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Global Routing Operations D. Plonka Network Working Group University of Wisconsin Request for Comments: 4085 June 2005 BCP: 105 Category: Best Current Practice

 Embedding Globally-Routable Internet Addresses Considered Harmful

Status of This Memo

 This document specifies an Internet Best Current Practices for the
 Internet Community, and requests discussion and suggestions for
 improvements.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).


 This document discourages the practice of embedding references to
 unique, globally-routable IP addresses in Internet hosts, describes
 some of the resulting problems, and considers selected alternatives.
 This document is intended to clarify best current practices in this

Table of Contents

 1. Introduction ....................................................2
 2. Problems ........................................................2
 3. Recommendations .................................................4
    3.1. Disable Unused Features ....................................4
    3.2. Provide User Interface for IP Features .....................4
    3.3. Use Domain Names as Service Identifiers ....................4
    3.4. Use Special-Purpose, Reserved IP Addresses When Available ..5
    3.5. Discover and Utilize Local Services ........................6
    3.6. Avoid Mentioning the IP Addresses of Services ..............6
 4. Security Considerations .........................................6
 5. Conclusion ......................................................7
 6. Acknowledgements ................................................7
 7. References ......................................................7
 Appendix A.  Background ............................................9

Plonka Best Current Practice [Page 1] RFC 4085 Embedding IP Addresses Considered Harmful June 2005

1. Introduction

 Some vendors of consumer electronics and network gear have
 unfortunately chosen to embed, or "hard-code", globally-routable
 Internet Protocol addresses within their products' firmware.  These
 embedded IP addresses are typically individual server IP addresses or
 IP subnet prefixes.  Thus, they are sometimes used as service
 identifiers, to which unsolicted requests are directed, or as subnet
 identifiers, specifying sets of Internet addresses that the given
 product somehow treats specially.
 One recent example was the embedding of the globally-routable IP
 address of a Network Time Protocol server in the firmware of hundreds
 of thousands of Internet hosts that are now in operation worldwide.
 The hosts are primarily, but are not necessarily, limited to low-cost
 routers and middleboxes for personal or residential use.  In another
 case, IP address prefixes that had once been reserved by the Internet
 Assigned Numbers Authority (IANA) were embedded in a router product
 so that it can automatically discard packets that appear to have
 invalid source IP addresses.
 Such "hard-coding" of globally-routable IP addresses as identifiers
 within the host's firmware presents significant problems to the
 operation of the Internet and to the management of its address space.
 Ostensibly, this practice arose as an attempt to simplify IP host
 configuration by pre-loading hosts with IP addresses.  Products that
 rely on such embedded IP addresses initially may appear to be
 convenient to the product's designer and to its operator or user, but
 this dubious benefit comes at the expense of others in the Internet
 This document denounces the practice of embedding references to
 unique, globally-routable IP addresses in Internet hosts, describes
 some of the resulting problems, and considers selected alternatives.
 It also reminds the Internet community of the ephemeral nature of
 unique, globally-routable IP addresses; the assignment and use of IP
 addresses as identifiers is temporary and therefore should not be
 used in fixed configurations.

2. Problems

 The embedding of IP addresses in products has caused an increasing
 number of Internet hosts to rely on a single central Internet
 service.  This can result in a service outage when the aggregate
 workload overwhelms that service.  When fixed addresses are embedded

Plonka Best Current Practice [Page 2] RFC 4085 Embedding IP Addresses Considered Harmful June 2005

