IFCONFIG(8)

NAME
ifconfig - configure network interface parameters

SYNOPSIS
ifconfig interface address_family [ address [dest_address]] [parameters]
ifconfig -a [-d] [-u] [address_family]
ifconfig -l [-d] [-u] [address_family]

DESCRIPTION
Ifconfig is used to assign an address to a network interface and/or configure network interface parameters. Ifconfig must be used at boot time to define the network address of each interface present on a machine; it may also be used at a later time to redefine an interface's address or other operating parameters.

The following options are available:

address
    For the DARPA-Internet family, the address is either a host name present in the host name data base, hosts(5), or a DARPA Internet address expressed in the Internet standard “dot notation”.

address_family
    Specify the address family which affects interpretation of the remaining parameters. Since an interface can receive transmissions in differing protocols with different naming schemes, specifying the address family is recommended. The address or protocol families currently supported are “inet”, “inet6”, “atalk”, and “ipx”.

dest_address
    Specify the address of the correspondent on the other end of a point to point link.

interface
    This parameter is a string of the form “name unit”, for example, “en0”.

The following parameters may be set with ifconfig:

add Another name for the ``alias'' parameter. Introduced for compatibility with BSD/OS.

alias
    Establish an additional network address for this interface. This is sometimes useful when changing network numbers, and one wishes to accept packets addressed to the old interface. If the address is on the same subnet as the first network address for this interface, a netmask of 0xffffffff has to be specified.

-alias
    Remove the network address specified. This would be used if you incorrectly specified an alias, or it was no longer needed. If you have incorrectly set an NS address having the side effect of specifying the host portion, removing all NS addresses will allow you to respecify the host portion.

anycast
    (Inet6 only) Specify that the address configured is an anycast address. Based on the current specification, only routers may configure anycast addresses. Anycast address will not be used as source address of any of outgoing IPv6 packets.

arp Enable the use of the Address Resolution Protocol in mapping between network level addresses and link level addresses (default). This is currently implemented for mapping between DARPA Internet addresses and 10Mb/s Ethernet addresses.

-arp
    Disable the use of the Address Resolution Protocol.

broadcast
    (Inet only) Specify the address to use to represent broadcasts to the network. The default broadcast address is the address with a host part of all 1's.

debug
    Enable driver dependent debugging code; usually, this turns on extra console error logging.

-debug
    Disable driver dependent debugging code.

delete
    Another name for the ``-alias'' parameter.

down
    Mark an interface ``down''. When an interface is marked ``down'', the system will not attempt to transmit messages through that interface. If possible, the interface will be reset to disable reception as well. This action does not automatically disable routes using the interface.

media type
    If the driver supports the media selection system, set the media type of the interface to type. Some interfaces support the mutually exclusive use of one of several different physical media connectors. For example, a 10Mb/s Ethernet interface might support the use of either AUI or twisted pair connectors. Setting the media type to “10base5/AUI” would change the currently active connector to the AUI port. Setting it to “10baseT/UTP” would activate twisted pair. Refer to the interfaces' driver specific documentation or man page for a complete list of the available types.

mediaopt opts
    If the driver supports the media selection system, set the specified media options on the interface. Opts is a comma delimited list of options to apply to the interface. Refer to the interfaces' driver specific man page for a complete list of available options.

-mediaopt opts
    If the driver supports the media selection system, disable the specified media options on the interface.

vlan vlan_tag
    If the interface is a vlan pseudo interface, set the vlan tag value to vlan_tag. This value is a 16-bit number which is used to create an 802.1Q vlan header for packets sent from the vlan interface. Note that vlan and vlandev must both be set at the same time.

vlandev iface
    If the interface is a vlan pseudo device, associate physical interface iface with it. Packets transmitted through the vlan interface will be diverted to the specified physical interface iface with 802.1Q vlan encapsulation. Packets with 802.1Q encapsulation received by the parent interface with the correct vlan tag will be diverted to the associated vlan pseudo-interface. The vlan interface is assigned a copy of the parent interface's flags and the parent's ethernet address. The vlandev and vlan must both be set at the same time. If the vlan interface already has a physical interface associated with it, this command will fail. To change the association to another physical interface, the existing association must be cleared first.

