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Apple’s iPad WiFi issues lie in the poor DHCP set up

by: Bradley Wint on April 19th, 2010 at 12:07 pm

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It seems the problems associated with the Apple iPad’s WiFi connection lies in the DHCP set up. Princeton University’s OIT Network Systems department decided to investigate the problems regarding the WiFi connectivity. They initially saw a number of iPads being blocked from the network but attributed the problem to personal user configuration. However, when the problem occurred over and over, they knew some other issue was to blame.

After a thorough investigation, they found out that the DHCP client (Dynamic Host Configuration Protocol) on the iPad’s end was not renewing its IP lease after the first few sessions. This meant that on the last lease, instead of renewing after the time period specified by the DHCP server, it continued without renewing until the lease ended. This cause the devices to get thrown off the network if the user did not end the session before. Also, Princeton’s servers reported problems of PING test responses when they tried to verify the IP addresses being used with the iPads. As a result of the failed PING tests, some of the iPads were blocked completely from the network for security purposes. The server would normally block devices which is has problems pinging with, because it may represent possible IP stealing.

Here is an excerpt from the report: “The iPad owner is often unaware of any problem, Nevertheless, it is an issue because it can interfere with service to other devices. Once the iPad has allowed its DHCP lease to expire, the DHCP server may lease the same IP address to another client.

“The DHCP servers try to reduce the impact of these malfunctioning clients. Before offering a client a new lease for a dynamically-assigned IP address, the servers perform a quick PING test to determine whether the IP address is unexpectedly in use. (For example, is some device “stealing” the IP address?) This quick test helps, but does not entirely work around the problem caused by the malfunctioning clients. (For example, sometimes the malfunctioning device may not respond to PING at the time the DHCP server checks before leasing the IP address to another client. And with some DHCP server implementations, the DHCP server may have limited time to perform the test, as other clients are waiting for responses from the DHCP server.)

“When a DHCP client malfunctions this way repeatedly, Princeton blocks the device from using those campus network services which rely on the device’s DHCP client respecting lease times. These include our wireless services. We do this to protect other customers of those services from the disruptions caused by the malfunctioning devices.”

Princeton’s IT division was able to replicate the problem multiple times with the same circumstances and felt that the fault lay in the iPad’s iPhone OS 3.2 software. They have since been working with Apple to correct the issue. This problem could explain why so many universities in the U.S., and entirely in Israel.

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