 in an ever-increasing number of client IP hosts, this practice runs
 directly counter to the design intent of hierarchically deployed
 services that would otherwise be robust solutions.
 The reliability, scalability, and performance of many Internet
 services require that the pool of users not access a service using
 its IP address directly.  Instead, they typically rely on a level of
 indirection provided by the Domain Name System, RFC 2219 [6].  When
 appropriately utilized, the DNS permits the service operator to
 reconfigure the resources for maintenance and to perform load
 balancing, without the participation of the users and without a
 requirement for configuration changes in the client hosts.  For
 instance, one common load-balancing technique employs multiple DNS
 records with the same name; the set of answers that is returned is
 rotated in a round-robin fashion in successive queries.  Upon
 receiving such a response to a query, resolvers typically will try
 the answers in order, until one succeeds, thus enabling the operator
 to distribute the user request load across a set of servers with
 discrete IP addresses that generally remain unknown to the user.
 Embedding globally-unique IP addresses taints the IP address blocks
 in which they reside, lessening the usefulness and mobility of those
 IP address blocks and increasing the cost of operation.  Unsolicited
 traffic may continue to be delivered to the embedded address well
 after the IP address or block has been reassigned and no longer hosts
 the service for which that traffic was intended.  Circa 1997, the
 authors of RFC 2101 [7] made this observation:
    Due to dynamic address allocation and increasingly frequent
    network renumbering, temporal uniqueness of IPv4 addresses is no
    longer globally guaranteed, which puts their use as identifiers
    into severe question.
 When IP addresses are embedded in the configuration of many Internet
 hosts, the IP address blocks become encumbered by their historical
 use.  This may interfere with the ability of the Internet Assigned
 Numbers Authority (IANA) and the Internet Registry (IR) hierarchy to
 usefully reallocate IP address blocks.  Likewise, to facilitate IP
 address reuse, RFC 2050 [1], encourages Internet Service Providers
 (ISPs) to treat address assignments as "loans".
 Because consumers are not necessarily experienced in the operation of
 Internet hosts, they cannot be relied upon to fix problems, if and
 when they arise.  Therefore, a significant responsibility lies with
 the manufacturer or vendor of an Internet host to avoid embedding IP
 addresses in ways that cause the aforementioned problems.

Plonka Best Current Practice [Page 3] RFC 4085 Embedding IP Addresses Considered Harmful June 2005

3. Recommendations

 Internet host and router designers, including network product
 manufacturers, should not assume that their products will be deployed
 and used in only the single global Internet that they happen to
 observe today.  A myriad of private or future internetworks in which
 these products will be used may not allow those hosts to establish
 communications with arbitrary hosts on the global Internet.  Since
 the product failure modes resulting from an unknown future
 internetwork environment cannot be fully explored, one should avoid
 assumptions regarding the longevity of our current Internet.
 The following recommendations are presented as best practice today.

3.1. Disable Unused Features

 Vendors should, by default, disable unnecessary features in their
 products.  This is especially true of features that generate
 unsolicited Internet traffic.  In this way, these hosts will be
 conservative regarding the unsolicited Internet traffic they produce.
 For instance, one of the most common uses of embedded IP addresses
 has been the hard-coding of addresses of well known public Simple
 Network Time Protocol (SNTP RFC 2030 [8]) servers in products.
 However, only a small fraction of users will benefit from these
 products having some notion of the current date and time.

3.2. Provide User Interface for IP Features

 Vendors should provide an operator interface for every feature that
 generates unsolicited Internet traffic.  A prime example is this:
 the Domain Name System resolver should have an interface enabling the
 operator to either explicitly set the choice of servers or enable a
 standard automated configuration protocol such as DHCP, defined by
 RFC 2132 [9].  These features should originally be disabled within
 the operator interface, and subsequently enabling these features
 should alert the operator that the feature exists.  This will make it
 more likely that the product's owner or operator can participate in
 problem determination and mitigation when problems arise.
 RFC 2606 [2] defines the IANA-reserved "", "",
 and "" domains for use in example configurations and
 documentation.  These are candidate examples to be used in user
 interface documentation.

3.3. Use Domain Names as Service Identifiers

 Internet hosts should use the Domain Name System to determine the IP
 addresses associated with the Internet services they require.

