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RFC 5227 - IPv4 Address Conflict Detection


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Network Working Group                                        S. Cheshire
Request for Comments: 5227                                    Apple Inc.
Updates: 826                                                   July 2008
Category: Standards Track

                    IPv4 Address Conflict Detection

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   When two hosts on the same link attempt to use the same IPv4 address
   at the same time (except in rare special cases where this has been
   arranged by prior coordination), problems ensue for one or both
   hosts.  This document describes (i) a simple precaution that a host
   can take in advance to help prevent this misconfiguration from
   happening, and (ii) if this misconfiguration does occur, a simple
   mechanism by which a host can passively detect, after the fact, that
   it has happened, so that the host or administrator may respond to
   rectify the problem.

Table of Contents

   1. Introduction ....................................................2
      1.1. Conventions and Terminology Used in This Document ..........4
      1.2. Relationship to RFC 826 ....................................5
           1.2.1. Broadcast ARP Replies ...............................7
      1.3. Applicability ..............................................7
   2. Address Probing, Announcing, Conflict Detection, and Defense ....9
      2.1. Probing an Address ........................................10
           2.1.1. Probe Details ......................................10
      2.2. Shorter Timeouts on Appropriate Network Technologies ......11
      2.3. Announcing an Address .....................................12
      2.4. Ongoing Address Conflict Detection and Address Defense ....12
      2.5. Continuing Operation ......................................14
      2.6. Broadcast ARP Replies .....................................14
   3. Why Are ARP Announcements Performed Using ARP Request
      Packets and Not ARP Reply Packets? .............................15
   4. Historical Note ................................................17
   5. Security Considerations ........................................17
   6. Acknowledgments ................................................18
   7. References .....................................................18
      7.1. Normative References ......................................18
      7.2. Informative References ....................................19

1.  Introduction

   Historically, accidentally configuring two Internet hosts with the
   same IP address has often been an annoying and hard-to-diagnose
   problem.

   This is unfortunate, because the existing Address Resolution Protocol
   (ARP) provides an easy way for a host to detect this kind of
   misconfiguration and report it to the user.  The DHCP specification
   [RFC2131] briefly mentions the role of ARP in detecting
   misconfiguration, as illustrated in the following three excerpts from
   RFC 2131:

   o the client SHOULD probe the newly received address, e.g., with ARP

   o The client SHOULD perform a final check on the parameters
     (e.g., ARP for allocated network address)

   o If the client detects that the address is already in use
     (e.g., through the use of ARP), the client MUST send a DHCPDECLINE
     message to the server

   Unfortunately, the DHCP specification does not give any guidance
   to implementers concerning the number of ARP packets to send, the
   interval between packets, the total time to wait before concluding
   that an address may safely be used, or indeed even which kinds
   of packets a host should be listening for, in order to make this
   determination.  It leaves unspecified the action a host should
   take if, after concluding that an address may safely be used, it
   subsequently discovers that it was wrong.  It also fails to specify
   what precautions a DHCP client should take to guard against
   pathological failure cases, such as a DHCP server that repeatedly
   OFFERs the same address, even though it has been DECLINEd multiple
   times.

   The authors of the DHCP specification may have been justified in
   thinking at the time that the answers to these questions seemed too
   simple, obvious, and straightforward to be worth mentioning, but
   unfortunately this left some of the burden of protocol design to each
   individual implementer.  This document seeks to remedy this omission
   by clearly specifying the required actions for:

   1. Determining whether use of an address is likely to lead to an
      addressing conflict.  This includes (a) the case where the address
      is already actively in use by another host on the same link, and
      (b) the case where two hosts are inadvertently about to begin
      using the same address, and both are simultaneously in the process
      of probing to determine whether the address may safely be used
      (Section 2.1.).

   2. Subsequent passive detection that another host on the network is
      inadvertently using the same address.  Even if all hosts observe
      precautions to avoid using an address that is already in use,
      conflicts can still occur if two hosts are out of communication
      at the time of initial interface configuration.  This could occur
      with wireless network interfaces if the hosts are temporarily out
      of range, or with Ethernet interfaces if the link between two
      Ethernet hubs is not functioning at the time of address
      configuration.  A well-designed host will handle not only
      conflicts detected during interface configuration, but also
      conflicts detected later, for the entire duration of the time
      that the host is using the address (Section 2.4.).

   3. Rate-limiting of address acquisition attempts in the case of
      an excessive number of repeated conflicts (Section 2.1.).

   The utility of IPv4 Address Conflict Detection (ACD) is not limited
   to DHCP clients.  No matter how an address was configured, whether
   via manual entry by a human user, via information received from a
   DHCP server, or via any other source of configuration information,

   detecting conflicts is useful.  Upon detecting a conflict, the
   configuring agent should be notified of the error.  In the case where
   the configuring agent is a human user, that notification may take the
   form of an error message on a screen, a Simple Network Management
   Protocol (SNMP) notification, or an error message sent via text
   message to a mobile phone.  In the case of a DHCP server, that
   notification takes the form of a DHCP DECLINE message sent to the
   server.  In the case of configuration by some other kind of software,
   that notification takes the form of an error indication to the
   software in question, to inform it that the address it selected is
   in conflict with some other host on the network.  The configuring
   software may choose to cease network operation, or it may
   automatically select a new address so that the host may re-establish
   IP connectivity as soon as possible.

