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Linux Network Administrators Guide


    broadcast option adapts to these strange environments. If a broadcast address has been set, ifconfig displays the BROADCAST flag.

  • irq

    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 network.[1] 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.)

  • arp

    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.

  • –arp

    This option disables the use of ARP on this interface.

  • promisc

    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.[2] This option corresponds to the PROMISC flag.

  • –promisc

    This option turns promiscuous mode off.

  • allmulti

    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.

  • –allmulti

    This option turns multicast addresses off.



RIP chooses the optimal route to a given host based on the "length" of the path. It is computed by summing up the individual metric values of each host-to-host link. By default, a hop has length 1, but this may be any positive integer less than 16. (A route length of 16 is equal to infinity. Such routes are considered unusable.) The metric parameter sets this hop cost, which is then broadcast by the routing daemon.


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