The venerable ifconfig is available on almost every unix I have encountered. In addition to reporting the IP addressing and usage statistics of an optionally specified interface, ifconfig can modify an interface's MTU and other flags and interface characteristics, bring up an interface and bring down an interface. This tool is the primary tool for manipulation of IP addressing on many linux distributions.
In its simplest use, ifconfig merely reports the IP interface and relevant statistics. For Ethernet devices, the hardware address, IP address, broadcast, netmask, IP interface states, and some other additional information is presented. For other interfaces, different information may be presented to the user, but the basic summary of IP addressing information will always be available. Be sure to read Section C.1.4, “Reading ifconfig output” also.
Example C.1. Viewing interface information with ifconfig
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It is fairly common to specify the name of an interface as an argument to ifconfig, which will restrict the output to the named interface. This is the only way to retrieve information from ifconfig about link layer devices which are available, but not in an UP state. See also Section B.3, “ip link” and Section C.2, “ip address”.
There are many other options available to the ifconfig command to control addressing and interface state. Contrary to the behaviour of most other standard unix command line utilities which operate on arguments and options, ifconfig operates on a grammar after the specified interface. Subsequent examples will demonstrate how this differs from conventional modern unix tools.
Let's look at some simple operations you can perform with ifconfig. Occasionally, you will need to bring down a network interface. For an introduction to this and its side effects, see Example 1.6, “Bringing down a network interface with ifconfig” and the list of side effects.
Example C.2. Bringing down an interface with ifconfig
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Naturally, when we view the active interfaces after downing the first Ethernet interface, we see that eth0 is no longer present. This is exactly what we had intended. Now to bring up the interface, we'll need the IP address and netmask information.
Bringing up an interface is slightly more complex than bringing an interface down because you need to have the IP addressing information handy in order to bring the interface back. For an introduction to the side effects of bringing up an IP address on an interface, see Example 1.7, “Bringing up an Ethernet interface with ifconfig” and the list of side effects.
Example C.3. Bringing up an interface with ifconfig
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The above operations are the simple operations one can perform with ifconfig. Let's examine the output a bit more closely now, with an eye toward the other flags and settings we can manually twiddle.
The first line of each interface definition represents data which cannot be altered with ifconfig. If we consider only Ethernet interfaces, the link encapsulation will always say "Ethernet", and the hardware address cannot be altered with ifconfig [45]. Below this, one line summarizes the IP information associated with this logical interface.
The third line indicates the current states of the interface, maximum transmission unit, and the metric for this interface. Possible state options are itemized in the table below. The maximimum transmission unit is routinely set to 1500 bytes for Ethernet and promptly forgotten. MTU suddenly becomes important when IP packets are forwarded across a link layer which requires a smaller MTU. Thus ifconfig provides a method to set the MTU on an interface. For more on MTU, see Section 4.10.1, “MTU, MSS, and ICMP”. The remaining lines of output are taken from the Ethernet driver. See further discussion of these statistics below.
It is a rare occasion on which the MTU needs to be changed, but when it needs to be changed, nothing else will suffice. Here's an example of setting the MTU on an interface to 1412 bytes.
Example C.4. Changing MTU with ifconfig
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Every device on a system has flags which indicate the state the device may be in. These flags can be altered by the ifconfig utility.
Table C.1. Interface Flags
| Flag | Description |
|---|---|
| UP | device is functioning |
| BROADCAST | device can send traffic to all hosts on the link |
| RUNNING | ??? |
| MULTICAST | device can perform and receive multicast packets |
| ALLMULTI | device receives all multicast packets on the link |
| PROMISC | device receives all traffic on the link |
I cannot confidently recommend believing the flags as reported by ifconfig output. Attestations from others and experimentation has proven to me that these flags (particularly the PROMISC flag) do not accurately represent the state of the device as reported in log files (by the kernel) and by the ip link show utility.
This does not mean, however, that the flags cannot be set with the ifconfig utility. Manipulation of the flags on an interface operates according to a peculiar grammar. To set the PROMISC flag, one issues a command with the promisc option from the grammar. If one wishes to remove the PROMISC flag from an interface, the -promisc option is required.
Example C.5. Setting interface flags with ifconfig
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Since linux 2.0 the kernel has supported multiple IP addresses hosted on the same device. By suffixing the real interface name with a colon and a non-negative integer, you can bring up additional IP adresses on the same device. Example alias names are eth0:0 eth0:7. See Section 9.4, “Multiple IPs on an Interface” for further details.
As you can see, ifconfig is both a powerful and idiosyncratic tool for controlling network interfaces and devices.
[45] If you need to change the hardware address of an Ethernet interface, you have a strange need, but you can accomplish this using the ip link set address command.