Packet Capture on Fuchsia

Packet capture is a fundamental tool for developing, debugging, and testing networking.

fx sniff is a development host command that:

• Runs the packet capture on the Fuchsia target device.
• Stores the packets in PCAPNG format on the Fuchsia development host.
• Streams out to a graphical user interface such as Wireshark.

tcpdump is a packet capturer with rich capture filter support. fx sniff internally invokes tcpdump with predefined capture filters that are necessary for Fuchsia's developer workflow. For use cases where fx sniff is not viable (e.g. when you have serial console access but without dev host connected), use tcpdump directly.

Prepare the image

Make sure to bundle tcpdump into your set of base packages.

$ fx set core.x64 --with-base //third_party/tcpdump
$ fx build

How-to (On Host)

Capture packets over WLAN interface

[host] $ fx sniff wlan

By default, this command captures packets for 30 seconds. To configure the duration, add the --time {sec} or -t {sec} option.

If you don't know the network interface name, run fx sniff without options. The error message shows you what interfaces are available. Alternatively, run:

[host] $ fx shell net if list

Show the hexdump of packets over the ethernet interface

[host] $ fx sniff --view hex eth

Capture WLAN packets and store them in a file

[host] $ fx sniff --file my_packets wlan

The captured packets are first stored in the target's /tmp/ directory. After the capture is complete, the files are moved to //out/my_packets.pcapng automatically.

Stream out to Wireshark in realtime

NOTE: Linux only.

[host] $ fx sniff --view wireshark wlan

Force stop

Packet capture runs for the specified duration (--time or -t option). If a user desires to stop early, presse one of the following keys:

c, q, C, Q

This will stop both a target side process and a host side process.

How-to (on target device)

Use tcpdump for debugging

fx sniff requires working ssh connectivity from the host to the target, which means that networking must be working to some degree. In some cases, networking might not be working at all. If you have access to the serial console while networking, including ssh, is not working, you must run tcpdump directly on the target. tcpdump provides a richer set of features than fx sniff.

Capture packets over the WLAN interface

[target] $ tcpdump -i wlan --no-promiscuous-mode

Stream out the binary dump in PCAPNG format

[target] $ tcpdump -i wlan --no-promiscuous-mode -w -

Capture packets and store them in a file

[target] $ tcpdump -i wlan --no-promiscuous-mode -w /tmp/my_packets.pcapng

Copy the dump file to the host

[host] $ cd ${FUCHSIA_OUT_DIR} && fx scp "[$(fx get-device-addr)]:/tmp/my_packets.pcapng"

tcpdump help

[target] $ tcpdump --help

Only Watch ARP, DHCP, and DNS packets

[target] $ tcpdump -i  wlan --no-promiscuous-mode "arp or port dns,dhcp" "$iface_filepath"

Filter syntax

tcpdump uses libpcap under the hood. See pcap-filter.

Reference: fx workflow packet signatures

There are many different kinds of services running between the Fuchsia development host and the target. Those are usually invoked by fx commands. Most of times, you are not interested in those packets generated by the fx workflows. The following table lists noteworthy signatures.

Troubleshooting

Q I get the error /boot/bin/sh: tcpdump not found

A The tcpdump package is not prepared. Make sure to bundle tcpdump in the image. See prepare the image.