Plonka Best Current Practice [Page 4] RFC 4085 Embedding IP Addresses Considered Harmful June 2005

 When using domain names as service identifiers in the configurations
 of deployed Internet hosts, designers and vendors are encouraged to
 introduce service names.  These names should be within a domain that
 they either control or are permitted to utilize by an agreement with
 its operator (such as for public services provided by the Internet
 community).  This is commonly done by introducing a service-specific
 prefix to the domain name.
 For instance, a vendor named "Example, Inc." with the domain
 "" might configure its product to find its SNTP server by
 the name "" or even by a name that is
 specific to the product and version, such as "sntp-".  Here the ""
 namespace is dedicated to that vendor's product configuration, with
 subdomains introduced as deemed necessary.  Such special-purpose
 domain names enable ongoing maintenance and reconfiguration of the
 services for their client hosts and can aid in the ongoing
 measurement of service usage throughout the product's lifetime.
 An alternative to inventing vendor-specific domain naming conventions
 for a product's service identifiers is to utilize SRV resource
 records (RRs), defined by RFC 2782 [10].  SRV records are a generic
 type of RR that uses a service-specific prefix in combination with a
 base domain name.  For example, an SRV-cognizant SNTP client might
 discover Example, Inc.'s suggested NTP server by performing an SRV-
 type query to lookup for "".
 However, note that simply hard-coding DNS name service identifiers
 rather than IP addresses is not a panacea.  Entries in the domain
 name space are also ephemeral and can change owners for various
 reasons, including acquisitions and litigation.  As such, developers
 and vendors should explore a product's potential failure modes
 resulting from the loss of administrative control of a given domain
 for whatever reason.

3.4. Use Special-Purpose, Reserved IP Addresses When Available

 Default configurations, documentation, and example configurations for
 Internet hosts should use Internet addresses that reside within
 special blocks that have been reserved for these purposes, rather
 than unique, globally-routable IP addresses.  For IPv4, RFC 3330 [3]
 states that the block has been assigned for use in
 documentation and example code.  The IPv6 global unicast address
 prefix 2001:DB8::/32 has been similarly reserved for documentation
 purposes RFC 3849 [4].  Private Internet Addresses, as defined by RFC
 1918 [5], should not be used for such purposes.

Plonka Best Current Practice [Page 5] RFC 4085 Embedding IP Addresses Considered Harmful June 2005

3.5. Discover and Utilize Local Services

 Service providers and enterprise network operators should advertise
 the identities of suitable local services, such as NTP.  Very often
 these services exist, but the advertisement and automated
 configuration of their use is missing.  For instance, the DHCP
 protocol, as defined by RFC 2132 [9], enables one to configure a
 server to answer client queries for service identifiers.  When local
 services, including NTP, are available but not pervasively advertised
 using such common protocols, designers are more likely to deploy ad
 hoc initialization mechanisms that unnecessarily rely on central

3.6. Avoid Mentioning the IP Addresses of Services

 Operators who provide public services on the global Internet, such as
 those in the NTP community, should deprecate the explicit
 advertisement of the IP addresses of public services.  These
 addresses are ephemeral.  As such, their widespread citation in
 public service indexes interferes with the ability to reconfigure the
 service when necessary to address unexpected, increased traffic and
 the aforementioned problems.

4. Security Considerations

 Embedding or "hard-coding" IP addresses within a host's configuration
 often means that a host-based trust model is being employed, and that
 the Internet host with the given address is trusted in some way.  Due
 to the ephemeral roles of globally-routable IP addresses, the
 practice of embedding them within products' firmware or default
 configurations presents a security risk in which unknown parties may
 be trusted inadvertently.
 Internet host designers may be tempted to implement some sort of
 remote control mechanism within a product, by which its Internet host
 configuration can be changed without reliance on, interaction with,
 or even the knowledge of, its operator or user.  This raises security
 issues of its own.  If such a scheme is implemented, its presence
 should be fully disclosed to the customer, operator, and user, so
 that an informed decision can be made, perhaps in accordance with
 local security or privacy policy.  Furthermore, the significant
 possibility of malicious parties exploiting such a remote control
 mechanism may completely negate any potential benefit of the remote
 control scheme.  Therefore, remote control mechanisms should be
 disabled by default, to be subsequently enabled and disabled by the

Plonka Best Current Practice [Page 6] RFC 4085 Embedding IP Addresses Considered Harmful June 2005

5. Conclusion

 When large numbers of homogeneous Internet hosts are deployed, it is
 particularly important that both their designers and other members of
 the Internet community diligently assess host implementation quality
 and reconfigurability.
 Implementors of host services should avoid any kind of use of unique
 globally-routable IP addresses within a fixed configuration part of
 the service implementation.  If there is a requirement for pre-
 configured state, then care should be taken to use an appropriate
 service identifier and to use standard mechanisms for dynamically
 resolving the identifier into an IP address.  Also, any such
 identifiers should be alterable in the field through a conventional
 command and control interface for the service.

