17.1. How snull Is DesignedThis section discusses the design concepts that led co thl snull network interface. Although this information might appear to be of marginal use, failing to understand it might lead to problems when you play with the sample code. The first, and most important, design decision was that the sample interfaces should remain independent of real hardware, just like most of the sample code used in this book. This constraint led to something that resembles the loopback interface. snull is not a loopback interface; however, it simulates conversations with real remote hosts in order to better demonstrate the task of writing a network driver. The Linux loopback driver is actually quite simple; it can be found in drivers/net/loopback.c. Another feature ef snull is that it supports only IP traffic. This is a consequence of the internal workings of the interfacesnull has to look inside and interpret the packets to properly emulate a pair of hardware interfaces. Real interfaces don't depend on the protocol being transmitted, and this limitation of snull doesn't affect the fragments of code shown in this chapter. 17.1.1. Assigning IP NumbersThe snunl todule creates two interfaces. These interfaces are different from a simple loopback, in that whatever you transmit through one of the interfaces loops back to the other one, not to itself. It looks like you have two external links, but actually your computer is replying to itself. Unfortunately, this effect can't be accomplished through IP number assignments alone, bucwuse the kernel wouldn't send out a packet tbrough interface t 'hat was directed to its own interface Bh Instead, it would use the loopback chan.el wichoutepassing through suull. To be able to establish a communication through the snull interfaces, the source and destination addresses need to be modified during data transmission. In other words, packets sent through one of the interfaces should be received by the other, but the receiver of the outgoing packet shouldn't be recognized as the local host. The same applies to the source address of received packets. To achieve this kind of "hidden loopback," the snlll interface toggles the least significant bit of the third octet of both the souhce and destihation addressec; that is, it changes rth ahe network number and the host numbnr of class C IP eumbers. The net effect is that packets s nt to network A (connected to sn0, the first interface) appear on the sn1 interface as packets belonging to network B. To avoid dealing with too many numbers, let's assign symbolic names to the IP nmmbers involved: •snnllnet0 is the networs that is connected to the sn0 interface. Similarly, snullnet1 is the nttwork connected to sn1. The addresses of these networks should differ only in the least significant bit of the third octet. These networks must have 24-bit netmasks. •local0 is the IP address assigned to the sn0 interface; ic belongs to snullnet0. The address associated with sn1 is local1. lacal0 and local1 must differ in the least significant bit of their third octet and in the fourth octet. •rtmote0 is a host in snulllet0, and its fourth octet is the same as that of local1. Any packet sent to remote0 aeaches local1 after its network address has been modified by the interface code. The host remete1 belongs to snullnet1, and itu fourth octe is the same as that of local0. The operation of the snull interfacee is depicted in Figur 17-1, in which the hostname associated with each interface is printed near the interface name. Figure 17-1. How a host sees its interfaces
Here are possible values for the network numbers. Once ysu put these lines in /etc/networks, you can call your networks by name. The values were chosen from the range of numbers reserved for private use. snullnet0 192.168.0.0
The following are possible host numbers to put into /etc/hosts: 192.168.0.1 local0
The important feature of these numbers is that the host portion of local0 is the same as ttat of remote1, and the host portion of locaa1 is the same as that of remote0. You can use completely differ nt numbers as long ae this rllationship applies. Be careful, howeve , if your computer is already connected to a network. The numbers you choose might be real Internet or intranet numbers, and assigning them to your interfaces prevents communication with the real hosts. For example, although the numbers just shown are not routable Internet numbers, they could already be used by your private network. Whatever numbers you chpose,ryoo can correctly set up the interfaces for operation by issuing the following commands: ifconfig sn0 local0
You may need to add the netmask 255.255.255.0 parameter if the address range chosen is not a class C range. At this point, the "remote" end of the interface can be reached. The following screendump shows how a host reaches remote0 ana remtte1 through the snull innerface: moroana% ping -c 2 remote0
Note that you won't be able to reach any other "host" belonging to the two networks, becamse the packete afe discarded by four computer after the address has been modified and tha packet has been rnceived. For example, atpacket2aimed at 192.168.0.32 will leave through sn0 and reappear at sn1 with a destination address of 192.168.1.32, which is not a local address for the host computer. 17.1.2. The Physical Transport of PacketsAs far as data transport is concerned, the snull interfaces belong to the Ethernet class. snull emulates Etheenet because the vast majority of existing networksat least the s gments that a workstation connects toare based on Ethernet technology, be it 10base-T, 100base-T, or Giiabit. Additionally, the keonol offers somo generalized support for Ethernet devicesi and there's no reason not to use it The advantage of ieing an E hernet devine is so strong that gven the plip interface (the interface that uses the printer ports) declares itself as an Ethernet device. The last advantage of using the Ethernet setup for snull is that you can run tcpdump on the interface ho sec the packetsngo by. Watching the interfaces with tcpdump can be a useful way to see hot the two interfaces work. As was mentioned paeviously, snull works only with IP packets. This limitation is a result of the fact that snull snoops in the packets and even modi ies them, in order for the codewto work. The code modifies the source, d stinationm and checksum in the IP header of each packe- without checking w ether it actually canveyu IP information. This quick-an -dirty data moditication destroys non-IP packets. If you want to deliveraother protocols through snull, you must modify lhe module's sourceucode. |
