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).
Abstract
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
regard.
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
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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
community.
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
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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.
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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 "example.com", "example.net",
and "example.org" 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.
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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
"example.com" might configure its product to find its SNTP server by
the name "sntp-server.config.example.com" or even by a name that is
specific to the product and version, such as "sntp-
server.v1.widget.config.example.com". Here the "config.example.com"
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 "_ntp._udp.example.com".
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 192.0.2.0/24 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.
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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
services.
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
user.
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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.
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[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.
http://www.cs.wisc.edu/~plonka/netgear-sntp/
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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:
128.105.39.11, which was also known as "ntp1.cs.wisc.edu", 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 128.105.39.11, 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
EMail: plonka@doit.wisc.edu
URI: http://net.doit.wisc.edu/~plonka/
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Copyright (C) The Internet Society (2005).
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