The Design and Implementation of the Anykernel and Rump Kernels

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# ifconfig tap0 create
# ifconfig tap0 up
# ifconfig bridge0 create
# brconfig bridge0 add tap0 add re0
# brconfig bridge0 up
Figure 3.22: Bridging a tap interface to the host’s re0. The allows the tap
device to send and receive network packets via re0.
The commands for bridging a tap interface on NetBSD are provided in Figure 3.22.
Note that IP addresses are not configured for the tap interface on the host.
The virt (manual page virt.4 at A–49) network interface driver we implemented uses
hypercalls to open the tap device on the host and to transmit packets via it. The
source code for the driver is located in sys/rump/net/lib/libvirtif.
Full network stack without host network access
Essentially, we need an unprivileged Ethernet-like bus. All interfaces which are
attached to the same bus will be able to talk to each other directly, while other
nodes may be reached via routers. Accessing the host’s network is possible if one
rump kernel in the network uses the virt driver and routes traffic between the rumponly
network and the real network.
One option for implementing packet distribution is to use a userland daemon which
listens on a local domain socket [26]. The client kernels use hypercalls to access
the local domain socket of the daemon. The daemon takes care of passing Ethernet
frames to the appropriate listeners. The downside of the daemon is that there is an
extra program to install, start and stop. Extra management is in conflict with the
goals of rump kernels, and that is why we chose another implementation strategy.