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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Network Interface / Internet "Layer Connection" Protocols
           9  Address Resolution and the TCP/IP Address Resolution Protocol (ARP)
                9  TCP/IP Address Resolution Protocol (ARP)

Previous Topic/Section
ARP Caching
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TCP/IP Address Resolution For IP Multicast Addresses
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Proxy ARP
(Page 2 of 2)

Proxy ARP Operation

The solution to this situation is called ARP proxying or Proxy ARP. In this technique, the router that sits between the local networks is configured to respond to device A's broadcast on behalf of device B. It does not send back to A the hardware address of device B; since they are not on the same network, A cannot send directly to B anyway. Instead, the router sends A its own hardware address. A then sends to the router, which forwards the message to B on the other network. Of course, the router also does the same thing on A's behalf for B, and for every other device on both networks, when a broadcast is sent that targets a device not on the same actual physical network as the resolution initiator. This is illustrated in Figure 50.


Figure 50: ARP Proxy Operation

In this small internetwork, a single router connects two LANs that are on the same IP network or subnet. The router will not pass ARP broadcasts, but has been configured to act as an ARP proxy. In this example, device A and device D are each trying to send an IP datagram to the other, and so each broadcasts an ARP Request. The router responds to the request sent by Device A as if it were Device D, giving to A its own hardware address (without propagating Device A’s broadcast.) It will forward the message sent by A to D on D’s network. Similarly, it responds to Device D as if it were Device A, giving its own address, then forwarding what D sends to it over to the network where A is located.

 


Proxy ARP provides flexibility for networks where hosts are not all actually on the same physical network but are configured as if they were at the network layer. It can be used to provide support in other special situations where a device cannot respond directly to ARP message broadcasts. It may be used when a firewall is configured for security purposes. A type of proxying is also used as part of the Mobile IP protocol, to solve the problem of address resolution when a mobile device travels away from its home network.

Key Concept: Since ARP relies on broadcasts for address resolution, and broadcasts are not propagated beyond a physical network, ARP cannot function between devices on different physical networks. When such operation is required, a device, such as a router, can be configured as an ARP proxy to respond to ARP requests on the behalf of a device on a different network.


Advantages and Disadvantages of Proxying

The main advantage of proxying is that it is transparent to the hosts on the different physical network segments. The technique has some drawbacks though. First, it introduces added complexity. Second, if more than one router connects two physical networks using the same network ID, problems may arise. Third, it introduces potential security risks; since it essentially means that a router “impersonates” devices in acting as a proxy for them, raising the potential for a device to spoof another. For these reasons, it may be better to redesign the network so routing is done between physical networks separated by a router, if possible.

 


Previous Topic/Section
ARP Caching
Previous Page
Pages in Current Topic/Section
1
2
Next Page
TCP/IP Address Resolution For IP Multicast Addresses
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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