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Versions: (draft-west-let-localhost-be-localhost) 00 01 02

DNSOP                                                            M. West
Internet-Draft                                               Google, Inc
Updates: 6761 (if approved)                            December 18, 2017
Intended status: Standards Track
Expires: June 21, 2018


                     Let 'localhost' be localhost.
             draft-ietf-dnsop-let-localhost-be-localhost-02

Abstract

   This document updates the treatment of the special-use domain name
   "localhost" as specified in RFC6761, Section 6.3, with the goal of
   ensuring that it can be safely relied upon as a name for the local
   host's loopback interface.  To that end, stub resolvers are required
   to resolve localhost names to loopback addresses.  Recursive DNS
   servers are required to return "NXDOMAIN" when queried for localhost
   names, making non-conformant stub resolvers more likely to fail and
   produce problem reports that result in updates.

   Together, these requirements would allow applications and
   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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on June 21, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   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  . . . . . . . . . . . . . . . . . .   4
   3.  The "localhost." Special-Use Domain Name  . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Domain Name Reservation Considerations  . . . . . . . . .   5
     4.2.  DNSSEC  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
     5.1.  Applications are encouraged to resolve localhost names
           themselves. . . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  'localhost' labels in subdomains  . . . . . . . . . . . .   6
   6.  Implementation Considerations . . . . . . . . . . . . . . . .   6
     6.1.  Non-DNS usage of localhost names  . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Changes from RFC 6761  . . . . . . . . . . . . . . .   7
   Appendix B.  Changes in this draft  . . . . . . . . . . . . . . .   8
     B.1.  draft-ietf-dnsop-let-localhost-be-localhost-02  . . . . .   8
     B.2.  draft-ietf-dnsop-let-localhost-be-localhost-01  . . . . .   8
     B.3.  draft-ietf-dnsop-let-localhost-be-localhost-00  . . . . .   9
     B.4.  draft-west-let-localhost-be-localhost-06  . . . . . . . .   9
     B.5.  draft-west-let-localhost-be-localhost-05  . . . . . . . .   9
     B.6.  draft-west-let-localhost-be-localhost-04  . . . . . . . .   9
     B.7.  draft-west-let-localhost-be-localhost-03  . . . . . . . .   9
     B.8.  draft-west-let-localhost-be-localhost-02  . . . . . . . .   9
     B.9.  draft-west-let-localhost-be-localhost-01  . . . . . . . .  10
     B.10. draft-west-let-localhost-be-localhost-00  . . . . . . . .  10
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The "127.0.0.0/8" IPv4 address block and "::1/128" IPv6 address block
   are reserved as loopback addresses.  Traffic to these blocks 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



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   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 "127.0.0.1" in order to obtain certainty regarding
   routing.  This causes problems in the transition from IPv4 to IPv6
   (see problem 8 in [I-D.ietf-sunset4-gapanalysis]).

   Second, HTTP user agents sometimes distinguish certain contexts as
   "secure"-enough to make certain features available.  Given the
   certainty that "127.0.0.1" cannot be maliciously manipulated or
   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 updates [RFC6761]'s recommendations regarding
   "localhost" by requiring that name resolution APIs and libraries
   themselves return a loopback address when queried for localhost
   names, bypassing lookup via recursive and authoritative DNS servers
   entirely.

   In addition, recursive and authoritative DNS servers are required to
   return "NXDOMAIN" for such queries.  This increases the likelihood
   that non-conformant stub resolvers will not go undetected.  Note that
   this does not have the result that such resolvers will fail safe--it
   just makes it more likely that they will be detected and fixed, since
   they will fail in the presence of conforming name servers.

   These changes are not sufficient to ensure that "localhost" can be
   assumed to actually refer to an address on the local machine.  This
   document therefore further requires that applications that wish to
   make that assumption handle the name "localhost" specially.




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2.  Terminology and notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   IPv4 loopback addresses are registered in Table 4 of Section 2.2.2 of
   [RFC6890] as "127.0.0.0/8".

   IPv6 loopback addresses are registered in Table 17 of Section 2.2.3
   of [RFC6890] as "::1/128".

   The domain "localhost.", and any names falling within ".localhost.",
   are known as "localhost names".

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

   Localhost names are special insofar as these names do not exist in
   the DNS, and querying the DNS for them is an error.  With that
   principle in mind, the considerations outlined in [RFC6761] can be
   answered as follows:

   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.

