So before going on the trip, I noticed the router I was using would occasionally drop off the network. The switch still reported the link as being up, but the router would not respond to pings from the internal network. If I SSHed into it from outside the network, and tried pinging internal IPs, it failed to ping them.
Something was up. After much debugging (and some arguments about upgrades), it was decided that the hardware was flakey. In that discussion, it was recommended that I have a look at PC Engines’ APU2 single board computer.
This is the only x86 computer I have seen with schematics and CoreBoot out-of-the-box, and it happens there’s a local supplier of them. For sure, this machine is overkill for the job, but it ticks nearly all the boxes.
The only one it didn’t tick was being able to run directly from the battery. As it happens, the unit only draws about 1.5A, and so a LM1085-12 LDO which can be sourced locally did the trick. I basically put 100µF capacitors on the input and output, bolted it to a small heatsink and threw it all into a salvaged case.
After hooking it up to a bench supply (disconnected from the APU2) and winding the voltage right up to the PSUs maximum, and observing that the voltage stayed at 12V, I decided to hook it up and see how it went. I plugged in my null modem cable, and sure enough, I was staring at CoreBoot.
I PXE-booted OpenBSD 6.3 and installed that onto the SD card, this was fairly painless and before long, the machine was booting on its own. I copied across the configuration settings from the old one, set up sniproxy, and I was in business, it was time to issue a `shutdown -p now` to both machines and for them to swap places.
Of course, a nicety of this box is there’s three Ethernet ports, so room for a move to another Internet connection, such as the HFC we’re supposed to be getting in this part of Brisbane (sadly, no thin pieces of glass for us), so in theory, I can run both in parallel and migrate between them.