   Allocation of IPv4 Link-Local Addresses [RFC3927] can be thought of
   as a special case of this mechanism, where the configuring agent is
   a pseudo-random number generator, and the action it takes upon being
   notified of a conflict is to pick a different random number and try
   again.  In fact, this is exactly how IPv4 Link-Local Addressing was
   implemented in Mac OS 9 back in 1998.  If the DHCP client failed to
   get a response from any DHCP server, it would simply make up a fake
   response containing a random 169.254.x.x address.  If the ARP module
   reported a conflict for that address, then the DHCP client would try
   again, making up a new random 169.254.x.x address as many times as
   was necessary until it succeeded.  Implementing ACD as a standard
   feature of the networking stack has the side effect that it means
   that half the work for IPv4 Link-Local Addressing is already done.

1.1.  Conventions and Terminology Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in "Key words for use in
   RFCs to Indicate Requirement Levels" [RFC2119].

   Wherever this document uses the term 'sender IP address' or 'target
   IP address' in the context of an ARP packet, it is referring to the
   fields of the ARP packet identified in the ARP specification [RFC826]
   as 'ar$spa' (Sender Protocol Address) and 'ar$tpa' (Target Protocol
   Address), respectively.  For the usage of ARP described in this
   document, each of these fields always contains an IPv4 address.

   In this document, the term 'ARP Probe' is used to refer to an ARP
   Request packet, broadcast on the local link, with an all-zero 'sender
   IP address'.  The 'sender hardware address' MUST contain the hardware
   address of the interface sending the packet.  The 'sender IP address'
   field MUST be set to all zeroes, to avoid polluting ARP caches in

   other hosts on the same link in the case where the address turns out
   to be already in use by another host.  The 'target hardware address'
   field is ignored and SHOULD be set to all zeroes.  The 'target IP
   address' field MUST be set to the address being probed.  An ARP Probe
   conveys both a question ("Is anyone using this address?") and an
   implied statement ("This is the address I hope to use.").

   In this document, the term 'ARP Announcement' is used to refer to an
   ARP Request packet, broadcast on the local link, identical to the ARP
   Probe described above, except that both the sender and target IP
   address fields contain the IP address being announced.  It conveys a
   stronger statement than an ARP Probe, namely, "This is the address I
   am now using."

   The following timing constants used in this protocol are referenced
   in Section 2, which describes the operation of the protocol in
   detail.  (Note that the values listed here are fixed constants; they
   are not intended to be modifiable by implementers, operators, or end
   users.  These constants are given symbolic names here to facilitate
   the writing of future standards that may want to reference this
   document with different values for these named constants; however,
   at the present time no such future standards exist.)

   PROBE_WAIT           1 second   (initial random delay)
   PROBE_NUM            3          (number of probe packets)
   PROBE_MIN            1 second   (minimum delay until repeated probe)
   PROBE_MAX            2 seconds  (maximum delay until repeated probe)
   ANNOUNCE_WAIT        2 seconds  (delay before announcing)
   ANNOUNCE_NUM         2          (number of Announcement packets)
   ANNOUNCE_INTERVAL    2 seconds  (time between Announcement packets)
   MAX_CONFLICTS       10          (max conflicts before rate-limiting)
   RATE_LIMIT_INTERVAL 60 seconds  (delay between successive attempts)
   DEFEND_INTERVAL     10 seconds  (minimum interval between defensive
                                    ARPs)
1.2.  Relationship to RFC 826

   This document does not modify any of the protocol rules in RFC 826.
   It does not modify the packet format, or the meaning of any of the
   fields.  The existing rules for "Packet Generation" and "Packet
   Reception" still apply exactly as specified in RFC 826.

   This document expands on RFC 826 by specifying:

   (1) that a specific ARP Request should be generated when an interface
       is configured, to discover if the address is already in use, and

   (2) an additional trivial test that should be performed on each
       received ARP packet, to facilitate passive ongoing conflict
       detection.  This additional test creates no additional packet
       overhead on the network (no additional packets are sent) and
       negligible additional CPU burden on hosts, since every host
       implementing ARP is *already* required to process every received
       ARP packet according to the Packet Reception rules specified in
       RFC 826.  These rules already include checking to see if the
       'sender IP address' of the ARP packet appears in any of the
       entries in the host's ARP cache; the additional test is simply to
       check to see if the 'sender IP address' is the host's *own* IP
       address, potentially as little as a single additional machine
       instruction on many architectures.

