dhcpd losing hostnames

Michael Vincent vyncebox at gmail.com
Wed Jul 13 06:31:51 UTC 2016

On Tue, Jul 12, 2016 at 2:57 PM, Shawn Routhier <sar at isc.org> wrote:
> 1) Do you know if anybody has created a bug ticket for this in the past?

I am not aware of anyone creating an isc-dhcpd bug ticket for this
issue so far. The first reference to this issue was in the pfSense 2.3
beta in February: https://forum.pfsense.org/index.php?topic=107235.0.
I'm fairly sure those betas were using isc-dhcpd 4.3.3 as well. Since
Chris just created a pfSense bug ticket a few days ago, I doubt any of
the pfSense devs were aware of this issue before then.

> 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?

I just booted a pfSense 2.2.6 livecd in a VM and it's using isc-dhcpd
4.2.8. This issue is not present in that version of pfSense.

I'm currently running an embedded install of pfSense, which makes it
difficult to roll back to earlier versions of isc-dhcpd. It looks like
pfSense jumped from 4.2.8 straight to 4.3.3. I'll see if I can maybe
just swap out the isc-dhcpd binary for an earlier version to test.

> 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.

Thanks, I'll give that a shot!

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

I have done some searching around the internet and have not come
across any other reports of similar issues. The way that pfSense
registers the client hostnames in dnsmasq or Unbound by parsing the
dhcpd.leases file may be somewhat unique. We only noticed the issue
when we were no longer able to reliably resolve dhcp client hostnames
after upgrading from pfSense 2.2 to 2.3. I wouldn't be surprised if
this behavior is present outside of pfSense, but doesn't really affect
anyone that doesn't parse dhcpd.leases for client hostnames.

> 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?

Yes, a couple of us have done packet captures over extended durations
and the client hostnames are *always* included in the DHCP request

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