Set up OSX to act as router via Internet Sharing

Question

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.)

Answer 1

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.

Answer 2

Try to enter each computer's address manually in the Network System Preference panel.

eg.

Machine A:
IP address: 192.168.1.1 
Subnet Mask: 255.255.255.0

Machine B: 
IP address: 192.168.1.2
Subnet Mask: 255.255.255.0