Reverse DNS in IPv6 for Internet Service Providers

Best practice is that "Every Internet-reachable host should have a name" [RFC1912] that is recorded with a PTR resource record in the .ARPA zone, and "PTR's should use official names and not aliases" [RFC1033]. Some network services perform a PTR lookup on the source address of incoming packets before performing services. Individual Internet users in the residential or consumer scale, including small and home businesses, are constantly joining or moving on the Internet. For large Internet service providers who serve residential users, maintenance of individual PTR records is impractical. Administrators at ISPs should consider the need for PTR records and evaluate methods for responding to reverse DNS queries in IPv6.

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 Example.com aggregated 192.0.2.0/24 at a network hub in Town in the province of AnyWhere, the reverse zone might look like: 1.2.0.192.IN-ADDR.ARPA. IN PTR 1.user.town.AW.example.com. 2.2.0.192.IN-ADDR.ARPA. IN PTR 2.user.town.AW.example.com. 3.2.0.192.IN-ADDR.ARPA. IN PTR 3.user.town.AW.example.com. . . . 254.2.0.192.IN-ADDR.ARPA. IN PTR 254.user.town.AW.example.com. The conscientious Example.com might then also have a zone: 1.user.town.AW.example.com. IN A 192.0.2.1 2.user.town.AW.example.com. IN A 192.0.2.2 3.user.town.AW.example.com. IN A 192.0.2.3 \. \. \. 254.user.town.AW.example.com. IN A 192.0.2.254 Many ISPs generate PTR records for all IP addresses used for customers, and many create the matching A record.

A sample entry for 2001:0db8:0f00:0000:0012:34ff:fe56:789a might be: a.9.8.7.6.5.e.f.f.f.4.3.2.1.0.0.0.0.0.0.0.0.f.0.8.b.d.0.1.0.0.2 .IP6.ARPA. IN PTR 1.user.town.AW.example.com. ISPs will often delegate an IPv6 prefix to their customers. Since 2^^80 possible addresses could be configured in an example 2001:db8:f00/48 zone alone, even with automation it is impractical to write a zone with every possible address entered. If 1000 entries could be written per second, the zone would still not be complete after 38 trillion years. Furthermore, it is often impossible to associate host names and addresses, since the 64 bits in the Interface Identifier portion of the address are frequently assigned using SLAAC [RFC4862] when the host comes online, and may be short-lived. [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] This document considers how to follow this advice for AAAA and PTR records.

Several options exist 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 systems.

Some ISP DNS administrators may choose to provide only a NXDomain response to PTR queries for subscriber addresses. In some ways, this is the most accurate response, since no name information is known about the host. Providing a negative response in response to PTR queries does not satisfy the expectation in [RFC1912] for entries to match. Users of services which are dependent on a successful lookup will have a poor experience. For instance, some web services and SSH connections wait for a DNS response, even NXDOMAIN, before responding. For best user experience, then, it is important to return a response, rather than have a lame delegation. On the other hand, external mail servers are likely to reject connections, which might be an advantage in fighting spam. DNS administrators should consider the uses for reverse DNS records and the number of services affecting the number of users when evaluating this option.

The use of wildcards in the DNS is described in [RFC4592], and their use in IPv6 reverse DNS is described in [RFC4472]. While recording all possible addresses is not scalable, it may be possible to record a wildcard entry for each prefix assigned to a customer. Consider also that "inclusion of wildcard NS RRSets in a zone is discouraged, but not barred." [RFC4035] This solution generally scales well. However, since the response will match any address in the wildcard range (/48, /56, /64, etc.), a forward DNS lookup on that response given will not be able to return the same hostname. This method therefore fails the expectation in [RFC1912] for forward and reverse to match. DNSsec [RFC4035] scalability is limited to signing the wildcard zone, which may be satisfactory.

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, but a single master server is not best practice. 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 TSIG [RFC2845], or at least ingress filtering so updates are only accepted from customer address space from internal network interfaces, rate limit the number of updates from a customer per second, and consider impacts on scalability. 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. Administrators should consider what domain will contain the records, and who will provide the names. If subscribers provide hostnames, they may provide inappropriate strings. Consider "ihate.example.com" or "badword.customer.example.com" or "celebrityname.committed.illegal.acts.example.com." There is no assurance of uniqueness if multiple hosts try to update with the same name ("mycomputer.familyname.org"). There is no standard way to indicate to a host what server it should send dDNS updates to.

