Internet Engineering Task Force                                L. Howard
Internet-Draft                                         Time Warner Cable
Intended status: BCP                                           A. Durand
Expires: January 7, 2010                                         Comcast
                                                            July 6, 2009

           Reverse DNS in IPv6 for Internet Service Providers

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   This Internet-Draft is submitted to IETF in full conformance with the
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   In IPv4, Internet Service Providers (ISPs) commonly provide IN-
   ADDR.ARPA. information by prepopulating the zone with one PTR record

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   for every available address.  This practice does not scale in IPv6.
   This document analyses different approaches to managing the
   zone for broadband customers. .

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Reverse DNS in IPv4  . . . . . . . . . . . . . . . . . . .  3
     1.2.  Reverse DNS Considerations in IPv6 . . . . . . . . . . . .  4
   2.  Recommended practice for IPv6 broadband providers  . . . . . .  5
   3.  Alternatives in IPv6 . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Wildcard match . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Dynamic DNS  . . . . . . . . . . . . . . . . . . . . . . .  6
       3.2.1.  Dynamic DNS from Individual Hosts  . . . . . . . . . .  6
       3.2.2.  Dynamic DNS through Residential Gateways . . . . . . .  7
       3.2.3.  Dynamic DNS Delegations  . . . . . . . . . . . . . . .  7
       3.2.4.  Generate Dynamic Records . . . . . . . . . . . . . . .  8
     3.3.  Delegate DNS . . . . . . . . . . . . . . . . . . . . . . .  8
     3.4.  Dynamically Generate PTR When Queried ("On the Fly") . . .  8
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
     4.1.  Using Reverse DNS for Security . . . . . . . . . . . . . .  9
     4.2.  DNS Security with Dynamic DNS  . . . . . . . . . . . . . .  9
     4.3.  Considerations for Other Uses of the DNS . . . . . . . . .  9
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11

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1.  Introduction

   Best practice [RFC1033] is that "Every Internet-reachable host should
   have a name" [RFC1912] that is recorded with a PTR resource record in
   the IN-ADDR.ARPA zone.  Many network services perform a PTR lookup on
   the source address of incoming packets before performing services.

   Some of the most common uses for reverse DNS include:

   o  Building trust.  An administrator who spends time and effort
      properly maintaining DNS records might be assumed to spend time
      and effort on other maintenance, so the network might be more

   o  Validating other data.  Information from reverse DNS may be
      compared to information higher in the stack (for instance, mail
      originator), with a lower trustworthiness if they are dissimilar.

   o  Some degree of location information can often be inferred, since
      most administrators create reverse zones corresponding to
      aggregation points, which often correspond with geographical
      areas.  This information is useful for geo-location services and
      for law enforcement.

   However, it should be noted that the information contained in the
   reverse DNS is only as trustworthy as the entity that manages the
   leaf zone.  There is no guaranty about the validity of the
   information.  For example, anybody managing a reverse zone can point
   a PTR record to or  As a
   consequence, no real security information can be derived from the
   absence or presence of PTR records.

   Given the above and the dynamic nature of the Internet, with users
   being added and moving constantly, as well as the size of large
   Internet service providers who serve residential users, maintenance
   of individual PTR records for is often impractical.  Administrators
   of ISPs should consider the requirements for reverse DNS when
   evaluating options for PTR records in IPv6.

1.1.  Reverse DNS in IPv4

   Internet service providers (ISPs) that provide access to many
   residential users typically assign one or a few IPv4 addresses to
   each of those users, and populate an IN-ADDR.ARPA zone with one PTR
   record for every IPv4 address.  Some ISPs also configure forward
   zones with matching A records, so that lookups match.  For instance,
   if an ISP aggregated at a network hub in
   Anytown in the province of AnyWhere, the reverse zone might look

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   The conscientious might then also have a zone:
  IN A
  IN A
  IN A



  IN A

   Most ISPs generate PTR records for all IP addresses used for
   customers, and many create the matching A record.

