When you activate any of the shared resources in the Sharing pane of System Preferences, you run a server of sorts: You make a certain kind of information available to other computers running the right sort of client software, provided that they can reach your Mac over a network (and, in most cases, have the necessary credentials). In fact, there may be any number of barriers preventing another computer from taking advantage of your shared resources — including a too-strict firewall setting on your Mac or a router or gateway that uses NAT (network address translation) and thereby hides your Mac's public IP address. Nevertheless, the fact remains that the more services you run and make available to other computers, the more potential paths exist by which someone could exploit bugs, hack into your system, steal data, and otherwise put your Mac to illicit use.
Therefore, let me begin this chapter by offering two pieces of blanket advice that apply to all these shared resources (and I echo this advice several other times in this chapter, just to make sure it sinks in):
Share resources only when necessary. All shared resources are disabled by default, but many Mac users like to turn some or all of them on just in case. I won't deny that there are rare situations in which having a shared resource turned on can save the day, but these situations occur far less frequently than hacking and exploit attempts. All things being equal, your odds of ...
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