169 lines
7.8 KiB
HTML
169 lines
7.8 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
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<!--Converted with LaTeX2HTML 96.1-c (Feb 29, 1996) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds -->
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<HTML>
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<HEAD>
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<TITLE>All About ifconfig</TITLE>
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</HEAD>
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<BODY LANG="EN">
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<A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A HREF="node74.html">Checking with netstat</A>
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<B>Up:</B> <A HREF="node58.html">Configuring TCP/IP Networking</A>
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<B> Previous:</B> <A HREF="node72.html">The Dummy Interface</A>
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<BR> <P>
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<H1><A NAME="SECTION007800000">All About ifconfig</A></H1>
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<A NAME="ifaceifconfig"></A>
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There are a lot more parameters to ifconfig than we have
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described above. Its normal invocation is this:
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<pre>
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ifconfig interface [[-net|-host] address [parameters]]
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</pre>
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interface is the interface name, and address is the
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IP-address to be assigned to the interface. This may either be an
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IP-address in dotted quad notation, or a name ifconfig will look
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up in /etc/hosts and /etc/networks. The -net and
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-host options force ifconfig to treat the address as
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network number or host address, respectively.
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<P>
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If ifconfig is invoked with only the interface name, it displays
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that interface's configuration. When invoked without any parameters, it
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displays all interfaces you configured so far; an option of -a
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forces it to show the inactive ones as well. A sample invocation for the
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Ethernet interface eth0 may look like this:
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<pre>
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# ifconfig eth0
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eth0 Link encap 10Mbps Ethernet HWaddr 00:00:C0:90:B3:42
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inet addr 191.72.1.2 Bcast 191.72.1.255 Mask 255.255.255.0
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UP BROADCAST RUNNING MTU 1500 Metric 0
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RX packets 3136 errors 217 dropped 7 overrun 26
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TX packets 1752 errors 25 dropped 0 overrun 0
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</pre>
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The MTU and Metric fields show the current MTU and metric
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value for that interface. The metric value is traditionally used by some
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operating systems to compute the cost of a route. doesn't use this
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value yet, but defines it for compatibility nevertheless.
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<P>
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The RX and TX lines show how many packets have been received or
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transmitted error free, how many errors occurred, how many packets
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were dropped, probably because of low memory, and how many were lost
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because of an overrun. Receiver overruns usually happen when packets
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come in faster than the kernel can service the last interrupt. The
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flag values printed by ifconfig correspond more or less to the
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names of its command line options; they will be explained below.
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<P>
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The following is a list of parameters recognized by ifconfig with
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the corresponding flag names are given in brackets. Options that simply
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turn on a feature also allow it to be turned off again by preceding the
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option name by a dash (-).
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<dl>
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<dt><b>up </b><dd>This marks an interface ``up'', i.e. accessible to the IP
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layer. This option is implied when an address is given on the
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command line. It may also be used to re-enable an interface
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that has been taken down temporarily using the down option.
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(This option corresponds to the flags UP RUNNING.)
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<dt><b>down</b><dd> This marks an interface ``down'', i.e. inaccessible to the IP
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layer. This effectively disables any IP traffic through the
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interface. Note that this does not delete all routing entries
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that use this interface automatically. If you take the inter-
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face down permanently, you should to delete these routing
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entries and supply alternative routes if possible.
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<dt><b> netmask mask </b><dd>This assigns a subnet mask to be used by the interface.
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It may be given as either a 32-bit hexadecimal number preceded
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by 0x, or as a dotted quad of decimal numbers.
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that involve only two hosts. This option is needed to config-
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ure, for example, SLIP or PLIP interfaces.
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<dt><b> pointopoint</b><dd> address This option is used for point-to-point IP links
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that involve only two hosts. This option is needed to config-
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ure, for example, SLIP or PLIP interfaces.
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(If a point-to-point address has been set, ifconfig displays the POINTO-
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POINT flag.)
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<dt><b> broadcast address</b><dd> The broadcast address is usually made up from the
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network number by setting all bits of the host part. Some IP
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implementations use a different scheme; this option is there
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to adapt to these strange environments.
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(If a broadcast address has been set, ifconfig displays the BROADCAST
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flag.)
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<dt><b> metric number </b><dd> This option may be used to assign a metric value to the
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routing table entry created for the interface. This metric is
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used by the Routing Information Protocol (RIP) to build rout-
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ing tables for the network.(8) The default metric used by
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ifconfig is a value of zero. If you don't run a RIP daemon,
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you don't need this option at all; if you do, you will rarely
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need to change the metric value.
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<dt><b> mtu bytes </b><dd> This sets the Maximum Transmission Unit, which is the maximum
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number of octets the interface is able to handle in one trans-
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action. For Ethernets, the MTU defaults to 1500; for SLIP
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interfaces, this is 296.
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<dt><b>arp</b><dd> This is an option specific to broadcast networks such as Eth-
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ernets or packet radio. It enables the use of ARP, the Address
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Resolution Protocol, to detect the physical addresses of hosts
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attached to the network. For broadcast networks, is on by
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default.
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<dt><b>-arp </b><dd>Disables the use of ARP on this interface.
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<dt><b>promisc</b><dd> Puts the interface in promiscuous mode. On a broadcast net-
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work, this makes the interface receive all packets, regardless
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of whether they were destined for another host or not. This
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allows an analysis of network traffic using packet filters and
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such, also called Ethernet snooping. Usually, this is a good
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technique of hunting down network problems that are otherwise
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hard to come by.
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On the other hand, this allows attackers to skim the
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traffic of your network for passwords and do other nasty
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things. One protection against this type of attack is not to
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let anyone just plug in their computers in your Ethernet.
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Another option is to use secure authentication protocols, such
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as Kerberos, or the SRA login suite.(9)
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(This option corresponds to the flag PROMISC.)
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<dt><b>-promisc</b><dd> Turns off promiscuous mode.
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<dt><b>allmulti</b><dd> Multicast addresses are some sort of broadcast to a group of
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hosts who don't necessarily have to be on the same subnet.
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Multicast addresses are not yet supported by the kernel.
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(This option corresponds to the flag ALLMULTI.)
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<dt><b>-allmulti</b><dd> Turns off multicast addresses.
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</dl>
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<P><A NAME="ifaceinterfaceifconfigmetric"></A><P><HR><A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A HREF="node74.html">Checking with netstat</A>
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<B>Up:</B> <A HREF="node58.html">Configuring TCP/IP Networking</A>
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<B> Previous:</B> <A HREF="node72.html">The Dummy Interface</A>
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<P><ADDRESS>
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<I>Andrew Anderson <BR>
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Thu Mar 7 23:22:06 EST 1996</I>
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</ADDRESS>
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