1.2.  Reverse DNS Considerations in IPv6

   The length of individual addresses makes manual zone entries
   cumbersome.  A sample entry for 2001:db8:f00::0012:34ff:fe56:789a
   might be:

         .IP6.ARPA.  IN PTR

   Since 2^^96 possible addresses could be configured in the 2001:db8:
   f00/48 zone alone, it is impractical to write a zone with every

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   possible address entered.  If 1000 entries could be written per
   second, the zone would still not be complete after two quintillion

   Furthermore, since the 64 bits in the host portion of the address are
   frequently assigned using SLAAC [RFC4826] when the host comes online,
   it is not possible to know which addresses may be in use ahead of

   [RFC1912]is an informational document that says "PTR records must
   point back to a valid A record" and further that the administrator
   should "Make sure your PTR and A records match."  [RFC1912] While it
   is possible to ignore this advice, and many administrators do ignore
   it, administrators of residential ISPs should consider how it may be
   followed for AAAA and PTR RRs in the residential ISP.

2.  Recommended practice for IPv6 broadband providers

   Considering the little real value of reverse DNS as a security tool,
   and considering the difficulties to pre-populate the entire reverse
   zone for a single /64 prefix, let alone for all customer /56 or /48
   prefixes, it is reasonable practice to not pre-populate those reverse
   zones at all.

   A service provider that would like to keep managing those zones can
   look at the alternative discussed below.

3.  Alternatives in IPv6

   Several options existing for providing reverse DNS in IPv6.  All of
   these options also exist for IPv4, but the scaling problem is much
   less severe in IPv4.  Each option should be evaluated for its scaling
   ability, its compliance with existing standards and best practices,
   and its availability in common DNS servers.

3.1.  Wildcard match

   The use of wildcards in the DNS is described in [RFC4592], and their
   use in IPv6 reverse DNS is described in [RFC4472].  However, using
   wildcards may sometime lead to surprising results, especially if
   other records exist in the zone.

   Note that this solution fails the expectation in [RFC1912] for
   forward and reverse to match.

   Also, [RFC4035] mentioned that wildcards introduce ambiguities and

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   complexities and that "Operationally, inclusion of wildcard NS RRSets
   in a zone is discouraged, but not barred."

   As such, a service provider considering deploying DNSsec should
   exercise caution before using this wildcard solution.

3.2.  Dynamic DNS

   One way to ensure forward and reverse records match is for hosts to
   update DNS servers dynamically, once interface configuration (whether
   SLAAC, DHCPv6, or other means) is complete, as described in
   [RFC4472].  Hosts would need to provide both AAAA and PTR updates,
   and would need to know which servers would accept the information.

   This option should scale as well or as poorly as IPv4 dynamic DNS
   does.  Dynamic DNS may not scale effectively in large ISP networks
   which have no single master name server.  The ISP's DNS system may
   provide a point for Denial of Service attacks, including many
   attempted dDNS updates.  Accepting updates only from authenticated
   sources may mitigate this risk, but only if authentication itself
   does not require excessive overhead.  No authentication of dynamic
   DNS updates is inherently provided; implementers should consider use
   of DNSsec [RFC2535], or at least ingress filtering so updates are
   only accepted from customer address space from internal network
   interfaces.  UDP is allowed per [RFC2136] so transmission control is
   not assured, though the host should expect an ERROR or NOERROR
   message from the server [RFC2136]; TCP provides transmission control,
   but the updating host would need to be configured to use TCP.

