So this is probably nothing that exciting for most people, but for me, I wanted to have a Terminal Server onto a DECnet network. Sure I could have probably just done one nice with tun/tap, dumped all the protocols on there, and called it even. But for some reason I wanted 2 NICs to keep the IP on one side, and DECnet on the other.

One thing I wanted was an internal bridge for DECnet only traffic, and since I just need MSRDP access, SLiRP can handle a single TCP port redirect.

The flags are as always pretty simple once you work them out:

qemu -vga std -cpu pentium -m 384 -vnc :0 -net none \
-hda nt4tse.vmdk \
-device pcnet,netdev=slback \
-device pcnet,netdev=decback \
-netdev tap,ifname=tap1,id=decback,script=/root/nt4tse-up,downscript=/root/nt4tse-down \
-netdev user,id=slback,hostfwd=tcp::3389-10.0.2.15:3389 \
-cdrom Windows\ NT\ 4\ All-In-One\ (Workstation\,\ Server\,\ Terminal\,\ Enterprise).iso

And the two network scripts starting with nt4tse-up:

#!/bin/bash
echo starting $1
ip tuntap add mode tap tap1
ifconfig tap1 up
ifconfig tap1
brctl addif decnet0 tap1
brctl show decnet0
echo done with tuntap

And the nt4tse-down:

#!/bin/bash
echo shutting down $1
ifconfig tap1 down
brctl delif decnet0 tap1
brctl show decnet0
ip tuntap del mode tap tap1
echo done shutting down $1

for completeness here is the bridge config in /etc/netplan/50-cloud-init.yaml

network:
    ethernets:
        ens3:
            addresses:
            - SOMETHING/24
            gateway4: SOMETHING
            match:
                macaddress: 00:f4:c1:56:40:7e
            nameservers:
                addresses:
                - 1.1.1.1
                - 8.8.8.8
    bridges:
      br0:
        dhcp4: no
        addresses: [192.168.23.1/24]
      decnet0:
        dhcp4: no
    version: 2

This way I have an IP bound bridge for things that talk IP, and a raw bridge, decnet0 that has my non IP decnet stuff on there. Naturally it’ll have my SIMH VAX on there:

# brctl show decnet0
bridge name     bridge id               STP enabled     interfaces
decnet0         8000.aede9f227e7b       no              tap0
                                                        tap1

Also the ability to mount directories as fake fat drives had it’s syntax change as well

 -drive file=fat:rw:win95cd

into something like this:

-drive file=fat:rw:dos,id=fat32,format=raw,if=none -device ide-hd,drive=fat32