   As already specified in RFC 826, an ARP Request packet serves two
   functions, an assertion and a question:

   * Assertion:
      The fields 'ar$sha' (Sender Hardware Address) and 'ar$spa' (Sender
      Protocol Address) together serve as an assertion of a fact: that
      the stated Protocol Address is mapped to the stated Hardware
      Address.

   * Question:
      The fields 'ar$tha' (Target Hardware Address, zero) and 'ar$tpa'
      (Target Protocol Address) serve as a question, asking, for the
      stated Protocol Address, to which Hardware Address it is mapped.

   This document clarifies what it means to have one without the other.

   Some readers pointed out that it is probably impossible to ask any
   truly pure question; asking any question necessarily invites
   speculation about why the interrogator wants to know the answer.
   Just as someone pointing to an empty seat and asking, "Is anyone
   sitting here?" implies an unspoken "... because if not then I will,"
   the same is true here.  An ARP Probe with an all-zero 'sender IP
   address' may ostensibly be merely asking an innocent question ("Is
   anyone using this address?"), but an intelligent implementation that
   knows how IPv4 Address Conflict Detection works should be able to
   recognize this question as the precursor to claiming the address.

   Consequently, if that implementation is also, at that exact moment,
   in the process of asking the very same question, it should recognize
   that they can't both sit in the same seat, so it would be prudent to
   ask about some other seat.

1.2.1.  Broadcast ARP Replies

   In some applications of IPv4 Address Conflict Detection (ACD), it may
   be advantageous to deliver ARP Replies using broadcast instead of
   unicast because this allows address conflicts to be detected sooner
   than might otherwise happen.  For example, "Dynamic Configuration of
   IPv4 Link-Local Addresses" [RFC3927] uses ACD exactly as specified
   here, but additionally specifies that ARP Replies should be sent
   using broadcast, because in that context the trade-off of increased
   broadcast traffic in exchange for improved reliability and fault-
   tolerance was deemed to be an appropriate one.  There may be other
   future specifications where the same trade-off is appropriate.
   Additional details are given in Section 2.6, "Broadcast ARP Replies".

   RFC 826 implies that replies to ARP Requests are usually delivered
   using unicast, but it is also acceptable to deliver ARP Replies using
   broadcast.  The Packet Reception rules in RFC 826 specify that the
   content of the 'ar$spa' field should be processed *before* examining
   the 'ar$op' field, so any host that correctly implements the Packet
   Reception algorithm specified in RFC 826 will correctly handle ARP
   Replies delivered via link-layer broadcast.

1.3.  Applicability

   This specification applies to all IEEE 802 Local Area Networks (LANs)
   [802], including Ethernet [802.3], Token-Ring [802.5], and IEEE
   802.11 wireless LANs [802.11], as well as to other link-layer
   technologies that operate at data rates of at least 1 Mb/s, have a
   round-trip latency of at most one second, and use ARP [RFC826] to map
   from IP addresses to link-layer hardware addresses.  Wherever this
   document uses the term "IEEE 802", the text applies equally to any of
   these network technologies.

   Link-layer technologies that support ARP but operate at rates below
   1 Mb/s or latencies above one second will still work correctly with
   this protocol, but more often may have to handle late conflicts
   detected after the Probing phase has completed.  On these kinds of
   links, it may be desirable to specify different values for the
   following parameters:

   (a) PROBE_NUM, PROBE_MIN, and PROBE_MAX, the number of, and interval
       between, ARP Probes, explained in Section 2.1.

   (b) ANNOUNCE_NUM and ANNOUNCE_INTERVAL, the number of, and interval
       between, ARP Announcements, explained in Section 2.3.

   (c) RATE_LIMIT_INTERVAL and MAX_CONFLICTS, controlling the maximum
       rate at which address claiming may be attempted, explained in
       Section 2.1.

   (d) DEFEND_INTERVAL, the time interval between conflicting ARPs below
       which a host MUST NOT attempt to defend its address, explained in
       Section 2.4.

   Link-layer technologies that do not support ARP may be able to use
   other techniques for determining whether a particular IP address is
   currently in use.  However, implementing Address Conflict Detection
   for such networks is outside the scope of this document.

   For the protocol specified in this document to be effective, it is
   not necessary that all hosts on the link implement it.  For a given
   host implementing this specification to be protected against
   accidental address conflicts, all that is required is that the peers
   on the same link correctly implement the ARP protocol as given in
   RFC 826.  To be specific, when a peer host receives an ARP Request
   where the Target Protocol Address of the ARP Request matches (one of)
   that host's IP address(es) configured on that interface, then as long
   as it properly responds with a correctly-formatted ARP Reply, the
   querying host will be able to detect that the address is already in
   use.

