Olaf Kirch - Linux Network Administrator Guide, Second Edition

# route add brew-net

Now that you've finished the basic configuration steps, we want to make sure that your Ethernet interface is indeed running happily. Choose a host from your Ethernet, for instance vlager , and type:

# ping vlager

PING vlager: 64 byte packets

64 bytes from 172.16.1.1: icmp_seq=0. time=11. ms

64 bytes from 172.16.1.1: icmp_seq=1. time=7. ms

64 bytes from 172.16.1.1: icmp_seq=2. time=12. ms

64 bytes from 172.16.1.1: icmp_seq=3. time=3. ms

^C

--vstout.vbrew.com PING Statistics-- 4 packets transmitted, 4 packets received, 0

round-trip (ms) min/avg/max = 3/8/12

If you don't see similar output, something is broken. If you encounter unusual packet loss rates, this hints at a hardware problem, like bad or missing terminators. If you don't receive any replies at all, you should check the interface configuration with netstat described later in "The netstat Command". The packet statistics displayed by ifconfig should tell you whether any packets have been sent out on the interface at all. If you have access to the remote host too, you should go over to that machine and check the interface statistics. This way you can determine exactly where the packets got dropped. In addition, you should display the routing information with route to see if both hosts have the correct routing entry. route prints out the complete kernel routing table when invoked without any arguments ( -n just makes it print addresses as dotted quad instead of using the hostname):

# route -n

Kernel routing table Destination Gateway Genmask Flags Metric Ref Use Iface

127.0.0.1 * 255.255.255.255 UH 1 0 112 lo

172.16.1.0 * 255.255.255.0 U 1 0 10 eth0

The detailed meaning of these fields is explained later in "The netstat Command". The Flags column contains a list of flags set for each interface. U is always set for active interfaces, and H says the destination address denotes a host. If the H flag is set for a route that you meant to be a network route, you have to reissue the route command with the -net option. To check whether a route you have entered is used at all, check to see if the Use field in the second to last column increases between two invocations of ping.

In the previous section, we covered only the case of setting up a host on a single Ethernet. Quite frequently, however, one encounters networks connected to one another by gateways. These gateways may simply link two or more Ethernets, but may also provide a link to the outside world, such as the Internet. In order to use a gateway, you have to provide additional routing information to the networking layer.

The Ethernets of the Virtual Brewery and the Virtual Winery are linked through such a gateway, namely the host vlager . Assuming that vlager has already been configured, we just have to add another entry to vstout 's routing table that tells the kernel it can reach all hosts on the Winery's network through vlager . The appropriate incantation of route is shown below; the gw keyword tells it that the next argument denotes a gateway:

# route add wine-net gw vlager

Of course, any host on the Winery network you wish to talk to must have a routing entry for the Brewery's network. Otherwise you would only be able to send data to the Winery network from the Brewery network, but the hosts on the Winery would be unable to reply.

This example describes only a gateway that switches packets between two isolated Ethernets. Now assume that vlager also has a connection to the Internet (say, through an additional SLIP link). Then we would want datagrams to any destination network other than the Brewery 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 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.

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] Note that pointopoint is not a typo. It's really spelled like this.

# 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] As a matter of caution, you should configure a PLIP or SLIP link only after you have completely set up the routing table entries for your Ethernets. With some older kernels, your network route might otherwise end up pointing at the point-to-point link.