dhcp and dynamic dns updates

Jeff Haran jharan at Brocade.COM
Tue Mar 3 18:38:55 UTC 2009

> -----Original Message-----
> From: dhcp-users-bounces at lists.isc.org 
> [mailto:dhcp-users-bounces at lists.isc.org] On Behalf Of Jiann-Ming Su
> Sent: Tuesday, March 03, 2009 8:15 AM
> To: dhcp-users at lists.isc.org
> Subject: dhcp and dynamic dns updates
> I'm running dhcp 3.0.5.  We ran into a problem yesterday where it
> appeared the dhcp process, while under load, was waiting on a response
> from the DNS server responsible for the dynamic dns domain before it
> would respond to a DHCPDISCOVER.  We did some traces and saw the dhcp
> discovers come into the server interface, but no offers were seen
> going out and nothing was logged in the dhcp server logs for those
> transactions.  We know there was a slight network misconfiguration on
> the DNS server which would have prevented the responses from coming
> back to the dhcp server.  We're trying to reproduce this problem in
> the lab, but with much lower load, it doesn't seem to be breaking in
> the same way.  Is there code in the dhcp server that would cause this
> type of blocking?  Thanks for any insight.
> -- 
> Jiann-Ming Su

For what it's worth, I've seen similar behavior with other servers when
DNS queries were not forthcoming. Some servers will do reverse DNS
lookups (gethostbyaddr()) on the IP addresses they get from clients,
often just for the purpose of logging a host name. Those calls can block
for quite a while when a DNS IP address is configured but the DNS server
is not responding.

Just guessin',

Jeff Haran

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