6. Acknowledgements

 The author thanks the following reviewers for their contributions to
 this document: Paul Barford, Geoff Huston, David Meyer, Mike
 O'Connor, Michael Patton, Tom Petch, and Pekka Savola.  Harald
 Alvestrand, Spencer Dawkins, Ted Hardie, David Kessens, and Thomas
 Narten provided valuable feedback during AD and IESG review.

7. References

7.1. Normative References

 [1]  Hubbard, K., Kosters, M., Conrad, D., Karrenberg, D., and J.
      Postel, "Internet Registry IP Allocation Guidelines", BCP 12,
      RFC 2050, November 1996.
 [2]  Eastlake 3rd, D. and A. Panitz, "Reserved Top Level DNS Names",
      BCP 32, RFC 2606, June 1999.
 [3]  IANA, "Special-Use IPv4 Addresses", RFC 3330, September 2002.
 [4]  Huston, G., Lord, A., and P. Smith, "IPv6 Address Prefix
      Reserved for Documentation", RFC 3849, July 2004.
 [5]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G., and E.
      Lear, "Address Allocation for Private Internets", BCP 5, RFC
      1918, February 1996.

7.2. Informative References

 [6]  Hamilton, M. and R. Wright, "Use of DNS Aliases for Network
      Services", BCP 17, RFC 2219, October 1997.

Plonka Best Current Practice [Page 7] RFC 4085 Embedding IP Addresses Considered Harmful June 2005

 [7]  Carpenter, B., Crowcroft, J., and Y. Rekhter, "IPv4 Address
      Behaviour Today", RFC 2101, February 1997.
 [8]  Mills, D., "Simple Network Time Protocol (SNTP) Version 4 for
      IPv4, IPv6 and OSI", RFC 2030, October 1996.
 [9]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
      Extensions", RFC 2132, March 1997.
 [10] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
      specifying the location of services (DNS SRV)", RFC 2782,
      February 2000.
 [11] Plonka, D., "Flawed Routers Flood University of Wisconsin
      Internet Time Server", August 2003.

Plonka Best Current Practice [Page 8] RFC 4085 Embedding IP Addresses Considered Harmful June 2005

Appendix A. Background

 In May 2003, the University of Wisconsin discovered that a network
 product vendor named NetGear had manufactured and shipped over
 700,000 routers with firmware containing a hard-coded reference to
 the IP address of one of the University's  NTP servers:, which was also known as "", a public
 stratum-2 NTP server.
 Due to that embedded fixed configuration and an unrelated bug in the
 SNTP client, the affected products occasionally exhibit a failure
 mode in which each flawed router produces one query per second
 destined for the IP address, and hence produces a large
 scale flood of Internet traffic from hundreds of thousands of source
 addresses, destined for the University's network, resulting in
 significant operational problems.
 These flawed routers are widely deployed throughout the global
 Internet and are likely to remain in use for years to come.  As such,
 the University of Wisconsin, with the cooperation of NetGear, will
 build a new anycast time service that aims to mitigate the damage
 caused by the misbehavior of these flawed routers.
 A technical report regarding the details of this situation is
 available on the world wide web: Flawed Routers Flood University of
 Wisconsin Internet Time Server [11].

Author's Address

 David Plonka
 University of Wisconsin - Madison

Plonka Best Current Practice [Page 9] RFC 4085 Embedding IP Addresses Considered Harmful June 2005

Full Copyright Statement

 Copyright (C) The Internet Society (2005).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Plonka Best Current Practice [Page 10]

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