I'll second pcengines. Just built a apu3a4 with openbsd as the firewall/nat for my gigabit connection and it was a fun little experience.

It was ridiculous how easy how few lines it took to get openbsd configured.

- Netgate has some intel atom based boards[1].

- pcengines (Alix, APU, APU2)

- Lanner

[1]: https://www.netgate.com/products/pfsense-appliances.html

http://espressobin.net I've been waiting for mine for months. The Ubiquiti ERLite has also been great though.

Possiby not what you want to hear, but personally I have always bought (usually for peanuts) used (fanless) "thin clients" and added a PCI NIC, they make very good routers on the cheap, and since (originally) the machines were sold for lots of money to the enterprise market, they are generally very good quality/well built, sturdy, suffer not from overheating, etc. If you want to buy "new" there are several suitable mini-itx boards available.

The Banana Pi router is interesting. It has several ethernet ports, has debian available for it and a SATA interface so it can double as a simple NAS:

https://www.amazon.com/s/ref=nb_sb_noss/144-3904771-9295165?...

I migrated about a year ago from using net4801s to Ubiquity EdgeRouter Lite devices. The EdgeRouter Lite has 3 Gigabit ethernet ports. I got to keep my network topology and the software was flexible enough to mostly configure it how I wanted. I had been using a "from scratch" custom built Linux on the net4801s, but it was becoming more and more painful to keep up to date.

You could get a USB network adapter as your second ethernet port.