Just a quick update for anyone who finds this page while searching for compatibility information:
The Barrier Breaker 14.07 file linked above is flashable to the E4200 (my procedure below); and is functional, with caveats:
i) 2.4GHz WiFi works but is very slow (single figures throughput); 5GHz does not start (is reported as generic 2.4GHz).
ii) VLAN configuration is available, accepts (& keeps) settings, but with the same configuration applied as on my Netgear WNDR-3700 for a DMZ (public IP) VLAN, I couldn't communicate with machines in that segment (I couldn't try from the machines themselves (i.e. traffic in the other direction) because I run them headless.
PPPoE comes up fine, and internet performance over Cat5 is OK.
Much more than that, I can't say; I was keen to get back to the WNDR3700(!); which brings me neatly around to flashing and de-flashing (i.e. reverting to stock).
In order to get OpenWrt on my E4200:
1) I started with Linksys stock firmware, reset to defaults; static IP set on host I'd be flashing from (192.168.1.2).
2) I flashed a DD-WRT 'trailed' mini build (I'm a recent arrival from DD-WRT and old habits die hard!).
3) I performed a 30/30/30 reset once the mini build finished its first boot.
4) I flashed the OpenWrt file linked in my post above from the DD-WRT firmware upgrade dialogue.
OpenWrt booted, and I was able to configure the router as previously described.
Flashing back to stock was interesting for a minute or two (this was the main thing I'd been cautious about since I haven't seen much about recovering E4200s). I offered the stock firmware file to OpenWrt's upgrade dialogue, and it reported that the file was unsuitable. After some quick reading, and taking a decision that I'd just give something a whirl...
1) I put the firmware file (renamed to e4200oem.bin) on the router in /tmp via SCP.
2) Connected to the router via SSH, navigated to /tmp.
3) On my initial attempt, mtd reported that the file was unsuitable for writing, so I [bravely!] decided to strip 32 bytes of header:
dd bs=32 skip=1 if=e4200oem.bin of=e4200oem.trx
4) Wrote the file to flash:
mtd -r write /tmp/e4200oem.trx firmware
5) mtd reported successful write and rebooted the router.
6) The router appeared to be booting normally (with OEM LED behaviour), but it continued to reboot (perhaps 30 seconds apart). I thought I'd give it a few minutes in case it was just settling down while I considered my next move. A few minutes later I thought I'd have a go at trying for a tftp recovery (who knows, maybe!), so I killed the power and booted up with the reset button held down. I noticed the Cisco logo behaving differently (constantly illuminated); I gave it 30 seconds and released reset. Before I'd got to trying to tftp, I saw ping responses coming in, and when I checked http://192.168.1.1 I found a first-booted Cisco welcome screen.
Anyway, I've got loads of screenshots, copies of logs, and notes on what I did, how I did it, and what resulted; is it worth putting any of it on the Wiki?
(Last edited by FactionOne on 15 Aug 2015, 05:08)