How to Configure Your NIC's IP Address

Now that you have a firm grasp of many of the most commonly used networking concepts, it is time to apply them to the configuration of your server. Some of these activities are automatically covered during a Linux installation, but you will often find yourself having to know how to modify these initial settings whenever you need to move your server to another network, add a new network interface card or use an alternative means of connecting to the Internet.
In Chapter 2, "", we started with an explanation of TCP/IP, so we'll start this Linux networking chapter with a discussion on how to configure the IP address of your server.
How to Configure Your NIC's IP Address You need to know all the steps needed to configure IP addresses on a NIC card. Web site shopping cart applications frequently need an additional IP address dedicated to them. You also might need to add a secondary NIC interface to your server to handle data backups. Last but not least, you might just want to play around with the server to test your skills.
This section shows you how to do the most common server IP activities with the least amount of headaches.
Determining Your IP Address Most modern PCs come with an Ethernet port. When Linux is installed, this device is called eth0. You can determine the IP address of this device with the ifconfig command.
[root@bigboy tmp]# ifconfig -a

eth0 Link encap:Ethernet HWaddr 00:08:C7:10:74:A8
BROADCAST MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
RX bytes:0 (0.0 b) TX bytes:0 (0.0 b)
Interrupt:11 Base address:0x1820

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:787 errors:0 dropped:0 overruns:0 frame:0
TX packets:787 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:82644 (80.7 Kb) TX bytes:82644 (80.7 Kb)

wlan0 Link encap:Ethernet HWaddr 00:06:25:09:6A:B5
inet addr:192.168.1.100 Bcast:192.168.1.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:47379 errors:0 dropped:0 overruns:0 frame:0
TX packets:107900 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
RX bytes:4676853 (4.4 Mb) TX bytes:43209032 (41.2 Mb)
Interrupt:11 Memory:c887a000-c887b000

wlan0:0 Link encap:Ethernet HWaddr 00:06:25:09:6A:B5
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
Interrupt:11 Memory:c887a000-c887b000

[root@bigboy tmp]#
In this example, eth0 has no IP address because this box is using wireless interface wlan0 as its main NIC. Interface wlan0 has an IP address of 192.168.1.100 and a subnet mask of 255.255.255.0
You can see that this command gives good information on the interrupts, or PCI bus ID, used by each card. On very rare occasions you might find that your NIC card doesn't work because it shares both an interrupt and memory access address with some other device. You can look at the contents of the /proc/interrupts file to get a listing of all the interrupt IRQs used by your system. In the example below we can see that there are no conflicts with each IRQ from 0 to 15 having only a single entry. Devices eth0 and eth1 use interrupts 10 and 5, respectively:
[root@bigboy tmp]# cat /proc/interrupts
CPU0
0: 2707402473 XT-PIC timer
1: 67 XT-PIC i8042
2: 0 XT-PIC cascade
5: 411342 XT-PIC eth1
8: 1 XT-PIC rtc
10: 1898752 XT-PIC eth0
11: 0 XT-PIC uhci_hcd
12: 58 XT-PIC i8042
14: 5075806 XT-PIC ide0
15: 506 XT-PIC ide1
NMI: 0
ERR: 43
[root@bigboy tmp]#If there are conflicts, you might need to refer to the manual for the offending device to try to determine ways to either use another interrupt or memory I/O location.
Changing Your IP Address If you wanted, you could give this eth0 interface an IP address using the ifconfig command.
[root@bigboy tmp]# ifconfig eth0 10.0.0.1 netmask 255.255.255.0 upThe "up" at the end of the command activates the interface. To make this permanent each time you boot up you'll have to add this command in your /etc/rc.local file which is run at the end of every reboot.
Fedora Linux also makes life a little easier with interface configuration files located in the /etc/sysconfig/network-scripts directory. Interface eth0 has a file called ifcfg-eth0, eth1 uses ifcfg-eth1, and so on. You can place your IP address information in these files, which are then used to auto-configure your NICs when Linux boots. See Figure 3-1 for two samples of interface eth0. One assumes the interface has a fixed IP address, and the other assumes it requires an IP address assignment using DHCP.


Figure 3-1 - File formats for network-scripts

Fixed IP Address

[root@bigboy tmp]# cd /etc/sysconfig/network-scripts
[root@bigboy network-scripts]# cat ifcfg-eth0

#
# File: ifcfg-eth0
#
DEVICE=eth0
IPADDR=192.168.1.100
NETMASK=255.255.255.0
BOOTPROTO=static
ONBOOT=yes
#
# The following settings are optional
#
BROADCAST=192.168.1.255
NETWORK=192.168.1.0

[root@bigboy network-scripts]#

Getting the IP Address Using DHCP

[root@bigboy tmp]# cd /etc/sysconfig/network-scripts
[root@bigboy network-scripts]# cat ifcfg-eth0

