option 51 ' dhcp-lease-time' vs other lease time statements

Simon Hobson dhcp1 at thehobsons.co.uk
Tue Sep 19 16:30:07 UTC 2006

Aaron Bennett wrote:

>Each scope from Win2K ( in isc terms, 'pool') defines option 51.  They
>call it "Lease" and describe it as "Client IP address lease time in
>seconds", but it's option value is 51 which is 'dhcp-lease-time' in isc
>and rfc world.  The isc dhcp.conf examples I've seen, rather then using
>option dhcp-lease-time, use 'default-lease-time,'  'max-lease-time,' and
>What's the difference?  What exactly does option 51 do and how does that
>compare to the other ways to specify a lease time?  What I'm looking for
>is the migrate the exact _intent_ of the Win2K configuration
>intelligently into isc-world.

Unless you really want an exact lease time then you can specify max 
and min values, the server will give out a lease for whatever time 
the client requests bounded by those values. If the client doesn't 
specify a desired lease time then the default value is used. If you 
set max and min to the same value then that has the same effect as 
setting lease time.

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