I finally get it to work.

It’s almost embarrassing to admit: I wrote a book about Leopard and I could never get Back to My Mac (BTMM) to work for me. I thought it was a problem with Leopard. Many others seemed to, too. I figured I’d wait for a fix.

But now I find that I need BTMM functionality. So I spent the better part of two days troubleshooting the problem. The result: I’ve found a solution for my situation. If you’ve been having BTMM problems, maybe this will fix you up, too.

What Is Back to My Mac?

In case you have no idea what I’m talking about, here’s a quick review.

BTMM is a Leopard feature that works with a .Mac account. With it properly configured and enabled (the tricky part), it enables you to connect to your Mac for file sharing or screen sharing from another Mac on the Internet. This gives you secure access to your computer’s hard disk contents from virtually anywhere in the world. But it also gives you access to applications on a Mac. So, for example, I could use screen sharing from my MacBook Pro in Washington to open a Word document on my iMac in Arizona and use the iMac’s faxmodem to fax that document anywhere I like.

As you can imagine, this can be very useful if you’re away from your home or office Mac — especially for an extended period of time. Since I’ll be away from my iMac from the end of May through the middle of September this year, I was highly motivated to make this work.

My Problem — Well, the One with BTMM, Anyway

My problem was that when I was at a remote location, I could see the icon for my iMac, but I could not connect to it for either file sharing or screen sharing. In other words, BTMM simply wasn’t able to connect, despite the fact that it “saw” the computer I wanted to connect to.

What really annoyed me was that I had BTMM set up according to the Back to My Mac Guide provided by Apple. I’d RTFM — why wasn’t it working?

I won’t go into details on basic BTMM configuration options. I wrote about them in “Going Back to Your Mac in Mac OS X Leopard” — which is even more embarrassing. These instructions should work for most BTMM users — unless you have a double-NAT problem. I suspect that the majority of Mac users who can’t get this to work have that problem. I did.

But, as it turned out, my problem was even weirder.

Drat, Drat, that Double-NAT!

I’m not a networking expert and I really don’t want to be. Here’s what I understand about NAT as it relates to BTMM.

For BTMM to work, you must have a NAT-enabled router. That includes any AirPort Extreme base station or AirPort Express. It also includes a wide variety of other routers that you may use. Basically, if it’s relatively new — within a few years or so — it probably supports NAT. Both the Back to My Mac guide and my article (referenced above) explain how to enable NAT on an AirPort base station.

But if the cablemodem or other device provided by your ISP also has NAT enabled, you have double-NAT. This possibility is discussed in a good amount of detail in the Apple technical note titled “Back to My Mac: ‘Double-NAT’ Configurations may prevent Back to My Mac connections.”

You can’t have double-NAT. That means you have to turn one of the NAT setups off. In the double-NAT article, Apple recommends turning off NAT on your cablemodem, which is usually possible via a logon with a Web browser. If you can’t do that for some reason, you can set Connection Sharing to Off (Bridge mode) on your AirPort base station. The article explains exactly how to do it.

Either technique should resolve the problem for 90% of the people who have followed all the basic instructions and still can’t get BTMM to work.

But it didn’t resolve my problem.

I’m Special

Unfortunately, life at the edge of nowhere means you often have to settle for non-standard services. That pretty much describes my Internet service, which is provided by Chandler, AZ-based Bluewire.

There’s no cable television or fiber-optic telephone lines where I live and work. If I want something faster than dial-up — let’s get real here: who can live with dial-up these days? — I had to resort to a wireless Internet connection. No, it’s not dial-up networking (DUN), which I have in my Treo for when I’m in the middle of nowhere. It’s an antenna on the roof of my house. It points at an antenna on an AM radio tower about 2 miles away. That antenna points to an antenna on a water tower in downtown Wickenburg. That’s connected to a router that’s connected to a T1 connection.

I’m not the only person using that T1. Anyone in Wickenburg who’s outside of the range of cable and DSL who is willing to spend $39.99 a month for something faster than dial-up is sucking Internet off the same system. The router at the water tower has NAT enabled and distributes IP addresses to all of us.

But because the router is centralized, I can’t reconfigure it. According to Apple’s double-NAT article, setting Connection Sharing to Off (Bridge Mode) should do the trick. But it didn’t. The Apple support guy I talked to, Daniel in Austin, says that’s because I’m not getting a 10.x.x.x or 192.168.x.x IP address from my ISP’s router. This is confusing the system to the point that I completely lose my Internet connection when Bridge mode is enabled.

I needed another solution.

Give Me Some Static!

Ira, at my ISP explained that if I had a static IP address, NAT would be disabled for my connection to his system. He gave me a static IP address to try.

Guess what? When everything was properly configured, it worked!

(That static IP address will cost me an extra $7/month. Hey Ira, how about forgetting to add that to the billing system? Aren’t I already paying enough? Have pity for me! I live in Wickenburg!)

Here’s the fix that worked for me. Remember, I’m using an AirPort Extreme base station. If you’re using a different router, these exact instructions won’t apply.

1. Open AirPort Utility and click the icon for your base station.
2. Click the Internet button in the toolbar and then click the Internet Connection button in the main window.
3. Choose Manually from the Configure IPv4 pop-up menu.
4. Enter the IP address, Subnet Mask, Router Address, and DNS Servers provided by your ISP in the appropriate boxes.
5. Make sure Connection Sharing is set to Share a public IP address. At this point, it should look something like this, but with real IP addresses:

   

6. Click the NAT button to display NAT options.
7. Make sure the check box for Enable NAT Port Mapping Protocol is turned on.
8. Click Update.
9. Wait while your AirPort base station is updated.

When the base station’s icon reappears in the AirPort utility window, you’re ready to test your Internet connection. Do this right away by checking your e-mail or opening a Web browser page that isn’t cached.

You shouldn’t have to change anything in your Network preferences pane. It should just take the IP address your AirPort base station sends it. So should all the other computers on the network.

Now go to another network and try accessing your computer via BTMM. In my case, the easiest way to do this was to create a DUN connection from my MacBook Pro’s Bluetooth connection to my Treo. (That sure doesn’t sound very easy, does it?) I was able to connect just like Steve Jobs said I should. Whew!

As soon as this is posted, I’ll head over to the local library and give it a try from their WiFi network. Cross your fingers for me!

I hope some of this information helps you fix your BTMM problems. Good luck!

4:00 PM Update: Just got back from a trip into town. Although BTMM would not work via the library’s WiFi connection, it did work from a nearby restaurant’s open WiFi connection that I latched onto from a parking space out front. (I must have looked pretty silly driving around town in my Jeep, parking in front of likely locations, and opening my MacBook Pro to search for WiFi.) I’m going to conclude that the library’s WiFi setup is somehow weird and different — which I wouldn’t doubt, since it was set up by the Town’s computer consultant who is also a bit weird and different. I’m going to call this a complete success. Best of luck to those of you still having problems.