During normal operations, iTunes communicates with the iPhone using something called “usbmux” – this is a system for multiplexing several “connections” over one USB pipe. Conceptually, it provides a TCP-like system – processes on the host machine open up connections to specific, numbered ports on the mobile device. (This resemblance is more than superficial – on the mobile device, usbmuxd actually makes TCP connections to localhost using the port number you give it.)

On the Mac, this is handled by /System/Library/PrivateFrameworks/MobileDevice.framework/Resources/usbmuxd, a daemon that is started by launchd (see /System/Library/LaunchDaemons/ com.apple.usbmuxd.plist). It creates a listening UNIX Domain Socket at /var/run/usbmuxd. usbmuxd then watches for iPhone connections via USB; when it detects an iPhone running in normal user mode (as opposed to recovery mode), it will connect to it and then start relaying requests that it receives via /var/run/usbmuxd – this is to say, usbmuxd is the only thing that actually speaks USB to the iPhone. This means that third-party applications which wish to talk to the iPhone must either do so through usbmuxd, or usbmuxd must be replaced.