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Little Snitch can block internet traffic of a particular app. Can I do something similar without (paying) Little Snitch?

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Little Snitch is completely worth the price. Buy it. You will thank me. –  Josh Jun 20 '13 at 18:44
    
@Josh Nice try, Little Snitch marketeer. ;) –  Mathias Bynens Jun 21 '13 at 6:00
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@MathiasBynens haha, I am not connected with them, just a long time user and being a developer myself, someone who can appreciate a well-built piece of software –  Josh Jun 21 '13 at 11:43

2 Answers 2

up vote 7 down vote accepted

TCPBlock

TCPBlock is a lightweight and fast application firewall ... you can prevent selected applications on your computer from opening connections to the network.

TCPBlock is currently free and can be downloaded here.

PF Firewall

OS X comes with the PF firewall, which can be configured to block all connections to certain servers in all apps. Blocking all connections from a single app is not possible as far as I am aware without external software.

You can use IceFloor as a GUI for the PF firewall.

IceFloor Black List

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Can you really block apps from IceFloor? –  dstonek Jan 21 at 21:44
    
I'm using IceFloor v2.0.2. Can you explain your answer a bit more on how to block a specific application from accessing internet? –  Manu yesterday

I think you can through the firewall as stated in this article:

  1. Click the System Preferences icon in the Dock.
  2. Click the Security icon.
  3. Click the Firewall tab.
  4. Click the Start button to turn on your firewall.
  5. Click the Advanced button.
  6. Click the Automatically Allow Signed Software to Receive Incoming Connections radio button to select it.

This enables the firewall. And, by default, virtually all incoming TCP/IP traffic is blocked. You must enable each sharing method that you want to be able to use. When you enable different sharing methods from the Sharing pane in System Preferences (such as File Sharing or FTP Access), you’ll notice that those types of traffic now appear in the Firewall list. (In other words, when you turn on a sharing method, the firewall automatically allows traffic for that sharing method, which Snow Leopard calls a service.)

Click the up/down arrow icon to the right of any service to specify whether the firewall should allow or block connections.

Sometimes, you might want to allow other traffic through your firewall that isn’t on the firewall list of recognized services and applications. At that point, you can click the Add button (which bears a plus sign) to specify the application that your firewall should allow. Snow Leopard presents you with the familiar Add dialog, and you can choose the application that needs access.

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It seems to me Firewall only blocks incoming traffic to an app but not outgoing traffic from an app. –  ohho Jun 19 '13 at 1:08
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Yes, the default firewall in OS X (as edited in this way) only blocks incoming traffic. It does not block outgoing connections from apps on your computer. The default OS X PF firewall blocks both ways (see my answer). –  George Garside Jun 20 '13 at 14:37

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