dhcpd losing hostnames

Shawn Routhier sar at isc.org
Tue Jul 12 19:57:02 UTC 2016

> On Jul 11, 2016, at 8:58 PM, Chris Buechler <cmb at pfsense.org> wrote:
> On Mon, Jul 11, 2016 at 9:52 PM, Michael Vincent <vyncebox at gmail.com> wrote:
>> A few pfSense 2.3.1 (based on FreeBSB 10.3) users have noticed an
>> issue with the dhcp server (ISC DHCP Server 4.3.3-P1) losing client
>> hostnames regularly. Packet captures have proven that while hostnames
>> are always provided by clients and generally recorded in dhcpd.leases,
>> the hostnames often disappear from dhcpd.leases after some period of
>> time. We haven't been able to conclusively prove that dhcpd itself is
>> to blame, but investigations of packet captures and logs are pointing
>> in that direction.
>> Background is available in this thread:
>> https://forum.pfsense.org/index.php?topic=110011.0 and a bug ticket
>> has been opened here: https://redmine.pfsense.org/issues/6589
> To elaborate on this a bit, the reason for the root problem is
> dhcpd.leases file doesn't always have client-hostname specified,
> though the client always sends it. We have a daemon that processes
> dhcpd.leases and registers hostnames in dnsmasq and/or Unbound from
> that (since they can't do proper DDNS), which is kind of a hack, but
> the only way to accommodate those in that circumstance.

I hope that should be read that dnsmasq and Unbound have issues with
proper DDNS?  The current servers should be doing DDNS correctly
and if they aren’t we’d like to know about issues.

> Prior to dhcpd 4.3, this was always fine, the client-hostname was
> always recorded in the lease in dhcpd.leases going back a number of
> versions across a decade or so. Now in 4.3.3 and 4.3.4, the presence
> of client-hostname is hit and miss for some people. With no apparent
> reason as to why some have the issue and others don't.
> So it appears there's a regression in 4.3.x with recording
> client-hostname to the leases file.

Some questions to help with isolating the problem
1) Do you know if anybody has created a bug ticket for this in the past?

2) You mention 4.3.3 and 4.3.4, do you know if the problem started showing
up earlier than that for example in 4.3.0?

3) From some of the descriptions on the first of the two links above it sounds
like dhcp-cache-threshold may be causing issues.  This was added in 4.3.0
and tries to avoid updating a lease if that isn’t necessary.  Instead it re-uses
the lease information (expiration time and etc) from the current lease and
thus avoids doing an fsync.
You can try to disable it by setting it to 0, add the following line in your config:
dhcp-cache-threshold 0;
and see if that affects the problem.

4) Have people seen this that aren’t using pfsense?

5) From the description in the pfsense chain it sounds like the client hostname
is always being included in the DHCP messages - is that correct?

Shawn Routhier
ISC DHCP lead engineer

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