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kbro
Asked: 2019-02-28 04:37:59 +0800 CST

How do I purge dhcpd.leases? [duplicate]

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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?

[edit] 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?

isc-dhcp

2 Answers

  1. Best Answer

    From the manual man dhcpd.leases:

    In order to prevent the lease database from growing without bound, the file is rewritten from time to time. First, a temporary lease database is created and all known leases are dumped to it. Then, the old lease database is renamed /var/lib/dhcpd/dhcpd.leases~. Finally, the newly written lease database is moved into place.

    AFAIK that may fail when file system permissions on the directory /var/lib/dhcpd prevent the unprivileged user dhcpd runs as from creating that temporary database there and then the leases file will continue to grow.

    Change ownership of /var/lib/dhcpd to the dhcpd user.

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  2. You do not need to delete it, only remove what's not needed.

    • Stop DHCPD server.
    • Edit your dhcpd.leases file and remove what entries you no longer need
    • Save the file and start DHCPD server.

    You can optimize this by editing the file (and not save until ready) and then perform DHCPD stop / file save / DHCPD start in a matter of seconds or even faster. It even worked under 1s for me; that's if you don't want downtime.

    • -2