[Dnsmasq-discuss] dhcp.leases file format?

Simon Kelley simon at thekelleys.org.uk
Thu Apr 27 07:41:55 BST 2006

William Trenker wrote:

> Thanks for the very helfpul and timely answer.  I should have
> mentioned that my interest in the dhcp.leases file is that I'm trying
> to find a way to force an expiration.  I was thinking perhaps I could
> simply remove the relevant line from the dhcp.leases file.  But of
> course I should have asked if that would even work.

Removing a lease line won't work: when the client renews, the line will 
just be re-instated. (and modifying the leases file whilst dnsmasq is 
running is completely pointless: the file is never read except at 
startup, and it's overwritten with dnsmasq's current idea of the state 
of the world whenever that changes.

There's normally no way to force a DHCP client to drop a lease before 
the expiry time it was given. Once the client needs to renew a lease, if 
you can configure dnsmasq to ignore the client

dhcp-host=<mac address>,ignore

then that will inhibit renewal.

There does exist a standard for forcing DHCP clients to give up or alter 
a release early, in RFC3203, but AKAIK, no clients implement it. dnsmasq 
doesn't, but it would be fairly trivial to do, if you can come up with a 
convincing case (ie, you can find a viable client).



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