DNSOP                                                            M. West
Internet-Draft                                               Google, Inc
Updates: 6761 (if approved)                               August 6, 2017
Intended status: Standards Track
Expires: February 7, 2018

                     Let 'localhost' be localhost.


   This document updates RFC6761 by requiring that the domain
   "localhost." and any names falling within ".localhost." resolve to
   loopback addresses.  This would allow other specifications to join
   regular users in drawing the common-sense conclusions that
   "localhost" means "localhost", and doesn't resolve to somewhere else
   on the network.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology and notation  . . . . . . . . . . . . . . . . . .   3
   3.  The "localhost." Special-Use Domain Name  . . . . . . . . . .   3
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Implementation Considerations . . . . . . . . . . . . . . . .   4
     5.1.  Security Decisions  . . . . . . . . . . . . . . . . . . .   4
     5.2.  Non-DNS usage of localhost names  . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Appendix A.  Changes from RFC 6761  . . . . . . . . . . . . . . .   6
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   The "" IPv4 address block and "::1/128" IPv6 address block
   are reserved as loopback addresses.  Traffic to this block is assured
   to remain within a single host, and can not legitimately appear on
   any network anywhere.  This turns out to be a very useful property in
   a number of circumstances; useful enough to label explicitly and
   interoperably as "localhost".  [RFC1537] suggests that this special-
   use top-level domain name has been implicitly mapped to loopback
   addresses for decades at this point, and that [RFC6761]'s assertion
   that developers may "assume that IPv4 and IPv6 address queries for
   localhost names will always resolve to the respective IP loopback
   address" is well-founded.

   Unfortunately, the rest of that latter document's requirements
   undercut the assumption it suggests.  Client software is empowered to
   send localhost names to DNS servers, and resolvers are empowered to
   return unexpectedly non-loopback results.  This divide between theory
   and practice has a few impacts:

   First, the lack of confidence that "localhost" actually resolves to
   the loopback interface encourages application developers to hard-code
   IP addresses like "" in order to obtain certainty regarding
   routing.  This causes problems in the transition from IPv4 to IPv6
   (see problem 8 in [draft-ietf-sunset4-gapanalysis]).

   Second, HTTP user agents sometimes distinguish certain contexts as
   "secure"-enough to make certain features available.  Given the
   certainty that "" cannot be maliciously manipulated or

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   monitored, [SECURE-CONTEXTS] treats it as such a context.  Since
   "localhost" might not actually map to the loopback address, that
   document declines to give it the same treatment.  This exclusion has
   (rightly) surprised some developers, and exacerbates the risks of
   hard-coded IP addresses by giving developers positive encouragement
   to use an explicit loopback address rather than a localhost name.

   This document hardens [RFC6761]'s recommendations regarding
   "localhost" by requiring that DNS resolution work the way that users
   assume: "localhost" is the loopback interface on the local host.
   Resolver APIs will resolve "localhost." and any names falling within
   ".localhost." to loopback addresses, and traffic to those hosts will
   never traverse a remote network.

2.  Terminology and notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   IPv4 loopback addresses are defined in Section 2.1 of [RFC5735] as

   IPv6 loopback addresses are defined in Section 3 of [RFC5156] as

3.  The "localhost."  Special-Use Domain Name

   The domain "localhost.", and any names falling within ".localhost.",
   are known as "localhost names".  Localhost names are special in the
   following ways:

   1.  Users are free to use localhost names as they would any other
       domain names.  Users may assume that IPv4 and IPv6 address
       queries for localhost names will always resolve to the respective
       IP loopback address.

   2.  Application software MAY recognize localhost names as special, or
       MAY pass them to name resolution APIs as they would for other
       domain names.

       Application software MUST NOT use a searchlist to resolve a
       localhost name.  That is, even if DHCP's domain search option
       [RFC3397] is used to specify a searchlist of "example.com" for a
       given network, the name "localhost" will not be resolved as
       "localhost.example.com", and "subdomain.localhost" will not be
       resolved as "subdomain.localhost.example.com".

