Internet Draft: DHC-IPV4-AUTOCONFIG                             R. Troll
Document: draft-ietf-dhc-ipv4-autoconfig-01.txt             October 1998
Expires: April 1999

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


Status of this memo

     This document is an Internet-Draft.  Internet-Drafts are working
     documents of the Internet Engineering Task Force (IETF), its areas,
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     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 automaticly 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 provided.

1. Introduction

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     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
     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 segments.

1.1 Conventions Used in the Document

     The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
     in this document are to be interpreted as defined in "Key Words for
     Use in RFCs to Indicate Requirement Levels" [KEYWORDS]

1.2 Terminology

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

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

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          DHCP Server    A DHCP Server is an Internet host that returns
                         configuration parameters to DHCP Clients.

1.3 Usage Clarification

     This document describes a method by which a host may automaticly
     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 decrease, 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].

2. To Choose or Not To Choose

     The first thing an Internet host should do is request 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

     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].

     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.

2.1 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 of this document.

3. Choosing an IP Address

     Once a DHCP Client has determined it must auto-configure an IP

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     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

     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
     being in use, and move on.

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 DHCP Client with an auto-
     configured IP address MUST keep checking for an active DHCP Server.
     To do this, the DHCP Client MUST attempt to fetch an IP address as

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     described in section 1 of this document.

     When rechecking, when the DHCP Client has determined no DHCP Server
     is responding, it MUST wait a period of time and try again.  For
     Ethernet implementations, the DHCP Client SHOULD check every 5

     If the DHCP Client 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.

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

     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

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     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 how 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 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

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     this document.

8. Copyright

     Copyright (C) The Internet Society 1998. 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 English.

     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

9. References

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


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


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


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     [IPv6SAC] Thomson, S. and Narten, T. "IPv6 Stateless Address
     Autoconfiguration", RFC 1971, August 1996


     [DHCPAC] Troll, R. "DHCP Option to Disable Stateless Auto-
     Configuration in IPv4 Clients", RFC XXXXX, November 1998


10. Author's Address

     Ryan Troll
     Network Development
     Carnegie Mellon
     5000 Forbes Avenue
     Pittsburgh, PA 15213

     Phone: (412) 268-8691

     This document will expire April 1999

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