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Basic Networking Configuration (205.1)

Candidates should be able to configure a network device to be able to connect to a local, wired or wireless, and a wide-area network. This objective includes being able to communicate between various subnets within a single network.

Key Knowledge Areas

  • Utilities to configure and manipulate ethernet network interfaces

  • Configuring wireless networks

Terms and Utilities

  • /sbin/route

  • /sbin/ifconfig

  • /sbin/ip

  • /usr/sbin/arp

  • /sbin/iw

  • /sbin/iwconfig

  • /sbin/iwlist

Configuring the network interface

Most network devices are supported by modern kernels. But you will need to configure your devices to fit into your network. They will need an IP address (IPv4, IPv6 or both), possibly a number of gateways/routers has to be made known to allow access to other networks and the default route needs to be set. Configuring Network Interface These tasks are usually performed from the routing table network-initialization script each time you boot the system. The basic tools for this process are ifconfig (where "if" stands ifconfig for interface) and route. route

The ifconfig command is still widely used. It configures an interface and makes it accessible to the kernel networking layer. An IP address, submask, broadcast address and various other parameters can be set. The tool can also be used to activate and de-activate the interface, also known as "bringing up" and "bringing down" the interface. An active interface will send and receive IP datagrams through the interface. The simplest way to IP invoke it is:

    # ifconfig interface ip-address

This command assigns ip-address to interface and activates it. All other parameters are set to default values. For instance, the default network mask is derived from the network class of the IP address, such as for a class B address.

route allows you to add or remove routes from the kernel routing table. It can be invoked as:

    # route {add|del} [-net|-host] target [if]

The add and del arguments determine whether to add or delete the route to target. The -net and -host arguments tell the route command whether the target is a network or a host (a host is assumed if you don't specify). The if argument specifies the interface and is optional, and allows you to specify to which network interface the route should be directed -- the Linux kernel makes a sensible guess if you don't supply this information.

The Loopback Interface

TCP/IP implementations include a virtual network interface that can be used to emulate network traffic between two processes on the same host. The loopback interface is not connected to any real network, it is implemented entirely within the operating system's networking software. Traffic sent to the loopback IP address (often the address is used) is simply passed back up the network software stack as if it had been received from another device. The IPv6 address used is ::1, and commonly the name "localhost" is used as hostname. It is the first interface to be activated during boot: loopback interface

    # ifconfig lo

Occasionally, you will see the dummy hostname localhost being used instead of the IP address. This requires proper configuration of the /etc/hosts file:

    # Sample /etc/hosts entry for localhost localhost

To view the configuration of an interface simply invoke ifconfig with the interface name as lo sole argument:

    $ ifconfig lo
    lo        Link encap:Local Loopback inet addr:
    Mask: UP LOOPBACK RUNNING MTU:3924 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

This example shows that the loopback interface has been assigned a netmask of by default. is a class A address.

These steps suffice to use networking applications on a stand-alone host. After adding these lines to your network initialization script and ensuring its execution at boot time by rebooting your machine you can test the loopback interface. For instance, telnet localhost should establish a telnet telnet connection to your host, giving you a login: prompt.

The loopback interface is often used as a test bed during development, but there are other applications. For example, all applications based on RPC use the loopback RPC interface to register themselves with the portmapper daemon at startup. These applications include NIS and NFS. NIS NFS Hence the loopback interface should always be configured, whether your machine is attached to a network or not.

Ethernet Interfaces

Configuring an Ethernet interface is pretty much the same as the ethernet interface loopback interface - it just requires a few more parameters when you use subnetting.

Suppose we have subnetted the IP network, which was originally a class B network, into class C subnetworks. To make the interface netmask recognize this, the ifconfig invocation would look like this:

    # ifconfig eth0 netmask

This command assigns the eth0 interface an IP address of If we had omitted the netmask, ifconfig would deduce the netmask from the IP network class, which would result in an incorrect netmask of Now a quick check shows:

    # ifconfig eth0
    eth0      Link encap 10Mps Ethernet HWaddr
              00:00:C0:90:B3:42 inet addr Bcast Mask
     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 sets the broadcast broadcast address address (the Bcast field) to the usual value, which is the host's network number with all the host bits set. Also, the maximum transmission unit (the maximum size of IP datagrams the MTU kernel will generate for this interface) has been set to the maximum size of Ethernet packets: 1,500 bytes. The defaults are usually what you will use, but all these values can be overridden if required.

Routing Through a Gateway

You do not need routing if your host is on a single Ethernet. Quite frequently however, networks are 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.

