Fedora™ Unleashed, 2008 edition

Change the broadcast	broadcast [address]	ifconfig eth0 broadcast 10.10.10.255
Take interface down	down	ifconfig eth0 down
Bring interface up	up (add IP address)	ifconfig eth0 up (ifconfig eth0 10.10.10.10)
Set NIC promiscuous	[-]promisc [ifconfig eth0 -promisc]	ifconfig eth0 promisc mode on [off]
Set multicasting mode	[-]allmulti	ifconfig eth0_on [off] allmulti [ifconfig eth0 - allmulti]
Enable [disable] [address]	[-]pointopoint eth0_pointopoint	ifconfig_point-to-point address 10.10.10.20 [ifconfig eth0 pointopoint_10.10.10.20]

The ifconfig man page shows other options that enable your machine to interface with a number of network types such as AppleTalk, Novell, IPv6, and others. Again, read the man page for details on these network types.

NOTE

Promiscuous mode causes the NIC to receive all packets on the network. It is often used to sniff a network. Multicasting mode enables the NIC to receive all multicast traffic on the network.

If no argument is given, ifconfig displays the status of active interfaces. For example, the output of ifconfig, without arguments and one active and configured NIC, looks similar to this:

# ifconfig
eth0 Link encap:Ethernet HWaddr 00:30:1B:0B:07:0D
     inet addr:192.168.0.7 Bcast:192.168.0.255 Mask:255.255.255.0
     UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
     RX packets:127948 errors:0 dropped:0 overruns:0 frame:0
     TX packets:172675 errors:0 dropped:0 overruns:0 carrier:0
     collisions:7874 txqueuelen:100
     RX bytes:19098389 (14.2 Mb) TX bytes:73768657 (70.3 Mb)
     Interrupt:11 Base address:0x2000
lo   Link encap:Local Loopback
     inet addr:127.0.0.1 Mask:255.0.0.0
     UP LOOPBACK RUNNING MTU:16436 Metric:1
     RX packets:215214 errors:0 dropped:0 overruns:0 frame:0
     TX packets:215214 errors:0 dropped:0 overruns:0 carrier:0
     collisions:0 txqueuelen:0
     RX bytes:68739080 (65.5 Mb) TX bytes:68739080 (65.5 Mb)

The output is easily understood. The inet entry displays the IP address for the interface. UP signifies that the interface is ready for use, BROADCAST denotes that the interface is connected to a network that supports broadcast messaging (ethernet), RUNNING means that the interface is operating, and LOOPBACK shows which device (lo) is the loopback address. The maximum transmission unit (MTU) on eth0 is 1500 bytes. This determines the size of the largest packet that can be transmitted over this interface (and is sometimes 'tuned' to other values for performance enhancement). Metric is a number from 0 to 3 that describes how much information from the interface is placed in the routing table. The lower the number, the smaller the amount of information.

The ifconfig command can be used to display information about or control a specific interface using commands as listed in Table 14.1. For example, to deactivate the first ethernet device on a host, use the ifconfig command, the interface name, and the command down, as follows:

# ifconfig eth0 down

You can also configure and activate the device by specifying a hostname or IP address and network information. For example, to configure and activate ('bring up') the eth0 inter face with a specific IP address, use the ifconfig command like this:

# ifconfig eth0 192.168.0.9 netmask 255.255.255.0 up

If you have a host defined in your system's /etc/hosts file (see the section 'Network Configuration Files' later in this chapter), you can configure and activate the interface according to the defined hostname, like this:

# ifconfig eth0 dogdog.hudson.com up

Read the next section to see how to configure your system to work with your LAN.

/sbin/route

The second command used to configure your network is the route command. It is used to build the routing tables (in memory) implemented for routing packets as well as displaying the routing information. It is used after ifconfig has initialized the interface. The route command is normally used to set up static routes to other networks via the gateway or to other hosts. The command configuration is like this:

# route [options] [commands] [parameters]

To display the routing table, use the route command with no options. The display will look similar to this:

# route
Kernel IP routing table
Destination   Gateway       Genmask         Flags Metric Ref Use Iface
149.112.50.64 *             255.255.255.192 U     0      0   0   eth0
208.59.243.0  *             255.255.255.0   U     0      0   0   eth0
127.0.0.0     *             255.0.0.0       U     0      0   0   lo
default       149.112.50.65 0.0.0.0         UG    0      0   0   eth0

In the first column, Destination is the IP address (or, if the host is in /etc/hosts or /etc/networks, the hostname) of the receiving host. The default entry is the default gateway for this machine. The Gateway column lists the gateway through which the packets must go to reach