What is Proxy ARP in Networking?

Introduction

If Someone is interested in networking, they must have heard the term Proxy ARP. But what does it really mean, and how does it work? Proxy ARP is a technique that allows a device to respond to the ARP requests on behalf of another device. This can be useful in situations where the devices want to communicate but are separated from the same network segment, i.e., on different subnets. In this blog, we will explain what Proxy ARP is, its purpose, how it works, along with its advantages and disadvantages.

Let’s begin by first understanding what a Proxy ARP is.

What is Proxy ARP?

Proxy ARP or Proxy Address Resolution Protocol is an extended version of ARP in which a device (usually a router) replies to the ARP requests for a given IP address that is not part of the local network. The router acts as a proxy for the destination device to which the host wants to communicate and provides its own MAC address as the reply. The sender then sends the packet to the router’s MAC address, and from there, it will be forwarded to the actual destination.

Let’s understand with the help of an example.

Suppose there is a device named A that wants to send a packet to another device named B. In order to send a packet, device A needs to know the MAC address of the device B. For this, device A broadcasts an ARP request on the network asking for a specific IP address (10.1.1.1). Device B, if it has the IP address, responds with an ARP reply by informing that it has an IP address (10.1.1.1) and MAC address (0053.FFFF.9999).

But as we all know, ARP only functions within the same subnet. If devices A and B are on different subnets, they need to use a router to communicate. Device A sends the packet to its default gateway (the router), which then forwards it to device B’s subnet. In this case, device A does not need to know device B’s MAC address, but only its default gateway’s MAC address.

Note: Proxy ARP can help devices on a network reach remote subnets without the need to configure routing or a default gateway.

Purpose of Proxy ARP

One of the purposes of Proxy ARP is to allow communication between devices that are not part of the same subnet but have incorrect (incorrect subnet masks) or incomplete (missing default gateways) routing information.

It can also be used in order to provide connectivity for devices that do not have any routing intelligence or do not support routing protocols.

Further, it can be useful in different scenarios. Some of these are:

• The router that is used to perform NAT can make use of Proxy ARP to reply to the ARP requests from devices that are part of the private network for public IP addresses. With the help of this, the devices that are part of the private network do not need to know the public IP address of the router or have a default gateway configured.
• Proxy ARP can also be used to extend the reach of a network without adding more routers or switches.
• With the help of Proxy ARP, hosts can be connected that use different network protocols, such as IPv4 and IPv6.

Now that we have a basic understanding of Proxy ARP along with its different purposes. Let’s now discuss how Proxy ARP works.

How Does Proxy ARP work?

Below, we have explained the functioning of Proxy ARP with the help of a topology.

In the above topology, we have two subnets, i.e., 200.10.2.0/24 and 200.10.1.0/24. There is a router in the middle connected to both of these subnets. On the bottom side, we have two hosts (PC0 and PC1); on the top side, we have a server named S1.

• In the beginning phase, PC0 tries to communicate with S1; it compares its IP address and subnet mask to S1’s IP address and decides that S1 is another subnet. After this, PC0 sends the packet to its default gateway having IP address 200.10.2.254/24 on subnet 200.10.2.0/24. PC0 sends an ARP request for 200.10.2.254, and in return, the router sends a reply with its MAC address on the interface FastEthernet 0/0.
• When PC1 tries to communicate with S1 the same way as PC0 does, it compares its IP address and subnet mask to S1’s IP address and decides that S1 is in the same subnet. PC1 then tries to send an ARP request for 200.10.1.100, which is S1’s IP address. But, as we know, S1 is not on the same subnet as 200.10.2.0/24, therefore routers do not forward broadcast traffic. This is the reason the ARP request never reaches S1.

This is where Proxy ARP comes into action.

• With Proxy ARP, the router sees the ARP request from PC1 on subnet 200.10.2.0/24 and sees that it is an ARP request for something in subnet 200.10.1.0/24. The router knows how to reach the subnet 200.10.1.0/24 and decides to respond to the ARP request on behalf of S1.
• To this, the router sends an ARP reply to PC1 with its MAC address on interface FastEthernet 0/0, pretending that it is S1’s Mac address.
• PC1 receives the ARP reply and then updates its ARP table with an entry for 200.10.1.100 with the router’s MAC address.
• Once the ARP table is updated, PC1 then sends the packet for S1 to the router’s MAC address.
• After receiving the packet from PC1, the router forwards it to S1 using its interface FastEthernet 0/1.
• In return, S1 receives the packet from PC1 and sends a reply back to PC1 using its default gateway.
• The router receives the reply from S1 and forwards it to PC1 using its interface FastEthernet 0/0.
• PC1 receives the reply from S1, and the communication is complete.

We now have a detailed knowledge of how it operates. Let’s now discuss its advantages and disadvantages.

Advantages of Proxy ARP

One of the main advantages of Proxy ARP is that it can easily be added to a single router on a network and doesn’t affect the routing tables of other routers.

Below, we have discussed some other advantages of it.

• By automating the configuration of subnet masks and default gateways, on each device it simplifies network setup and management.
• It ensures compatibility with devices that lack support for routing protocols or dynamic address allocation.
• It facilitates communication between devices that have conflicting or overlapping IP addresses.

Apart from all the advantages that we have discussed, Proxy ARP also has some drawbacks. Let’s understand them in detail.

Disadvantages of Proxy ARP

The main disadvantage of Proxy ARP is that it can lead to security and performance issues on the network. Some of these are discussed below.

• It can lead to increased network congestion due to ARP requests and replies.
• It poses a security risk by making the network vulnerable to ARP spoofing attacks. In attacks, malicious devices can impersonate proxies. Intercept or modify traffic between devices.
• It may introduce confusion and inconsistency into the network’s topology. Addressing scheme by concealing device locations and identities.
• It may cause complications for applications and protocols that rely on IP, MAC mapping, or subnet information.

Frequently Asked Questions

Q1. Should I disable proxy ARP?

Yes, disabling proxy ARP is necessary for security reasons. One can intercept traffic on your network with proxy ARP enabled. The command for disabling Proxy ARP is no ip proxy-arp.

Q2. How do I enable IP proxy ARP?

In order to enable IP proxy ARP, one can use the command ‘ip proxy-arp’ in interface configuration mode. With the help of this command, the router will respond to ARP requests on behalf of other hosts.

Q3. What is ARP used?

ARP, or Address Resolution Protocol, is mainly used to map network layer addresses to link layer addresses. With the help of ARP, different devices can communicate on the same network.

Q4. What is the difference between proxy ARP and gratuitous ARP?

Proxy ARP is when a device responds to ARP requests on behalf of another device. Gratuitous ARP is a broadcast request in order to get information regarding the IP address of the router.

Conclusion

In this blog, we have explained how proxy ARP works. It is a technique that allows communication between two devices on different subnets. We also have discussed the purpose, advantages, disadvantages, and functioning of Proxy ARP. We understand that it can be helpful in some situations, but along with that, it also has some drawbacks. Hence, it is always suggested to use it whenever necessary.