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Networking with QEMU User

This page is intended to serve as a starting point for anyone interested in using QEMU SLIRP with Zephyr.

Introduction

SLIRP is a network backend which provides the complete TCP/IP stack within QEMU and uses that stack to implement a virtual NAT’d network. As there are no dependencies on the host, SLIRP is simple to setup.

By default, QEMU uses the 10.0.2.X/24 network and runs a gateway at 10.0.2.2. All traffic intended for the host network has to travel through this gateway, which will filter out packets based on the QEMU command line parameters. This gateway also functions as a DHCP server for all GOS, allowing them to be automatically assigned with an IP address starting from 10.0.2.15.

More details about User Networking can be obtained from here: https://wiki.qemu.org/Documentation/Networking#User_Networking_.28SLIRP.29

Using SLIRP with Zephyr

In order to use SLIRP with Zephyr, the user has to set the Kconfig option to enable User Networking.

CONFIG_NET_QEMU_USER=y

Once this configuration option is enabled, all QEMU launches will use SLIRP. In the default configuration, Zephyr only enables User Networking, and does not pass any arguments to it. This means that the Guest will only be able to communicate to the QEMU gateway, and any data intended for the host machine will be dropped by QEMU.

In general, QEMU User Networking can take in a lot of arguments including,

  • Information about host/guest port forwarding. This must be provided to create a communication channel between the guest and host.

  • Information about network to use. This may be valuable if the user does not want to use the default 10.0.2.X network.

  • Tell QEMU to start DHCP server at user-defined IP address.

  • ID and other information.

As this information varies with every use case, it is difficult to come up with good defaults that work for all. Therefore, Zephyr Implementation offloads this to the user, and expects that they will provide arguments based on requirements. For this, there is a Kconfig string which can be populated by the user.

CONFIG_NET_QEMU_USER_EXTRA_ARGS="net=192.168.0.0/24,hostfwd=tcp::8080-:8080"

This option is appended as-is to the QEMU command line. Therefore, any problems with this command line will be reported by QEMU only. Here’s what this particular example will do,

  • Make QEMU use the 192.168.0.0/24 network instead of the default.

  • Enable forwarding of any TCP data received from port 8080 of host to port 8080 of guest, and vice versa.

Limitations

If the user does not have any specific networking requirements other than the ability to access a web page from the guest, user networking (slirp) is a good choice. However, it has several limitations

  • There is a lot of overhead so the performance is poor.

  • The guest is not directly accessible from the host or the external network.

  • In general, ICMP traffic does not work (so you cannot use ping within a guest).

  • As port mappings need to be defined before launching qemu, clients which use dynamically generated ports cannot communicate with external network.

  • There is a bug in the SLIRP implementation which filters out all IPv6 packets from the guest. See https://bugs.launchpad.net/qemu/+bug/1724590 for details. Therefore, IPv6 will not work with User Networking.