# ISC dhcpd - how can I disable use of 'old IP address'



## Rudy (Dec 21, 2012)

In man dhcpd.conf, there is this paragraph:

```
When the DHCP server allocates a new address for  a  client  (remember,
       this  only  happens  if  the  client has sent a DHCPDISCOVER), it first
       looks to see if the client already has a valid lease on an IP  address,
       or  [B]if there is an old IP address the client had before[/B] that hasn't yet
       been reassigned.
```

Is there any way to make a lease _really expire_ so that dhcpd does *not* look to see 'if there is an old IP address the client had before'?

I'd like to have users who turn off their computer and come back 12 hours later to get a different IP from the DHCP pool.  To reflect this, I have set the lease time to 3 hours, and then I noticed people still got their same IP the next day.  RTFM told me why.    Anyhow, I view this as a little step in reducing tracking and helping with privacy for users.

If there is anyone out there familiar with the isc-dhcp41-server source that knows what file to edit, let me know!  If I find it, I'll post a reply later.


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## Rudy (Dec 21, 2012)

*grace period*

A configuration option of *max-grace-time* for 'old' leases would be handy.  Due to the tricky nature of DHCP (read source file comments for dhcp.c and dhcpd.c), the minimum should be 120 seconds, just in case.  If *max-grace-time* is 0, the leases should last forever in the 'old' state - (as they do now), if set it to 3600, then the 'old' lease would be in a grace period and reusable.


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