If the A R P request is sent from a host to another network, then the router connecting the two networks can answer the request, this process is called a delegate A R, A r P agent (Proxyarp) . This can spoof the transmitting end of the A rp request, making it mistakenly think that the router is the destination host, and the destination host is "the other side" on the router. The function of the router is equivalent to the agent of the destination host, forward the packet from other hosts to it.
An example is the best way to explain the A R p agent. As shown in Figure 3 - 1 0, the system S u n is connected to two Ethernet. However, we also pointed out that this is not the case, please compare it with the inside 2 1. In S U N and subnet 1 4 0. 2 5 2. 1 actually presents a router, that is, the router having the A R p agent function makes S u n as in the subnet 1 4 0. 2 5 2. 1 The same is true. The specific placement is shown in Figure 4 - 6, the router Telebit Netblazer is named N E T B, between the subnet and the host S u n.
When the subnet 1 4 0. 2 5 2. 1 (referred to as G E m i N i) has a part of the I P Data report to 1 4 0. 2 5 2. 1. 2 9 When S u n, the G e m I n i compare the network number (1 4 0. 2 5 2) and subnet number (1) because they are all identical, so I send I in the Ethernet of Figure 4-6 P Address 1 4 0. 2 5 2. A r p request. The router N e t B identifies a coamenter host that belongs to it, so that its Ethernet interface address 1 4 0. 2 5 2. 1 is answered as the hardware address. Host g e m i n i transmits IP data by Ethernet to N e T B, N e t b forwards the dialog to S u n by dialing S1 P link. This process is transparent to all 1 4 0. 2 5 2. 1 subnet, the host S u n is actually configured behind the router N e t B.
Figure 4-6 Example of ARP Agent
If the A r p command is executed on the host G e m i n i, after the host S u n is communicated with the host S u n, we have found that the hardware addresses of the I P address mapped in the same subnet 1 4 0. 2 5 2. 1 are the same. This is usually used using a clue of delegate A r p. Gemini% ARP -A is a subnet 1 4 0. 2 5 2. 1 Other host output line NetB (140.252.1.183) at: 80: AD: 3: 6A: 80 Sun (140.252.1.29) AT 0: 80: AD: 3: 6A: 80 Figure 4 - 6 Another detail to explain is the following (SLIP link) under the router Netb (SLIP link) obviously missing an IP address. Why is there only one I P address on both ends of the dial S L i P link, but both ends between B S D i and S L i p, have an I P address? At 3.8, we have pointed out that the I f C O N f i g command can display the destination address of the dial S L I P link, which is 1 4 0. 2 5 2. 1. 1 8 3. N E T B L A Z e r does not need to know the I P address of each end of the dial S L I P link (so will use more I P addresses). Instead, it determines the dial-up host that is transmitted by grouping the serial line interface that is reached, so that each dial host connected to the router does not require unique I P addresses. All dial hosts use the same I P address 1 4 0. 2 5 2. 1. 1 8 3 as the destination address of the S L i P link.
The A R p agent can transmit the data report to the router S u n, but the subnet 1 4 0. 2 5 2. How is other hosts on 1 3? The circuitry must make the data to reach other hosts. It requires special processing here, and the entry in the selection path must be set in a certain place of the network 1 4 0. 2 5 2, the destination end of all datagrams is either a subnet 1 4 0. 2 5 2. 1 3, Either a host on the subnet, which points to the router Netb. The router N e T b knows how to report data to the final destination, that is, pass the router S u n. The A R p agent is also referred to as mixing A R p (P R O M I S C u O U S A R) or ARP Rent. These names come from other purposes of A RP proxy: the router between the two physical networks can hide the physical network to each other. In this case, the two physical networks can use the same network number, as long as the intermediate router is set to a A R p agent to respond to a network to another network host A r P request. This technique is used to hide a group of hosts running old version T c P / I P on different physical cables. Separate these old hosts There are two common reasons, one of which cannot handle subnet division, and the other of them uses old broadcast addresses (all host numbers 0, not all bit specifics for current use 1 The host number).
