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Static ARP configuration

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Static ARP Configuration refers to setting up a host with predefined ARP entries that can't be changed. This is useful in preventing ARP poisoning and can be done at the operating system level.

Reading your ARP Tables

In linux, you can type the following :

root ~ # arp -n

To receive something like :

 Address                  HWtype  HWaddress           Flags Mask            Iface              ether   5c:da:d4:4d:85:b4   C                     ra0              ether   00:24:21:f5:fa:c4   C                     eth0

On a router. In this case, the router's ip addresses are:

 root ~ # ifconfig|grep inet
         inet addr:  Bcast:  Mask:
         inet6 addr: fe80::92e6:baff:fe76:243a/64 Scope:Link
         inet addr:  Mask:
         inet6 addr: ::1/128 Scope:Host
         inet addr:  Bcast:  Mask:
         inet6 addr: fe80::226:82ff:fe20:6559/64 Scope:Link

As we all know, localhost is, so its routing traffic for the subnet reserved in RFC 1918 and then pushing it through the gateway. You can see this by typing:

 root ~ # route -n

Which will give you:

 Kernel IP routing table
 Destination     Gateway         Genmask         Flags Metric Ref    Use Iface   U     1      0        0 eth0   U     2      0        0 ra0     U     1000   0        0 eth0         UG    0      0        0 ra0

In any case, we can see that the previous output of `arp -n`:

 Address                  HWtype  HWaddress           Flags Mask            Iface              ether   5c:da:d4:4d:85:b4   C                     ra0              ether   00:24:21:f5:fa:c4   C                     eth0

Shows the hardware address or MAC address of the gateway and a client on the subnet.



Proxy ARP (Address Resolution Protocol) is a technique by which a device on a given network answers the ARP queries for a network address that is not on that network. The ARP Proxy is aware of the location of the traffic's destination, and offers its own MAC address in reply, effectively saying, "send it to me, and I'll get it to where it needs to go." Serving as an ARP Proxy for another host effectively directs LAN traffic to the Proxy. The "captured" traffic is then typically routed by the Proxy to the intended destination via another interface or via a tunnel.

The process which results in the node responding with its own MAC address to an ARP request for a different IP address for proxying purposes is sometimes referred to as 'publishing'.A Proxy ARP Simulation visualizes how a router responds to ARP request on behalf of the target host at different networks. See ARP cache updates at host, router, remote host.


Below are some typical uses for proxy ARP:

Joining a broadcast LAN with serial links (e.g., dialup or VPN connections).
Assume an Ethernet broadcast domain (e.g., a group of stations connected to the same hub) using a certain IPv4 address range (e.g.,, where - are assigned to wired nodes). One or more of the nodes is an access router accepting dialup or VPN connections. The access router gives the dial-up nodes IP addressses in the range -; for this example, assume a dial-up node gets IP address
The access router uses Proxy ARP to make the dial-up node present in the subnet without being wired into the Ethernet: the access server 'publishes' its own MAC address for Now, when another node wired into the Ethernet wants to talk to the dial-up node, it will ask on the network for the MAC address of and find the access server's MAC address. It will therefore send its IP packets to the access server, and the access server will know to pass them on to the particular dial-up node. All dial-up nodes therefore appear to the wired Ethernet nodes as if they are wired into the same Ethernet subnet.
Taking multiple addresses from a LAN
Assume a station (e.g., a server) with an interface ( connected to a network ( Certain applications may require multiple IP addresses on the server. Provided the addresses have to be from the range, the way the problem is solved is through Proxy ARP. Additional addresses (say, are aliased to the loopback (RFC 1918) interface of the server (or assigned to special interfaces, the latter typically being the case with VMware/UML/jails/vservers/other virtual server environments) and 'published' on the interface (although many operating systems allow direct allocation of multiple addresses to one interface, thus eliminating the need for such tricks).
On a firewall
In this scenario a firewall can be configured with a single IP address. One simple example of a use for this would be placing a firewall in front of a single host or group of hosts on a subnet. Example- A network ( has a server which should be protected ( a proxy-arp firewall can be placed in front of the server. In this way the server is put behind a firewall without making any changes to the network at all.
In case of Mobile-IP the Home Agent uses ProxyARP in order to receive messages on behalf of the Mobile Node, so that it can forward the appropriate message to the actual mobile node's address (Care Of Address).
Transparent subnet gatewaying
A setup that involves two physical segments sharing the same IP subnet and connected together via a router. This use is documented in RFC 1027.


The advantage of Proxy ARP over other networking schemes is simplicity. A network can be extended using this technique without the knowledge of the upstream router.

For example, suppose a host, say A, wants to contact another host B, where B is on a different subnet/broadcast domain than A. For this, host A will send an ARP request with a Destination IP address of B in its ARP packet. The multi-homed router which is connected to both the subnets, responds to host A's request with its MAC address instead of host B's actual MAC address, thus proxying for host B. In the due course of time, when host A sends a packet to the router which is actually destined to host B, the router just forwards the packet to host B. The communication between host A and B is totally unaware of the router proxying for each other.


Disadvantage of Proxy ARP include scalability (ARP resolution is required for every device routed in this manner) and reliability (no fallback mechanism is present, and masquerading can be confusing in some environments). ARP manipulation techniques, however, are the basis for protocols providing redundancy on broadcast networks (e.g., Ethernet), most notably CARP and Virtual Router Redundancy Protocol.

Proxy ARP can create DoS attacks on networks if misconfigured. For example a misconfigured router with proxy ARP has the ability to receive packets destined for other hosts (as it gives its own MAC address in response to ARP requests for other hosts/routers), but may not have the ability to correctly forward these packets on to their final destination, thus blackholing the traffic.

Further reading

  • RFC 925 - Multi-LAN Address Resolution
  • RFC 1027 - Using ARP to Implement Transparent Subnet Gateways
  • W. Richard Stevens - The Protocols (TCP/IP Illustrated, Volume 1). Addison-Wesley Professional; 1st edition (December 31, 1993). ISBN 0-201-63346-9