The "source" code for nas
in DD-WRT can be found here: http://svn.dd-wrt.com/browser#src/router/nas
OpenWRT is what you have already seen.
To me it appears that nas
is a (possibly closed source?) binary that has been repackaged into these two distributions as a binary blob, obtained from somewhere else.
After a bit more digging I found source code for what appears to be the same program on GitHub, which hopefully answers your question: https://github.com/jcolp/rtn66u/tree/master/release/src/router/nas
TBH, I guessed this by running strings
on the nas
binary and discovered the C source files nas_wksp.c
and nas_wksp_radius.c
were named probably for debugging.
There is version buried inside the Tomato firmware as well: http://repo.or.cz/w/tomato.git/tree/620424a4333f6eeaf57d8b76ee61a287b6c01683:/release/src-rt/wl/nas
Why the source is not referenced DD-WRT and OpenWRT I have no idea...