Internet DRAFT - draft-schoen-intarea-unicast-127
draft-schoen-intarea-unicast-127
Internet Engineering Task Force S.D. Schoen
Internet-Draft J. Gilmore
Updates: 1122, 1812, 2827, 3704 (if approved) D. Täht
Intended status: Standards Track IPv4 Unicast Extensions Project
Expires: 31 August 2024 28 February 2024
Unicast Use of the Formerly Special-Cased 127/8
draft-schoen-intarea-unicast-127-05
Abstract
This document redefines the IPv4 local loopback network as consisting
only of the 65,536 addresses 127.0.0.0 to 127.0.255.255
(127.0.0.0/16). It asks implementers to make addresses in the prior
loopback range 127.1.0.0 to 127.255.255.255 fully usable for unicast
use on the Internet.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 31 August 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Change in Status of Addresses Within 127/8 . . . . . . . . . 4
4. Compatibility and Interoperability . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Implementation Status . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
With ever-increasing pressure to conserve IP address space on the
Internet, it makes sense to consider where relatively minor changes
can be made to fielded practice to improve numbering efficiency. One
such change, proposed by this document, is to allow the unicast use
of more than 16 million addresses historically allocated for loopback
in the middle of the IPv4 address space.
This document provides history and rationale to reduce the size of
the IPv4 local loopback network ("localnet") from /8 to /16, freeing
up over 16 million IPv4 addresses for other possible uses.
When all of 127.0.0.0/8 was allocated for loopback addressing, IPv4
addresses were not yet recognized as scarce. Today, there is no
justification for allocating 1/256 of all IPv4 addresses for this
purpose, when only one of these addresses is commonly used and only a
handful are regularly used at all. Reallocating the majority of
these addresses provides a large number of additional IPv4 host
addresses for possible use, alleviating some of the pressure of IPv4
address exhaustion.
1.1. Requirements Language
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 RFC 2119 [RFC2119].
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2. Background
The IPv4 network 127/8 was first reserved for a then-unspecified
purpose by Jon Postel in 1981 [RFC0776]. Postel's policy was to
reserve the first and last network of each class, and it does not
appear that he had a specific plan for how to use 127/8. Apparently,
the first operating systems to support a loopback interface as we
understand it today were experimental Berkeley Unix releases by Bill
Joy and Sam Leffler at the University of California at Berkeley. The
choice of 127.0.0.1 as loopback address was made in 1983 by Joy and
Leffler in the code base that was eventually released as 4.2BSD.
Their earliest experimental code bases used 254.0.0.0 and 127.0.0.0
as loopback addresses. Three years later, Postel and Joyce Reynolds
documented the loopback function in November 1986 [RFC0990], and it
was codified as a requirement for all Internet hosts three years
after that, in [RFC1122]. The substantive interpretation of these
addresses has remained unchanged since RFC 990 indicated that the
| network number 127 is assigned the "loopback" function, that is, a
| datagram sent by a higher level protocol to a network 127 address
| should loop back inside the host. No datagram "sent" to a network
| 127 address should ever appear on any network anywhere.
Many decisions about IPv4 addressing contemporaneous with this one
underscore the lack of concern about address scarcity. It was common
in the early 1980s to allocate an entire /8 to an individual
university, company, government agency, or even to a research
project.
By contrast, IPv6, despite its vastly larger pool of available
address space, allocates only a single local loopback address (::1)
[RFC4291]. This appears to be an architectural vote of confidence in
the idea that Internet protocols do not require millions of distinct
loopback addresses.
Most applications use only the single loopback address 127.0.0.1
("localhost") for IPv4 loopback purposes, although there are
exceptions. For example, the systemd-resolved service on Linux
provides a stub DNS resolver at 127.0.0.53.
In theory, having multiple local loopback addresses might be useful
for increasing the number of distinct IPv4 sockets that can be used
for inter-process communication within a host. The local loopback
/16 network retained by this document will still permit billions of
distinct concurrent loopback TCP connections within a single host,
even if both the IP address and port number of one endpoint of each
connection are fixed.
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3. Change in Status of Addresses Within 127/8
The purpose of this document is to reduce the size of the special-
case allocation of 127/8, so that only 127.0/16 is allocated as the
local loopback network.
Other IPv4 addresses whose first octet is 127 (that is, the addresses
127.1.0.0 to 127.255.255.255) are no longer allocated and are now
available for general Internet unicast use, treated identically to
other IPv4 addresses, and subject to potential future allocation.
