Tun/tap and Mac OS - HECnet

23 May 2023

Much of my DECnet work involves either SIMH or PyDECnet, both of which can do Ethernet
over UDP, with a Billquist bridge (or Python equivalent) providing the bridging.  So while
I've used TAP some years ago, it hasn't been a concern lately.

Then came the time to find an Alpha emulator to run VMS, and it turns out SIMH only has
the skeleton of one, not even close to ready for real use.  But there are others, and
after some searching I landed on AXPbpx.  That works well; VMS 8.4 installs on it without
problems especially with some help from various Wiki pages.  But the obvious complication
is that its Ethernet emulation talks to an interface, not UDP (not as far as I can tell). 
So I looked for TAP as the way to do that since my laptop's physical interface (being
WiFi) is not useable for the job.

Surprise: no more Tun/Tap for Mac OS.  Or at most, something that's buried in a corner
of some other package and at risk of being broken by Apple any day now.

But some Googling for options turned up something very simple that seems to work.  Mac OS,
in its network settings, lets you define a "virtual interface" bridge.  A
typical use for that is to share a physical interface, but it doesn't actually need to
be connected to a physical interface to work.  I tried that.

In System Preferences, Network, at the bottom left is a button bar with + - O, that last
one a circle with some dots in it and a downarrow.  Click that, last option is Manage
virtual interfaces.  It lists your bridges, if any.  Click on + to add one, give it a
name.  It will show up in the virtual interfaces list with a Unix-style interface name
like "bridge1".

Then you can connect things to that bridge, for example an AXPbox emulated Ethernet, or a
PyDECnet circuit via pcap.  Wireshark will happily display the packets on that bridged
network.  And a software bridge can then connect that LAN to other things, for example
over UDP.

Enjoy.

	paul