Asus RT-N66U Enable SSH Access

are they using some custom firmware?

Yes, most owners of this router use custom firmware like tomato or mods thereof.

Even though this is a year old I wanted to chime in with what I found because I was just now trying to get sshd working on the same router after reading similar pages.

I'm using version 3.0.0.4.376_3861 and they still haven't added in sshd. I got a chuckle out of the hidden ssh options in the menu. They are still there, but don't work. I telnetted in and did nvram show | grep ssh. Nothing was already there, but I tried shinji14's answer anyway. I tried with reboot too, but nmap still showed no SSH on a port scan. I then telnetted back in and tried ls -alR --color=always | more and after scanning the results I saw that they are using BusyBox. I couldn't remember if BusyBox had sshd built in so I did busybox --help and didn't see it in the list.

BusyBox's site mentions Dropbear for sshd here, so it looks like they are likely using a WRT variant like 0xC0000022L mentioned or they may be putting Dropbear on it. I'll probably end up trying Dropbear myself. If that works then I'd say it'd be the way to go because one could pretty much keep it stock and then just put it back on anytime an official update is installed.