Imagine two ethernets linked through such a gateway, the host romeo. Assuming that romeo has already been configured, we just have to add an entry to the routing table telling the kernel all hosts on the other network can be reached through romeo. The appropriate invocation of route is shown below; the gw keyword tells it that the next argument denotes a gateway:

    # route add -net netmask gw romeo

Of course, any host on the other network you wish to communicate with must have a routing entry for our network. Otherwise you would only be able to send data to the other network, but the hosts on the other network would be unable to reply.

This example only describes a gateway that switches packets between two isolated ethernets. Now assume that romeo also has a connection to the Internet (say, through an additional PPP link). In this case, we want datagrams to any destination network to be handed to romeo. This can be accomplished by making it the default gateway: default gateway

    # route add default gw romeo

Something that is frequently misunderstood is that only ONE default gateway can be configured. You may have many gateways, but only one can be the default.

The network name default is a shorthand for, which denotes the default route. The default route default route matches every destination and will be used if a more specific route is not available.

The ip command

In recent years many people have advocated the use of the newer command /sbin/ip. It too can be used to show or manipulate routing and network devices, and also can be used to configure or show policy routing and tunnels. However, the old tools ifconfig and route can be used too if that is more convenient. A use case for the ip would be to show the IP addresses used on the network interfaces in a more concise way compared to ifconfig:

    # ip addr show
    1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN 
        link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
        inet scope host lo
        inet6 ::1/128 scope host 
           valid_lft forever preferred_lft forever
    2: p8p1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
        link/ether 00:90:f5:b6:91:d1 brd ff:ff:ff:ff:ff:ff
        inet brd scope global p8p1
        inet6 fe80::290:f5ff:feb6:91d1/64 scope link 
           valid_lft forever preferred_lft forever
    3: wlan0: <BROADCAST,MULTICAST> mtu 1500 qdisc mq state DOWN qlen 1000
        link/ether 88:53:2e:02:df:14 brd ff:ff:ff:ff:ff:ff
    4: vboxnet0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN qlen 1000
        link/ether 0a:00:27:00:00:00 brd ff:ff:ff:ff:ff:ff

in contrast to ifconfig

    # ifconfig -a
    lo        Link encap:Local Loopback  
              inet addr:  Mask:
              inet6 addr: ::1/128 Scope:Host
              UP LOOPBACK RUNNING  MTU:16436  Metric:1
              RX packets:48 errors:0 dropped:0 overruns:0 frame:0
              TX packets:48 errors:0 dropped:0 overruns:0 carrier:0
              collisions:0 txqueuelen:0 
              RX bytes:4304 (4.2 KiB)  TX bytes:4304 (4.2 KiB)

    p8p1      Link encap:Ethernet  HWaddr 00:90:F5:B6:91:D1  
              inet addr:  Bcast:  Mask:
              inet6 addr: fe80::290:f5ff:feb6:91d1/64 Scope:Link
              UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
              RX packets:6653 errors:0 dropped:0 overruns:0 frame:0
              TX packets:7609 errors:0 dropped:0 overruns:0 carrier:0
              collisions:0 txqueuelen:1000 
              RX bytes:3843849 (3.6 MiB)  TX bytes:938833 (916.8 KiB)

    vboxnet0  Link encap:Ethernet  HWaddr 0A:00:27:00:00:00  
              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:1000 
              RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)

    wlan0     Link encap:Ethernet  HWaddr 88:53:2E:02:DF:14  
              BROADCAST MULTICAST  MTU:1500  Metric:1
              RX packets:2 errors:0 dropped:0 overruns:0 frame:0
              TX packets:11 errors:0 dropped:0 overruns:0 carrier:0
              collisions:0 txqueuelen:1000 
              RX bytes:330 (330.0 b)  TX bytes:1962 (1.9 KiB)

An example of using ip as an alternative to ifconfig when configuring a network interface (p8p1):

    # ifconfig p8p1 netmask broadcast

can be replaced by:

                ip addr add broadcast dev p8p1

The ip can also be used as alternative for the route command:

    # ip route add dev p8p1

    # ip route show dev p8p1  proto kernel  scope link  src  metric 1 
    default via dev p8p1  proto static

The output of the route command in this case would be:

    # route
    Destination     Gateway         Genmask         Flags Metric Ref    Use Iface   *        U     1      0        0 p8p1
    default         UG    0      0        0 p8p1

As another example, to add a static route to network over eth0, use:

    # ip route add dev eth0

For more information please read the manual pages of ip.

ARP, Address Resolution Protocol

In the ISO network model there are seven layers. The Internet Protocol is a layer 3 protocol and the NIC is a layer 2 device. In a local network (layer 2) devices know each other by the MAC (Media Access Control) address. In the IP network (layer 3) devices know each other by their IP address.

To allow transfer from data to and from layer 3 IP communication requires a protocol to map between layer 2 and layer 3. This protocol is known as ARP - the Address Resolution Protocol. ARP creates a mapping between an IP address and the MAC address where the IP address is configured.