    Note: if the link0 flag is set on the vlan interface, the vlan pseudo interface's behavior changes: the link0 tells the vlan interface that the parent interface supports insertion and extraction of vlan tags on its own (usually in firmware) and that it should pass packets to and from the parent unaltered.

-vlandev iface
    If the driver is a vlan pseudo device, disassociate the physical interface iface from it. This breaks the link between the vlan interface and its parent, clears its vlan tag, flags and its link address and shuts the interface down.

metric n
    Set the routing metric of the interface to n, default 0. The routing metric is used by the routing protocol (routed(8)). Higher metrics have the effect of making a route less favorable; metrics are counted as addition hops to the destination network or host.

mtu n
    Set the maximum transmission unit of the interface to n, default is interface specific. The mtu is used to limit the size of packets that are transmitted on an interface. Not all interfaces support setting the mtu, and some interfaces have range restrictions.

netmask mask
    (Inet only) Specify how much of the address to reserve for subdividing networks into sub-networks. The mask includes the network part of the local address and the subnet part, which is taken from the host field of the address. The mask can be specified as a single hexadecimal number with a leading 0x, with a dot-notation Internet address, or with a pseudo-network name listed in the network table networks(5). The mask contains 1's for the bit positions in the 32-bit address which are to be used for the network and subnet parts, and 0's for the host part. The mask should contain at least the standard network portion, and the subnet field should be contiguous with the network portion.

prefixlen len
    (Inet6 only) Specify that len bits are reserved for subdividing networks into sub-networks. The len must be integer, and for syntactical reason it must be between 0 to 128. It is almost always 64 under the current IPv6 assignment rule. If the parameter is ommitted, 64 is used.

range
    Under appletalk, set the interface to respond to a netrange. of the form startnet-endnet. Appletalk uses this scheme instead of netmasks though FreeBSD implements it internally as a set of netmasks.

remove
    Another name for the ``-alias'' parameter. Introduced for compatibility with BSD/OS.

phase
    The argument following this specifies the version (phase) of the Appletalk network attached to the interface. Values of 1 or 2 are permitted.

link[0-2]
    Enable special processing of the link level of the interface. These three options are interface specific in actual effect, however, they are in general used to select special modes of operation. An example of this is to enable SLIP compression, or to select the connector type for some Ethernet cards. Refer to the man page for the specific driver for more information.

-link[0-2]
    Disable special processing at the link level with the specified interface.

up  Mark an interface “up”. This may be used to enable an interface after an “ifconfig down”. It happens automatically when setting the first address on an interface. If the interface was reset when previously marked down, the hardware will be re-initialized.

Ifconfig displays the current configuration for a network interface when no optional parameters are supplied. If a protocol family is specified, ifconfig will report only the details specific to that protocol family.

If the driver does supports the media selection system, the supported media list will be included in the output.

If -L flag is supplied, address lifetime is dislayed for IPv6 addresses, as time offset string.

Optionally, the -a flag may be used instead of an interface name. This flag instructs ifconfig to display information about all interfaces in the system. The -d flag limits this to interfaces that are down, and -u limits this to interfaces that are up.

The -l flag may be used to list all available interfaces on the system, with no other additional information. Use of this flag is mutually exclusive with all other flags and commands, except for -d (only list interfaces that are down) and -u (only list interfaces that are up).

Only the super-user may modify the configuration of a network interface.

NOTES
The media selection system is relatively new and only some drivers support it (or have need for it).

DIAGNOSTICS
Messages indicating the specified interface does not exist, the requested address is unknown, or the user is not privileged and tried to alter an interface's configuration.

BUGS
IPv6 link-local addresses are required for several basic communication between IPv6 node. If they are deleted by ifconfig manually, the kernel might show very strange behavior. So, such manual deletions are strongly discouraged.

SEE ALSO
netstat(1), netintro(4), rc(8), routed(8)

HISTORY
The ifconfig command appeared in 4.2BSD.