MacOS – Why can’t I share the Internet connection with another Mac

The answer is yes.

I'm typing you this answer through an InternetSharing connexion provided by an iMac running MacOS Lion (10.7.3) where no one is connected:

me -> MBP    ………Wi-Fi………   iMac   ………Ethernet………   Internet 
      10.6.8               10.7.3

I'm using such a connection for professionnal use since MacOS 10.4 with no one connected (on the Mac running InternetSharing). This connection was used by 5 other Mac (10.4, 10.5, 10.6) and might be considered as a nearly professional grade network.

When you login when your InternetSharing is faulty, could you type in a Terminal or xterm window:

ifconfig -a
ps ax | egrep '(bootp|natp)'

My first blind guess is that your main Ethernet connection was shut down under the responsability of your Energy Saver tuning.

To try to make things clear let's say you have 2 Mac named:

• Alex
• Bob

Here is the scheme of your small home network :

Internet --- USB --- Alex --- WLAN --- Bob

If when Bob try to join the wireless network (AirPort or WI-Fi or better WLAN) initiated from Alex, it gets an immediate rejection, this may be due to a network access with the wrong security setting.

To check this hypothesis, on Bob, simply go within:

System Preferences… > Network > WiFi > Advanced…

and look at the column Security.

If it is anything else than WPA2 Personal, double click on this network setting to fix it.

If it is already WPA2 Personal, double click on this network setting to check if the registered password is OK.