I'm running DHCPD (isc-dhcp-4.2.4) on an Ubuntu 14.04 system. The var/lib/dhcp/dhcpd.leases file is getting pretty huge (350MB). Looking in it, there's historical data going back many months about leases that have long since expired. How do I purge the old info? Presumably stopping dhcpd and deleting the file is a bad idea because then it will forget recent leases that might still be relevant?
 Apparently dhcpd is supposed to auto-purge the leases file once an hour, but this isn't happening for me. Apparently this is down to a permissions problem - the /var/lib/dhcp directory and contents are ownwd by root:root but the server runs as dhcpd:dhcpd. I tried changing the ownership as follows:-
sudo service isc-dhcp-server stop sudo chown -R dhcpd:dhcpd /var/lib/dhcp/ sudo service isc-dhcp-server start
but after this the ownership of the directory, dhcpd.leases and dhcpd.leases~ reverted to root:root.
So how do I sort out directory ownership vs the user:group running the dhcpd process to get auto-purge to work?