All host and router software SHOULD treat 127.1.0.0 to
127.255.255.255 as a global unicast address range.
Clients for autoconfiguration mechanisms such as DHCP [RFC2131]
SHOULD accept a lease or assignment of addresses within 127.1/16 to
127.255/16 whenever the underlying operating system is capable of
accepting it. Servers for these mechanisms SHOULD assign this
address when so configured.
4. Compatibility and Interoperability
Many deployed systems follow older Internet standards in rejecting
externally-originating packets from addresses in 127/8, and in not
generating packets addressed to them). RFC 3704 [RFC3704] (BCP 84)
cites RFC 2827 [RFC2827] (BCP 38) to this effect:
| RFC 2827 recommends that ISPs police their customers' traffic by
| dropping traffic entering their networks that is coming from a
| source address not legitimately in use by the customer network.
| The filtering includes but is in no way limited to the traffic
| whose source address is a so-called "Martian Address" - an address
| that is reserved, including any address within 0.0.0.0/8,
| 10.0.0.0/8, 127.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16,
| 224.0.0.0/4, or 240.0.0.0/4.
In this context, RFC 3704 specifies filtering of these addresses as
source (not destination) addresses at a network ingress point as a
countermeasure against forged source addresses, limiting forwarded
packets' source addresses to only the set which have been actually
assigned to the customer's network. The RFC's mention of these
"Martian Addresses" is based on the assumption that they could never
be legitimately in use by the customer network.
Because the 127/8 address space is no longer allocated to loopback as
a whole, an address within this space, other than those within
127/16, is no longer inherently a "Martian" address. Both hosts and
routers MUST NOT hard-code a policy of always rejecting such
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addresses. Hosts and routers SHOULD NOT be configured to apply
Martian address filtering to any packet solely on the basis of its
reference to a source or destination address in 127/8 (other than
those in 127/16). Maintainers of lists of "Martian addresses" MUST
NOT designate addresses from the 127/8 range (other than those within
127/16) as "Martian".
The filtering recommended by RFC 3704 is designed for border routers,
not for hosts. To the extent that an ISP had validly allocated an
address range from within 127/8 to its customer, RFC 3704 would
already not require packets with those source addresses to be
filtered out by the ISP's border router.
Since deployed implementations' willingness to accept 127/8 addresses
as valid unicast addresses varies, a host to which a unicast address
from this range has been assigned may also have a varying ability to
communicate with other hosts.
Such a host might be inaccessible by some devices either on its local
network segment or elsewhere on the Internet, due to a combination of
host software limitations and reachability limitations in network
routers. IPv4 unicast interoperability with 127/8 can be expected to
improve over time following the publication of this document. Before
or after allocations are eventually made within this range,
"debogonization" efforts for allocated ranges can improve
reachability to the whole address block. Similar efforts have
already been done by Cloudflare on 1.1.1.1 [Cloudflare], and by RIPE
Labs on 1/8 [RIPElabs18], 2a10::/12 [RIPElabs2a1012], and 128.0/16
[RIPElabs128016]. The Internet community can use network probing
with any of several measurement-oriented platforms to investigate how
usable these addresses are at any particular point in time, as well
as to localize medium-to-large-scale routing problems. (Examples are
described in [Huston], [NLNOGRing], and [Atlas].) Any network
operator to whom such addresses are made available by a future
allocation will have to examine the situation in detail to determine
how well its interoperability requirements will be met.
5. IANA Considerations
This memo removes the loopback allocation of a portion of 127/8. It
therefore requests IANA to update the IPv4 Special-Purpose Address
registry [IANA4SP] by replacing the entry for 127.0.0.0/8 with
127.0.0.0/16, with authority of this document.
IANA is also requested to update the IPv4 Address Space Registry
[IANA4] by changing the entry for 127/8 (IANA - Loopback) to read
127/16, and by adding a new entry 127.1/16-127.255/16 Unallocated
[Date of this document] [blank] [blank] UNALLOCATED
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Finally, IANA is requested to prepare for this address space to be
addressed in in the reverse DNS space in in-addr.arpa.
This memo does not effect a registration, transfer, allocation, or
authorization for use of the newly unallocated addresses by any
specific entity. This memo's scope is to require IPv4 software
implementations to support the ordinary unicast use of addresses in
the newly unallocated range 127.1.0.0 through 127.255.255.255.
