View source for Address Resolution Protocol

{{Short description|Protocol used for discovering the network layer address associated with an internet layer address}}

{{IPstack}}

The '''Address Resolution Protocol''' ('''ARP''') is a [[communication protocol]] for discovering the [[link layer]] address, such as a [[MAC address]], associated with a [[internet layer]] address, typically an [[IPv4 address]]. The protocol, part of the [[Internet protocol suite]], was defined in 1982 by {{IETF RFC|826}}, which is [[Internet Standard]] STD 37.

ARP enables a host to send an IPv4 packet to another node in the local network by providing a protocol to get the MAC address associated with an IP address. The host broadcasts a request containing the node's IP address, and the node with that IP address replies with its MAC address.

ARP has been implemented with many combinations of network and data link layer technologies, such as [[IPv4]], [[Chaosnet]], [[DECnet]] and Xerox [[PARC Universal Packet]] (PUP) using [[IEEE 802]] standards, [[FDDI]], [[X.25]], [[Frame Relay]] and [[Asynchronous Transfer Mode]] (ATM).

In [[Internet Protocol Version 6]] (IPv6) networks, the functionality of ARP is provided by the [[Neighbor Discovery Protocol]] (NDP).

==Operating scope==
The Address Resolution Protocol is a [[request-response]] protocol. Its messages are directly encapsulated by a link layer protocol. It is communicated within the boundaries of a single [[subnetwork]] and is never [[routed]].

==Packet structure==
The Address Resolution Protocol uses a simple message format containing one address resolution request or response. The packets are carried at the [[data link layer]] of the underlying network as raw payload. In the case of Ethernet, a {{mono|0x0806}} [[EtherType]] value is used to identify ARP frames.

The size of the ARP message depends on the link layer and network layer address sizes. The message [[Header (computing)|header]] specifies the types of network in use at each layer as well as the size of addresses of each. The message header is completed with the operation code for request (1) and reply (2). The payload of the packet consists of four addresses, the hardware and protocol address of the sender and receiver hosts.

The principal packet structure of ARP packets is shown in the following table which illustrates the case of IPv4 networks running on Ethernet. In this scenario, the packet has 48-bit fields for the sender hardware address (SHA) and target hardware address (THA), and 32-bit fields for the corresponding sender and target protocol addresses (SPA and TPA). The ARP packet size in this case is 28 bytes.
{{APHD|start|title=Internet Protocol (IPv4) over Ethernet ARP packet}}
{{APHD|0|bits1=16|field1=Hardware Type|value1={{Mono|1}}|hint1=A value of 1 indicates Ethernet.|bits2=16|field2=Protocol Type|hint2=0x800 indicates IPv4|value2={{Mono|0x0800}}}}
{{APHD|4|bits1=8|field1=Hardware Length|value1={{Mono|6}}|bits2=8|field2=Protocol Length|value2={{Mono|4}}|bits3=16|field3=Operation}}
{{APHD|8|bits1=32|border1=bottom|field1=Sender Hardware Address}}
{{APHD|12|bits1=16|border1=top|field1={{Nbsp}}|bits2=16|border2=right|field2=Sender Protocol Address}}
{{APHD|16|bits1=16|border1=left|field1=Sender Protocol Address (cont.)|bits2=16|border2=bottom|field2=Target Hardware Address}}
{{APHD|20|bits1=32|border1=top|field1={{Nbsp}}}}
{{APHD|24|bits1=32|field1=Target Protocol Address}}
{{APHD|end}}
;{{APHD|def|name=Hardware Type|short=HTYPE|length=16 bits|text=This field specifies the network link protocol type.<ref name="IANA">{{Cite web|url=https://www.iana.org/assignments/arp-parameters/arp-parameters.xhtml|title=Address Resolution Protocol (ARP) Parameters|website=www.iana.org|access-date=2018-10-16}}</ref> In this example, a value of {{Mono|1}} indicates [[Ethernet]].}}
;{{APHD|def|name=Protocol Type|short=PTYPE|length=16 bits|text=This field specifies the internetwork protocol for which the ARP request is intended. For IPv4, this has the value {{mono|0x0800}}. The permitted PTYPE values share a numbering space with those for [[EtherType]].<ref name="IANA" />{{Ref RFC|9542}}}}
;{{APHD|def|name=Hardware Length|short=HLEN|length=8 bits|text=Length (in [[octet (computing)|octet]]s) of a hardware address. For Ethernet, the address length is {{Mono|6}}.}}
;{{APHD|def|name=Protocol Length|short=PLEN|length=8 bits|text=Length (in octets) of internetwork addresses. The internetwork protocol is specified in PTYPE. In this example: IPv4 address length is {{Mono|4}}.}}
;{{APHD|def|name=Operation|short=OPER|length=16 bits|text=Specifies the operation that the sender is performing: 1 for request, 2 for reply.}}
;{{APHD|def|name=Sender Hardware Address|short=SHA|length=48 bits|text=Media address of the sender. In an ARP request this field is used to indicate the address of the host sending the request. In an ARP reply this field is used to indicate the address of the host that the request was looking for.}}
;{{APHD|def|name=Sender protocol address|short=SPA|length=32 bits|text=Internetwork address of the sender.}}
;{{APHD|def|name=Target hardware address|short=THA|length=48 bits|text=Media address of the intended receiver. In an ARP request this field is ignored. In an ARP reply this field is used to indicate the address of the host that originated the ARP request.}}
;{{APHD|def|name=Target protocol address|short=TPA|length=32 bits|text=Internetwork address of the intended receiver.}}
ARP parameter values have been standardized and are maintained by the [[Internet Assigned Numbers Authority]] (IANA).<ref name="IANA" />

