said by graysonf:
There is no packet capture capability in m0n0wall, at least not in the as delivered product. Anything is possible though if one would go thru the trouble of altering the sources and rebuilding it from scratch.
Have you tried to bring up a shell and see if tcpdump is installed? Eg. tcpdump -i your_int -vvn icmp6
It's totally normal to see an RA say every 1000 seconds (default on IOS retrans time I think). The default reachable time on an RA is 9000 seconds so you will at least see the retrans timer under every 9000 seconds.
This is why I'm thinking it's just some over aggressive debug logging. I'd submit a bug report if you haven't yet.
I'd also suggest everyone read RFC 2461 if you haven't yet, it's a good read!