   The specifications in this document allow hosts to detect conflicts
   between two hosts using the same address on the same physical link.
   ACD does not detect conflicts between two hosts using the same
   address on different physical links, and indeed it should not.
   For example, the address 10.0.0.1 [RFC1918] is in use by countless
   devices on countless private networks throughout the world, and this
   is not a conflict, because they are on different links.  It would
   only be a conflict if two such devices were to be connected to the
   same link, and when this happens (as it sometimes does), this is a
   perfect example of a situation where ACD is extremely useful to
   detect and report (and/or automatically correct) this error.

   For the purposes of this document, a set of hosts is considered to be
   "on the same link" if:

   -  when any host, A, from that set, sends a packet to any other host,
      B, in that set, using unicast, multicast, or broadcast, the entire
      link-layer packet payload arrives unmodified, and

   -  a broadcast sent over that link by any host from that set of hosts
      can be received by every other host in that set.

   The link-layer *header* may be modified, such as in Token Ring Source
   Routing [802.5], but not the link-layer *payload*.  In particular, if
   any device forwarding a packet modifies any part of the IP header or
   IP payload, then the packet is no longer considered to be on the same
   link.  This means that the packet may pass through devices such as
   repeaters, bridges, hubs, or switches and still be considered to be
   on the same link for the purpose of this document, but not through a
   device such as an IP router that decrements the TTL or otherwise
   modifies the IP header.

   Where this document uses the term "host", it applies equally to
   interfaces on routers or other multi-homed hosts, regardless of
   whether the host/router is currently forwarding packets.  In many
   cases a router will be critical network infrastructure with an IP
   address that is locally well known and assumed to be relatively
   constant.  For example, the address of the default router is one of
   the parameters that a DHCP server typically communicates to its
   clients, and (at least until mechanisms like DHCP Reconfigure
   [RFC3203] become widely implemented) there isn't any practical way
   for the DHCP server to inform clients if that address changes.
   Consequently, for such devices, handling conflicts by picking a new
   IP address is not a good option.  In those cases, option (c) in
   Section 2.4 ("Ongoing Address Conflict Detection and Address
   Defense") applies.

   However, even when a device is manually configured with a fixed
   address, having some other device on the network claiming to have the
   same IP address will pollute peer ARP caches and prevent reliable
   communication, so it is still helpful to inform the operator.  If a
   conflict is detected at the time the operator sets the fixed manual
   address, then it is helpful to inform the operator immediately; if a
   conflict is detected subsequently, it is helpful to inform the
   operator via some appropriate asynchronous communication channel.
   Even though reliable communication via the conflicted address is not
   possible, it may still be possible to inform the operator via some
   other communication channel that is still operating, such as via some
   other interface on the router, via a dynamic IPv4 link-local address,
   via a working IPv6 address, or even via some completely different
   non-IP technology such as a locally-attached screen or serial
   console.

2.  Address Probing, Announcing, Conflict Detection, and Defense

   This section describes initial probing to safely determine whether an
   address is already in use, announcing the chosen address, ongoing
   conflict checking, and optional use of broadcast ARP Replies to
   provide faster conflict detection.

2.1.  Probing an Address

   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.  This also applies when a network
   interface transitions from an inactive to an active state, when a
   computer awakes from sleep, when a link-state change signals that an
   Ethernet cable has been connected, when an 802.11 wireless interface
   associates with a new base station, or when any other change in
   connectivity occurs where a host becomes actively connected to a
   logical link.

   A host MUST NOT perform this check periodically as a matter of
   course.  This would be a waste of network bandwidth, and is
   unnecessary due to the ability of hosts to passively discover
   conflicts, as described in Section 2.4.

2.1.1.  Probe Details

   A host probes to see if an address is already in use by broadcasting
   an ARP Request for the desired address.  The client MUST fill in the
   'sender hardware address' field of the ARP Request with the hardware
   address of the interface through which it is sending the packet.  The
   'sender IP address' field MUST be set to all zeroes; this is to avoid
   polluting ARP caches in other hosts on the same link in the case
   where the address turns out to be already in use by another host.
   The 'target hardware address' field is ignored and SHOULD be set to
   all zeroes.  The 'target IP address' field MUST be set to the address
   being probed.  An ARP Request constructed this way, with an all-zero
   'sender IP address', is referred to as an 'ARP Probe'.

   When ready to begin probing, the host should then wait for a random
   time interval selected uniformly in the range zero to PROBE_WAIT
   seconds, and should then send PROBE_NUM probe packets, each of these
   probe packets spaced randomly and uniformly, PROBE_MIN to PROBE_MAX
   seconds apart.  This initial random delay helps ensure that a large
   number of hosts powered on at the same time do not all send their
   initial probe packets simultaneously.

   If during this period, from the beginning of the probing process
   until ANNOUNCE_WAIT seconds after the last probe packet is sent, the
   host receives any ARP packet (Request *or* Reply) on the interface
   where the probe is being performed, where the packet's 'sender IP
   address' is the address being probed for, then the host MUST treat
   this address as being in use by some other host, and should indicate
   to the configuring agent (human operator, DHCP server, etc.) that the
   proposed address is not acceptable.

