Dynamic Host Configuration WG                                 Ryan Troll
Document: draft-ietf-dhc-ipv4-autoconfig-04.txt            @Home Network
Expires October 19, 1999                                  April 14, 1999

       Automatically Choosing an IP Address in an Ad-Hoc IPv4 Network


Status of this Memo

    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC2026.  Internet-Drafts are
    working documents of the Internet Engineering Task Force (IETF), its
    areas, and its working groups.  Note that other groups may also
    distribute working documents as Internet-Drafts.

    Internet-Drafts are draft documents valid for a maximum of six
    months and may be updated, replaced, or obsoleted by other documents
    at any time.  It is inappropriate to use Internet- Drafts as
    reference material or to cite them other than as "work in progress."

    To view the list Internet-Draft Shadow Directories, see

    Distribution of this memo is unlimited.


    With operating systems appearing in more and more devices, as well
    as computers appearing in more and more aspects of everyday life,
    communication between networked devices is increasingly important.
    The communication mechanism between these devices must be able to
    not only support the office LAN environment, but must also scale to
    larger WANS and the internet.

    This draft describes a method by which a host may automatically give
    itself a link-local IPv4 address, so that it will be able to use IP
    applications in the absence of an IP address management mechanism,
    such as DHCP.  This mechanism is in use today by a few operating
    systems, and additional information on those implementations is also

1.  Introduction

    Now that networked applications are becoming more prevalent,
    operating systems are migrating towards more scalable network
    protocols such as IP, allowing them to work in all sizes of

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    environments.  However, there is a price to pay for this migration
    -- IP requires configuration that other protocols (IPX, Appletalk)
    do not require.

    Dynamic creation of usable ad-hoc networks is very useful when there
    are only a few machines on the entire network.  (For example, a
    dentist's office may only have a couple of machines.)  In order to
    allow a site such as this to use IP, the machines must each be
    configured with an IP address.  OS's wish to retain the minimal
    configuration that was necessary under their non-IP network stacks.

    Dynamic configuration protocols such as DHCP [DHCP] allow a site
    administrator to take care of the network configuration for a
    machine remotely.  By requesting network parameters via DHCP, the
    site administrator may provide all information necessary without the
    host's owner having to do anything.  However, not all sites have a
    central administrator to take care of this.

    To accommodate unmanaged networks, the OS may decide to
    intelligently choose an IP address for itself.  These addresses are
    only valid for the local network.

    This document describes a method by which an OS may determine
    whether or not to autoconfigure itself an IP address, as well as how
    to inter-operate cleanly with an existing managed infrastructure,
    allowing a host to easily move between managed and unmanaged network

1.1.  Conventions Used in the Document

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    document are to be interpreted as described in [KEYWORDS].

1.2.  Terminology

    Site Administrator
                   A Site Administrator is the person or organization
                   responsible for handing out IP addresses to client

    DHCP Client    A DHCP Client is an Internet host using DHCP to
                   obtain configuration parameters such as a network

    DHCP Server    A DHCP Server is an Internet host that returns

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                   configuration parameters to DHCP Clients.

1.3.  Usage Clarification

    This document describes a method by which a host may automatically
    choose an IPv4 address in the absence of a central service to
    maintain and hand out addresses.  This is not designed to replace
    this functionality, but to basicly provide it in small networks.

    This SHOULD NOT be used for large-scale networks.  As more and more
    machines begin to use this mechanism on a network, startup times for
    these machines will begin to increase, as the chance of collisions
    will rise.

    Addresses allocated by this mechanism MUST NOT be routed by any
    network device.  The addresses are designed to be link local
    addresses.  Link local address are to be, by definition, restricted
    to the local network segment.  Allocation of link-local addresses in
    an IPv6 network is described in [IPv6SAC].

    Finally, it should be noted that this functionality is designed to
    work on a host that is running a DHCP Client.  Hosts that are not
    running a DHCP Client are not considered in this draft, and MUST NOT
    attempt to use these mechanisms to configure an IP address, as these
    no-dhcp-client hosts will not interoperate with the existing
    infrastructure correctly.  (This mechanism relies on the DHCP Server
    for some information, and if a host can not receive this
    information, it will not be fully informed.)