3.2.1.  Dynamic DNS from Individual Hosts

   In the simplest case, a residential user will have a single host
   connected to the ISP.  Since the typical residential user cannot
   configure IPv6 addresses and resolving name servers on their hosts,
   the ISP should provide address information conventionally (i.e.,
   their normal combination of RAs, SLAAC, DHCP, etc.), and should
   provide a DNS Recursive Name Server and Domain Search List via DHCPv6
   as described in [RFC3646].  Note that the Domain Search List is
   commonly used as a domain name suffix for hosts, but this is an
   overloading of the parameter: hosts may need to search for
   unqualified names in multiple domains, without necessarily being a
   member of those domains.  Administrators should consider whether the
   domain search list actually provides an appropriate DNS suffix(es)
   when considering use of this option.  For purposes of dynamic DNS,
   the host should concatenate its local hostname (e.g., "hostname")
   plus the domain(s) in the Domain Search List (e.g.,
   ""), as in ""

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   Once it learns its address, and has a resolving name server (the
   Recursive Name Server learned via DHCPv6), the host must perform an
   MNAME lookup to find the primary master server in; note that
   many recursive lookups may be required to find the longest prefix
   which has been delegated.  Since ISPs are most commonly allocated /32
   prefixes, a host may start at the 4th byte of the address and
   increment or decrement by nybbles until the longest match is found.
   The DNS administrator must designate the Primary Master Server for
   the longest match required.  Once found, the host sends dynamic AAAA
   and PTR updates using the concatenation defined above

   In order to use this alternative, hosts must be configured to use
   dynamic DNS.  This is not default behavior for many hosts, which is
   an inhibitor for the large ISP.

   Given the number of assumptions made to make this work, this solution
   is not recommended.

3.2.2.  Dynamic DNS through Residential Gateways

   Residential customers may have a gateway, which may provide DHCPv6
   service to hosts from a delegated prefix.  ISPs should provide a DNS
   Recursive Name Server and Domain Search List to the gateway, as
   described above and in [RFC3646].  The gateway must either provide
   the same information in DHCPv6 responses to local hosts
   (recommended), or relay dynamic updates provided to it by hosts.
   Host behavior is unchanged; they should provide updates to the ISP's
   servers as described above.  For the same reasons mentioned above,
   this solution is not recommended.

3.2.3.  Dynamic DNS Delegations

   An ISP may delegate authority for a subdomain such as
   "" or
   "" to the customer's gateway.  Each domain
   thus delegated must be unique within the DNS.  However, individual
   hosts connected directly to the ISP rarely have the capability to run
   DNS for themselves; therefore, an ISP can only delegate to customers
   with gateways capable of being authoritative name servers.  If a
   device requests a DHCPv6 Prefix Delegation, that may be considered a
   reasonably reliable indicator that it is a gateway.  It is not
   necessarily an indicator that the gateway is capable of providing DNS
   services, and therefore cannot be relied upon as a way to test
   whether this option is feasible.

   If the customer's gateway is the name server, it provides its own
   information to hosts on the network, as normally done for enterprise

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   networks, and as described in [RFC2136].

   An ISP may elect to provide authoritative responses as a secondary
   server to the customer's primary server.

   Since few residential gateways are authoritative name servers capable
   of dynamic DNS updates, this method is not recommended to residential

3.2.4.  Generate Dynamic Records

   An ISP's name server that receives a dynamic forward or reverse DNS
   update may create a matching entry.  Since a host capable of updating
   one is generally capable of updating the other, the should not be
   required, but redundant record creation will ensure a record exists.
   ISPs implementing this method should check whether a record already
   exists before accepting or creating updates.

   This method is also dependent on hosts being capable of providing
   dynamic DNS updates, which is not default behavior for many hosts.

   Note also that this solution would have a severe impact on any DNSsec

   As for the previous variation of dynamic DNS updates, this method is
   not recommended.

3.3.  Delegate DNS

   For customers who are able to run their own DNS servers, such as
   commercial customers, often the best option is to delegate the
   reverse DNS zone to them, as described in [RFC2317].

   This is a general case of the specific case described in
   Section 3.2.3.  All of the same considerations still apply.

   Since most residential users have neither the equipment nor the
   expertise to run DNS servers, this method is not recommended to
   residential ISPs.

3.4.  Dynamically Generate PTR When Queried ("On the Fly")

   Common practice in IPv4 is to provide PTR records for all addresses,
   regardless of whether a host is actually using the address.  In IPv6,
   ISPs may generate PTR records for all IPv6 addresses as the records
   are requested.  Configuring records "on the fly" may consume more
   processor resource than other methods, but only on demand.  A denial
   of service is therefore possible, but with rate-limiting and normal

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   countermeasures, this risk is no higher than with other options.