   In addition, if during this period the host receives any ARP Probe
   where the packet's 'target IP address' is the address being probed
   for, and the packet's 'sender hardware address' is not the hardware
   address of any of the host's interfaces, then the host SHOULD
   similarly treat this as an address conflict and signal an error to
   the configuring agent as above.  This can occur if two (or more)
   hosts have, for whatever reason, been inadvertently configured with
   the same address, and both are simultaneously in the process of
   probing that address to see if it can safely be used.

   NOTE: The check that the packet's 'sender hardware address' is not
   the hardware address of any of the host's interfaces is important.
   Some kinds of Ethernet hub (often called a "buffered repeater") and
   many wireless access points may "rebroadcast" any received broadcast
   packets to all recipients, including the original sender itself.  For
   this reason, the precaution described above is necessary to ensure
   that a host is not confused when it sees its own ARP packets echoed
   back.

   A host implementing this specification MUST take precautions to limit
   the rate at which it probes for new candidate addresses: if the host
   experiences MAX_CONFLICTS or more address conflicts on a given
   interface, then the host MUST limit the rate at which it probes for
   new addresses on this interface to no more than one attempted new
   address per RATE_LIMIT_INTERVAL.  This is to prevent catastrophic ARP
   storms in pathological failure cases, such as a defective DHCP server
   that repeatedly assigns the same address to every host that asks for
   one.  This rate-limiting rule applies not only to conflicts
   experienced during the initial probing phase, but also to conflicts
   experienced later, as described in Section 2.4 "Ongoing Address
   Conflict Detection and Address Defense".

   If, by ANNOUNCE_WAIT seconds after the transmission of the last ARP
   Probe no conflicting ARP Reply or ARP Probe has been received, then
   the host has successfully determined that the desired address may be
   used safely.

2.2.  Shorter Timeouts on Appropriate Network Technologies

   Network technologies may emerge for which shorter delays are
   appropriate than those required by this document.  A subsequent IETF
   publication may be produced providing guidelines for different values
   for PROBE_WAIT, PROBE_NUM, PROBE_MIN, and PROBE_MAX on those
   technologies.

   If the situation arises where different hosts on a link are using
   different timing parameters, this does not cause any problems.  This
   protocol is not dependent on all hosts on a link implementing the

   same version of the protocol; indeed, this protocol is not dependent
   on all hosts on a link implementing the protocol at all.  All that is
   required is that all hosts implement ARP as specified in RFC 826, and
   correctly answer ARP Requests they receive.  In the situation where
   different hosts are using different timing parameters, all that will
   happen is that some hosts will configure their interfaces more
   quickly than others.  In the unlikely event that an address conflict
   is not detected during the address probing phase, the conflict will
   still be detected by the Ongoing Address Conflict Detection described
   below in Section 2.4.

2.3.  Announcing an Address

   Having probed to determine that a desired address may be used safely,
   a host implementing this specification MUST then announce that it
   is commencing to use this address by broadcasting ANNOUNCE_NUM ARP
   Announcements, spaced ANNOUNCE_INTERVAL seconds apart.  An ARP
   Announcement is identical to the ARP Probe described above, except
   that now the sender and target IP addresses are both set to the
   host's newly selected IPv4 address.  The purpose of these ARP
   Announcements is to make sure that other hosts on the link do not
   have stale ARP cache entries left over from some other host that may
   previously have been using the same address.  The host may begin
   legitimately using the IP address immediately after sending the first
   of the two ARP Announcements; the sending of the second ARP
   Announcement may be completed asynchronously, concurrent with other
   networking operations the host may wish to perform.

2.4.  Ongoing Address Conflict Detection and Address Defense

   Address Conflict Detection is not limited to only the time of initial
   interface configuration, when a host is sending ARP Probes.  Address
   Conflict Detection is an ongoing process that is in effect for as
   long as a host is using an address.  At any time, if a host receives
   an ARP packet (Request *or* Reply) where the 'sender IP address' is
   (one of) the host's own IP address(es) configured on that interface,
   but the 'sender hardware address' does not match any of the host's
   own interface addresses, then this is a conflicting ARP packet,
   indicating some other host also thinks it is validly using this
   address.  To resolve the address conflict, a host MUST respond to a
   conflicting ARP packet as described in either (a), (b), or (c) below:

   (a) Upon receiving a conflicting ARP packet, a host MAY elect to
       immediately cease using the address, and signal an error to the
       configuring agent as described above.

   (b) If a host currently has active TCP connections or other reasons
       to prefer to keep the same IPv4 address, and it has not seen any
       other conflicting ARP packets within the last DEFEND_INTERVAL
       seconds, then it MAY elect to attempt to defend its address by
       recording the time that the conflicting ARP packet was received,
       and then broadcasting one single ARP Announcement, giving its own
       IP and hardware addresses as the sender addresses of the ARP,
       with the 'target IP address' set to its own IP address, and the
       'target hardware address' set to all zeroes.  Having done this,
       the host can then continue to use the address normally without
       any further special action.  However, if this is not the first
       conflicting ARP packet the host has seen, and the time recorded
       for the previous conflicting ARP packet is recent, within
       DEFEND_INTERVAL seconds, then the host MUST immediately cease
       using this address and signal an error to the configuring agent
       as described above.  This is necessary to ensure that two hosts
       do not get stuck in an endless loop with both hosts trying to
       defend the same address.

