Next: Routing through a Gateway
Up: Interface Configuration for IP
Previous: The Loopback Interface
Configuring an Ethernet interface goes pretty much the same as
with the loopback interface, it just requires a few more parameters
when you are using sub-netting.
At the Virtual Brewery, we have sub netted the IP network, which was
originally a class-B network, into class-C subnetworks. To make the
interface recognize this, the ifconfig incantation would look
like this:
# ifconfig eth0 vstout netmask 255.255.255.0
This assigns the eth0 interface the IP-address of vstout
(191.72.1.2). If we had omitted the netmask, ifconfig
would have deduced the the netmask from the IP-network class, which
would have resulted in a netmask of 255.255.0.0.
Now a quick check shows:
# ifconfig eth0
eth0 Link encap 10Mps Ethernet HWaddr 00:00:C0:90:B3:42
inet addr 191.72.1.2 Bcast 191.72.1.255 Mask 255.255.255.0
UP BROADCAST RUNNING MTU 1500 Metric 1
RX packets 0 errors 0 dropped 0 overrun 0
TX packets 0 errors 0 dropped 0 overrun 0
You can see that ifconfig automatically set the broadcast
address (the Bcast field above) to the usual value, which is
the hosts network number with the host bits all set. Also, the message
transfer unit (the maximum size of Ethernet frames the kernel will
generate for this interface) has been set to the maximum value of 1500
bytes. All these values can be overridden with special options that
will be described later.
Quite similar to the loopback case, you now have to install a routing
entry that informs the kernel about the network that can be reached
through eth0. For the Virtual Brewery, you would invoke
route as
# route add -net 191.72.1.0
At first, this looks a little like magic, because it's not really
clear how route detects which interface to route through.
However, the trick is rather simple: the kernel checks all interfaces
that have been configured so far and compares the destination address
(191.72.1.0 in this case) to the network part of the interface
address (that is, the bitwise and of the interface address and the
netmask). The only interface that matches is eth0.
Now, what's that -net option for? This is used because
route can handle both routes to networks and routes to single
hosts (as you have seen above with localhost). When being given an
address in dotted quad notation, it attempts to guess whether it is a
network or a hostname by looking at the host part bits. If the address'
host part is zero, route assumes it denotes a network, otherwise
it takes it as a host address. Therefore, route would think that
191.72.1.0 is a host address rather than a network number,
because it cannot know that we use sub-netting. We therefore have to tell
it explicitly that it denotes a network, giving it the -net
flag.
Of course, the above route command is a little tedious to type,
and it's prone to spelling mistakes. A more convenient approach is to
use the network names we have defined in /etc/networks above.
This makes the command much more readable; even the -net flag
can now be omitted, because route now knows that
191.72.1.0 denotes a network.
# route add brew-net
Now that you've finished the basic configuration steps, we want to
make sure 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 191.72.1.1: icmp seq=0. time=11. ms
64 bytes from 191.72.1.1: icmp seq=1. time=7. ms
64 bytes from 191.72.1.1: icmp seq=2. time=12. ms
64 bytes from 191.72.1.1: icmp seq=3. time=3. ms
^C
----vstout.vbrew.com PING Statistics----
4 packets transmitted, 4 packets received, 0% packet loss
round-trip (ms) min/avg/max = 3/8/12
If you don't see any output similar to this, then something is broken,
obviously. If you encounter unusual packet loss rates, this hints at
a hardware problem, like bad or missing terminators, etc. If you
don't receive any packets at all, you should check the interface
configuration with netstat. 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,
too. In 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 (the -n option only makes it print
addresses as dotted quad instead of using the hostname):
# route -n
Kernel routing table
Destination Gateway Genmask Flags Metric Ref Use
127.0.0.1 * 255.255.255.255 UH 1 0
191.72.1.0 * 255.255.255.0 U 1 0
The detailed meaning of these fields is explained below in
section-
. The Flag 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, then you have to specify the -net option with the
route command. To check whether a route you have entered is used
at all, check if the Use field in the second to last column
increases between two invocations of ping.
Next: Routing through a Gateway
Up: Interface Configuration for IP
Previous: The Loopback Interface
Andrew Anderson
Thu Mar 7 23:22:06 EST 1996
