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Versions: 00 01 02 03                                                   
Internet Draft                                              Philip Hazel
draft-ietf-dnsop-dontpublish-unreachable-01.txt  University of Cambridge
Valid for six months                                      September 2001
Category: Best Current Practice

          IP Addresses that should never appear in the public DNS

      Copyright (C) The Internet Society (2001).  All Rights Reserved.

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   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."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at


   This document specifies an Internet Best Current Practice for the
   Internet Community. It has two themes. Firstly, it reinforces the
   prohibition in [RFC 1918] about the appearance of private IP
   addresses in publicly visible DNS records. Secondly, the document
   discusses the problems that can be caused by the appearance of public
   addresses, or indirect references to them, when the service implied
   by the address or reference is inaccessible from the public Internet.
   Specifying a blanket prohibition in the second case is difficult
   because inaccessibility may arise from many causes, some possibly
   legitimate. Instead, the document points out some of the problems
   that can arise, and suggests that other means of achieving the
   desired effects should be used wherever possible.

1. Introduction

   The increasing use of firewalls, NAT boxes, and similar technology
   has resulted in the fragmentation of the Internet into regions whose
   boundaries do not allow general connectivity. There are two primary
   reasons for this:

   (1) The perceived shortage of IPv4 addresses has caused increasing
   use of private IP network addresses such as on LANs. A
   number of such private address ranges are designated in [RFC 1918],
   and others may be also assigned by IANA.

[Note: For example, there's 169.254/16, which is mentioned in
draft-ietf-zeroconf-ipv4-linklocal-04.txt, but since that's still a
draft, I can't cite it.]

   Hosts using private addresses that wish to communicate with the
   public Internet must do so via an address translation mechanism such
   as a NAT box. This allows a host with a private address to send
   packets to public Internet hosts, and to receive replies. However,
   unsolicited incoming packets cannot reach these hosts from outside
   their own private network.

   (2) Increasing security concerns have caused many sites to install
   firewalls or to implement restrictions in their boundary routers in
   order to lock out certain kinds of connection to their hosts, even
   when the hosts are using public Internet addresses, though in many
   cases firewalls also provide NAT functionality.

   Thus, there are two classes of host which some or all types of
   unexpected incoming packet from the public Internet cannot reach.

   A number of instances have been observed where IP addresses that are
   never accessible from the public Internet have nevertheless been
   inserted into resource records in the public DNS. This document seeks
   to prohibit such behaviour in the case of truly private addresses,
   and to discourage it in the case of public, but unreachable,

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC 2119].

   The phrase "address record" means an A record or an AAAA record, or
   any other kind of name-to-address record that may come into use.

2. Private network addresses

   Examples of [RFC 1918] private host addresses are and Packets cannot be routed to such addresses from the
   public Internet. [RFC 1918] explains this in section 3, from where
   this paragraph is taken:

      Because private addresses have no global meaning, routing
      information about private networks shall not be propagated on
      inter-enterprise links, and packets with private source or
      destination addresses should not be forwarded across such links.
      Routers in networks not using private address space, especially
      those of Internet service providers, are expected to be
      configured to reject (filter out) routing information about
      private networks.

   Because the same private addresses are in use in many different
   organizations, they are ambiguous. The appearance of private
   addresses in the DNS could therefore lead to unpredictable and
   unwanted behaviour. Consider this set of entries:

      @       IN      MX  10  smtp
      smtp    IN      A
      smtp    IN      A

   Zones set up in this way have been seen, and some administrators
   apparently believe this is useful, because it allows mail on their
   local network to be delivered straight to the internal server (the
   one with address However, it all breaks down when a host
   on a foreign network that is also using the address
   attempts to send mail to the domain.

   In section 5 of [RFC 1918] there is a prohibition of the appearance of
   private addresses in publicly visible DNS records. It says:

      If an enterprise uses the private address space, or a mix of
      private and public address spaces, then DNS clients outside of
      the enterprise should not see addresses in the private address
      space used by the enterprise, since these addresses would be

   The wording "should not" is not a very strong prohibition,
   considering the interworking problems that ignoring it can cause.
   Therefore, this document makes a stronger statement:

   Public DNS zones MUST NOT contain [RFC 1918] addresses, or any other
   addresses designated by IANA as private, in any resource records.

3. Public network addresses that are inacessible

   The situation with public network addresses is more complicated
   because the Internet cannot in general be cleanly divided into
   "public" and "private" parts in this case. Examples of situations
   where the division is fuzzy are:

   (1) A host with a public address that is behind a firewall
   may be accessible for SSH sessions, but not for SMTP sessions. That
   is, the blocking may apply only to certain ports.

   (2) A host with a public address may make certain services available
   only to specific client hosts, for example, those in partner

   (3) A host might respond to incoming packets only if the client host
   is using IPsec.

   When a host is providing any service at all over the public Internet,
   a publicly visible address record is of course required to give
   access to the host.

   However, for some protocols and services, additional DNS records
   are defined that reference hosts' address records. These are the MX
   record for SMTP, and the SRV record for other services. The existence
   of such indirect records advertises the availability of the relevant

   If these services are always inaccessible over the public Internet,
   it is bad practice to include the MX or SRV records in public DNS
   zones, for the following reason:

   A host that tries to connect to an unreachable address (or port)
   may not receive an immediate rejection; in many cases the connection
   will fail only after a timeout expires. The wasted effort ties up
   resources on the calling host and the network, possibly for some
   considerable time (SMTP timeouts, for example, are of the order of
   minutes). It may also cause a gratuitous slowing down of the

   Furthermore, in the case of dial-up connections, ISDN, or other kinds
   of usage-based charged network connection, the wasted network
   resources may cost real money.