During a significant transition period, it would only be prudent for
the global Internet to use those addresses for experimental purposes
such as debogonization and testing. After that transition period, a
responsible entity such as IETF or IANA could later consider whether,
how and when to allocate those addresses to entities or to other
protocol functions.
6. Security Considerations
The behavior change specified by this document could produce security
concerns where two devices, or two different parts of the software on
a host, or a software application and a human user, follow divergent
interpretations of an address that was formerly a loopback address.
For example, this could lead to errors in the specification or
enforcement of rules about Internet hosts' connectivity to one
another, or their right to access resources. It could also lead to
an application connecting to the local host when it expected to
connect to a remote host, or vice versa.
One undesired case would arise where a local process on a host
accepts connections on what it believes is a loopback address, in
order to receive commands from other software on the same host, yet
the bound address is actually reachable from outside that host. The
traditional socket API present on most operating systems does not
make this especially likely, since a listening process typically
binds to either INADDR_ANY (which includes both loopback and
nonloopback interfaces) or INADDR_LOOPBACK (which includes only the
single address 127.0.0.1). The existence of an additional interface
with a remotely addressable unicast address like 127.8.9.10 would
not, in itself, change which hosts can communicate with either of
these sockets. Nonetheless, an operating system or software library
that provides some other interface with its own means of scoping the
receipt of incoming connections must take care not to leave an
ambiguity between host-only and non-host-only address scopes as a
result of the change specified by this document.
The importance of the distinction just mentioned is underscored by
practical examples of vulnerabilities when specific software relaxed
the distinction between loopback and non-loopback addresses in a
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different way. A 2017 vulnerability [CVE-2016-1551] related to the
reference implementation of the Network Time Protocol v4 [RFC5905],
and an analogous 2020 vulnerability [CVE-2020-8558] in the Kubernetes
cluster management software, both involved the use of a Linux kernel
option that removed the prohibition on sending or receiving packets
over the wire with a 127/8 destination address. This, however,
allowed other devices to reach and communicate with server processes
that had deliberately listened on what they otherwise expected to be
loopback addresses.
The change requested by this document does not have the same effect,
because loopback addresses in the reduced 127.0/16 loopback range are
still not permitted to appear on the wire, and incoming packets to or
from such addresses should still be rejected by implementations. The
ability to enforce the inaccessibility of loopback addresses by other
hosts remains necessary for security. In particular, treating all of
127/8 as globally routable address space is not a safe behavior.
Operating systems SHOULD continue to treat 127.0/16 as loopback-only
and never route packets between 127.0/16 loopback addresses and any
other interface. Addresses in 127.0/16 still SHOULD NOT appear on
any network link and SHOULD NOT be accepted or generated over a
network link. Applications MUST NOT use 127.1/16 to 127.255/16 for
loopback purposes and MUST NOT assume that connections from these
addresses necessarily originated from software on the local host.
Apart from that, firewall rules that assume that 127.1/16 through
127.255/16 are unroutable and/or local SHOULD be updated to take into
account that they may be routable and/or non-local.
Software that assumes that all of 127/8, either as a source or a
destination, refers to the local host SHOULD be updated to make that
inference only for 127/16. Communications to or from 127.1/16
through 127.255/16 SHOULD NOT be treated as inherently more trusted
than communications to or from the public Internet as a whole.
7. Acknowledgements
This document directly builds on prior work by Dave Täht and John
Gilmore as part of the IPv4 Unicast Extensions Project.
Members of the Internet History Mailing List helped us clarify the
early history of 127/8.
8. References
8.1. Normative References
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[IANA4] Internet Assigned Numbers Authority, "IANA IPv4 Address
Space Registry", <https://www.iana.org/assignments/ipv4-
address-space/ipv4-address-space.xhtml>.
[IANA4SP] Internet Assigned Numbers Authority, "IANA IPv4 Special-
Purpose Address Registry",
<https://www.iana.org/assignments/iana-ipv4-special-
registry/iana-ipv4-special-registry.xhtml>.
[RFC0776] Postel, J., "Assigned numbers", RFC 776,
DOI 10.17487/RFC0776, January 1981,
<https://www.rfc-editor.org/info/rfc776>.
[RFC0990] Reynolds, J. and J. Postel, "Assigned numbers", RFC 990,
DOI 10.17487/RFC0990, November 1986,
<https://www.rfc-editor.org/info/rfc990>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.
