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(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 As broadcast.) It will forward the message sent by A to D on Ds 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
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.
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Version 3.0 - Version Date: September 20, 2005
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