   (c) If a host has been configured such that it should not give up its
       address under any circumstances (perhaps because it is the kind
       of device that needs to have a well-known stable IP address, such
       as a link's default router or a DNS server) then it MAY elect to
       defend its address indefinitely.  If such a host receives a
       conflicting ARP packet, then it should take appropriate steps to
       log useful information such as source Ethernet address from the
       ARP packet, and inform an administrator of the problem.  The
       number of such notifications should be appropriately controlled
       to prevent an excessive number of error reports being generated.
       If the host has not seen any other conflicting ARP packets
       recently, within the last DEFEND_INTERVAL seconds, then it MUST
       record the time that the conflicting ARP packet was received, and
       then broadcast one single ARP Announcement, giving its own IP and
       hardware addresses.  Having done this, the host can then continue
       to use the address normally without any further special action.
       However, if this is not the first conflicting ARP packet the host
       has seen, and the time recorded for the previous conflicting ARP
       packet is within DEFEND_INTERVAL seconds, then the host MUST NOT
       send another defensive ARP Announcement.  This is necessary to
       ensure that two misconfigured hosts do not get stuck in an
       endless loop flooding the network with broadcast traffic while
       they both try to defend the same address.

   A host wishing to provide reliable network operation MUST respond to
   conflicting ARP packets as described in (a), (b), or (c) above.
   Ignoring conflicting ARP packets results in seemingly random network
   failures that can be hard to diagnose and very frustrating for human
   users.

   Forced address reconfiguration may be disruptive, causing TCP (and
   other transport-layer) connections to be broken.  However, such
   disruptions should be exceedingly rare, and if inadvertent address
   duplication happens, then disruption of communication is inevitable.
   It is not possible for two different hosts using the same IP address
   on the same network to operate reliably.

   Before abandoning an address due to a conflict, hosts SHOULD actively
   attempt to reset any existing connections using that address.  This
   mitigates some security threats posed by address reconfiguration, as
   discussed in Section 5.

   For most client machines that do not need a fixed IP address,
   immediately requesting the configuring agent (human user, DHCP
   client, etc.) to configure a new address as soon as the conflict is
   detected is the best way to restore useful communication as quickly
   as possible.  The mechanism described above of broadcasting a single
   ARP Announcement to defend the address mitigates the problem
   somewhat, by helping to improve the chance that one of the two
   conflicting hosts may be able to retain its address.

2.5.  Continuing Operation

   From the time a host sends its first ARP Announcement, until the
   time it ceases using that IP address, the host MUST answer ARP
   Requests in the usual way required by the ARP specification [RFC826].
   Specifically, this means that whenever a host receives an ARP
   Request, that's not a conflicting ARP packet as described above in
   Section 2.4, where the 'target IP address' of the ARP Request is (one
   of) the host's own IP address(es) configured on that interface, the
   host MUST respond with an ARP Reply as described in RFC 826.  This
   applies equally for both standard ARP Requests with non-zero sender
   IP addresses and Probe Requests with all-zero sender IP addresses.

2.6.  Broadcast ARP Replies

   In a carefully-run network with manually-assigned addresses, or
   a network with a reliable DHCP server and reliable DHCP clients,
   address conflicts should occur only in rare failure scenarios, so
   the passive monitoring described above in Section 2.4 is adequate.
   If two hosts are using the same IP address, then sooner or later one
   host or the other will broadcast an ARP Request, which the other will
   see, allowing the conflict to be detected and consequently resolved.

   It is possible, however, that a conflicting configuration may persist
   for a short time before it is detected.  Suppose that two hosts, A
   and B, have been inadvertently assigned the same IP address, X.
   Suppose further that at the time they were both probing to determine

   whether the address could safely be used, the communication link
   between them was non-functional for some reason, so neither detected
   the conflict at interface-configuration time.  Suppose now that the
   communication link is restored, and a third host, C, broadcasts an
   ARP Request for address X.  Unaware of any conflict, both hosts A and
   B will send unicast ARP Replies to host C.  Host C will see both
   Replies, and may be a little confused, but neither host A nor B will
   see the other's Reply, and neither will immediately detect that there
   is a conflict to be resolved.  Hosts A and B will continue to be
   unaware of the conflict until one or other broadcasts an ARP Request
   of their own.

