zebra barcode printer c# 11: Setting Up the Mac OS X Firewall in Objective-C

Generator Data Matrix 2d barcode in Objective-C 11: Setting Up the Mac OS X Firewall

CHAPTER 11: Setting Up the Mac OS X Firewall
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to keep them from taking creative assets out of your environment. All of this must be configured using the command line. Some outgoing network traffic blocking can be configured using a firewall device at the edge of your network, but not all. Even if it is possible at the firewall-device level, it s wise to impose a rule at both the software level and the hardware level to circumvent a breach at one of your layers of defense. This layered, or tiered, approach to security helps keep your network as secure as possible. But little of this would be possible without ipfw. Another reason to use ipfw to configure your firewall is to block outgoing traffic for certain ports, a feature not available otherwise. NOTE: Later in this chapter we will discuss blocking outgoing traffic, but it is important to point out that you can block outgoing traffic only by using either the command line or a third-party application. You can block incoming traffic by using the built-in firewall in System Preferences.
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The firewall located on the Sharing pane in Tiger is based on the open source program ipfw. Ipfw is short for Internet Protocol Firewall and can also be used with Leopard and Snow Leopard. The ipfw program has the ability to configure the port-based firewall in OS X, different from the application layer firewall in that rather than relying on application signatures it monolithically blocks specified ports. However, there is a rather steep learning curve when configuring the firewall with the ipfw interface for the first time, especially if you aren t very experienced with BSD. But learning curves are good. They offer you a chance to know more about the firewall on your system, what it is doing, and the key factors you should consider to make your network more secure. You also have the opportunity to get to know more about the inner workings of your system and how the different pieces to the security puzzle fit together. Because ipfw works at the kernel level, or innermost level, of the operating system, it has particular security requirements and requires that you run it from a superuser account. This is accomplished by using the sudo command to issue commands and create rules for it. Applications such as LimeWire or Samba (the Windows file-sharing component of Mac OS X) cannot override rules enforced by ipfw because of the secure nature of the command. The official syntax for the base ipfw toolkit is as follows:
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ipfw [-cq] add rule ipfw [-acdefnNStT] {list | show} [rule | first-last ...] ipfw [-f | -q] flush ipfw [-q] {delete | zero | resetlog} [set] [number ...] ipfw enable {firewall | one_pass | debug | verbose | dyn_keepalive} ipfw disable {firewall | one_pass | debug | verbose | dyn_keepalive}
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CHAPTER 11: Setting Up the Mac OS X Firewall
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To understand how to make the firewall perform many of its more complex features, you first need to understand what the firewall is doing and how it interacts with rules and the rule set. An ipfw rule is a line of code that informs your firewall about what is acceptable to allow and what to do when a pattern that you indicate is encountered. The ipfw rule set is the combination of all the ipfw rules that need to be invoked to provide the proper configuration for the policy item. Ipfw applies this rule set as criteria for whether network traffic that is being scanned is acceptable to enter the network. This can include which rules to log, what to do when a pattern matches a rule, and how much bandwidth is possible on a rule (limiting bandwidth is possible using dummynet). Rules in an ipfw rule set are processed based on their numerical priority. Each rule requires a unique five-digit number in the first column of the rule with a maximum value of 65535 that tells ipfw what priority to assign it in a rule set. They are then processed in order from highest to lowest. This is an example of an ipfw rule:
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