Monday, April 19, 2010

iPad DHCP Issue

Apparently the iPad and iPhone OS version 3.2 has some nasty DHCP behavior.

the iPad uses DHCP to obtain a lease, renews the lease zero or more times (as expected), but then continues using the IP address without renewing the lease further. The iPad allows the DHCP lease to expire, but it continues using the IP address after allowing the lease to expire.

Kudos to Princeton network administrators to taking a pro-active monitoring approach. Spending the time up-front to monitor the network can save countless hours on the back-end troubleshooting emergencies.

Princeton detected this issue because we take a very pro-active stance to monitor for certain kinds of common network problems, including this one. Our network monitoring includes comparing actual IP address usage to DHCP server lease assignments on a daily basis. This allows us to detect some devices using IP addresses not assigned for their use. This is a degree of monitoring that many sites do not perform. We also monitor our DHCP servers very closely for any problems they detect, including when they DHCP-leased IP addresses in-use when they should not be.
As a result, Princeton tends to learn about some kinds of bugs in DHCP client implementations sooner and more often than do many other sites.

Although much less severe, this issue is reminiscent of the early Apple iPhone issue experienced most notably by Duke University. That turned out to be caused by a Cisco bug which caused an ARP storm between wireless mobility peers. However, this iPad issue appears to clearly be a client-related.


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