ifconfig

Kevin O'BrienWashtenaw Linux Users Group

http://www.lugwash.org

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ifconfig

● ... stands for interface configurator● ... is part of the original internet toolkit

http://www.faqs.org/docs/linux_network/x-087-2-iface.interface.html

● ... has versions available for Linux, BSD, Solaris, and Mac OSX

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man page

● Ifconfig is used to configure the kernel-resident network interfaces. It is used at boot time to set up interfaces as necessary. After that, it is usually only needed when debugging or when system tuning is needed.

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man page 2

● If no arguments are given, ifconfig displays the status of the currently active interfaces. If a single interface argument is given, it displays the status of the given interface only; if a single -a argument is given, it displays the status of all interfaces, even those that are down. Otherwise, it configures an interface.

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Sample output

eth0 Link encap 10Mbps Ethernet HWaddr 00:0F:20:CF:8B:42inet addr 172.16.1.2 Bcast 172.16.1.255 Mask 255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU 1500 Metric 0RX packets 3136 errors217 dropped 7 overrun 26 TX packets 1752 errors 25 dropped 0 overrun 0

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What is it saying?

● eth0 is the interface it is reporting on. It is the first of the ethernet interfaces on this computer.

● The Hardware Address is 00:0F:20:CF:8B:42. This is sometimes called the MAC address (Media Access Control), and is a hexadecimal number assigned to the ethernet card at the factory.

● The internet address is 172.16.1.2. This is the address assigned to this computer.

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What is it saying? 2

● The Broadcast Address is 172.16.1.255. This is an address that can be used to send messages to every computer on the same subnet as this computer.

● The Subnet Mask is 255.255.255.0. This can be thought of as defining or identifying the subnet this computer is part of.

● The rest of it tells about packets sent, received, dropped, etc.

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Use for information

● The first thing most beginners will want to do is use this for information.

● If you find you are not connected to the Internet, use this command to see if your interface is configured and active.

● You can use this to check on your IP address and subnet mask.

● There is an equivalent for wireless interfaces, called iwconfig.

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Parallels for newbies

● Windows 95 through ME had a command called winipcfg

● Windows NT through Windows Vista have ipconfig

● These are essentially similar to the Unix command ifconfig

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Syntax

● ifconfig interface [address [parameters]]● interface is the interface name, such as eth0

(first ethernet device) or lo (local host)● address is the IP address assigned to the

interface. This can be specified as a dotted quad address, or as a name that can be looked up in /etc/hosts.

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Parameters

● up – Makes an interface accessible to the IP layer.

● down – Makes an interface inaccessible to the IP layer, i.e. stops all traffic through this interface.

● netmask mask – Sets the subnet mask to be used by the interface.

● broadcast address – Sets the broadcast address.

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Warning!

● If you are not a network administrator, be very careful about setting addresses from the command line. Even people with some experience can get confused setting a subnet mask. You don't want your interface to stop working in some mysterious way.

● Using this command to get information is completely safe.

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Warning! 2

● If you do want to experiment (a very good way to learn about these things), just make sure you are not using a mission-critical computer. Whatever you screw up you will need to unscrew.☺

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More on this command

● There are other parameters and options available for this command, but they go beyond the requirements for this presentation. Consult Google or the man page for more information.

● A good readable guide to the basics can be found at http://tldp.org/LDP/nag2/x-087-2-iface.ifconfig.html

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Examples

● ifconfig eth0 – View the network settings on the first Ethernet adapter installed in the computer.

● ifconfig -a – Display information on all network interfaces on the computer, active or inactive.

● ifconfig eth0 down – Would take down the interface, and no packets would be sent or received.

● ifconfig eth0 up – Would bring the interface back up so it could send and receive data.

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Examples 2

● ifconfig eth0 192.168.1.102 netmask 255.255.255.0 broadcast 192.168.1.255 – This would assign these values to the first Ethernet device installed in the computer. Be careful here!