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@ -34,46 +34,50 @@
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</legalnotice>
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</articleinfo>
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<para>Proxy ARP (RFC 1027) is a way to make a machine physically located on
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one network appear to be logically part of a different physical network
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<section>
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<title>Overview</title>
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<para>Proxy ARP (RFC 1027) is a way to make a machine physically located
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on one network appear to be logically part of a different physical network
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connected to the same router/firewall. Typically it allows us to hide a
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machine with a public IP address on a private network behind a router, and
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still have the machine appear to be on the public network "in front of" the
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router. The router "proxys" ARP requests and all network traffic to and from
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the hidden machine to make this fiction possible.</para>
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still have the machine appear to be on the public network "in front of"
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the router. The router "proxys" ARP requests and all network traffic to
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and from the hidden machine to make this fiction possible.</para>
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<para>Consider a router with two interface cards, one connected to a public
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network PUBNET and one connected to a private network PRIVNET. We want to
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hide a server machine on the PRIVNET network but have it accessible from the
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PUBNET network. The IP address of the server machine lies in the PUBNET
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network, even though we are placing the machine on the PRIVNET network
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behind the router.</para>
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<para>Consider a router with two interface cards, one connected to a
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public network PUBNET and one connected to a private network PRIVNET. We
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want to hide a server machine on the PRIVNET network but have it
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accessible from the PUBNET network. The IP address of the server machine
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lies in the PUBNET network, even though we are placing the machine on the
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PRIVNET network behind the router.</para>
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<para>By enabling proxy ARP on the router, any machine on the PUBNET network
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that issues an ARP "who has" request for the server's MAC address will get a
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proxy ARP reply from the router containing the router's MAC address. This
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tells machines on the PUBNET network that they should be sending packets
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destined for the server via the router. The router forwards the packets from
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the machines on the PUBNET network to the server on the PRIVNET
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network.</para>
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<para>By enabling proxy ARP on the router, any machine on the PUBNET
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network that issues an ARP "who has" request for the server's MAC address
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will get a proxy ARP reply from the router containing the router's MAC
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address. This tells machines on the PUBNET network that they should be
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sending packets destined for the server via the router. The router
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forwards the packets from the machines on the PUBNET network to the server
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on the PRIVNET network.</para>
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<para>Similarly, when the server on the PRIVNET network issues a "who has"
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request for any machines on the PUBNET network, the router provides its own
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MAC address via proxy ARP. This tells the server to send packets for
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request for any machines on the PUBNET network, the router provides its
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own MAC address via proxy ARP. This tells the server to send packets for
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machines on the PUBNET network via the router. The router forwards the
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packets from the server on the PRIVNET network to the machines on the PUBNET
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network.</para>
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packets from the server on the PRIVNET network to the machines on the
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PUBNET network.</para>
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<para>The proxy ARP provided by the router allows the server on the
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PRIVNETnetwork to appear to be on the PUBNET network. It lets the router
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pass ARP requests and other network packets in both directions between the
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server machine and the PUBNET network, making the server machine appear to
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be connected to the PUBNET network even though it is on the PRIVNET network
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hidden behind the router.</para>
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be connected to the PUBNET network even though it is on the PRIVNET
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network hidden behind the router.</para>
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<para>Before you try to use this technique, I strongly recommend that you
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read the <ulink url="shorewall_setup_guide.htm">Shorewall Setup
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Guide</ulink>.</para>
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</section>
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<section id="Example">
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<title>Example</title>
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Tom Eastep