Linux Network Administrator Guide, Second Edition

172.16.1.65 actually refers to itself! Consequently, the kernel discards the datagram and returns an error to the application.

This is where the dummy device steps in. It solves the dilemma by simply serving as the alter ego of the loopback interface. In the case of vlite, you simply give it the address 172.16.1.65 and add a host route pointing to it. Every datagram for 172.16.1.65 is then delivered locally. The proper invocation is:[35]

# ifconfig dummy vlite
# route add vlite

IP Alias

New kernels support a feature that can completely replace the dummy interface and serve other useful functions. IP Alias allows you to configure multiple IP addresses onto a physical device. In the simplest case, you could replicate the function of the dummy interface by configuring the host address as an alias onto the loopback interface and completely avoid using the dummy interface. In more complex uses, you could configure your host to look like many different hosts, each with its own IP address. This configuration is sometimes called 'Virtual Hosting,' although technically it is also used for a variety of other techniques.[36]

To configure an alias for an interface, you must first ensure that your kernel has been compiled with support for IP Alias (check that you have a /proc/net/ip_alias file; if not, you will have to recompile your kernel). Configuration of an IP alias is virtually identical to configuring a real network device; you use a special name to indicate it's an alias that you want. For example:

# ifconfig lo:0 172.16.1.1

This command would produce an alias for the loopback interface with the address 172.16.1.1. IP aliases are referred to by appending:n to the actual network device, in which 'n' is an integer. In our example, the network device we are creating the alias on is lo, and we are creating an alias numbered zero for it. This way, a single physical device may support a number of aliases.

Each alias may be treated as though it is a separate device, and as far as the kernel IP software is concerned, it will be; however, it will be sharing its hardware with another interface.

All About ifconfig

There are many more parameters to ifconfig than we have described so far. Its normal invocation is this:

ifconfig interface [address [parameters]]

interface is the interface name, and address is the IP address to be assigned to the interface. This may be either an IP address in dotted quad notation or a name that ifconfig will look up in /etc/hosts.

If ifconfig is invoked with only the interface name, it displays that interface's configuration. When invoked without any parameters, it displays all interfaces you have configured so far; a -a option forces it to show the inactive ones as well. A sample invocation for the Ethernet interface eth0 may look like this:

# ifconfig eth0

eth0 Link encap 10Mbps Ethernet HWaddr 00:00:C0:90:B3:42
   inet addr 172.16.1.2 Bcast 172.16.1.255 Mask 255.255.255.0
   UP BROADCAST RUNNING MTU 1500 Metric 0
   RX packets 3136 errors 217 dropped 7 overrun 26
   TX packets 1752 errors 25 dropped 0 overrun 0

The MTU and Metric fields show the current MTU and metric value for that interface. The metric value is traditionally used by some operating systems to compute the cost of a route. Linux doesn't use this value yet, but defines it for compatibility, nevertheless.

The RX and TX lines show how many packets have been received or transmitted error free, how many errors occurred, how many packets were dropped (probably because of low memory), and how many were lost because of an overrun. Receiver overruns usually occur when packets come in faster than the kernel can service the last interrupt. The flag values printed by ifconfig roughly correspond to the names of its command-line options; they will be explained later.

The following is a list of parameters recognized by ifconfig with the corresponding flag names. Options that simply turn on a feature also allow it to be turned off again by preceding the option name by a dash (-).

up

This option makes an interface accessible to the IP layer. This option is implied when an address is given on the command line. It may also be used to reenable an interface that has been taken down temporarily using the down option.

This option corresponds to the flags UP and RUNNING.

down

This option marks an interface inaccessible to the IP layer. This effectively disables any IP traffic through the interface. Note that this option will also automatically delete all routing entries that use this interface.

netmask mask

This option assigns a subnet mask to be used by the interface. It may be given as either a 32-bit hexadecimal number preceded by 0x, or as a dotted quad of decimal numbers. While the dotted quad format is more common, the hexadecimal representation is often easier to work with. Netmasks are essentially binary, and it is easier to do binary-to-hexadecimal than binary-to-decimal conversion.

pointopoint address

This option is used for point-to-point IP links that involve only two hosts. This option is needed to configure SLIP or PLIP interfaces, for example. If a point-to-point address has been set, ifconfig displays the POINTOPOINT flag.

broadcast address

The broadcast address is usually made up from the network number by setting all bits of the host part. Some IP implementations (systems derived from BSD 4.2, for instance) use a different scheme in which all host part bits are cleared instead. The broadcast option adapts to these strange environments. If a broadcast address has been set, ifconfig displays the BROADCAST flag.

irq

This option allows you to set the IRQ line used by certain devices. This is especially useful for PLIP, but may also be useful for certain Ethernet cards.

metric number

This option may be used to assign a metric value to the routing table entry created for the interface. This metric is used by the Routing Information Protocol (RIP) to build routing tables for the network.[37] The default metric used by ifconfig is zero. If you don't run a RIP daemon, you don't need this option at all; if you do, you will rarely need to change the metric value.

mtu bytes

This sets the Maximum Transmission Unit, which is the maximum number of octets the interface is able to