   Public DNS zones SHOULD NOT contain MX or SRV records that point to
   hosts for which the relevant services are never accessible over the
   public Internet. In other words, if there is no host that is able to
   make use of the service using the public Internet, the service SHOULD
   NOT be publicly advertised.

4. Loopback addresses

   The loopback addresses ( for IPv4 and ::1 for IPv6) are
   another form of private address. There has been a practice of including
   them in DNS zones for two entirely different reasons.

4.1 The name "localhost"

   Some hostmasters include records of this type in their zones:

     localhost.some.domain.example.  A

   The reason for doing this is so that other hosts in the domain
   that use the DNS for all their name resolution can make use of the
   unqualified name "localhost". This works because DNS resolvers
   normally add the local enclosing domain to unqualified names.

   DNS zones MAY make use of this technique for the name "localhost"
   only, if it is required in their environment, but SHOULD avoid it
   if possible.

4.2 DNS "black lists"

   There is an  increasingly popular practice of creating "black
   lists" of misbehaving hosts (for example, open mail relays) in
   the DNS. The first of these was the "Realtime Blackhole List"
   (RBL). Such lists make use of addresses in the
   network in DNS address records to give information about listed
   hosts (which are looked up via their inverted IP addresses).

   Such records are in specific "black list" domains, and are well
   understood not to be invitations to attempt connections to the
   addresses they publish.

   DNS zones MAY continue to make use of this technique.

4.3 Other uses of loopback networks

   Apart from the exceptions mentioned in 4.1 and 4.2 above, the
   loopback addresses MUST NOT appear in address records in the public

4.4 References to loopback addresses

   When address records that contain loopback addresses do exist,
   DNS zones MUST NOT contain indirect records (MX or SRV) that
   reference them.

5. Alternative techniques

5.1 Splitting DNS zones

   A site that is using private addresses may well want to use DNS
   lookups for address resolution on its hosts. The lazy way approach is
   simply to put the data into the public DNS zone, as in the example
   shown in section 2 above. Because this can cause problems for
   external hosts, this MUST NOT be done.

   One approach that is commonly taken is to run a so-called "split
   DNS". Two different authoritative servers are created: one containing
   all the zone data is accessible only from within the private network.
   External DNS queries are directed to the second server, which
   contains a filtered version of the zone, without the private

5.2 SMTP servers behind firewalls

   The complication of a split DNS is not normally needed if it is only
   SMTP traffic that is being blocked to a public address on a host
   behind a firewall. Setting up MX records like this:

     plc.example.   MX   5   mail.plc.example.
                    MX  10   public.plc.example.

   where both hosts have public IP addresses, but the first is blocked
   at the firewall, SHOULD NOT be done. Only the publicly accessible
   host should be used:

     plc.example.   MX  10   public.plc.example.

   If a split DNS is in use, the host public.plc.example can use the
   internal version to route the mail onwards. However, most MTAs have
   configuration facilities to allow for explicit routing of mail, without
   the need to use a DNS lookup.

5.3 Specification of no SMTP service

   MX records that point to host names whose address records specify the
   loopback address have been seen in the DNS. This seems to be a
   misguided attempt to specify "no SMTP service for this domain".

   If such a facility is required, it SHOULD instead be done by
   arranging for the hosts in question to return

     554 No SMTP service here

   to all SMTP connections.

6. Security Considerations

   This document is not known to create new security issues in the DNS,
   mail agents, etc. In some sense, it may reduce security exposure by
   insisting that a site's inappropriate internal data not be exposed.

7. IANA Considerations

   No IANA actions are required by this document.

8. Acknowledgements

   Randy Bush read an early draft of this document and suggested several

   Draft 01 has benefitted from comments made by Daniel Senie, John
   Schnizlein, Robert Elz, Bert Hubert, and Stuart Cheshire.

9. Author's Address

   Philip Hazel
   University of Cambridge Computing Service
   New Museums Site, Pembroke Street
   Cambridge CB2 3QH, England

   Phone: + 44 1223 334714
   Email: ph10@cam.ac.uk

10. References

   [RFC 1918]  Rekhter, Y. et al "Address allocation for Private
               Internets", BCP 5, RFC 1918, February 1996.

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

11. Changes made during development of this document

   This section is provided for the convenients of those tracking the
   document. It will be removed from the final draft.

11.1 Changes made to the -00 version

   . While leaving the MUSTs in for truly private addresses, I've tried
   to be more "educational" about the case of public addresses that are
   inaccessible, and backed down to SHOULD in those cases.

   . I've pointed out the lack of a clear-cut public/private boundary,
   and tried to make the case for not advertising unavailable services
   without being so probititive in the wording. This includes using
   "never accessible" instead of "not accessible".

   . Changed "hostmaster" to "zone" in a couple of cases.

   . Included an example of bad MX practice with an [RFC 1918] address.

   . Noted that [RFC 1918] is not the only list of private addresses.

   . General tidying of the wording and rearrangement of the material.

   . The Post Office changed our postcode!

Full Copyright Statement

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

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