When IP enabled devices want to communicate, the kernel of the originating device hands the IP packet to the network interface driver software and requests to deliver the IP packet to the recipient. The only way to communicate on a local Ethernet is by means of a MAC address, IP addresses are of no use there. To find out the MAC address of the recipient with the proper IP address, the network driver for the interface on the origination side will send an ARP request. An ARP request is a broadcast: it is sent to any computer on the local network. The computer that has the requested IP address will now answer back with its MAC address. The sender then has al the information needed to transmit the packet. Also, the MAC and IP addres are stored in a local cache for future reference.

The command arp can be used to show the ARP cache. Example:

    # arp
    Address                  HWtype  HWaddress           Flags Mask            Iface               ether   00:19:bb:2e:df:73   C                     wlan0

The cache can be manipulated manually, for example if a host is brought down you might want to remove it's arp entry. Normally you do not need to bother as the cache is not overly persistent. See the man pages for more information on the arp command.

Note Additionally, there exists the reverse ARP protocol (RARP). This protocol is used to allow an Ethernet (layer 2) device which IP address(es) it has configured. ARP: broadcast IP and receive MAC. RARP: broadcast MAC and receive IP.

Wireless networking


iw is used to configure wireless devices. It only supports the nl80211 (netlink) standard. So if iw doesnt see your device, this might be the reason. You should use iwconfig (from the wireless_tools package) and iwlist to configure the wireless device. These are using the WEXT standard. wireless_tools is deprecated, but still widely supported.


            # iw command
            # iw [options] [object] command

Some common options:


  • This is an object and the name op the wireless device should follow after this option. with the command iw dev you can see the name of your device.


  • This is a command and gets the link status of your wireless device.


  • This is a command and scans the network for available access points.


  • This is a command which lets you connect to an access point (essid), you can specify a channel behind it and/or your password.


  • This is a command that lets you set a different interface/mode. For instance ibss if you want to set the operation mode to Ad-Hoc. Or set the power save state of the interface.


        # iw dev wlan0 link                 
        # iw dev wlan0 scan                 
        # iw dev wlan0 connect "Access Point"         
        # iw dev wlan0 connect "Access Point" 2432        
        # iw dev wlan0 connect "Access Point"  0:"Your Key"
        # iw dev wlan0 set type ibss            
        # iw dev wlan0 ibss join "Access Point" 2432
        # iw dev wlan0 ibss leave
        # iw dev wlan0 set power_save on


iwconfig iwconfig is similar to ifconfig, but is dedicated to the wireless interfaces. It is used to set the parameters of the network interface which are specific to the wireless operation (for example : the frequency). iwconfig may also be used to display those parameters, and the wireless statistics.

All parameters and statistics are device dependent. Each driver will provide only some of them depending on hardware support, and the range of values may change. Please refer to the man page of each device for details.


    # iwconfig [interface]
    # iwconfig interface [options]

Some common options:

  • essid

    • Set the ESSID (or Network Name - in some products it may also be called Domain ID). With some cards, you may disable the ESSID checking (ESSID promiscuous) with off or any (and on to reenable it). If the ESSID of your network is one of the special keywords (off, on or any), you should use -- to escape it.
  • mode

    • Set the operating mode of the device, which depends on the network topology. The mode can be Ad-Hoc (the network is composed of one cell only and is without an Access Point), Managed (the node connects to a network composed of multiple Access Points, with roaming), Master (the node is the synchronisation master or acts as an Access Point), Repeater (the node forwards packets between other wireless nodes), Secondary (the node acts as a backup master/repeater), Monitor (the node is not associated with any cell and passively monitors all packets on the frequency) or Auto.


    # iwconfig wlan0 essid any
    # iwconfig wlan0 essid "Access Point"
    # iwconfig wlan0 essid -- "any"
    # iwconfig wlan0 mode Ad-Hoc


iwlist is used to scan for available wireless networks and display additional information about them. The syntax is as follows:

    # iwlist [interface] command

iwlist can display ESSID's, frequency/channel information, bit-rates, encryption type, power management information of other wireless nodes in range. Which information is displayed is hardware dependent.

Some useful options are:

  • scan[ning]

  • Returns a list of ad-hoc networks and access points. Depending on the type of card, more information is shown, i.e. ESSID, signal strength, frequency. Scanning can only be done by root. When a non-root users issues the scan command, results from the last scan are returned, if available. This can also be achieved by adding the option last. Furthermore, the option essid can be used to scan for a specific ESSID. Depending on the driver, more options may be available.

  • keys/enc[ryption]

  • List the encryption key sizes supported and list all the encryption keys set in the device.