Set up OSX to act as router via Internet Sharing I have the following network setup:

LAN====OSX++++Machine A
        +
        +
        ++++++Machine B


OSX is currently sharing an ethernet connection with A and B via an ad-hoc wireless network.  This works great, except that Machine A and Machine B can't see each other.  They can see the LAN and OSX, and OSX can see both of them.  I'm convinced some combination of ipfw and/or natd configuration options can solve this problem, but my networking knowledge is limited enough that I can't seem to Google effectively for a solution.  What do I want ipfw/natd to do for me?  How do I get them to do it?

(Obviously, I could solve this problem much more easily by the expedient of purchasing a switch, but I feel like I should know how to do this sort of things regardless.)
You need to enable routing.  I'm pretty sure on OS X this is a sysctl(like FreeBSD):

http://securityreliks.securegossip.com/2010/10/enabling-ip-forwarding-on-mac-osx/

doing:

# sysctl -w net.inet.ip.forwarding=1


as root should enable this.