176 lines
4.4 KiB
HTML
176 lines
4.4 KiB
HTML
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>Side Effects and Fringe Benefits</TITLE
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>Chapter 11. IP Masquerade and Network Address Translation</TD
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NAME="X-087-2-MASQ.SIDE.EFFECTS"
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>11.1. Side Effects and Fringe Benefits</A
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></H1
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><P
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> The IP masquerade facility comes with its own set of side effects, some of
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which are useful and some of which might become bothersome.</P
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><P
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>None of the hosts on the supported network behind the masquerade router
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are ever directly seen; consequently, you need only one valid and
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routable IP address to allow all hosts to make network connections out
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onto the Internet. This has a downside; none of those hosts are
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visible from the Internet and you can't directly connect to them from
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the Internet; the only host visible on a masqueraded network is the
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masquerade machine itself. This is important when you consider
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services such as mail or FTP. It helps determine what services should
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be provided by the masquerade host and what services it should proxy
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or otherwise treat specially.</P
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><P
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>Second, because none of the masqueraded hosts are visible, they are relatively
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protected from attacks from outside; this could simplify or even remove
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the need for firewall configuration on the masquerade host. You shouldn't rely
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too heavily on this, though. Your whole network will be only as safe as
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your masquerade host, so you should use firewall to protect it if security is
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a concern.</P
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><P
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>Third, IP masquerade will have some impact on the performance of your
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networking. In typical configurations this will probably be barely measurable.
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If you have large numbers of active masquerade sessions, though, you may find
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that the processing required at the masquerade machine begins to impact
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your network throughput. IP masquerade must do a good deal of work for
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each datagram compared to the process of conventional routing. That
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386SX16 machine you have been planning on using as a masquerade machine
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supporting a dial-up link to the Internet might be fine, but don't expect
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too much if you decide you want to use it as a router in your corporate
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network at Ethernet speeds.</P
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><P
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>Last, some network services just won't work through masquerade, or at least
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not without a lot of help. Typically, these are services that rely on incoming
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sessions to work, such as some types of Direct Communications Channels (DCC),
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features in IRC, or certain types of video and audio multicasting services.
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Some of these services have specially developed kernel modules to provide
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solutions for these, and we'll talk about those in a moment. For others, it
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is possible that you will find no support, so be aware,it won't be suitable
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in all situations.</P
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>IP Masquerade and Network Address Translation</TD
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>Configuring the Kernel for IP Masquerade</TD
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