The iPhone manufacturer posted small descriptions of what com.apple.mobile.pcapd, com.apple.mobile.file_relay, and com.apple.mobile.house_arrest, three of the processes highlighted as potential backdoors by Jonathan Zdziarski in his paper titled Identifying back doors, attack points, and surveillance mechanisms in iOS devices.
Here is Apple's [Q&A] in full:
iOS: About diagnostic capabilities
iOS offers the following diagnostic capabilities to help enterprise IT departments, developers, and AppleCare troubleshoot issues.
Each of these diagnostic capabilities requires the user to have unlocked their device and agreed to trust another computer. Any data transmitted between the iOS device and trusted computer is encrypted with keys not shared with Apple. For users who have enabled iTunes Wi-Fi Sync on a trusted computer, these services may also be accessed wirelessly by that computer.
1. com.apple.mobile.pcapd
pcapd supports diagnostic packet capture from an iOS device to a trusted computer. This is useful for troubleshooting and diagnosing issues with apps on the device as well as enterprise VPN connections. You can find more information at developer.apple.com/library/ios/qa/qa1176.
2. com.apple.mobile.file_relay
file_relay supports limited copying of diagnostic data from a device. This service is separate from user-generated backups, does not have access to all data on the device, and respects iOS Data Protection. Apple engineering uses file_relay on internal devices to qualify customer configurations. AppleCare, with user consent, can also use this tool to gather relevant diagnostic data from users' devices.
3. com.apple.mobile.house_arrest
house_arrest is used by iTunes to transfer documents to and from an iOS device for apps that support this functionality. This is also used by Xcode to assist in the transfer of test data to a device while an app is in development.
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