   If quicker conflict detection is desired, this may be achieved by
   having hosts send ARP Replies using link-level broadcast, instead of
   sending only ARP Requests via broadcast, and Replies via unicast.
   This is NOT RECOMMENDED for general use, but other specifications
   building on IPv4 ACD may choose to specify broadcast ARP Replies if
   appropriate.  For example, "Dynamic Configuration of IPv4 Link-Local
   Addresses" [RFC3927] specifies broadcast ARP Replies because in that
   context, detection of address conflicts using IPv4 ACD is not merely
   a backup precaution to detect failures of some other configuration
   mechanism; detection of address conflicts using IPv4 ACD is the sole
   configuration mechanism.

   Sending ARP Replies using broadcast does increase broadcast traffic,
   but in the worst case by no more than a factor of two.  In the
   traditional usage of ARP, a unicast ARP Reply only occurs in response
   to a broadcast ARP Request, so sending these via broadcast instead
   means that we generate at most one broadcast Reply in response to
   each existing broadcast Request.  On many networks, ARP traffic is
   such an insignificant proportion of the total traffic that doubling
   it makes no practical difference.  However, this may not be true of
   all networks, so broadcast ARP Replies SHOULD NOT be used
   universally.  Broadcast ARP Replies should be used where the benefit
   of faster conflict detection outweighs the cost of increased
   broadcast traffic and increased packet processing load on the
   participant network hosts.

3.  Why Are ARP Announcements Performed Using ARP Request Packets and
    Not ARP Reply Packets?

   During IETF deliberation of IPv4 Address Conflict Detection from 2000
   to 2008, a question that was asked repeatedly was, "Shouldn't ARP
   Announcements be performed using gratuitous ARP Reply packets?"

   On the face of it, this seems reasonable.  A conventional ARP Reply
   is an answer to a question.  If in fact no question had been asked,
   then it would be reasonable to describe such a reply as gratuitous.

   The term "gratuitous reply" would seem to apply perfectly to an ARP
   Announcement: an answer to an implied question that in fact no one
   asked.

   However reasonable this may seem in principle, in practice there are
   two reasons that swing the argument in favor of using ARP Request
   packets.  One is historical precedent, and the other is pragmatism.

   The historical precedent is that (as described above in Section 4)
   Gratuitous ARP is documented in Stevens Networking [Ste94] as using
   ARP Request packets.  BSD Unix, Microsoft Windows, Mac OS 9, Mac OS
   X, etc., all use ARP Request packets as described in Stevens.  At
   this stage, trying to mandate that they all switch to using ARP Reply
   packets would be futile.

   The practical reason is that ARP Request packets are more likely to
   work correctly with more existing ARP implementations, some of which
   may not implement RFC 826 entirely correctly.  The Packet Reception
   rules in RFC 826 state that the opcode is the last thing to check in
   packet processing, so it really shouldn't matter, but there may be
   "creative" implementations that have different packet processing
   depending on the 'ar$op' field, and there are several reasons why
   these are more likely to accept gratuitous ARP Requests than
   gratuitous ARP Replies:

   * An incorrect ARP implementation may expect that ARP Replies are
     only sent via unicast.  RFC 826 does not say this, but an incorrect
     implementation may assume it; the "principle of least surprise"
     dictates that where there are two or more ways to solve a
     networking problem that are otherwise equally good, the one with
     the fewest unusual properties is the one likely to have the fewest
     interoperability problems with existing implementations.  An ARP
     Announcement needs to broadcast information to all hosts on the
     link.  Since ARP Request packets are always broadcast, and ARP
     Reply packets are not, receiving an ARP Request packet via
     broadcast is less surprising than receiving an ARP Reply packet via
     broadcast.

   * An incorrect ARP implementation may expect that ARP Replies are
     only received in response to ARP Requests that have been issued
     recently by that implementation.  Unexpected unsolicited Replies
     may be ignored.

   * An incorrect ARP implementation may ignore ARP Replies where
     'ar$tha' doesn't match its hardware address.

   * An incorrect ARP implementation may ignore ARP Replies where
     'ar$tpa' doesn't match its IP address.

   In summary, there are more ways that an incorrect ARP implementation
   might plausibly reject an ARP Reply (which usually occurs as a result
   of being solicited by the client) than an ARP Request (which is
   already expected to occur unsolicited).

4.  Historical Note

   Some readers have claimed that "Gratuitous ARP", as described in
   Stevens [Ste94], provides duplicate address detection, making ACD
   unnecessary.  This is incorrect.  What Stevens describes as
   Gratuitous ARP is the exact same packet that this document refers to
   by the more descriptive term 'ARP Announcement'.  This traditional
   Gratuitous ARP implementation sends only a single ARP Announcement
   when an interface is first configured.  The result is that the victim
   (the existing address holder) logs an error, and the offender
   continues operation, often without even detecting any problem.  Both
   machines then typically proceed to try to use the same IP address,
   and fail to operate properly because they are each constantly
   resetting the other's TCP connections.  The human administrator is
   expected to notice the log message on the victim machine and repair
   the damage after the fact.  Typically this has to be done by
   physically going to the machines in question, since in this state
   neither is able to keep a TCP connection open for long enough to do
   anything useful over the network.

