HP printers losing DNS entries


Users started complaining about not being able to use several HP printers in their company. Whenever that happened, they were not able to resolve their names. They were still able to log in to the printer via the IP address.

The company has an IPAM solution that sends DDNS updates to the DNS server, which is administered by a third party so we were not able to have a look at the DNS policies.

What we noticed after a while was that even though our standard DHCP lease was 7 days, HP printers would get 30 days! It turns out that the parameter ”default lease time” does not mean ”maximum lease time”. If a device asks for a longer lease, it gets it via DHCP option 51. Aaargh!

Once we found this, we asked the DNS admins what the scavenging policy was. It turned out that the DNS server considered dynamic entries as stale if they were not refreshed within 8 days. The math was easy: the lease was refreshed at 50% of the lease, so between the 9th day and 15th days of the lease, DNS could scavenge the DNS entry. The DNS admins were reluctant to say what the scavenging interval was.

The solution was to change the parameter „default lease time” to „maximum lease time” on the DHCP server.

Incidentally, the same applies to iphones so make sure you don’t have hundreds of 30-day leases for your BYOD guests. It’s quite easy to run out of IP addresses…


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