It's pretty simple in concept - I take FreeBSD-HEAD, build it with some cut-down options, create a custom filesystem image with some custom boot scripts and a custom configuration file, and provide an image that you can TFTP (using a serial console and ethernet cable) or upload directly to the AP if it supports it.
The supported hardware list is here:
Now, it's not a huge list like OpenWRT, but that's mostly because I don't have an infinite supply of Atheros MIPS based routers. I think I'll get some of the TP-Link Archer series stuff next.
Building it is pretty simple:
You checkout the build repo, check out FreeBSD-HEAD, install a couple of packages, and run the build for your board. Once it's done, the images for your board appear in ../tftpboot/. There's a wiki page for each of the supported boards with a walkthrough with how to get FreeBSD going on it.
It comes up on 192.168.1.20/24 with 'user' and 'root' users, with no password. So, the first thing you should do after installation is telnet in, configure /etc/cfg/rc.conf with your actual LAN IPs, set the user/root passwords, and then 'cfg_save' to save things. Then, reboot and voila!
The configuration file format looks like FreeBSD but it isn't. I'm keeping it somewhat hierarchical-looking in naming but flat in implementation so I can migrate it to something like a sqlite or luci backend in the future.
It's good enough for me to be able to set up an AP to be a bridge with a management IP address and configure the ethernet switch. Others have added ipfw support to do NAT and firewalling - I'm going to add configuration rules for NAT, IPFW and routing soon so it's all integrated.
It's FreeBSD, all the way through:
$ uname -a FreeBSD tl-wdr3600 11.0-CURRENT FreeBSD 11.0-CURRENT #0 r282406M: Wed May 6 22:27:16 PDT 2015 adrian@lucy-11i386:/usr/home/adrian/work/freebsd/head-embedded/obj/mips/mips.mips/usr/home/adrian/work/freebsd/head-embedded/src/sys/TL-WDR4300 mips $ ifconfig wlan0 list sta ADDR AID CHAN RATE RSSI IDLE TXSEQ RXSEQ CAPS FLAG 18:ee:69:15:f4:12 2 1 26M 37.0 45 2703 51888 EPS AQEHTRM RSN HTCAP WME 04:e5:36:0d:1b:0d 1 1 19M 23.0 15 1524 47072 EPS AQEPHTR RSN HTCAP WME cc:3a:61:0e:33:a0 3 1 19M 32.0 30 2585 43072 EPS AQEPHTR RSN HTCAP WME 40:0e:85:1a:f1:69 4 1 19M 25.0 30 1138 54800 EPS AQEPHTR RSN HTCAP WME 00:0f:13:97:14:54 5 1 54M 30.0 45 1808 57424 EPS AE RSN 00:22:fa:c2:d1:20 6 1 26M 24.5 0 574 57776 EPS AQEHTRS RSN HTCAP WME
So if you'd like a FreeBSD based device to act as your home gateway, this is where you can start. It's not pfsense, but it's designed to run on things much smaller than pfsense supports and it's a good introduction into the world of FreeBSD embedded.