2.  To Choose or Not To Choose

    The first thing an Internet host should do is attempt to get an
    administratively assigned address using DHCP.  Only after the DHCP
    process fails should a host use the methods later for

2.1.  Obtaining an Address via DHCP

    A host begins by requesting an IP address via DHCP [DHCP].  This is
    done by sending out a DHCPDISCOVER message, with various tags set
    indicating what options the DHCP Client would like to receive
    information for [DHCPOPT].  The DHCP Client SHOULD also send the
    DHCP AutoConfigure option described in [DHCPAC].

    According to [DHCP], Section 4.4.1, the amount of time over which a
    DHCP Client should listen for DHCPOFFERS is implementation
    dependant.  During this time, if a DHCPOFFER is received, network
    configuration MUST occur as described in [DHCP] and [DHCPAC].

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    If, during this time, no valid DHCPOFFERS are received, the DHCP
    Client is free to autoconfigure an IP address according to section 3
    of this document.

2.2.  Rebinding an Existing IP Address

    If the DHCP Client already has an existing IP address, it MUST
    follow the instructions outlined in [DHCP].  If the client winds up
    back in the INIT state, refer to section 2.1 of this document.

3.  Choosing an IP Address

    Once a DHCP Client has determined it must auto-configure an IP
    address, it chooses an address.  The algorithm for choosing an
    address is implementation dependant.  The address range to use MUST
    be "169.254/16", which is registered with the IANA as the LINKLOCAL

    If choosing an address in this range, the DHCP Client MUST NOT use
    the first 256 or the last 256, as these are reserved for future use.

    When an address is chosen, the DHCP Client MUST test to see if the
    address is already in use.  If the network address appears to be in
    use, the client MUST choose another address, and try again.  The
    client MUST keep choosing addresses until it either finds one, or it
    has tried more then the autoconfig-retry count.  The autoconfig-
    retry count is implementation specific, and should be based on the
    algorithm used for choosing an IP address.  This retry count is
    present to make sure that DHCP Clients auto-configuring on busy
    auto-configured network segments do not loop infinitely looking for
    an IP address.

3.1.  Determining Whether or Not an Address is in Use

    If the client is on a network that supports ARP, the client may
    issue an ARP request for the suggested address.  When broadcasting
    an ARP request for the suggested address, the client MUST fill in
    its own hardware address as the sender's hardware address, and all
    0s as the sender's IP address, to avoid confusing ARP caches in
    other hosts on the same subnet.  This ARP request with a sender IP
    address of all 0s is referred to as an "ARP probe".

    While waiting for a possible response to this request, the client
    MUST also listen for other ARP probes for the same address (but not
    from its own hardware address).  This will occur if two (or more)
    hosts are attempting to autoconfigure the exact same address.  If
    the client receives a response to the ARP request, or sees another
    ARP probe for the same address, it MUST consider the address as

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    being in use, and move on.

    Clients SHOULD randomize their IP address selection algorithm, to
    help keep clients from becoming stuck in a loop with other clients
    while trying to allocate an address.  Algorithm designers MUST make
    sure that their algorithm will not cause the client IP selection
    process to become stuck when interoperating with any other possible

4.  Ongoing Checks for a DHCP Server

    When the client originally sent out it's request, there may have
    been a network problem stopping the DHCP Server from responding.  To
    make sure this is not the case, a host with an auto-configured IP
    address MUST keep checking for an active DHCP Server.  To do this,
    the host MUST attempt to fetch an IP address as described in section
    1 of this document.

    When rechecking, if the host has determined no DHCP Server is
    responding, it MUST wait a period of time and try again.  This
    interval is tunable.  The suggested default for Ethernet
    implementations is to check every 5 minutes.

    If the host receives a response from a DHCP Server, it MUST respond
    and attempt to obtain a lease from the server (per the DHCP
    specification).  If the client is successful in obtaining a new
    lease, and the internet host does not support multiple addresses on
    the interface being configured, it MUST drop any existing auto-
    configured IP address, and all active connections, while moving to
    the new address.  If the internet host does support multiple
    addresses on the interface, it MAY keep the auto-configured address

    If the DHCP response is an AutoConfigure [DHCPAC] response set to
    "DoNOTAutoConfigure", the host MUST drop all connections, give up
    any existing auto-configured IP address, and continue checking for a
    DHCP server.

    WARNING: If the client drops all connections, the user may lose data
    being access over the existing connections.  The operating system
    SHOULD do everything it can to allow existing clients to finish
    using all existing connections before closing them.  It SHOULD NOT
    allocate any further connections using the auto-configured IP

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5.  Current Vendor Implementations

    As of this writing, Microsoft and Apple have operating systems that
    contain this functionality.  Descriptions of the implementation
    dependant parts are listed below.

