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

Generator Data Matrix ECC200 in Objective-C 11: Setting Up the Mac OS X Firewall

CHAPTER 11: Setting Up the Mac OS X Firewall
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Figure 11 6. Accepting an application ad-hoc
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Once you have configured which applications can and cannot communicate with computers outside of your system, it s time to configure the Advanced settings on the Security pane s Firewall button. NOTE: If you encounter services that cannot function once you have enabled the firewall, you can quickly determine whether the firewall is the problem. Try disabling the firewall to see whether it is blocking those services (click the Stop button on the Firewall tab of the Sharing pane). If the problem persists, then the firewall is not the culprit. If this resolves the issue, then the firewall is most likely the problem.
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Setting Advanced Features
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There are also a few more advanced features. These include blocking all incoming connections, automatically allowing signed software to create connections and enabling stealth mode. From the Security System Preferences pane, click on the Firewall tab and then on Advanced to access these options.
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Blocking Incoming Connections
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The Block all incoming connections option will disable all non-essential incoming connections on the computer. Because they are required, DHCP, Bonjour registration, and other services that will lead to instability will be left enabled. However, all sharing services will be disabled, as will any third-party applications that have been allowed. To disable incoming connections, check the Block all incoming connections box in the Advanced options in the Firewall System Preference pane.
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CHAPTER 11: Setting Up the Mac OS X Firewall
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Figure 11 7. The Block all incoming connections option in the Firewall pane
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Notice in Figure 11 7 that once this feature has been enabled, you cannot configure any other features of the firewall in Mac OS X. Once enabled, sharing services will be disabled, and you ll be unable to configure any of the other options as well.
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Allowing Signed Software to Receive Incoming Connections
Perhaps one of the most dangerous options in the Firewall is to allow all signed applications access to receive incoming connections. This option, shown in Figure 11 7, will rely on application signatures, which is good, but will allow all signed applications to communicate, which is bad. An application is considered signed if the application bundle has a signature, a designated requirement, and if the signature matches the requirement. The signature is then used when the application is opened to verify that it has not been altered since it was signed and to detect that it is the same application if it is a new version of the same application. To determine if an application has a signature, browse to the application in question and then Ctrl-click (or right-click if you have a multi-button mouse) on it and then click on Show Contents. From here, open the CodeSignature folder and check that there is a CodeResources file (see Figure 11 8). If so, then there is a signature. But who signed the app And when was it signed In some cases, this method of verifying the signature will not work as intended and so the codesign command, covered in more detail in 6, can be used as well.
CHAPTER 11: Setting Up the Mac OS X Firewall
Figure 11 8. Looking inside the Safari bundle
NOTE: All applications built for the iPhone must be signed, and someday we hope to see this replicated in Mac OS X as well! To check the signature of the application and obtain more granular information about it, Apple has provided the codesign application. While this tool is covered more granularly in 6, a good example of its use would be to obtain very verbose output of the Safari application with the following command:
codesign -vvv Safari.app
The output would simply indicate that the Safari.app checks clean against the tests performed:
/Applications/Safari.app: valid on disk /Applications/Safari.app: satisfies its Designated Requirement
Going Stealthy
When Computer A sends a request to Computer B, Computer B determines how to respond. Responses contain either the data requested or a rejection to the request, which is sent back to the requesting computer. If a process is running and traffic is allowed, then Computer A will get a message from Computer B allowing it to connect. If a process is not running or a firewall rule is active that prohibits Computer A from connecting to a given port on Computer B, then Computer A will get a deny message from Computer B. This process occurs before a web page is loaded or before a password is requested. This seemingly innocuous process is the basis for much of the process by which computers communicate, and can also be used to expose rather important details about your system. By analyzing the patterns in delivery times and the actual content of responses to requests for traffic, it is possible to determine the operating system you are
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