On the mac they have this way to share an internet connection to other computers.

The closest I've come up with is a script which sets up forwarding or a bridge between two devices, taking the second device from a user input.

But it is so simple and obvious that someone must have written an app just for this that is much more elegant.

Anyone found one?

Maybe you could explain what "this way" does. Your link just indicates how to enable it not what it does.

Why do you want your LAN to use public IP addresses? Do you have a public IP address assigned for each one?

You misunderstand - what is described is what I am looking for. I am not trying to set up a home LAN - though the ability to act as a subnet at the drop of a hat would be good.

I know how to use iptables to bridge or forward packets between two interfaces.
I can set up a script to do this, and the ability to launch it from a desktop or a menu.
I'm not asking how to do the technical side.

What I am asking about is a turnkey solution where this is already set up.
Install the app, and you get a menu item which facilitates sharing an existing internet connection over a nominated interface - by any means. I don't care how it gets this done. Its an abstraction layer.

The specific context is the case of a kid in Africa with an XO. I want them to have the option to use their machine to share its connection with another kids without having the kid do any scripting.

If you act as a dhcp server and perform NAT masquerading for any hosts on the lan, additional hosts added to the lan simply need to opt to use DHCP to configure their devices. I can't imagine anything easier than that. For many distro's this is just a matter of selecting that you want to share your internet connection in a network device configuration wizard. IMHO, bridging is the wrong method to use.

If you have only one interface, then you need to use a virtual NIC device with a LAN address. There is the disadvantage that Internet traffic will be exposed on the switch or wireless LAN.

you might wanna check out firestarter
http://www.fs-security.com/
this should do what you want

One possible situation is that an xo (olpc) has access to a mesh network and thus the internet but another computer cannot use the mesh method. The xo also has a wired nic. OR - perhaps it can access a household wireless network and the other computer does not have wifi.

Conceptually, the xo can share its access through the nic.

Its not something that comes up very often, but I have seen it done at a conference where a mac shared its access with several ubuntu computers by acting as a wap.

I concur that it is probably more robustly set up not to use bridging, but to allow each xo to act as a dhcp server - setting up forwarding with ip masquerade in iptables.

This produces the function I'm looking for.

What I was looking for was an existing solution which does this in a manner similar to the mac. For instance, I'd have got what I wanted if a user can click a menu item or a launcher, get a dialog where they select how to share their existing connection, and then the connection is shared.

Its probably not all that hard to impliment - I can probably write it myself. But I don't want to do that if there is already something that works.

firestarter I have seen.

It does do this, and quite a bit more. The process seems fairly complex compared with the mac experience. I could mess about with a couple of laptops here to see what can be leveraged.

For that matter, there are mac users around here - there may be limitations to this connection sharing on the mac I don't know about.

I do recall there are HW limits to using a laptop as a wap. (Some wifi cards do not support this.)