The [[EtherType]] for ARP is {{mono|0x0806}}. This appears in the Ethernet frame header when the payload is an ARP packet and is not to be confused with PTYPE, which appears within this encapsulated ARP packet.

==Layering==
ARP's placement within the [[Internet protocol suite]] and the [[OSI model]] may be a matter of confusion or even of dispute. {{IETF RFC|826}} places it into the [[Link Layer]] and characterizes it as a tool to inquire about the "higher level layer", such as the Internet layer.{{Ref RFC|826|section=Network monitoring and debugging}} {{IETF RFC|1122}} also discusses ARP in its link layer section.{{Ref RFC|1122}}
Richard Stevens places ARP in OSI's data link layer<ref>W. Richard Stevens, ''TCP/IP Illustrated, Volume 1: The Protocols'', Addison Wesley, 1994, ISBN 0-201-63346-9.</ref> while newer editions associate it with the network layer or introduce an intermediate OSI layer 2.5.<ref>W. Richard Stevens, ''TCP/IP Illustrated, Volume 1: The Protocols'', Addison Wesley, 2011, ISBN 0-321-33631-3, page 14</ref>

==Example==
Two computers, ''A'' and ''B'', are connected to the same [[local area network]] with no intervening [[Gateway (telecommunications)|gateway]] or [[Router (computing)|router]]. ''A'' has a packet to send to IP address {{IPaddr|192.168.0.55}} which happens to be the address of ''B''.

Before sending the packet to ''B'', ''A'' broadcasts an ARP request message {{endash}} addressed with the broadcast MAC address {{MACaddr|FF:FF:FF:FF:FF:FF}} and requesting response from the node with IP address {{IPaddr|192.168.0.55}}. All nodes of the network receive the message, but only ''B'' replies since it has the requested IP address. ''B'' responds with an ARP response message containing its MAC addresses which ''A'' receives. ''A'' sends the data packet on the link addressed with ''B''{{'}}s MAC address.

Typically, network nodes maintain a [[lookup table|lookup]] cache that associates IP and MAC addressees. In this example, if ''A'' had the lookup cached, then it would not need to broadcast the ARP request. Also, when ''B'' received the request, it could cache the lookup to ''A'' so that if ''B'' needs to send a packet to ''A'' later, it does not need to use ARP to lookup its MAC address. Finally, when ''A'' receives the ARP response, it can cache the lookup for future messages addressed to the same IP address.<ref>{{cite book|last1=Chappell|first1=Laura A.|last2=Tittel|first2=Ed|title=Guide to TCP/IP|edition=Third|publisher=Thomson Course Technology|year=2007|pages=115–116|isbn=9781418837556}}</ref>

