.edu>, "Jason Richardson" writes:
> Quick poll for what people are doing with DHCP lease times on their
> networks, especially res nets if your network is divided like that and
> you have different policies applying to student resident and admin
> machines.  We are considering moving to a three month or longer lease
> time to make it easier to pinpoint IP usage to specific users/machines.

Three-month leases will be really inconvenient if you have significant 
turnover in DHCP clients.  (Unless you have a really big address pool).

For public areas, such as wireless nets, with high turnover, we use 
shor leases (less than an hour), and rely on the DHCP server logs to 
match client systems to IP addresses for a given time.  This means 
you need reasonably accurate timestamps in order to investigate any 
suspect activity, but the same applies to any other dynamically addressed 
connection, such as dial-up.

BTW, the ISC DHCP server keeps track of leases even after they expire:  
when a client reconnects, it'll get the same address as before if 
someone else hasn't grabbed it first.  So if turnover isn't too high, 
frequent users will tend to get the same IP address all the time.

George C. Kaplan                            gckaplan at ack.berkeley.edu
Communication & Network Services            510-643-0496
University of California at Berkeley

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