       If application software wishes to make security decisions based
       upon the assumption 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 MUST
       directly translate localhost names to a loopback address, and
       MUST NOT rely upon name resolution APIs to do so.

       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." but as "localhost.", and
       "subdomain.localhost" will not be resolved as
       "subdomain.localhost.example.com." but as "subdomain.localhost.".

   3.  Name resolution APIs and libraries MUST recognize localhost names
       as special, and MUST always return an appropriate IP loopback



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       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 recursive DNS
       server(s).

       As for application software, name resolution APIs and libraries
       MUST NOT use a searchlist to resolve a localhost name.

   4.  (Caching) recursive 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

4.1.  Domain Name Reservation Considerations

   This document requests that IANA updates the "localhost."
   registration in the registry of Special-Use Domain Names [RFC6761] to
   reference this document rather than [RFC6761].

   Considerations for this reservation are detailed in Section 3.

4.2.  DNSSEC

   The ".localhost" TLD is already assigned to IANA, as per [RFC2606],
   but does not have an entry in the root-zone.  This means that the
   root will return an NXDOMAIN response along with NSEC records
   constituting a secure denial of existence if queried.  That's
   consistent with the general principle that localhost names do not
   exist in the DNS, and the subsequent requirements to return NXDOMAIN
   that are laid out in Section 3.






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5.  Security Considerations

5.1.  Applications are encouraged to resolve localhost names themselves.

   Applications that attempt to use the local resolver to query
   "localhost" do not fail safely.  If an attacker sets up a malicious
   DNS server which returns a non-loopback address when queried for
   localhost names, such applications will connect to that remote server
   assuming it is local.  This risk drives the requirement that
   applications resolve localhost names themselves if they intend to
   make security decisions based on the assumption that localhost names
   resolve locally.

   There may be cases in which the target runtime environment can be
   safely assumed to do the right thing with localhost names.  In this
   case, the requirement that the application resolve localhost names on
   its own may be safe to ignore, but only if all the requirements under
   point 2 of Section 3 are known to be followed by the resolver that is
   known to be present in the target environment.

5.2.  'localhost' labels in subdomains

   Hosts like "localhost.example.com" and
   "subdomain.localhost.example.com" contain a "localhost" label, but
   are not themselves localhost names, as they do not fall within
   "localhost.".  Therefore, they are not directly affected by the
   recommendations in this document.  They have no resolution guarantees
   one way or another, and should not be given special treatment, either
   in DNS or in client software.

   Note, however, that the admonition against searchlist usage could
   affect their resolution in practice, as discussed in Section 3.  For
   example, even with a searchlist of "example.com" in place for a given
   network, the name "localhost" will not be resolved as
   "localhost.example.com." but as "localhost.", and
   "subdomain.localhost" will not be resolved as
   "subdomain.localhost.example.com." but as "subdomain.localhost.".

6.  Implementation Considerations

6.1.  Non-DNS usage of localhost names

   Some application software differentiates between the hostname
   "localhost" and the IP address "127.0.0.1".  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.




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7.  References

7.1.  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, <https://www.rfc-
              editor.org/info/rfc2119>.

   [RFC2606]  Eastlake 3rd, D. and A. Panitz, "Reserved Top Level DNS
              Names", BCP 32, RFC 2606, DOI 10.17487/RFC2606, June 1999,
              <https://www.rfc-editor.org/info/rfc2606>.

   [RFC6761]  Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
              RFC 6761, DOI 10.17487/RFC6761, February 2013,
              <https://www.rfc-editor.org/info/rfc6761>.

   [RFC6890]  Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
              "Special-Purpose IP Address Registries", BCP 153,
              RFC 6890, DOI 10.17487/RFC6890, April 2013,
              <https://www.rfc-editor.org/info/rfc6890>.

7.2.  Informative References

   [I-D.ietf-sunset4-gapanalysis]
              LIU, W., Xu, W., Zhou, C., Tsou, T., Perreault, S., Fan,
              P., Gu, R., Xie, C., and Y. Cheng, "Gap Analysis for IPv4
              Sunset", draft-ietf-sunset4-gapanalysis-09 (work in
              progress), August 2017.

   [RFC1537]  Beertema, P., "Common DNS Data File Configuration Errors",
              RFC 1537, DOI 10.17487/RFC1537, October 1993,
              <https://www.rfc-editor.org/info/rfc1537>.

