Recording a Packet Trace

A packet trace is a record of traffic traveling across the network. It’s useful for investigating complex network problems related to both correctness and performance.

Once you start a packet trace on a network interface, it records all traffic passing through that interface until you stop the trace. Packet traces are usually quite short — perhaps recording the traffic associated with one specific connection — but there are situations where you might want to run the trace for hours or even days.

Both macOS and iOS have built-in support for packet traces. This article explains how to record a packet trace on both platforms. Even if your primary focus is iOS, you should start by running some tests on macOS to familiarize yourself with the tools before you attempt to debug your actual problem.

The first step in recording a packet trace on the Mac is to choose the correct interface. If you choose the wrong interface, you may end up recording an empty packet trace. For example, if you use the en0 interface on a Mac that has built-in Ethernet but is connected to the Internet over Wi-Fi, your packet trace will include all the traffic over the built-in Ethernet, that is, nothing.

Determine the correct interface name by running the networksetup command-line tool with the -listallhardwareports argument. This prints a list of network interfaces, including both the user-visible name and the short interface name needed by packet trace tools. For example:

$ networksetup -listallhardwareports

Hardware Port: Ethernet
Device: en0
Ethernet Address: 54:45:5b:01:ca:89

Hardware Port: Wi-Fi
Device: en1
Ethernet Address: 78:a1:3c:02:2b:da

… and so on …

In this example, the built-in Ethernet is en0 and the built-in Wi-Fi is en1, but this will change from machine to machine. Look through this list for the user-visible name of the interface you want to trace (these match the names shown in Network preferences), then use the associated short interface name.

Working with packet traces usually involves recording a packet trace to a file and analyzing that file. It’s possible to do both steps at once, and it’s a good idea to do that when you’re just getting started. The following Terminal command starts a packet trace and prints information about each packet as it’s transferred.

sudo tcpdump -i en0 -n

In this example:

• tcpdump is the name of macOS’s built-in packet trace tool.

• The sudo command causes tcpdump to run with privileges, which is necessary in order to record packets.

• The -i en0 option tells tcpdump to record packets on the default Ethernet-like interface. Replace en0 with the short interface name you determined in Choose the Correct Interface.

• The -n option tells tcpdump not to attempt to use reverse DNS to map IP addresses to names; such mapping is rarely useful on the modern Internet and it radically slows things down.

When you run tcpdump in this way, you see something like this:

17:49:26.500970 IP 192.168.1.187.64917 > 192.168.1.39.22: Flags [S], seq 3769365868, win 65535, options [mss 1460,nop,wscale 5,nop,nop,TS val 1478872373 ecr 0,sackOK,eol], length 0
17:49:26.503325 IP 192.168.1.39.22 > 192.168.1.187.64917: Flags [S.], seq 405178393, ack 3769365869, win 65535, options [mss 1460,nop,wscale 5,nop,nop,TS val 72353448 ecr 1478872373,sackOK,eol], length 0
17:49:26.503413 IP 192.168.1.187.64917 > 192.168.1.39.22: Flags [.], ack 1, win 4117, options [nop,nop,TS val 1478872375 ecr 72353448], length 0
17:49:26.504887 IP 192.168.1.39.22 > 192.168.1.187.64917: Flags [.], ack 1, win 4117, options [nop,nop,TS val 72353449 ecr 1478872375], length 0
17:49:26.526134 IP 192.168.1.39.22 > 192.168.1.187.64917: Flags [P.], seq 1:22, ack 1, win 4117, options [nop,nop,TS val 72353470 ecr 1478872375], length 21
17:49:26.526228 IP 192.168.1.187.64917 > 192.168.1.39.22: Flags [.], ack 22, win 4117, options [nop,nop,TS val 1478872396 ecr 72353470], length 0

There’s a line of output for each packet seen on the network. On each line there’s a timestamp and a lot of information about that packet. This specific example shows the start of a connection from an SSH client at 192.168.1.187 to an SSH server listening on port 22 of 192.168.1.39.

Packet traces can be quite overwhelming. Rather than trying to interpret the packet trace in real time, use the -w option to write the trace to a file and then do your analysis later on.

sudo tcpdump -i en0 -w trace.pcap

This example records a packet trace and writes the results to a file call trace.pcap. You can save that file to analyze later on using tcpdump itself, or one of the tools listed in Taking Advantage of Third-Party Network Debugging Tools. You can also submit it to Apple for analysis (see Submitting a Packet Trace to Apple).

If you’re having problems recording a packet trace, see Troubleshooting Packet Traces.

iOS doesn’t let you record a packet trace directly. However, you can use your Mac to record a packet trace on an attached iOS device using the Remote Virtual Interface (RVI) mechanism. To get started, first connect your iOS device to your Mac via USB. Next run the rvictl command in Terminal.

rvictl -s b0e8fe73db17d4993bd549418bfbdba70a4af2b1

In this example:

• rvictl is the name of the command that manipulates RVIs.

• -s tells rvictl to set up a new RVI.

• b0e8fe73db17d4993bd549418bfbdba70a4af2b1 is the UDID of the iOS device to target. This UDID is just an example; you can find your device’s UDID in the Devices and Simulators window in Xcode.

This command prints the following output.

$ rvictl -s b0e8fe73db17d4993bd549418bfbdba70a4af2b1

Starting device b0e8fe73db17d4993bd549418bfbdba70a4af2b1 [SUCCEEDED] with interface rvi0

This output includes the interface name of the newly-created RVI, rvi0 in this example. Supply this interface name to your favorite packet trace tool to record a trace of the traffic on your iOS device. For example, use the following command to record a packet trace on rvi0 and write it to trace.pcap.

sudo tcpdump -i rvi0 -w trace.pcap

If you’re having problems working with RVIs, see Troubleshooting Packet Traces for some troubleshooting tips.

An RVI represents the entire networking stack of the iOS device; you cannot target a specific interface on the device as you would on the Mac. However, information about the interface is recorded in the packet metadata. You can use your packet trace tool to display the interface for each packet and filter the trace based on that interface. For the tcpdump tool, use the -k option to display packet metadata and tcpdump -Q to filter on it. See the tcpdump man page for details.