[DHCP] Clean up of the dhcpd.leases file

Simon Hobson dhcp1 at thehobsons.co.uk
Mon Mar 19 19:29:17 UTC 2012

Ashley M. Kirchner wrote:

>     That's actually exactly what I do, just delete the file.  Static 
>IPs are defined in dhcp.conf, not in .leases.  The .leases file just 
>gets updated every time there's a connection change.  I have yet to 
>see any ill behavior from deleting it.  All the systems on the 
>network will at some point renew without a problem.

Please do not suggest that.
Because you haven't seen a problem does **NOT** mean it is a safe or 
sensible thing to do.

Deleting the leases file fundamentally breaks things - because you 
now have a server that has made promises to clients that it no knows 
nothing about.

Two of the issues it can cause :

1) First and foremost, the server will now most likely attempt to 
lease out addresses which are already in use by other devices.

If the device is responding top pings, then the "ping before offer" 
"last chance" safety net will result in that address being marked as 
abandoned. Abandoned leases are bad because, unless you have a lot of 
spare addresses, they diminish the size of the active pool (they are 
only use as a last resort) and will increase address churn.

If the device doesn't respond to pings (as is common these days) then 
the lease will be issued, and depending on client behaviour you will 
either have two devices fighting over the address (neither will work) 
or the device will detect the issue and decline the lease. I'm not 
sure what the server does - I think it will try and lease the address 
again to another client.

2) If you are using dynamic DNS updates, the server will no longer 
know which DNS entries it has created. Most devices will get their 
DNS "sorted" next time they renew, but you will also probably end up 
with orphaned DNS entries because the server will not know about DNS 
entries it needs to remove.

Simon Hobson

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