==ARP probe==
An '''ARP probe''' in IPv4 is an ARP request constructed with the SHA of the probing host, an SPA of all 0s, a THA of all 0s, and a TPA set to the IPv4 address being probed for.  If some host on the network regards the IPv4 address (in the TPA) as its own, it will reply to the probe (via the SHA of the probing host) thus informing the probing host of the address conflict.  If instead there is no host which regards the IPv4 address as its own, then there will be no reply.  When several such probes have been sent, with slight delays, and none receive replies, it can reasonably be expected that no conflict exists.  As the original probe packet contains neither a valid SHA/SPA nor a valid THA/TPA pair, there is no risk of any host using the packet to update its cache with problematic data.  Before beginning to use an IPv4 address (whether received from manual configuration, DHCP, or some other means), a host implementing this specification must test to see if the address is already in use, by broadcasting ARP probe packets.{{Ref RFC|5227}}<ref>{{cite web |last1=Harmoush |first1=Ed |title=ARP Probe and ARP Announcement |url=https://www.practicalnetworking.net/series/arp/arp-probe-arp-announcement |website=Practical Networking |publisher=PracticalNetworking .net |access-date=3 August 2022}}</ref>

==ARP announcements==
ARP may also be used as a simple announcement protocol. This is useful for updating other hosts' mappings of a hardware address when the sender's IP address or MAC address changes. Such an announcement, also called a '''gratuitous ARP''' (GARP) message, is usually broadcast as an ''ARP request'' containing the SPA in the target field (TPA=SPA), with THA set to zero. An alternative way is to broadcast an ''ARP reply'' with the sender's SHA and SPA duplicated in the target fields (TPA=SPA, THA=SHA).

The ''ARP request'' and ''ARP reply'' announcements are both standards-based methods,{{Ref RFC|5944|rsection=4.6}} but the ''ARP request'' method is preferred.{{Ref RFC|5227|rsection=3|quote=Why Are ARP Announcements Performed Using ARP Request Packets and Not ARP Reply Packets?}} Some devices may be configured for the use of either of these two types of announcements.<ref>{{cite web
 | url = http://support.citrix.com/article/CTX112701
 | title = FAQ: The Firewall Does not Update the Address Resolution Protocol Table
 | publisher = [[Citrix]]
 | date = 2015-01-16
 | quote =  [...] garpReply enabled [...] generates ARP packets that [...] are of OPCODE type REPLY, rather than REQUEST.
}}</ref>

An ARP announcement is not intended to solicit a reply; instead, it updates any cached entries in the ARP tables of other hosts that receive the packet. The operation code in the announcement may be either request or reply; the ARP standard specifies that the opcode is only processed after the ARP table has been updated from the address fields.<ref>{{cite web|url=http://www1.ietf.org/mail-archive/web/dhcwg/current/msg03797.html|title=Gratuitous ARP in DHCP vs. IPv4 ACD Draft|archive-url=https://web.archive.org/web/20071012093401/http://www1.ietf.org/mail-archive/web/dhcwg/current/msg03797.html |archive-date=October 12, 2007 }}</ref>{{Ref RFC|5944|rsection=4.6}}{{Ref RFC|2131|rsection=4.4.1}}

Many operating systems issue an ARP announcement during startup. This helps to resolve problems that would otherwise occur if, for example, a [[network card]] was recently changed (changing the IP-address-to-MAC-address mapping) and other hosts still have the old mapping in their ARP caches.

ARP announcements are also used by some network interfaces to provide load balancing for incoming traffic. In a [[NIC teaming|team]] of network cards, it is used to announce a different MAC address within the team that should receive incoming packets.

ARP announcements can be used in the [[Zeroconf]] protocol to allow automatic assignment of a [[link-local address]] to an interface where no other IP address configuration is available. The announcements are used to ensure an address chosen by a host is not in use by other hosts on the network link.{{Ref RFC|3927}}

This function can be dangerous from a cybersecurity viewpoint since an attacker can obtain information about the other hosts of its subnet to save in their ARP cache ([[ARP spoofing]]) an entry where the attacker MAC is associated, for instance, to the IP of the [[default gateway]], thus allowing them to [[Man-in-the-middle attack|intercept]] all the traffic to external networks.