   Gratuitous ARP does not in fact provide effective duplicate address
   detection and (as of January 2008) many of the top results for a
   Google search for the phrase "Gratuitous ARP" are articles describing
   how to disable it.

   However, implementers of IPv4 Address Conflict Detection should be
   aware that, as of this writing, Gratuitous ARP is still widely
   deployed.  The steps described in Sections 2.1 and 2.4 of this
   document help make a host robust against misconfiguration and address
   conflicts, even when the other host is *not* playing by the same
   rules.

5.  Security Considerations

   IPv4 Address Conflict Detection (ACD) is based on ARP [RFC826] and it
   inherits the security vulnerabilities of that protocol.  A malicious
   host may send fraudulent ARP packets on the network, interfering with
   the correct operation of other hosts.  For example, it is easy for a
   host to answer all ARP Requests with Replies giving its own hardware
   address, thereby claiming ownership of every address on the network.

   This specification makes this existing ARP vulnerability no worse,
   and in some ways makes it better: instead of failing silently with no
   indication why, hosts implementing this specification either attempt
   to reconfigure automatically, or at least inform the human user of
   what is happening.

   If a host willingly selects a new address in response to an ARP
   conflict, as described in Section 2.4, subsection (a), this
   potentially makes it easier for malicious attackers on the same link
   to hijack TCP connections.  Having a host actively reset any existing
   connections before abandoning an address helps mitigate this risk.

6.  Acknowledgments

   This document arose as a result of Zeroconf Working Group discussions
   on IPv4 Link-Local Addressing [RFC3927], where it was not clear to
   many participants which elements of link-local address management
   were specific to that particular problem space (e.g., random
   selection of an address), and which elements were generic and
   applicable to all IPv4 address configuration mechanisms (e.g., the
   detection of address conflicts).  The following people made valuable
   comments in the course of that work and/or the subsequent editing of
   this document: Bernard Aboba, Randy Bush, Jim Busse, James Carlson,
   Alan Cox, Spencer Dawkins, Pavani Diwanji, Ralph Droms, Donald
   Eastlake III, Alex Elder, Stephen Farrell, Peter Ford, Spencer
   Giacalone, Josh Graessley, Erik Guttman, Myron Hattig, Mike Heard,
   Hugh Holbrook, Richard Johnson, Kim Yong-Woon, Marc Krochmal, Rod
   Lopez, Rory McGuire, Satish Mundra, Thomas Narten, Erik Nordmark,
   Randy Presuhn, Howard Ridenour, Pekka Savola, Daniel Senie, Dieter
   Siegmund, Valery Smyslov, Mark Townsley, Oleg Tychev, and Ryan Troll.

7.  References

7.1.  Normative References

   [RFC826]  Plummer, D., "An Ethernet Address Resolution Protocol
             -- or -- Converting Network Protocol Addresses to 48.bit
             Ethernet Address for Transmission on Ethernet Hardware",
             STD 37, RFC 826, November 1982.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

7.2.  Informative References

   [802]     IEEE Standards for Local and Metropolitan Area Networks:
             Overview and Architecture, ANSI/IEEE Std 802, 1990.

   [802.3]   ISO/IEC 8802-3 Information technology - Telecommunications
             and information exchange between systems - Local and
             metropolitan area networks - Common specifications - Part
             3:  Carrier Sense Multiple Access with Collision Detection
             (CSMA/CD) Access Method and Physical Layer Specifications,
             (also ANSI/IEEE Std 802.3-1996), 1996.

   [802.5]   ISO/IEC 8802-5 Information technology - Telecommunications
             and information exchange between systems - Local and
             metropolitan area networks - Common specifications -
             Part 5: Token ring access method and physical layer
             specifications, (also ANSI/IEEE Std 802.5-1998), 1998.

   [802.11]  Information technology - Telecommunications and information
             exchange between systems - Local and metropolitan area
             networks - Specific Requirements Part 11:  Wireless LAN
             Medium Access Control (MAC) and Physical Layer (PHY)
             Specifications, IEEE Std. 802.11-1999, 1999.

   [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
             and E. Lear, "Address Allocation for Private Internets",
             BCP 5, RFC 1918, February 1996.

   [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
             March 1997.

   [RFC3203] T'Joens, Y., Hublet, C., and P. De Schrijver, "DHCP
             reconfigure extension", RFC 3203, December 2001.

   [RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
             Configuration of IPv4 Link-Local Addresses", RFC 3927, May
             2005.

   [Ste94]   W. Stevens, "TCP/IP Illustrated, Volume 1: The Protocols",
             Addison-Wesley, 1994.

Author's Address

   Stuart Cheshire
   Apple Inc.
   1 Infinite Loop
   Cupertino
   California 95014
   USA

   Phone: +1 408 974 3207
   EMail: rfc@stuartcheshire.org

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