   An ISP using this option should generate a PTR record on demand, and
   cache or prepopulate the forward (AAAA) entry for the duration of the
   time-to-live of the PTR.  This option has the advantage of assuring
   matching forward and reverse entries, while being simpler than
   dynamic DNS.  Administrators should consider whether the lack of
   user-specified hostnames is a drawback.

   This method may not scale well in conjunction with DNSsec [RFC4035],
   because the keys and records must be generated on the fly for the
   specific record requested, and possibly for each hexadecimal digit in
   the address.  As such, this method is not recommended.

4.  Security Considerations

4.1.  Using Reverse DNS for Security

   Some people think the existence of reverse DNS records, or matching
   forward and reverse DNS records, provides useful information about
   the hosts with those records.  For example, one might infer that the
   administrator of a network with properly configured DNS records was
   better-informed, and by further inference more responsible, than the
   administrator of a less-thoroughly configured network.  For instance,
   most email providers will not accept incoming connections on port 25
   unless forward and reverse DNS entries match.  If they match, but
   information higher in the stack (for instance, mail source) is
   inconsistent, the packet is questionable.  These records may be
   easily forged though, unless DNSsec or other measures are taken.  The
   string of inferences is questionable.

   Providing location information in PTR records is useful for
   troubleshooting, law enforcement, and geo-location services, but for
   the same reasons can be considered sensitive information.

4.2.  DNS Security with Dynamic DNS

   Security considerations of using dynamic DNS are described in
   [RFC3007].  DNS Security Extensions are documented inRFC2535

   Interactions with DNSsec are described throughout this document.

4.3.  Considerations for Other Uses of the DNS

   Several methods exist for providing encryption keys in the DNS.  Any
   of the options presented here may interfere with these key

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5.  IANA Considerations

   There are no IANA considerations or implications that arise from this

6.  References

6.1.  Normative References

   [RFC1033]  Lottor, M., "Domain Administrators Operators Guide",
              November 1987.

   [RFC1034]  Mockapetris, P., "Domain Names - Concepts and Facilities",
              November 1987.

   [RFC1912]  Barr, D., "Common DNS Operational and Configuration
              Errors", February 1996.

   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              April 1917.

   [RFC3007]  Wellington, B., "Secure Domain Name System (DNS) Dynamic
              Update", November 2000.

   [RFC3646]  Droms, R., Ed., "DNS Configuration options for Dynamic
              Host Configuration Protocol for IPv6 (DHCPv6)",
              December 2003.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", March 2005.

   [RFC4592]  Lewis, E., "The Role of Wildcards in the Domain Name
              System", July 2006.

   [RFC4826]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", September 2007.

   [RFC883]   Mockapetris, P., "Domain names: Implementation
              specification", November 1983,

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6.2.  Informative References

   [RFC2317]  Eidnes, H., de Groot, G., and P. Vixie, "Classless IN-
              ADDR.ARPA delegation", March 1998.

   [RFC2535]  Eastlake, D., "Domain Name System Security Extensions",
              March 1999.

   [RFC2672]  Crawford, M., "Non-Terminal DNS Name Redirection",
              August 1999.

   [RFC4339]  Jeong, J., Ed., "IPv6 Host Configuration of DNS Server
              Information Approaches", February 2006.

   [RFC4472]  Durand, A., Ihren, J., and P. Savola, "Operational
              Considerations and Issues with IPv6 DNS", April 2006.

              Senie, D., "draft-ietf-dnsop-inaddr-required-07",
              August 2005.

              Senie, D. and A. Sullivan,
              March 2008.

Authors' Addresses

   Lee Howard
   Time Warner Cable
   13820 Sunrise Valley Drive
   Herndon, VA  20171

   Phone: +1 703 345 3513

   Alain Durand
   1, Comcast center
   Philadelphia, PA  19333


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