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
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.
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.
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
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 handle in one transaction. For Ethernets, the MTU
defaults to 1,500 (the largest allowable size of an Ethernet packet); for SLIP
interfaces, it is 296. (There is no constraint on the MTU of SLIP links; this
value is a good compromise.)
This is an option specific to broadcast networks such as Ethernets or packet
radio. It enables the use of the Address Resolution Protocol (ARP) to detect
the physical addresses of hosts attached to the network. For broadcast
networks, it is on by default. If ARP is disabled, ifconfig displays the NOARP flag.
This option disables the use of ARP on this interface.
This option puts the interface in promiscuous mode. On a broadcast
network, this makes the interface receive all packets, regardless of
whether they were destined for this host or not. This allows network
traffic analysis using packet filters and such, also called
Ethernet snooping. Usually, this is a good
technique for hunting down network problems that are otherwise hard to
detect. Tools such as tcpdump rely on this.
On the other hand, this option allows attackers to do nasty things,
such as skim the traffic of your network for passwords. You can
protect against this type of attack by prohibiting just anyone from
plugging their computers into your Ethernet. You could also use
secure authentication protocols, such as Kerberos or the secure shell
login suite. This option corresponds to the PROMISC flag.
This option turns promiscuous mode off.
Multicast addresses are like Ethernet broadcast addresses, except that
instead of automatically including everybody, the only people who
receive packets sent to a multicast address are those programmed to
listen to it. This is useful for applications like Ethernet-based
videoconferencing or network audio, to which only those interested can
listen. Multicast addressing is supported by most, but not all,
Ethernet drivers. When this option is enabled, the interface receives
and passes multicast packets for processing. This option corresponds to the ALLMULTI flag.
This option turns multicast addresses off.