Monitor Network Traffic

It is useful to be able to monitor the network traffic especially when debugging a connectivity issues or when developing new protocol support in Zephyr. This page describes how to set up a way to capture network traffic so that user is able to use Wireshark or similar tool in remote host to see the network packets sent or received by a Zephyr device.

See also the Network Packet Capture sample application from the Zephyr source distribution for configuration options that need to be enabled.

Host Configuration

The instructions here describe how to setup a Linux host to capture Zephyr network RX and TX traffic. Similar instructions should work also in other operating systems. On the Linux Host, fetch the Zephyr net-tools project, which is located in a separate Git repository:

git clone

The net-tools project provides a configure file to setup IP-to-IP tunnel interface so that we can transfer monitoring data from Zephyr to host.

In terminal #1, type:

./ -c zeth-tunnel.conf

This script will create following IPIP tunnel interfaces:

Interface name



IPv6-over-IPv4 tunnel


IPv4-over-IPv4 tunnel


IPv4-over-IPv6 tunnel


IPv6-over-IPv6 tunnel

Zephyr will send captured network packets to one of these interfaces. The actual interface will depend on how the capturing is configured. You can then use Wireshark to monitor the proper network interface.

After the tunneling interfaces have been created, you can use for example script from net-tools project to print or save the captured network packets. The provides an UDP listener, it can print the captured data to screen and optionally can also save the data to a pcap file.

$ ./ -i zeth-ip6ip -w capture.pcap
[20210408Z14:33:08.959589] Ether / IP / ICMP > echo-request 0 / Raw
[20210408Z14:33:08.976178] Ether / IP / ICMP > echo-reply 0 / Raw
[20210408Z14:33:16.176303] Ether / IPv6 / ICMPv6 Echo Request (id: 0x9feb seq: 0x0)
[20210408Z14:33:16.195326] Ether / IPv6 / ICMPv6 Echo Reply (id: 0x9feb seq: 0x0)
[20210408Z14:33:21.194979] Ether / IPv6 / ICMPv6ND_NS / ICMPv6 Neighbor Discovery Option - Source Link-Layer Address 02:00:5e:00:53:3b
[20210408Z14:33:21.217528] Ether / IPv6 / ICMPv6ND_NA / ICMPv6 Neighbor Discovery Option - Destination Link-Layer Address 00:00:5e:00:53:ff
[20210408Z14:34:10.245408] Ether / IPv6 / UDP 2001:db8::2:47319 > 2001:db8::1:4242 / Raw
[20210408Z14:34:10.266542] Ether / IPv6 / UDP 2001:db8::1:4242 > 2001:db8::2:47319 / Raw

The has following command line options:

Listen captured network data from Zephyr and save it optionally to pcap file.
./ \
     -i | --interface <network interface>
             Listen this interface for the data
     [-p | --port <UDP port>]
             UDP port (default is 4242) where the capture data is received
     [-q | --quiet]
             Do not print packet information
     [-t | --type <L2 type of the data>]
             Scapy L2 type name of the UDP payload, default is Ether
     [-w | --write <pcap file name>]
             Write the received data to file in PCAP format

Instead of the script, you can for example use netcat to provide an UDP listener so that the host will not send port unreachable message to Zephyr:

nc -l -u 2001:db8:200::2 4242 > /dev/null

The IP address above is the inner tunnel endpoint, and can be changed and it depends on how the Zephyr is configured. Zephyr will send UDP packets containing the captured network packets to the configured IP tunnel, so we need to terminate the network connection like this.

Zephyr Configuration

In this example, we use native_posix board. You can also use any other board that supports networking.

In terminal #3, type:

west build -b native_posix samples/net/capture -- -DCONFIG_NATIVE_UART_AUTOATTACH_DEFAULT_CMD=\""gnome-terminal -- screen %s"\"

To see the Zephyr console and shell, start Zephyr instance like this:

build/zephyr/zephyr.exe -attach_uart

Any other application can be used too, just make sure that suitable configuration options are enabled (see samples/net/capture/prj.conf file for examples).

The network capture can be configured automatically if needed, but currently the capture sample application does not do that. User has to use net-shell to setup and enable the monitoring.

The network packet monitoring needs to be setup first. The net-shell has net capture setup command for doing that. The command syntax is

net capture setup <remote-ip-addr> <local-ip-addr> <peer-ip-addr>
     <remote> is the (outer) endpoint IP address
     <local> is the (inner) local IP address
     <peer> is the (inner) peer IP address
     Local and Peer IP addresses can have UDP port number in them (optional)
     like or [2001:db8:100::2]:4242

In Zephyr console, type:

net capture setup 2001:db8:200::1 2001:db8:200::2

This command will create the tunneling interface. The is the remote host where the tunnel is terminated. The address is used to select the local network interface where the tunneling interface is attached to. The 2001:db8:200::1 tells the local IP address for the tunnel, the 2001:db8:200::2 is the peer IP address where the captured network packets are sent. The port numbers for UDP packet can be given in the setup command like this for IPv6-over-IPv4 tunnel

net capture setup [2001:db8:200::1]:9999 [2001:db8:200::2]:9998

and like this for IPv4-over-IPv4 tunnel

net capture setup

If the port number is omitted, then 4242 UDP port is used as a default.

The current monitoring configuration can be checked like this:

uart:~$ net capture
Network packet capture disabled
                Capture  Tunnel
Device          iface    iface   Local                  Peer
NET_CAPTURE0    -        1      [2001:db8:200::1]:4242  [2001:db8:200::2]:4242

which will print the current configuration. As we have not yet enabled monitoring, the Capture iface is not set.

Then we need to enable the network packet monitoring like this:

net capture enable 2

The 2 tells the network interface which traffic we want to capture. In this example, the 2 is the native_posix board Ethernet interface. Note that we send the network traffic to the same interface that we are monitoring in this example. The monitoring system avoids to capture already captured network traffic as that would lead to recursion. You can use net iface command to see what network interfaces are available. Note that you cannot capture traffic from the tunnel interface as that would cause recursion loop. The captured network traffic can be sent to some other network interface if configured so. Just set the <remote-ip-addr> option properly in net capture setup so that the IP tunnel is attached to desired network interface. The capture status can be checked again like this:

uart:~$ net capture
Network packet capture enabled
                Capture  Tunnel
Device          iface    iface   Local                  Peer
NET_CAPTURE0    2        1      [2001:db8:200::1]:4242  [2001:db8:200::2]:4242

After enabling the monitoring, the system will send captured (either received or sent) network packets to the tunnel interface for further processing.

The monitoring can be disabled like this:

net capture disable

which will turn currently running monitoring off. The monitoring setup can be cleared like this:

net capture cleanup

It is not necessary to use net-shell for configuring the monitoring. The network capture API functions can be called by the application if needed.

Wireshark Configuration

The Wireshark tool can be used to monitor the captured network traffic in a useful way.

You can monitor either the tunnel interfaces or the zeth interface. In order to see the actual captured data inside an UDP packet, see Wireshark decapsulate UDP document for instructions.