This was inspired by tunbridge(1).
The reason I can't just use a bridge(4) device is because some
properties of packets are preserved in copying them around the kernel
and a userland set of tun(4) devices connected to each other avoids this
nicely.
If something else does this same task 'better' let me know, until then,
there is tuncat(1).
tuncat(1) is developed on OpenBSD but it should work on other BSDs.
HOW TO INSTALL
% make
# make install
HOW TO SET UP
The system must be configured to send packets to the tun device and
properly handle reinjected packets. Care should be taken not to
create a loop for reinjected packets.
The following example shows a simple pf.conf(5)
to redirect packets destined to a specific host to the emulated link.
tun_if = "tun0"
int_if = "em0"
pass in quick on $int_if route-to $tun_if from any to 192.0.2.1
In order to forward reinjected packets, packet forwarding needs to
be enabled, which usually needs the following line in /etc/sysctl.conf.
net.inet.ip.forwarding=1
HOW TO USE
The following exmaples demonstrates how to use tuncat(1).
To emulate a link.
tuncat -n /dev/tun0 -o /dev/tun1
CHANGES
2005/05/31 version 0.1 for beta test