#
# File: ifcfg-eth0
#
DEVICE=eth0
BOOTPROTO=dhcp
ONBOOT=yes

[root@bigboy network-scripts]#

As you can see eth0 will be activated on booting, because the parameter ONBOOT has the value yes and not no. You can read more about netmasks and DHCP in Chapter 2, "", that acts as an introduction to networking.
The default RedHat/Fedora installation will include the broadcast and network options in the network-scripts file. These are optional.
After you change the values in the configuration files for the NIC you have to deactivate and activate it for the modifications to take effect. The ifdown and ifup commands can be used to do this:
[root@bigboy network-scripts]# ifdown eth0
[root@bigboy network-scripts]# ifup eth0Your server will have to have a default gateway for it to be able to communicate with the Internet. This will be covered later in the chapter.
How DHCP Affects the DNS Server You Use Your DHCP server not only supplies the IP address your Linux box should use, but also the desired DNS servers. When using DHCP for an interface, make sure your /etc/resolv.conf file has the servers configuration lines commented out to prevent any conflicts.
Multiple IP Addresses on a Single NIC In the previous section "Determining Your IP Address" you may have noticed that there were two wireless interfaces: wlan0 and wlan0:0. Interface wlan0:0 is actually a child interface wlan0, a virtual subinterface also known as an IP alias. IP aliasing is one of the most common ways of creating multiple IP addresses associated with a single NIC. Aliases have the name format parent-interface-name:X, where X is the sub-interface number of your choice.
The process for creating an IP alias is very similar to the steps outlined for the real interface in the previous section, "Changing Your IP Address":

• First ensure the parent real interface exists
• Verify that no other IP aliases with the same name exists with the name you plan to use. In this we want to create interface wlan0:0.
• Create the virtual interface with the ifconfig command

[root@bigboy tmp]# ifconfig wlan0:0 192.168.1.99 netmask 255.255.255.0 up

• You should also create a /etc/sysconfig/network-scripts/ifcfg-wlan0:0 file so that the aliases will all be managed automatically with the ifup and ifdown commands. Here is a sample configuration:

DEVICE=wlan0:0
ONBOOT=yes
BOOTPROTO=static
IPADDR=192.168.1.99
NETMASK=255.255.255.0The commands to activate and deactivate the alias interface would therefore be:
[root@bigboy tmp]# ifup wlan0:0
[root@bigboy tmp]# ifdown wlan0:0Note: Shutting down the main interface also shuts down all its aliases too. Aliases can be shutdown independently of other interfaces.
After completing these four simple steps you should be able to ping the new IP alias from other servers on your network.
IP Address Assignment for a Direct DSL Connection If you are using a DSL connection with fixed or static IP addresses, then the configuration steps are the same as those outlined earlier. You plug your Ethernet interface into the DSL modem, configure it with the IP address, subnet mask, broadcast address, and gateway information provided by your ISP and you should have connectivity when you restart your interface. Remember that you might also need to configure your DNS server correctly.
If you are using a DSL connection with a DHCP or dynamic IP address assignment, then the process is different. Your ISP will provide you with a PPP authentication over Ethernet (PPPoE) username and password which will allow your computer to login transparently to the Internet each time it boots up. Fedora Linux installs the rp-pppoe RPM software package required to support this.
Note: Unless you specifically request static IP addresses, your ISP will provide you with a DHCP based connection. The DHCP IP address assigned to your computer and/or Internet router will often not change for many days and you may be fooled into thinking it is static.
Downloading and installing RPMs isn't hard. If you need a refresher, Chapter 6, "", on RPMs, covers how to do this in detail. When searching for the file, remember that the PPPoE RPM's filename usually starts with the word rp-pppoe followed by a version number like this: rp-pppoe-3.5-8.i386.rpm.
After installing the RPM, you need to go through a number of steps to complete the connection. The PPPOE configuration will create a software-based virtual interface named ppp0 that will use the physical Internet interface eth0 for connectivity. Here's what you need to do:

• Make a backup copy of your ifcfg-eth0 file.

[root@bigboy tmp]#
[root@bigboy tmp]# cd /etc/sysconfig/network-scripts/
[root@bigboy network-scripts]# ls ifcfg-eth0
ifcfg-eth0
[root@bigboy network-scripts]# cp ifcfg-eth0 DISABLED.ifcfg-eth0

• Edit your ifcfg-eth0 file to have no IP information and also to be deactivated on boot time.

DEVICE=eth0
ONBOOT=no

• Shutdown your eth0 interface.

[root@bigboy network-scripts]# ifdown eth0
[root@bigboy network-scripts]#

• Run the adsl-setup configuration script

[root@bigboy network-scripts]# adsl-setup It will prompt you for your ISP username, the interface to be used (eth0) and whether you want to the connection to stay up indefinitely. We'll use defaults wherever possible. Welcome to the ADSL client setup.