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   3.  Name resolution APIs and libraries MUST recognize localhost names
       as special, and MUST always return an appropriate IP loopback
       address for IPv4 and IPv6 address queries and negative responses
       for all other query types.  Name resolution APIs MUST NOT send
       queries for localhost names to their configured caching DNS

       Name resolution APIs and libraries MUST NOT use a searchlist to
       resolve a localhost name.

   4.  Caching DNS servers MUST respond to queries for localhost names
       with NXDOMAIN.

   5.  Authoritative DNS servers MUST respond to queries for localhost
       names with NXDOMAIN.

   6.  DNS server operators SHOULD be aware that the effective RDATA for
       localhost names is defined by protocol specification and cannot
       be modified by local configuration.

   7.  DNS Registries/Registrars MUST NOT grant requests to register
       localhost names in the normal way to any person or entity.
       Localhost names are defined by protocol specification and fall
       outside the set of names available for allocation by registries/
       registrars.  Attempting to allocate a localhost name as if it
       were a normal DNS domain name will not work as desired, for
       reasons 2, 3, 4, and 5 above.

4.  IANA Considerations

   IANA is requested to update the "localhost." registration in the
   registry of Special-Use Domain Names [RFC6761] to reference this

5.  Implementation Considerations

5.1.  Security Decisions

   If application software wishes to make security decisions based upon
   the fact that localhost names resolve to loopback addresses (e.g. if
   it wishes to ensure that a context meets the requirements laid out in
   [SECURE-CONTEXTS]), then it SHOULD avoid relying upon name resolution
   APIs, instead performing the resolution itself.  If it chooses to
   rely on name resolution APIs, it MUST verify that the resulting IP
   address is a loopback address before making a decision about its
   security properties.

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5.2.  Non-DNS usage of localhost names

   Some application software differentiates between the hostname
   "localhost" and the IP address "".  MySQL, for example, uses
   a unix domain socket for the former, and a TCP connection to the
   loopback address for the latter.  The constraints on name resolution
   APIs above do not preclude this kind of differentiation.

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5156]  Blanchet, M., "Special-Use IPv6 Addresses", RFC 5156,
              DOI 10.17487/RFC5156, April 2008,

   [RFC5735]  Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
              RFC 5735, DOI 10.17487/RFC5735, January 2010,

   [RFC6761]  Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
              RFC 6761, DOI 10.17487/RFC6761, February 2013,

6.2.  Informative References

              Perreault, S., Tsou, T., Zhou, C., and P. Fan, "Gap
              Analysis for IPv4 Sunset", n.d.,

   [RFC1537]  Beertema, P., "Common DNS Data File Configuration Errors",
              RFC 1537, DOI 10.17487/RFC1537, October 1993,

   [RFC3397]  Aboba, B. and S. Cheshire, "Dynamic Host Configuration
              Protocol (DHCP) Domain Search Option", RFC 3397,
              DOI 10.17487/RFC3397, November 2002,

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              West, M., "Secure Contexts", n.d.,

Appendix A.  Changes from RFC 6761

   Section 3 of this document updates the requirements in section 6.3 of
   [RFC6761] in a few substantive ways:

   1.  Application software and name resolution APIs and libraries are
       prohibited from using searchlists when resolving localhost names.

   2.  Name resolution APIs and libraries are required to resolve
       localhost names to loopback addresses, without sending the query
       on to caching DNS servers.

   3.  Caching and authoritative DNS servers are required to respond to
       resolution requests for localhost names with NXDOMAIN.

Appendix B.  Acknowledgements

   Ryan Sleevi and Emily Stark informed me about the strange state of
   localhost name resolution.  Erik Nygren poked me to take another look
   at the set of decisions we made in [SECURE-CONTEXTS] around
   "localhost."; this document is the result, and his feedback has been
   very helpful.

Author's Address

   Mike West
   Google, Inc

   Email: mkwst@google.com
   URI:   https://mikewest.org/

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