by Glenn Fleishman, Macworld.com   Sep 21, 2011 9:15 am

Back in July, I explained how Lion’s new AirDrop feature lets you exchange files simply between two computers with up-to-date Wi-Fi hardware. As I wrote then, AirDrop is a breeze to use if you have the right Mac. You’re out of luck if your computer doesn’t have the right hardware—specifically, if it doesn’t have Wi-Fi chips capable of personal area networking (PAN) for peer-to-peer connections. Many Macs, even many of relatively recent vintage and many that can run Lion, don’t have those chips and so can’t use AirDrop. (Apple provides a list of AirDrop-capable Macs here.)

But, it turns out, there’s a workaround. An anonymous Mac OS X Hints reader found that, if you have one of those older Macs, you can add a setting to AirDrop’s defaults that allows AirDrop to work over regular networks, not just PANs.

The change is a one-liner: Open Terminal and, at the command line, type:

<code>defaults write com.apple.NetworkBrowser BrowseAllInterfaces 1</code>

Hit Return, then go back to your Desktop and hold down the Option key as you Control-click (or right-click) on the Finder icon in the Dock. Select Relaunch from the contextual menu; relaunching the Finder activates the code you entered above. When the Finder finishes its restart, you should see an AirDrop entry in the Finder sidebar that wasn’t there before. (You can reverse the process by using the same command with 0 instead of 1, and relaunching the Finder again.)

How it works

There’s an interesting reason this switch works. AirDrop uses Bonjour, the technology that lets Macs and other devices on a network announce their presence and the services they provide, so other computers and devices on that network can find them without a lot of tedious configuration on your part. AirDrop is just another service that announces itself over Bonjour, but with a difference: Lion only looks for incoming AirDrop messages on the Wi-Fi PAN interface; it doesn’t look on the Wi-Fi wireless local area network (WLAN) interface as a whole, and Ethernet and other local networking types don’t register either. What the tweak above does is tell Lion to remove that restriction and look for AirDrop announcements on any network interface.

Doing so actually removes one of AirDrop’s key advantages: In its default mode, it can connect Macs that don’t already have a network connection. The pair may be connected to separate networks, or not connected to any network at all. If their Wi-Fi chips are active, though, AirDrop can still connect them. The setting above connects Macs only if they’re on the same local network, regardless of how they’re connected (Wi-Fi, Ethernet, or both).

How to use it

In testing, I found that you can make this change on just one single machine and still reap the benefits, but it’s best to enable it on all the computers you’ll use together.

For example, I have a Mac Pro and a MacBook Air, and the Pro isn’t capable of AirDrop. I made the setting change initially just on the Pro. It could then “see” the MacBook Air in its list of AirDrop devices (because it was now receiving all Bonjour messages on the local network); the Pro wasn’t visible from the Air (which was still receiving messages on the PAN interface only). However, I could still transfer files to the Air by dragging them onto the Air’s icon in AirDrop on my Mac Pro. When I clicked OK to start the transfer, a new connection was initiated from the Pro to the Air, which received it properly and displayed the Mac Pro in the AirDrop window when the request to transfer dialog appeared. The Mac Pro remained visible during the transfer and for a few seconds afterward. After applying the settings change to the Air, too, the Pro was continuously visible and accessible.

I tested with other machines, using Ethernet and Wi-Fi in various combinations, and as long as both machines were on the same local network using any interfaces, they could see each other and transfer files.

The only downside to this tweak is that Apple enables strong encryption over the PAN connection, ensuring that a separate layer of security wraps files being sent back and forth. With the LAN or WLAN transfers, Apple doesn’t add any such security, and you’re at the mercy of whatever protections are in place. You might avoid using AirDrop on public Wi-Fi networks or campus Ethernet, or use a program like Truecrypt to encipher files on a disk image before sending them over an insecure network.

There’s no knowing how long this feature will remain active. Apple has a habit of disabling hidden features (often retained for debugging purposes) that it doesn’t want to support. In the meantime, you can get all the advantages of Lion’s AirDrop over a local network with the flip of a single switch.