5.1.  Microsoft Windows 98

    With the initial release of Windows 98, Microsoft introduced auto-
    configuration functionality.  When developed, the AutoConfig
    [DHCPAC] specification did not exist, so the initial release does
    not contain this functionality.

    The Win98 DHCP Client sends out a total of 4 DHCPDISCOVERs, with an
    inter-packet interval of 6 seconds.  When no response is received
    after all 4 packets (24 seconds), it will auto-configure an address.

    The auto-configure retry count for Windows 98 is 10.  After trying
    10 auto-configured IP addresses, and finding all are taken, the host
    will boot without an IP address.

5.2.  Apple MacOS 8.5

    MacOS 8.5 sends three DHCPDISCOVER packets, with timeouts of 4, 8,
    and then 16 seconds.  When no response is received from all of these
    requests (28 seconds), it will auto-configure.

    The auto-configure retry count for MacOS 8.5 is 10.  After trying 10
    auto-configured IP addresses, and finding all are taken, the host
    will boot without an IP address.

6.  Security Considerations

    The use of this functionality may open a network host to new Denial
    Of Service (DOS) attacks.  In particular, a host that previously did
    not have an IP address, and no IP stack running, was not succeptable
    to IP based DOS attacks, as there was no IP stack configured to
    interpret these packets.

    However, the addition of this functionality to an OS may cause IP
    stacks to be capable of receiving and interpreting information that
    the host was not previously configured to receive.  As this host is
    now interpreting IP communications, it is now open to IP based DOS

    Another security concern is the DOS attack that may be made on the
    local subnet which stops all machines from being able to allocate an
    IP address.  A malicious host on the local wire may listen for ARP

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    probes, and respond with it's own ARP probe.  This will stop the
    auto-configuring machine from using that address, and it will move
    on to the next one.  Eventually, it will run out of addresses to
    attempt, and will give up.  The use of DHCP removes this attack,
    leaving only the concerns described in [DHCP].

    Finally, machines that rely on this for communication over a large
    network may allocate the same address if the network itself is
    segmented when the machines boot.  If the link between two machines
    is down when they boot, they may both auto-configure the same
    address.  However, when the network link returns, there will be
    numerous problems (ARP caches, etc.)  There is currently no way to
    solve this auto-configuration problem without causing all hosts
    involved to re-autoconfigure IP addresses.  The use of DHCP to
    configure hosts on a subnet will solve this, and hosts that
    implement this configuration mechanism will behave appropriately on
    a DHCP managed network in which the DHCP server is not initially

7.  Acknowledgments

    I'd like to thank Microsoft and Apple for their help in writing this

8.  Copyright

    Copyright (C) The Internet Society 1999. All Rights Reserved.

    This document and translations of it may be copied and furnished to
    others, and derivative works that comment on or otherwise explain it
    or assist in its implementation may be prepared, copied, published
    and distributed, in whole or in part, without restriction of any
    kind, provided that the above copyright notice and this paragraph
    are included on all such copies and derivative works.  However, this
    document itself may not be modified in any way, such as by removing
    the copyright notice or references to the Internet Society or other
    Internet organizations, except as needed for the purpose of
    developing Internet standards in which case the procedures for
    copyrights defined in the Internet Standards process must be
    followed, or as required to translate it into languages other than

    The limited permissions granted above are perpetual and will not be
    revoked by the Internet Society or its successors or assigns.

    This document and the information contained herein is provided on an

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9.  References

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


    [DHCPOPT] Alexander, S. and  Droms,  R.,  "DHCP  Options  and  BOOTP
        Vendor Extension", RFC 2132, March 1997


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


    [IPv6SAC] Thomson,  S.  and  Narten,  T.,  "IPv6  Stateless  Address
        Autoconfiguration", RFC 2462, December 1998


    [DHCPAC]  Troll,  R.,  "DHCP  Option  to  Disable  Stateless   Auto-
        Configuration  in  IPv4  Clients", RFC Work in progress, January

        <ftp://ds.internic.net/rfc/rfcWork in progress.txt>

10.  Author's Address

    Ryan Troll
    @Home Network
    425 Broadway
    Redwood City, CA 94063

    Phone: (650) 569-6031
    EMail: rtroll@corp.home.net

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