Networking with QEMU

This page describes how to set up a virtual network between a (Linux) host and a Zephyr application running in a QEMU virtual machine (built for Zephyr targets such as qemu_x86 and qemu_cortex_m3).

In this example, the Socket Echo Server sample application from the Zephyr source distribution is run in QEMU. The QEMU instance is connected to a Linux host using a serial port, and SLIP is used to transfer data between the Zephyr application and Linux (over a chain of virtual connections).


On the Linux Host, fetch the Zephyr net-tools project, which is located in a separate Git repository:

git clone
cd net-tools


If you get an error about AX_CHECK_COMPILE_FLAG, install package autoconf-archive package on Debian/Ubuntu.

Basic Setup

For the steps below, you will need at least 4 terminal windows:

  • Terminal #1 is your usual Zephyr development terminal, with the Zephyr environment initialized.

  • Terminals #2, #3, and #4 are terminal windows with net-tools being the current directory (cd net-tools)

Step 1 - Create helper socket

Before starting QEMU with network emulation, a Unix socket for the emulation should be created.

In terminal #2, type:


Step 2 - Start TAP device routing daemon

In terminal #3, type:

sudo ./

For applications requiring DNS, you may need to restart the host’s DNS server at this point, as described in Setting up Zephyr and NAT/masquerading on host to access Internet.

Step 3 - Start app in QEMU

Build and start the echo_server sample application.

In terminal #1, type:

west build -b qemu_x86 samples/net/sockets/echo_server
west build -t run

If you see an error from QEMU about unix:/tmp/slip.sock, it means you missed Step 1 above.

Step 4 - Run apps on host

Now in terminal #4, you can run various tools to communicate with the application running in QEMU.

You can start with pings:

ping6 2001:db8::1

You can use the netcat (“nc”) utility, connecting using UDP:

echo foobar | nc -6 -u 2001:db8::1 4242
echo foobar | nc -u 4242

If echo_server is compiled with TCP support (now enabled by default for the echo_server sample, CONFIG_NET_TCP=y):

echo foobar | nc -6 -q2 2001:db8::1 4242


Use Ctrl+C to exit.

You can also use the telnet command to achieve the above.

Step 5 - Stop supporting daemons

When you are finished with network testing using QEMU, you should stop any daemons or helpers started in the initial steps, to avoid possible networking or routing problems such as address conflicts in local network interfaces. For example, stop them if you switch from testing networking with QEMU to using real hardware, or to return your host laptop to normal Wi-Fi use.

To stop the daemons, press Ctrl+C in the corresponding terminal windows (you need to stop both and

Exit QEMU by pressing CTRL+A x.

Setting up Zephyr and NAT/masquerading on host to access Internet

To access the internet from a Zephyr application, some additional setup on the host may be required. This setup is common for both application running in QEMU and on real hardware, assuming that a development board is connected to the development host. If a board is connected to a dedicated router, it should not be needed.

To access the internet from a Zephyr application using IPv4, a gateway should be set via DHCP or configured manually. For applications using the “Settings” facility (with the config option CONFIG_NET_CONFIG_SETTINGS enabled), set the CONFIG_NET_CONFIG_MY_IPV4_GW option to the IP address of the gateway. For apps not using the “Settings” facility, set up the gateway by calling the net_if_ipv4_set_gw() at runtime.

To access the internet from a custom application running in QEMU, NAT (masquerading) should be set up for QEMU’s source address. Assuming is used, the following command should be run as root:

iptables -t nat -A POSTROUTING -j MASQUERADE -s

Additionally, IPv4 forwarding should be enabled on the host, and you may need to check that other firewall (iptables) rules don’t interfere with masquerading. To enable IPv4 forwarding the following command should be run as root:

sysctl -w net.ipv4.ip_forward=1

Some applications may also require a DNS server. A number of Zephyr-provided samples assume by default that the DNS server is available on the host (IP, which, in modern Linux distributions, usually runs at least a DNS proxy. When running with QEMU, it may be required to restart the host’s DNS, so it can serve requests on the newly created TAP interface. For example, on Debian-based systems:

service dnsmasq restart

An alternative to relying on the host’s DNS server is to use one in the network. For example, is a publicly available DNS server. You can configure it using CONFIG_DNS_SERVER1 option.

Network connection between two QEMU VMs

Unlike the VM-to-Host setup described above, VM-to-VM setup is automatic. For sample applications that support this mode (such as the echo_server and echo_client samples), you will need two terminal windows, set up for Zephyr development.

Terminal #1:

west build -b qemu_x86 samples/net/sockets/echo_server

This will start QEMU, waiting for a connection from a client QEMU.

Terminal #2:

west build -b qemu_x86 samples/net/sockets/echo_client

This will start a second QEMU instance, where you should see logging of data sent and received in both.

Running multiple QEMU VMs of the same sample

If you find yourself wanting to run multiple instances of the same Zephyr sample application, which do not need to talk to each other, use the QEMU_INSTANCE argument.

Start socat and tunslip6 manually (instead of using the scripts) for as many instances as you want. Use the following as a guide, replacing MAIN or OTHER.

Terminal #1:

socat PTY,link=/tmp/slip.devMAIN UNIX-LISTEN:/tmp/slip.sockMAIN
$ZEPHYR_BASE/../net-tools/tunslip6 -t tapMAIN -T -s /tmp/slip.devMAIN \
# Now run Zephyr
make -Cbuild run QEMU_INSTANCE=MAIN

Terminal #2:

socat PTY,link=/tmp/slip.devOTHER UNIX-LISTEN:/tmp/slip.sockOTHER
$ZEPHYR_BASE/../net-tools/tunslip6 -t tapOTHER -T -s /tmp/slip.devOTHER \
make -Cbuild run QEMU_INSTANCE=OTHER