[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>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://www.rfc-editor.org/info/rfc2131>.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
May 2000, <https://www.rfc-editor.org/info/rfc2827>.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
2004, <https://www.rfc-editor.org/info/rfc3704>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
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8.2. Informative References
[Atlas] RIPE Network Coordination Centre, "RIPE Atlas",
<https://atlas.ripe.net/>.
[Cloudflare]
Strong, M., "Fixing reachability to 1.1.1.1, GLOBALLY!", 4
April 2018, <https://blog.cloudflare.com/fixing-
reachability-to-1-1-1-1-globally/>.
[CVE-2016-1551]
NIST National Vulnerability Database, "CVE-2016-1551",
January 2017,
<https://nvd.nist.gov/vuln/detail/CVE-2016-1551>.
[CVE-2020-8558]
NIST National Vulnerability Database, "CVE-2020-8558",
July 2020,
<https://nvd.nist.gov/vuln/detail/CVE-2020-8558>.
[Huston] Huston, G., "Detecting IP Address Filters", 13 January
2012, <https://labs.ripe.net/author/gih/detecting-ip-
address-filters/>.
[NLNOGRing]
NLNOG RING, "10 Years of NLNOG RING",
<https://ring.nlnog.net/post/10-years-of-nlnog-ring/>.
[Patch] IPv4 Unicast Extensions Project, "Pending Patches",
<https://github.com/schoen/unicast-extensions/tree/master/
pending-patches/linux>.
[RIPElabs128016]
Aben, E., "The Curious Case of 128.0/16", 6 December 2011,
<https://labs.ripe.net/author/emileaben/the-curious-case-
of-128016/>.
[RIPElabs18]
Schwarzinger, F., "Pollution in 1/8", 3 February 2010,
<https://labs.ripe.net/author/franz/pollution-in-18/>.
[RIPElabs2a1012]
Aben, E., "The Debogonisation of 2a10::/12", 17 January
2020, <https://labs.ripe.net/author/emileaben/the-
debogonisation-of-2a1012/>.
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Appendix A. Implementation Status
The behavior specified by this document is similar to the default
behavior of BSD operating systems' TCP/IP implementation, which
typically provides a single loopback address by default and limits
special-case treatment of the remainder of 127/8. This has been the
case for many years. On OpenBSD or NetBSD, the 127/8 addresses are
not blocked from use as host address by the OS kernel, but are
covered by a reject route created from userspace at boot time, which
causes packets addressed to them to be discarded. Removing this
route allows an individual system to use these addresses at least
within the LAN, although the system would not be willing to forward
packets to them when acting as a gateway.
This behavior is disabled by default in FreeBSD, but enabled by
"sysctl net.inet.ip.loopback_prefixlen=16" (or other values between 8
and 31), available in FreeBSD 14.0, released in November 2023. It
has been available in development releases since July 13, 2022.
The authors have prepared and tested small patches [Patch] to the
Linux kernel for this behavior. We achieved interoperability between
the patched version of Linux and the OpenBSD and FreeBSD
implementations, when interfaces are numbered with unicast 127/8
addresses. The patched systems were otherwise normally usable.
The behavior specified in this document contrasts with that of the
existing route_localnet option in Linux. The route_localnet option
is a Linux kernel feature which a user can enable in order to make
all of 127/8 simultaneously addressable in both host and global
address scopes, which, as described in the Security Considerations
section, has had undesirable security consequences. This
specification, and our patches, instead retain 127.0/16 as an
exclusive loopback address range, continuing to forbid it from
appearing on the wire at all.
Support for assigning addresses within 127/8 may be typical of many
DHCP implementations (because the 127/8 address assignment special
case has often been handled at the kernel level). If the underlying
operating system supports unicast 127/8 assignment to an interface,
the final official ISC DHCP release (4.4.3) supports 127/8 allocation
as both client and server, as do Busybox DHCP udhcpc/udhcpd (release
1.1.15), and ISC Kea (which currently includes only a DHCP server
implementation).
Authors' Addresses
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Seth David Schoen
IPv4 Unicast Extensions Project
San Francisco, CA
United States of America
Email: schoen@loyalty.org
John Gilmore
IPv4 Unicast Extensions Project
PO Box 170640-rfc
San Francisco, CA 94117-0640
United States of America
Email: gnu@rfc.toad.com
David M. Täht
IPv4 Unicast Extensions Project
Half Moon Bay, CA
United States of America
Email: dave@taht.net
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