To learn how iTunes and Xcode sync with iPhones, I enabled a hidden option in macOS’s usbmuxd daemon that logs how applications communicate with iOS devices over USB.

What the config option does

It causes usbmuxd to log which processes are accessing connected iOS devices.

usbmuxd is the system daemon on macOS that handles communications to iOS devices over USB (or Wi-Fi sync, if you have that enabled).

You can view the logs by opening the Console app and filtering for “usbmuxd”:

output of the log when opening Xcode with an iOS device attached

In the above log, I opened Xcode’s Devices window, which triggered Xcode to connect to Lockdownd on my iPad at port 62078, likely to start some sort of developer service. Xcode then connected to the newly launched service through port 59426.

How to enable it

You need a /Library/Preferences/com.apple.usbmuxd.plist config file.

Download this configuration file for usbmuxd:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>DebugLevel</key>
	<integer>7</integer>
</dict>
</plist>

and copy it to your /Library/Preferences/ directory:

sudo cp ~/Downloads/com.apple.usbmuxd.plist /Library/Preferences

usbmuxd should pick up the configuration file and print a message to the console:

usbmuxd	notice    log filter changed from 5 to 7

How I found it

After I loaded usbmuxd into IDA Free, I noticed that it was calling asl_set_filter to filter out debug logs:

function calling asl_set_filter

This function is called by two functions.

xrefs of asl_set_filter

The _start function handles the command line parameter to enable verbose logging.

The other function, sub_10000236E, processes usbmuxd’s config file:

usbmuxd's config path

There’s quite a few configuration options that usbmuxd supports. The one that sounded promising was DebugLevel:

usbmuxd settings' names

I created the /Library/Preferences/com.apple.usbmuxd.plist file and increased the debug level until debug messages started appearing.

Additional resources

Here’s some sources I consulted during this research that you might find useful:

Libimobiledevice - open source tools to communicate with iOS devices over USB
Usbmuxd protocol documentation on iPhone Wiki - documents both the usbmuxd protocol and the Lockdownd protocol (which controls USB services on the device)
Discovering the iOS Instruments Server - Troy Bowman’s presentation on how Xcode communicates with iOS devices

What I learned

Where system daemons on macOS store their preferences
Logging levels in the Apple System Logger framework
I can write a short article