Internet DRAFT - draft-west-let-localhost-be-localhost
draft-west-let-localhost-be-localhost
DNSOP M. West
Internet-Draft Google, Inc
Updates: 6761 (if approved) August 31, 2017
Intended status: Standards Track
Expires: March 4, 2018
Let 'localhost' be localhost.
draft-west-let-localhost-be-localhost-06
Abstract
This document updates RFC6761 with the goal of ensuring that
"localhost" 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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on March 4, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
4.2.1. Option 1: Explicit delegation . . . . . . . . . . . . 5
4.2.2. Option 2: Implicit failure . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5.1. Applications are encouraged to resolve localhost names
themselves. . . . . . . . . . . . . . . . . . . . . . . . 6
6. Implementation Considerations . . . . . . . . . . . . . . . . 6
6.1. Non-DNS usage of localhost names . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Changes from RFC 6761 . . . . . . . . . . . . . . . 8
Appendix B. Changes in this draft . . . . . . . . . . . . . . . 8
B.1. draft-west-let-localhost-be-localhost-06 . . . . . . . . 8
B.2. draft-west-let-localhost-be-localhost-05 . . . . . . . . 8
B.3. draft-west-let-localhost-be-localhost-04 . . . . . . . . 8
B.4. draft-west-let-localhost-be-localhost-03 . . . . . . . . 9
B.5. draft-west-let-localhost-be-localhost-02 . . . . . . . . 9
B.6. draft-west-let-localhost-be-localhost-01 . . . . . . . . 9
B.7. draft-west-let-localhost-be-localhost-00 . . . . . . . . 9
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
The "127.0.0.0/8" 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-
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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 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.
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", and "subdomain.localhost" will not be
resolved as "subdomain.localhost.example.com".
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 recursive DNS
server(s).
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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
IANA is requested to update the "localhost." registration in the
registry of Special-Use Domain Names [RFC6761] to reference the
domain name reservations considerations section of this document.
4.1. Domain Name Reservation Considerations
This document requests that IANA update the "localhost." registration
in the registry of Special-Use Domain Names [RFC6761] to reference
the domain name reservation considerations defined in Section 3.
4.2. DNSSEC
(Ed note: The following options seem reasonable. I personally prefer
the latter, but could be convinced that the former is reasonable if
that's the way the working group's concensus trends.)
4.2.1. Option 1: Explicit delegation
The ".localhost" TLD is already assigned to IANA, as per [RFC2606].
This document requests that a DNSSEC insecure delegation (that is, a
delegation with no DS records) be inserted into the root-zone,
delegated to "blackhole-[12].iana.org".
This request for an insecure delegation relies on the rationale
spelled out in section 4 of [I-D.wkumari-dnsop-internal], which
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discusses the DNSSEC considerations for the ".internal" TLD. The
same considerations apply to this document's discussion of localhost
names.
4.2.2. Option 2: Implicit failure
The ".localhost" TLD is already assigned to IANA, as per [RFC2606],
but does not have an entry in the DNSSEC 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 requirements to return NXDOMAIN that are laid out
in Section 3.
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.
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.
7. References
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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>.
[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.
[I-D.wkumari-dnsop-internal]
Kumari, W., "The .internal TLD.", draft-wkumari-dnsop-
internal-00 (work in progress), July 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>.
[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>.
[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/>.
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Appendix A. Changes from RFC 6761
Section 3 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,
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-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.2. 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.3. 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].
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B.4. 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.5. draft-west-let-localhost-be-localhost-02
o Pulled in definitions for IPv4 and IPv6 loopback addresses.
B.6. draft-west-let-localhost-be-localhost-01
o Added a requirement that caching DNS servers MUST generate an
immediate negative response.
B.7. 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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