Advanced Network Debugging in Xcode

Hit the run tab in the left column and hit the + button to add an environment variable.  Add the CFNETWORK_DIAGNOSTICS flag and set the value to 0, 1, 2, or 3 based upon how verbose you want your logs to be.

From here, you should be all set to run your application and see the networking logs.  In the following screen shots, I have log priority set to 3.  Which will be the most verbose and will provide the most useful information when debugging network traffic.  

In the first screen shot below you can see the GET request going out to http://jsonplaceholder.typicode.com/posts/28.  There are few a few thing happening in these logs.  The first is HTTPCookieStorage has set a cookie in an array to the request URL: http://jsonplaceholder.typicode.com/posts/28 and displayed all of the values for the cookie.  Next, CFNetwork is taking the CFURLRequest object and putting it enqueue to be sent out to the server.

In this next screen shot below you can see that the request has graduated to being a full blown socket connection and communication is taking place between the iOS client and the server.  In this screen shot below you can see the data transmissions being sent and received.  Notice the first two transmissions (284) are iOS writing the request and the cookie data to the writeStream of the connection that is being sent to the server.  The third transmission (725) is the actual raw response headers from the readStream responding from the server in regards to our first two transmissions.  

In this last screen shot below you can see the entire response received from the server (minus the JSON payload).  This response includes all of the headers and a record of the transmission read from and written from the socket connection.  These are the CFNumber values for 284 and 725.  These values allow you to verify that all data was sent and received properly based upon what the client and server have negotiated.

Profiling Network Connections with Instruments 💻

Xcode Instruments allows you to profile network connections in your application just like you would profile memory leaks or CPU usage.  Xcode Instruments can provide an informative look into the state of network connections by process or network interface on a device.  One thing that Instruments does not do though is that it does not provide raw packet information like CFNetwork Diagnostic logging or Wireshark would.  Instruments only provides a summary of all packets sent and received for the used connections, as well as high level information like host, port, protocol, interface, and remote addresses.  In some cases Instruments may provide information on whether packets for a connection are re-transmitted, duplicated, or out-of-order, but doe go in-depth like Wireshark does for a specific tcp stream.

One other thing to note is that Xcode also provides an overview of active network connections running in your application right in the debug navigator without using Instruments.  Below is a screen shot taken directly from Xcode.  This is very similar to information that is provided in Instruments, but just for the running application only.  You will find very similar information here, but just not as verbose as the overall picture in Instruments.  Very handy for a quick look.

To get started using Instruments to profile network connections, simply hit Product -> Profile from the Xcode menu.

From here the Instruments process will start up and present you with a list of items you can choose to profile.  Select Network and click "choose."

Once you select Network, the network connections UI will open and in the top left of the screen choose the device you want to profile the network connections on.  In this case it is Matt's iPhone 7+ (11.3).  Next, to the right of your device target the application.  Once your device and application is configured, select how you want to capture your connections; by interface, by process, or by active connections only.  In the case of this example I am capturing by process.  Next hit the red button to start your capturing session.

Once my capturing session is started I am not able to see my process for Background URLSession, mDNSResponder, apsd, and swcd all capturing active connections.  Here in the screen shot below you can also see packets going in and out as well as data being read and wrote to the byte streams of the connection.

Lastly, let's take a look at the selected connection for Background URLSession.  At first there may not appear to be much more information here than what is available on the active connections per process view, but if you take a look at the duplicates, out-of-order, and retransmitted columns, these are the key pieces of information that make this profile valuable.  These values can tell you a lot about the state of the connection and if you are receiving packet loss or dropped connections.  Also, notice how there are two packets out and one packet in, matching the CFNetwork Diagnostic transmission logs from above.

In Summary ⌛️

In summary, I hope you have gained some more insight about how to debug network traffic from Xcode a bit more efficiently.  I know that CFNetwork Diagnostic Logging has saved me personally a lot of time just being able to see the data packets being transferred back and forth between the client and the server unencrypted.   This traditionally is not the case when you do a tcpdump or grab a pcap with Wireshark.  Also, utilizing Instruments to profile network connections can be very handy when reviewing individual connections to see if there have been any re-transmissions, out-of-order, or duplicate packets sent or received on your connection.  This is also something that you will have to evaluate manually when using Wireshark or tcpdump. I hope you have enjoyed this article and please let me know if you have any questions comments or concerns as I would love to discuss more!