Warning: This tutorial is targeted for advanced users. You should have knowledge in networking and Mac OS X, beside handy with UNIX commandline when using this tutorial.

Couple weeks ago I just got the error message when synchronizing my iPhone with iTunes in my Mac. To sync-up apps from the AppStore, both iPhone and iTunes should be authorized with the same account. But, I always failed the authorization and got the iTunes Store error 50 message.

There are many possibilities that can cause that error message. But, mostly the network problem. Then I cross checked with another Mac within the same network, just fine! So, I have to suspect and focus to my Mac.

After a whole day in front of my Mac, then I found something odd: Built-in Ethernet of my Mac is on "en2", not "en0" as default. I thought I was forget that I have some modification with my OSX network configuration. You can check it using Terminal.app with typing:

ifconfig -a

It will bring all network interfaces on the Mac. But just focus on ethernets, it has "en-" prefix. Generally, Mac only has one ethernet. So it would be only one "en-". In my case, I got "en2":

ether 00:1c:c0:0e:f9:ea
inet6 fe80::21c:c0ff:fe0e:f9ea%en0 prefixlen 64 scopeid 0x4
inet netmask 0xffffff00 broadcast
media: autoselect (100baseTX <full-duplex>
status: active

Yes, We have to bring back "en2" to "en0". Don't go anywhere, stay at the Terminal.app. We have to edit a file. I prefer to use command line.

sudo nano /Library/Preferences/SystemConfiguration/NetworkInterfaces.plist

Then find some lines that have these patterns: "<string>enX</string>" and "<integer>X</integer>". Where X is the identification number of ethernet, in my case, X is 2. So, I find "<string>en2</string>" and "<integer>2</integer>"

I changed those to "<string>en0</string>" and "<integer>0</integer>". Save file (Ctrl+O) and exit nano (Ctrl+X). Quit Terminal.app (CMD+Q). Then, restart the Mac.

After your Mac have done restart, then open "System Preferences", click "Networks". You have to unlock to make changes. Click the "+" button on lower left to add network interface. Choose "Ethernet" for "Interface", and give it some name on "Service Name", for example: "Built-in Ethernet". Configure your ethernet network if necessary, or leave it if your network has automatic configuration (DHCP).

In my case, I used Snow Leopard 10.6.3. The last step might be slightly different when you are not using Snow Leopard.
I originally posted this article for MakeMac in Bahasa Indonesia: iTunes Store Error -50 dan Ethernet OSX.