Linux Network Administrator Guide, Second Edition

to be handed to vlager. This action can be accomplished by making it the default gateway for vstout:

# route add default gw vlager

The network name default is a shorthand for 0.0.0.0, which denotes the default route. The default route matches every destination and will be used if there is no more specific route that matches. You do not have to add this name to /etc/networks because it is built into route.

If you see high packet loss rates when pinging a host behind one or more gateways, this may hint at a very congested network. Packet loss is not so much due to technical deficiencies as to temporary excess loads on forwarding hosts, which makes them delay or even drop incoming datagrams.

Configuring a Gateway

Configuring a machine to switch packets between two Ethernets is pretty straightforward. Assume we're back at vlager, which is equipped with two Ethernet cards, each connected to one of the two networks. All you have to do is configure both interfaces separately, giving them their respective IP addresses and matching routes, and that's it.

It is quite useful to add information on the two interfaces to the hosts file as shown in the following example, so we have handy names for them, too:

172.16.1.1 vlager.vbrew.com vlager vlager-if1
172.16.2.1 vlager-if2

The sequence of commands to set up the two interfaces is then:

# ifconfig eth0 vlager-if1
# route add brew-net
# ifconfig eth1 vlager-if2
# route add wine-net

If this sequence doesn't work, make sure your kernel has been compiled with support for IP forwarding enabled. One good way to do this is to ensure that the first number on the second line of /proc/net/snmp is set to 1.

The PLIP Interface

A PLIP link used to connect two machines is a little different from an Ethernet. PLIP links are an example of what are called point-to-point links, meaning that there is a single host at each end of the link. Networks like Ethernet are called broadcast networks. Configuration of point- to-point links is different because unlike broadcast networks, point-to-point links don't support a network of their own.

PLIP provides very cheap and portable links between computers. As an example, we'll consider the laptop computer of an employee at the Virtual Brewery that is connected to vlager via PLIP. The laptop itself is called vlite and has only one parallel port. At boot time, this port will be registered as plip1. To activate the link, you have to configure the plip1 interface using the following commands:[33]

# ifconfig plip1 vlite pointopoint vlager
# route add default gw vlager

The first command configures the interface, telling the kernel that this is a point-to-point link, with the remote side having the address of vlager. The second installs the default route, using vlager as gateway. On vlager, a similar ifconfig command is necessary to activate the link (a route invocation is not needed):

# ifconfig plip1 vlager pointopoint vlite

Note that the plip1 interface on vlager does not need a separate IP address, but may also be given the address 172.16.1.1. Point-to-point networks don't support a network directly, so the interfaces don't require an address on any supported network. The kernel uses the interface information in the routing table to avoid any possible confusion.[34]

Now we have configured routing from the laptop to the Brewery's network; what's still missing is a way to route from any of the Brewery's hosts to vlite. One particularly cumbersome way is to add a specific route to every host's routing table that names vlager as a gateway to vlite:

# route add vlite gw vlager

Dynamic routing offers a much better option for temporary routes. You could use gated, a routing daemon, which you would have to install on each host in the network in order to distribute routing information dynamically. The easiest option, however, is to use proxy ARP (Address Resolution Protocol). With proxy ARP, vlager will respond to any ARP query for vlite by sending its own Ethernet address. All packets for vlite will wind up at vlager, which then forwards them to the laptop. We will come back to proxy ARP in the section 'Checking the ARP Tables'.

Current net-tools releases contain a tool called plipconfig, which allows you to set certain PLIP timing parameters. The IRQ to be used for the printer port can be set using the ifconfig command.

The SLIP and PPP Interfaces

Although SLIP and PPP links are only simple point-to-point links like PLIP connections, there is much more to be said about them. Usually, establishing a SLIP connection involves dialing up a remote site through your modem and setting the serial line to SLIP mode. PPP is used in a similar fashion. We discuss SLIP and PPP in detail in Chapter 7 and Chapter 8, The Point-to-Point Protocol.

The Dummy Interface

The dummy interface is a little exotic, but rather useful nevertheless. Its main benefit is with standalone hosts and machines whose only IP network connection is a dialup link. In fact, the latter are standalone hosts most of the time, too.

The dilemma with standalone hosts is that they only have a single network device active, the loopback device, which is usually assigned the address 127.0.0.1. On some occasions, however, you must send data to the 'official' IP address of the local host. For instance, consider the laptop vlite, which was disconnected from a network for the duration of this example. An application on vlite may now want to send data to another application on the same host. Looking up vlite in /etc/hosts yields an IP address of 172.16.1.65, so the application tries to send to this address. As the loopback interface is currently the only active interface on the machine, the kernel has no idea that