==ARP mediation==
'''ARP mediation''' refers to the process of resolving Layer-2 addresses through a [[virtual private wire service]] (VPWS) when different resolution protocols are used on the connected circuits, e.g., [[Ethernet]] on one end and [[Frame Relay]] on the other.  In [[IPv4]], each [[provider edge]] (PE) device discovers the IP address of the locally attached [[customer edge]] (CE) device and distributes that IP address to the corresponding remote PE device.  Then each PE device responds to local ARP requests using the IP address of the remote CE device and the hardware address of the local PE device.  In [[IPv6]], each PE device discovers the IP address of both local and remote CE devices and then intercepts local [[Neighbor Discovery]] (ND) and [[Inverse Neighbor Discovery]] (IND) packets and forwards them to the remote PE device.<ref>{{cite IETF | rfc = 6575 | author = Shah, H. | title = Address Resolution Protocol (ARP) Mediation for IP Interworking of Layer 2 VPNs | publisher = Internet Engineering Task Force | date = June 2012 | display-authors = etal}}</ref>

=={{anchor|INARP}}Inverse ARP and Reverse ARP==
'''Inverse Address Resolution Protocol''' ('''Inverse ARP''' or '''InARP''') is used to obtain [[network layer]] addresses (for example, [[IP address]]es) of other nodes from [[data link layer]] (Layer 2) addresses. Since ARP translates layer-3 addresses to layer-2 addresses, InARP may be described as its inverse. In addition, InARP is implemented as a protocol extension to ARP: it uses the same packet format as ARP, but different operation codes.

InARP is primarily used in [[Frame Relay]] ([[DLCI]]) and ATM networks, in which layer-2 addresses of [[virtual circuit]]s are sometimes obtained from layer-2 signaling, and the corresponding layer-3 addresses must be available before those virtual circuits can be used.{{Ref RFC|2390}}

The [[Reverse Address Resolution Protocol]] (Reverse ARP or RARP), like InARP, translates layer-2 addresses to layer-3 addresses. However, in InARP the requesting station queries the layer-3 address of another node, whereas RARP is used to obtain the layer-3 address of the requesting station itself for address configuration purposes. RARP is obsolete; it was replaced by [[BOOTP]], which was later superseded by the [[Dynamic Host Configuration Protocol]] (DHCP).{{Ref RFC|903}}

==ARP spoofing and proxy ARP==
{{main|ARP spoofing|Proxy ARP}}
[[Image:ARP Spoofing.svg|right|thumb|200px|A successful [[ARP spoofing]] attack allows an attacker to perform a [[man-in-the-middle attack]].]]
Because ARP does not provide methods for authenticating ARP replies on a network, ARP replies can come from systems other than the one with the required Layer 2 address.  An ARP ''proxy'' is a system that answers the ARP request on behalf of another system for which it will forward traffic, normally as a part of the network's design, such as for a dialup internet service. By contrast, in ARP ''spoofing'' the answering system, or ''spoofer'', replies to a request for another system's address with the aim of intercepting data bound for that system. A malicious user may use ARP spoofing to perform a [[man-in-the-middle]] or [[denial-of-service]] attack on other users on the network. Various software exists to both detect and perform ARP spoofing attacks, though ARP itself does not provide any methods of protection from such attacks.<ref name="grc">{{cite web | url = http://www.grc.com/nat/arp.htm | author = Steve Gibson | title = ARP Cache Poisoning | publisher = [[Gibson Research Corporation|GRC]] | date = 2005-12-11}}</ref>

==Alternatives==
IPv6 uses the [[Neighbor Discovery Protocol]] and its extensions such as [[Secure Neighbor Discovery]], rather than ARP.

