arp(8)
NAME
arp - manipulate the system ARP cache
SYNOPSIS
arp [-vn] [-H type] [-i if] [-ae] [hostname] arp [-v] [-i if] -d hostname [pub] arp [-v] [-H type] [-i if] -s hostname hw_addr [temp] arp [-v] [-H type] [-i if] -s hostname hw_addr [netmask nm] pub arp [-v] [-H type] [-i if] -Ds hostname ifname [netmask nm] pub arp [-vnD] [-H type] [-i if] -f [filename]
DESCRIPTION
Arp manipulates or displays the kernel's IPv4 network neighbour cache. It can add entries to the table, delete one or display the current content. ARP stands for Address Resolution Protocol, which is used to find the media access control address of a network neighbour for a given IPv4 Address.
MODES
arp with no mode specifier will print the current content of the table. It is possible to limit the number of entries printed, by specifying an hardware address type, interface name or host address. arp -d address will delete a ARP table entry. Root or netadmin privilege is required to do this. The entry is found by IP address. If a hostname is given, it will be resolved before looking up the entry in the ARP table. arp -s address hw_addr is used to set up a new table entry. The format of the hw_addr parameter is dependent on the hardware class, but for most classes one can assume that the usual presentation can be used. For the Ethernet class, this is 6 bytes in hexadecimal, separated by colons. When adding proxy arp entries (that is those with the publish flag set) a netmask may be specified to proxy arp for entire subnets. This is not good practice, but is supported by older kernels because it can be useful. If the temp flag is not supplied entries will be permanent stored into the ARP cache. To simplify setting up entries for one of your own network interfaces, you can use the arp -Ds address ifname form. In that case the hardware address is taken from the interface with the specified name.
OPTIONS
-v, --verbose
Tell the user what is going on by being verbose.
-n, --numeric
shows numerical addresses instead of trying to determine
symbolic host, port or user names.
-H type, --hw-type type, -t type
When setting or reading the ARP cache, this optional parameter
tells arp which class of entries it should check for. The
default value of this parameter is ether (i.e. hardware code
0x01 for IEEE 802.3 10Mbps Ethernet). Other values might
include network technologies such as ARCnet (arcnet) , PROnet
(pronet) , AX.25 (ax25) and NET/ROM (netrom).
-a Use alternate BSD style output format (with no fixed columns).
-e Use default Linux style output format (with fixed columns).
-D, --use-device
Instead of a hw_addr, the given argument is the name of an
interface. arp will use the MAC address of that interface for
the table entry. This is usually the best option to set up a
proxy ARP entry to yourself.
-i If, --device If
Select an interface. When dumping the ARP cache only entries
matching the specified interface will be printed. When setting a
permanent or temp ARP entry this interface will be associated
with the entry; if this option is not used, the kernel will
guess based on the routing table. For pub entries the specified
interface is the interface on which ARP requests will be
answered.
NOTE: This has to be different from the interface to which the
IP datagrams will be routed. NOTE: As of kernel 2.2.0 it is no
longer possible to set an ARP entry for an entire subnet. Linux
instead does automagic proxy arp when a route exists and it is
forwarding. See arp(7) for details. Also the dontpub option
which is available for delete and set operations cannot be used
with 2.4 and newer kernels.
-f filename, --file filename
Similar to the -s option, only this time the address info is
taken from file filename. This can be used if ARP entries for a
lot of hosts have to be set up. The name of the data file is
very often /etc/ethers, but this is not official. If no filename
is specified /etc/ethers is used as default.
The format of the file is simple; it only contains ASCII text
lines with a hostname, and a hardware address separated by
whitespace. Additionally the pub, temp and netmask flags can be
used.
In all places where a hostname is expected, one can also enter an IP
address in dotted-decimal notation.
As a special case for compatibility the order of the hostname and the
hardware address can be exchanged.
Each complete entry in the ARP cache will be marked with the C flag.
Permanent entries are marked with M and published entries have the P
flag.
EXAMPLES
/usr/sbin/arp -i eth0 -Ds 10.0.0.2 eth1 pub This will answer ARP requests for 10.0.0.2 on eth0 with the MAC address for eth1. /usr/sbin/arp -i eth1 -d 10.0.0.1 Delete the ARP table entry for 10.0.0.1 on interface eth1. This will match published proxy ARP entries and permanent entries.
FILES
/proc/net/arp /etc/networks /etc/hosts /etc/ethers
SEE ALSO
rarp(8), route(8), ifconfig(8), netstat(8)
AUTHORS
Fred N. van Kempen <waltje@uwalt.nl.mugnet.org>, Bernd Eckenfels <net-tools@lina.inka.de>.
Opportunity
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
Free Books
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
