Proxy ARP functions and principles

Proxy ARP is a variant of ARP. For computers that do not have a default gateway configured to communicate with computers in other networks, the gateway will use its MAC address to respond to the ARP request from the source computer from the IP address of the target computer. Proxy ARP is to respond to the ARP of another host. It can add a new Router without affecting the route table, making the subnet more transparent to the host. In addition to ARP spoofing and increasing ARP in a network segment, the most important thing is that the network topology cannot be summarized. Proxy ARP is generally used on a network without the default gateway and routing policy configured. Proxy ARP: A proxy ARP uses a host (usually a router) to respond to ARP requests from another device as a specified device. Working principle: This host A sends data packets to host D. The chart shows that host A uses A 16-bit mask. (Note this !) The destination CIDR block of host a is directly connected to 172.16.0.0. Therefore, host A directly sends an ARP request to the target site. Host A needs to obtain the MAC address of host D, so host A broadcasts ARP requests: host A broadcasts its MAC address as the source address FFFF. FFFF. FFFF as the destination address. However, the E0 port of the router does not support forwarding broadcast by default. Therefore, host D cannot respond to this ARP request. The router knows that host D is in another subnet, So it uses its MAC address to respond to the A00-00-0c-94-36-ab172.16.20.20000-00-0c-94-36-aa172.16.10.100 router with its own interface MAC address as the source address to reply ARP to host. This ARP response always uses unicast to reply. After receiving the ARP request, host A updates its MAC Address Table 172.16.000020000-00-0c-94-36-ab. If host A sends A packet to host D, it sends the data to MAC 00-00-0c-94-36-ab. The router forwards the data to host D. Therefore, data destined for subnet B is sent to the router. The ARP Address Table of all hosts in subnet A shows that the MAC address to subnet B is the MAC address of the router interface. This router forwards other data packets to subnet B. The ARP Address Table 172.16.000020000-large of host A is mapped to one MAC address. This indicates that proxy-arp is used on the vro. (Check the host's arp table.) Disable proxy ARP: The cisco switch should be configured to support proxy arp. It is enabled by default. If you want to disable it, use no ip proxy-arp to disable it in interface mode. Router # configure terminal Enter configuration commands, one per line. end with CNTL/Z. router (config) # interface ethernet 0 Router (config-if) # no ip proxy-arp Router (config-if) # ^ Z Router # What are the advantages of proxy ARP? The primary advantage is that a new router can be added to the network without affecting the route tables of other routers, in this way, the subnet change is transparent to the host. Which of the following negative effects of proxy ARP on the network where no default gateway is configured or no routing policy is configured on the host? 1. added ARP traffic for a certain network segment. the host needs a larger ARP table to map IP addresses to MAC addresses. security issues, such as ARP spoofing (spoofing) 4. it does not work for networks that do not use ARP to parse addresses. 5. cannot summarize and promote network topology FAQ: 1) What is ARP proxy? When the router receives an ARP Request and finds that the destination IP address to be queried is in different subnets, the router will assume the ARP role of the proxy and answer the question, tell the queryer about the MAC address (using the MAC address of the inter-Interface). 2) Why is there an ARP proxy? An important function of a vro is to restrict the LAN broadcast packets to this network, so as not to spread. Otherwise, a network storm may occur. ARP Request is a broadcast packet. If the Request object is in the same LAN, it will answer. But what if the query object is not in the same lan? To solve this problem, the router provides a service: proxy ARP. 3) Is there any way to solve cross-lan address query? If the host is configured with the default gateway, you can submit the query task to the default gateway when the MAC address object is located outside the LAN. 4) under what circumstances will ARP proxy be used? The host and router in the network have ARP cache. The host is usually configured with the default gateway, which queries the MAC address outside the LAN with the default gateway. When a vroarp needs to query the MAC address in a remote CIDR Block, the vro connected to it plays the role of proxy ARP when receiving the ARP Request. Additionally, ARP (Address Resolution Protocol) is a Protocol used to resolve an IP Address to an Ethernet MAC Address (or physical Address. In a LAN, when a host or other network device has data to send to another host or device, it must know the peer network layer address (that is, IP address ). However, IP addresses alone are not enough, because IP data packets must be encapsulated into frames to be sent through the physical network. Therefore, the sending station must also have the physical address of the receiving station, therefore, a ing from the IP address to the physical address is required. APR is the protocol that implements this function. Assume that host A and host B are in the same CIDR block. host a must send messages to host B. The specific address parsing process is as follows: (1) host A first checks its ARP table and determines whether it contains the ARP table corresponding to host B. If the corresponding MAC address is found, host A uses the MAC address in the ARP table to encapsulate the IP packet frames and send the packet to host B. (2) If host A cannot find the corresponding MAC address in the ARP table, the data packet will be cached and an ARP request packet will be sent in broadcast mode. The sending IP address and MAC address in the ARP request packet are the IP address and MAC address of host A, and the target IP address and the target MAC address are the IP address of host B and the whole 0 MAC address. As arp request packets are sent in broadcast mode, all hosts in the network segment can receive the request, but only the requested host (that is, host B) will process the request. (3) host B compares its own IP address and the target IP address in the ARP request packet. when both are processed as follows) the IP address and MAC address are stored in your ARP table. Then, an ARP response packet is sent to host A in Unicast mode, which contains its own MAC address. (4) After receiving the ARP response packet, host A adds the MAC address of host B to its ARP table for subsequent packet forwarding. At the same time, host A encapsulates the IP packet and sends it out. When host A and host B are not in the same CIDR Block, host A sends an ARP request to the Gateway. The destination IP address in the ARP request packet is the IP address of the gateway. When host A obtains the MAC address of the gateway from the received response message, it encapsulates the packet and sends it to the gateway. If the gateway does not have an ARP entry for host B, the gateway broadcasts an ARP request. The target IP address is the IP address of host B. After the gateway obtains the MAC address of host B from the response message, the gateway can send packets to host B. If the gateway already has an ARP entry for host B, the gateway directly sends the packets to host B.