Computers can maintain lists of known addresses, rather than using an active protocol. In this model, each computer maintains a database of the mapping of [[Layer 3]] addresses (e.g., [[IP addresses]]) to [[Layer 2]] addresses (e.g., [[Ethernet]] [[MAC addresses]]). This data is maintained primarily by interpreting ARP packets from the local network link. Thus, it is often called the ''[[ARP cache]]''. Since at least the 1980s,<ref>{{cite web
| url = http://www.freebsd.org/cgi/man.cgi?query=arp&apropos=0&sektion=0&manpath=2.10+BSD&arch=default&format=html
| author = University of California, Berkeley
| title = BSD manual page for arp(8C) command
| access-date= 2011-09-28}}</ref> networked computers have a utility called ''arp'' for interrogating or manipulating this database.<ref>{{cite web
 |url=http://manpages.ubuntu.com/manpages/lucid/man8/arp.8.html 
 |author=Canonical 
 |title=Ubuntu manual page for arp(8) command 
 |access-date=2011-09-28 
 |url-status=dead 
 |archive-url=https://web.archive.org/web/20120316213518/http://manpages.ubuntu.com/manpages/lucid/man8/arp.8.html 
 |archive-date=2012-03-16 
}}</ref><ref>{{cite web
| url = https://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man8/arp.8.html
| author = Apple Computer
| title = Mac OS X manual page for arp(8) command
| access-date= 2011-09-28}}</ref><ref>{{cite web
| url = https://technet.microsoft.com/en-us/library/cc786759%28WS.10%29.aspx
| author = Microsoft
| title = Windows help for arp command
| access-date= 2011-09-28}}</ref>

Historically, other methods were used to maintain the mapping between addresses, such as static configuration files,<ref>{{cite web
| url = http://www.freebsd.org/cgi/man.cgi?query=ethers&sektion=5&apropos=0&manpath=SunOS+4.1.3
| author = Sun Microsystems
| title = SunOS manual page for ethers(5) file
| access-date= 2011-09-28}}</ref> or centrally maintained lists.

==ARP stuffing==
Embedded systems such as networked cameras<ref>{{cite web
| url = http://www.axis.com/files/manuals/ig_p13Series_38731_en_1006.pdf
| author = Axis Communication
| title = Axis P13 Network Camera Series Installation Guide
| access-date= 2011-09-28}}</ref> and networked power distribution devices,<ref>
{{cite web
| url = http://www.apcmedia.com/salestools/ASTE-6Z6K56_R0_EN.pdf
| author = American Power Corporation
| title = Switched Rack Power Distribution Unit Installation and Quick Start Manual
| access-date = 2011-09-28
| archive-url = https://web.archive.org/web/20111125012617/http://www.apcmedia.com/salestools/ASTE-6Z6K56_R0_EN.pdf
| archive-date = 2011-11-25
| url-status = dead
}}
</ref> which lack a user interface, can use so-called ''ARP stuffing'' to make an initial network connection, although this is a misnomer, as ARP is not involved.

ARP stuffing is accomplished as follows:
# The user's computer has an IP address ''stuffed'' manually into its address table (normally with the ''arp'' command with the MAC address taken from a label on the device)
# The computer sends special packets to the device, typically a [[Ping (networking utility)|ping]] packet with a non-default size.
# The device then adopts this IP address
# The user then communicates with it by [[telnet]] or [[http|web]] protocols to complete the configuration.

Such devices typically have a method to disable this process once the device is operating normally, as the capability can make it vulnerable to attack.

==Standards documents==
* {{Sum RFC|826}}
* {{Sum RFC|903}}
* {{Sum RFC|2390}}
* {{Sum RFC|5227}}

==See also==
* {{Annotated link|Arping}}
* {{Annotated link|Arptables}}
* {{Annotated link|Arpwatch}}
* {{Annotated link|Bonjour Sleep Proxy}}
* {{Annotated link|Cisco HDLC}}

==References==
{{Reflist}}

==External links==
{{Wikiversity | Address Resolution Protocol}}
* {{cite web |url=http://www.eventhelix.com/RealtimeMantra/Networking/Arp.pdf |title=ARP Sequence Diagram (pdf) |archive-url=https://web.archive.org/web/20210301060206/http://www.eventhelix.com/RealtimeMantra/Networking/Arp.pdf |archive-date=2021-03-01}}
* [http://wiki.wireshark.org/Gratuitous_ARP Gratuitous ARP]
* [https://gitlab.com/wireshark/wireshark/-/wikis/AddressResolutionProtocol Information and sample capture from Wireshark]
* [https://web.archive.org/web/20090903074149/http://sid.rstack.org/arp-sk/ ARP-SK ARP traffic generation tools]

{{IETF RFC standards}}

[[Category:Address Resolution Protocol| ]]
[[Category:Internet Standards]]