Traditional IPv4 Routing
You should now understand the fundamental thought behind traditional IPv4 routing. However, at this point, you need to step back and look into the methods of implementation. First consider UNIX commands. These commands are traditional both in function and in age. Once you have learned how to use them under one UNIX, including Linux, you will know essentially how to use them in any UNIX. This section details how to use these commands under Linux to configure a basic two-network router with a single default route. You will also add additional specific routes to illustrate the meaning behind the fundamental thought of traditional routing.
The two primary utilities used in configuring IPv4 routing under UNIX are ifconfig and route. You will step through the use of both of these utilities to configure a simple dual connected router under Linux.
The ifconfig utility has been used with UNIX systems for quite some time and is the original utility for configuring network interfaces under UNIX. The utility is appropriately named InterFace CONFIGuration, or ifconfig. On many UNIXes as well as Linux, ifconfig supports multiple protocol suites, or address families. The complete syntax and supported address families can be found in the man pages. This book is only concerned with the inet family, which is the address family for IPv4. There is also an inet6 family for IPv6 but we will not cover that usage in this book. In Chapter 9, "IPv6," you will learn how the ip utility is used for Policy Routing in IPv6.
The basic ifconfig syntax for an IPv4 interface under Linux is as follows:
ifconfig interface IP-address netmask netmask broadcast broadcast
The italicized words will be replaced with the relevant information to configure the interface.
On most UNIXes you can find out the names of the available interfaces through ifconfig by asking it to list all of the interfaces it knows about. Type ifconfig -a and you will see a list of all the interfaces on the system, regardless of whether they are configured. You can also call ifconfig with the name of a single interface to obtain information about that interface only. This feature helps when you are on a strange system and need some quick information.
In UNIX there is an IP interface that should be present whenever the system is enabled for TCP/IP communication. This is the loopback interface, usually specified by the name lo or lo0. By conventional standard this interface has the IPv4 address 127.0.0.1/8 assigned to it. This interface is very important to the correct operation of the TCP/IP networking subsystems. In Chapters 28, as the details of the Linux Policy Routing structures are illustrated and then exemplified, you will see that this interface has a myriad of uses.
The other interfaces that may be on the system will have names that vary according to the UNIX flavor. I will use the Linux conventions herein and refer primarily to using Ethernet (ethxx) and Token Ring (trxx) interfaces.
Consider a router that has one Ethernet interface, eth0, and one Token Ring interface, tr0. The appropriate information is as follows:
eth0 - 192.168.1.254/24 tr0 - 10.1.1.254/24 Default Gateway - 10.1.1.1/24
Two networks are represented here, and from the default gateway address you can deduce that the Ethernet network is a stub network off of the main network. The Token Ring could be considered the backbone network if you prefer that terminology because it contains the default gateway.
To set up the interfaces using ifconfig on a typical Linux system, do the following from the command line:
ifconfig eth0 192.168.1.254 netmask 255.255.255.0 broadcast 192.168.1.255 ifconfig tr0 10.1.1.254 netmask 255.255.255.0 broadcast 10.1.1.255
You could then list the interfaces and get information about them using the straight ifconfig command by itself.
Now you have a router with three IPv4 configured interfaces. Do not forget about the loopback interface when counting the interfaces! You should now set up your router to actually be able to route between the locally connected networks. In order for the router to do this it needs to have all three of the locally connected networks in the routing table. Here you call upon the route command to populate the routing table.
In general usage the route command is called as follows:
route add -net|host IPv4 Address netmask netmask gw gateway dev interface
Note that there are many other options that can be specified to the route command. Many of the options are UNIX flavor specific. The preceding options are fairly general and should work on most UNIXes. Also the route command, when issued by itself on the command line, will give different outputs depending on the UNIX flavor. In Linux, issuing a route command by itself will dump the main kernel routing table. However, on most UNIXes you will need to use the netstat -r command in order to dump the routing table.
Linux 2.0 Versus Linux 2.2 Routing Changes
In Linux there are some fundamental differences in the IPv4 configuration process between the 2.0 series of kernels and the 2.1 and newer kernels. When configuring an interface for IPv4 using ifconfig under a 2.0 series kernel, only the interface is configured. But the kernel does not know how to get to this address or to the network it defines. You have to add a network route manually in order for the kernel to see where to send the packet.
In kernel 2.1 and newer, the situation is different. When Alexey Kuznetsov rewrote the network routing code for Linux during the 2.1 development kernel series, he added automatic route creation for directly connected networks. Thus, if you run the same ifconfig configuration command on a 2.1 or higher series kernel, you will be able to connect immediately to the network defined by the interface.
Having the route automatically added by the kernel helps users unfamiliar with networking immensely in normal operation of a Linux network. If you want to configure an interface, the kernel will automatically create a connection to the interface address and network. Thus, as soon as you are done configuring the interface, you are ready to use it. However, for advanced usage this can become a nuisance.
For advanced usage you can override this behavior by explicitly configuring the interface with the true host address, which is a netmask of all ones. If you want to have the same behavior in 2.1 and higher that you had in 2.0 for ifconfig, use ifconfig interface IPv4 Address netmask 255.255.255.255 broadcast broadcast, and the automatic route will not be created.
Now that you have seen the syntax for basic routing configuration, it's time to apply it to the router configured in the ifconfig section.
route add -net 127.0.0.0 netmask 255.0.0.0 dev lo route add -net 192.168.1.0 netmask 255.255.255.0 dev eth0 route add -net 10.1.1.0 netmask 255.255.255.0 dev tr0
Note that the loopback interface route is explicitly included here for completeness. You did not see the actual ifconfig statement in the previous section; it is left as an exercise for the reader to determine what command was needed. Note that in all cases where you would want to ensure that your system is correctly configured, you will want to configure all the interfaces, including the loopback interface, manually or through your own set of scripts. Under Linux, and in UNIX in general, any system interface including loopback is not automatically configured.
The router at this point is ready to route all traffic between the 192.168.1.0/24 network and the 10.1.1.0/24 network. But if there is any traffic from either of those networks with a destination address that does not belong to either 192.168.1.0/24 or 10.1.1.0/24, the router will return an error. This is because it has no default route at this point in time.
The subject of default routes contains an explicit security concern. Essentially, a default route enables a router to trust all traffic to another router. In many cases this may not be a necessary or wise decision. For example, in the network structure considered here there may only be one or two other networks of interest. Alternatively, you may not want the systems on 192.168.1.0/24 to see any other networks. The route setup at this point would ensure, in the absence of source-routing capabilities, that the systems on 192.168.1.0/24 would only be able to talk externally to systems on 10.1.1.0/24. When we consider the theory of Policy Routing in Chapters 2 and 3 and the actual implementation usage in Chapters 58, you will see various methods of considering this fundamental security problem.
But to consider standard and traditional usage you should install a default route into your router. This default route provides your router with a router to which all unknown destination addressed traffic should be forwarded. In this case you have a gateway that has been specified for this route. You can then add it in with the following command:
route add -net 0.0.0.0 netmask 0.0.0.0 gw 10.1.1.1
Note that under Linux and under many other UNIXes this command can also be expressed as route add default gw 10.1.1.1, where the default keyword stands for the -net 0.0.0.0 netmask 0.0.0.0 parts of the given example.
You now have a router that will route traffic between the 192.168.1.0/24 and 10.1.1.0/24 networks and will also provide connectivity to any other network destination from the 192.168.1.0/24 network. Of course that does not mean that those networks can get back to the 192.168.1.0/24 network, but that is a different problem.