Linux Network Administrators Guide
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Chapter 5. Configuring TCP/IP Networking
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5.8. 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
- down
- netmask mask
- pointopoint address
- broadcast address
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 .
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.
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.
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.
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
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