   [RFC3397]  Aboba, B. and S. Cheshire, "Dynamic Host Configuration
              Protocol (DHCP) Domain Search Option", RFC 3397,
              DOI 10.17487/RFC3397, November 2002, <https://www.rfc-
              editor.org/info/rfc3397>.

   [SECURE-CONTEXTS]
              West, M., "Secure Contexts", n.d.,
              <http://w3c.github.io/webappsec-secure-contexts/>.

Appendix A.  Changes from RFC 6761

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




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   1.  Application software and name resolution APIs and libraries are
       prohibited from using searchlists when resolving localhost names,
       and encouraged to bypass resolution APIs and libraries altogether
       if they intend to make security decisions based on the
       "localhost" name.

   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.  Changes in this draft

B.1.  draft-ietf-dnsop-let-localhost-be-localhost-02

   o  Based on some feedback from Suzanne Woolf, this draft:

      *  Clarified the abstract
         (https://github.com/mikewest/internetdrafts/
         commit/837b89f35e08e98b0e02df87032c4ccc19cd06eb )

      *  Addressed nits in the "IANA considerations" section
         (https://github.com/mikewest/internetdrafts/commit/
         d65d4fbaec6afbbae70496ffb98dfb60e8d3e2eb )

      *  Reworded the "Non-TLD localhost" section
         (https://github.com/mikewest/internetdrafts/
         commit/44b1d7d4cfcb65aab3c46ff1c436a75a2fb3403f )

      *  Made the reference to [RFC2606] normative
         (https://github.com/mikewest/internetdrafts/commit/
         cd94988a966b93d2239de03d54513031a5823c0a )

B.2.  draft-ietf-dnsop-let-localhost-be-localhost-01

   o  Explicit adoption of the principle Wes Hardaker proposed in
      https://www.ietf.org/mail-archive/web/dnsop/current/msg21039.html
      , and that Warren Kumari reiterated in https://www.ietf.org/mail-
      archive/web/dnsop/current/msg21129.html : localhost names do not
      exist in the DNS, there is no authoritative source for these
      names, and querying resolvers for them is an error.

   o  Slight tightening of the admonition against search lists.

   o  Addressed "localhost" labels in non-localhost names.




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B.3.  draft-ietf-dnsop-let-localhost-be-localhost-00

   o  No change since draft-west-let-localhost-be-localhost-06, just
      renaming the document after DNSOP adopted it.

B.4.  draft-west-let-localhost-be-localhost-06

   o  Incorporated Ted Lemon's further feedback from
      https://www.ietf.org/mail-archive/web/dnsop/current/msg20769.html

   o  Explicitly waffling on DNSSEC.

B.5.  draft-west-let-localhost-be-localhost-05

   o  Updated obsolete references to RFC 5735 and 5156 in favor of
      [RFC6890].

   o  Clarify that non-caching recursive DNS servers are also addressed
      by #4 in Section 3.

   o  Reformulating the abstract and introduction based on feedback like
      Ted Lemon's in https://www.ietf.org/mail-
      archive/web/dnsop/current/msg20757.html

   o  Added a request that an insecure delegation for "localhost." be
      added to the root-zone.

B.6.  draft-west-let-localhost-be-localhost-04

   o  Restructured the draft as a stand-alone document, rather than as
      set of monkey-patches against [RFC6761].

B.7.  draft-west-let-localhost-be-localhost-03

   o  Explicitly referenced [I-D.ietf-sunset4-gapanalysis].

   o  Added a prohibition against using searchlists to resolve localhost
      names.

   o  Noted that MySQL has special behavior differentiating the
      connection mechanism used for "localhost" and "127.0.0.1".

B.8.  draft-west-let-localhost-be-localhost-02

   o  Pulled in definitions for IPv4 and IPv6 loopback addresses.






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B.9.  draft-west-let-localhost-be-localhost-01

   o  Added a requirement that caching DNS servers MUST generate an
      immediate negative response.

B.10.  draft-west-let-localhost-be-localhost-00

   First draft.

Appendix C.  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.  They, along with Warren
   Kumari, Ted Lemon, John Levine, Mark Andrews, and many other members
   of DNSOP offered substantive feedback that markedly improved the
   quality of this document.

Author's Address

   Mike West
   Google, Inc

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

























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