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Unicast Use of the Formerly Special-Cased 127/8

Document Type Active Internet-Draft (individual)
Authors Seth David Schoen , John IETF Gilmore , David M. Täht
Last updated 2023-08-31
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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: 3 March 2024                                     31 August 2023

            Unicast Use of the Formerly Special-Cased 127/8


   This document redefines the IPv4 local loopback network as consisting
   only of the 65,536 addresses to
   (  It asks implementers to make addresses in the prior
   loopback range to 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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 3 March 2024.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   described in Section 4.e of the Trust Legal Provisions and are
   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 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",
   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 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 and
   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

   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
   ("localhost") for IPv4 loopback purposes, although there are
   exceptions.  For example, the systemd-resolved service on Linux
   provides a stub DNS resolver at

   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 to 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 to 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,
   |, or

   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 [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 with, 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

   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 through
   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  The existence of an additional interface
   with a remotely addressable unicast address like 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", <

   [IANA4SP]  Internet Assigned Numbers Authority, "IANA IPv4 Special-
              Purpose Address Registry",

   [RFC0776]  Postel, J., "Assigned numbers", RFC 776,
              DOI 10.17487/RFC0776, January 1981,

   [RFC0990]  Reynolds, J. and J. Postel, "Assigned numbers", RFC 990,
              DOI 10.17487/RFC0990, November 1986,

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,

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

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,

   [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, <>.

   [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
              2004, <>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <>.

   [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,

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8.2.  Informative References

   [Atlas]    RIPE Network Coordination Centre, "RIPE Atlas",

              Strong, M., "Fixing reachability to, GLOBALLY!", 4
              April 2018, <

              NIST National Vulnerability Database, "CVE-2016-1551",
              January 2017,

              NIST National Vulnerability Database, "CVE-2020-8558",
              July 2020,

   [Huston]   Huston, G., "Detecting IP Address Filters", 13 January
              2012, <

              NLNOG RING, "10 Years of NLNOG RING",

              Aben, E., "The Curious Case of 128.0/16", 6 December 2011,

              Schwarzinger, F., "Pollution in 1/8", 3 February 2010,

              Aben, E., "The Debogonisation of 2a10::/12", 17 January
              2020, <

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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.

   The behavior specified here also been implemented in FreeBSD-current
   since July 13, 2022.  This behavior is disabled by default, but
   enabled by "sysctl net.inet.ip.loopback_prefixlen=16" (or other
   values between 8 and 31).  As of September 2023, this support has not
   yet appeared in a numbered release of FreeBSD; it is scheduled to be
   included in FreeBSD 14.0 in October 2023.

   The authors have prepared and tested small patches 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

Authors' Addresses

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   Seth David Schoen
   IPv4 Unicast Extensions Project
   San Francisco, CA
   United States of America

   John Gilmore
   IPv4 Unicast Extensions Project
   PO Box 170640-rfc
   San Francisco, CA 94117-0640
   United States of America

   David M. Täht
   IPv4 Unicast Extensions Project
   Half Moon Bay, CA
   United States of America

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