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, DHCP, etc.), and should provide a DNS Recursive Name Server and Domain Search List as described in [RFC3646] or [RFC6106]. In determining its Fully Qualified Domain Name, a host will typically use a domain from the Domain Search List. This is an overloading of the parameter; multiple domains could be listed, since 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 would concatenate its local hostname (e.g., "hostname") plus the domain(s) in the Domain Search List (e.g., "customer.example.com"), as in "hostname.customer.example.com." Once it learns its address, and has a resolving name server, the host must perform an SOA lookup on the ip6.arpa record to be added, to find the owner, eventually to find the server authoritative for the zone (which might accept dynamic updates). Several recursive lookups may be required to find the longest prefix which has been delegated. 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 ("hostname.customer.example.com"). 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. This option may be scalable, although registration following an outage may cause significant load, and hosts using privacy extensions [RFC4941] may update records daily. It is up to the host to provide matching forward and reverse records, and to update them when the address changes.

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] and [RFC6106]. There are two options for how the gateway uses this information. The first option is for the gateway to respond to DHCPv6 requests with the same DNS Recursive Name Server and Domain Search List provided by the ISP. The alternate option is for the gateway to relay dynamic DNS updates from hosts to the servers and domain provided by the ISP. Host behavior is unchanged; the host sends the same dynamic updates, either to the ISP's server (as provided by the gateway), or to the gateway for it to forward.

An ISP may delegate authority for a subdomain such as "customer12345.town.AW.customer.example.com" or "customer12345.example.com" to the customer's gateway. Each domain thus delegated must be unique within the DNS. The ISP may also then delegate the ip6.arpa zone for the prefix delegated to the customer, as in (for 2001:db8:f00::/48) "0.0.f.0.8.b.d.0.1.0.0.2.ip6.arpa." Then the customer could provide updates to their own gateway, with forward and reverse. 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, rather than an individual host. 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. In fact, this kind of delegation will not work for devices complying with [RFC6092], which includes the requirement, "By DEFAULT, inbound DNS queries received on exterior interfaces MUST NOT be processed by any integrated DNS resolving server." If the customer's gateway is the name server, it provides its own information to hosts on the network, as often done for enterprise networks, and as described in [RFC2136]. An ISP could provide authoritative responses as a secondary server to the customer's master server. For instance, the home gateway name server could be the master server, with the ISP providing the only published NS authoritative servers. To implement this alternative, users' residential gateways must be capable of acting as authoritative name servers capable of dynamic DNS updates. There is no mechanism for an ISP to dynamically communicate to a user's equipment that a zone has been delegated, so user action would be required. Most users have neither the equipment nor the expertise to run DNS servers, so this option is unavailable to the residential ISP.

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, this 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.

A ISP's DHCPv6 server may populate the forward and reverse zones when the DHCP request is received, if the request contains enough information. [RFC4704] However, this method will only work for a single host address (IA_NA); the ISP's DHCP server would not have enough information to update all records for a prefix delegation. If the zone authority is delegated to a home gateway which used this method, the gateway could update records for residential hosts. To implement this alternative, users' residential gateways would have to support the FQDN DHCP option, and would have to either have the zones configured, or send dDNS messages to the ISP's name server.

A user may receive an address or prefix from a RADIUS [RFC2865] server, the details of which may be recorded via RADIUS Accounting [RFC2866] data. The ISP may populate the forward and reverse zones from the accounting data if it contains enough information. This solution allows the ISP to populate data concerning allocated prefixes (as per 2.2 (wildcards)) and CPE endpoints, but as with 2.3.5 does not allow the ISP to populate information concerning individual hosts.

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] (for IPv4). However, since most residential users have neither the equipment nor the expertise to run DNS servers, this method is unavailable to residential ISPs. This is a general case of the specific case described in Section 2.3.3. All of the same considerations still apply.

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, which may be mitigated with rate-limiting and normal countermeasures. 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. Similarly, the ISP would prepopulate the PTR following a AAAA query. Alternatively, if an algorithm is used to generate unique name, it can be employed on the fly in both directions. 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 of the additional load, but since keys may be pregenerated for zones, and not for each record, the risk is moderate. Signing records on the fly may increase load, and may not scale; unsigned records can indicate that these records are less trusted, which might be acceptable. Another consideration is that the algorithm used for generating the record must be the same on all servers for a zone. In other words, any server for the zone must produce the same response for a given query. Administrators managing a variety of rules within a zone might find it difficult to keep those rules synchronized on all servers.

There are five common uses for PTR lookups: Rejecting mail: A PTR with a certain string or missing may indicate "This host is not a mail server," which may be useful for rejecting probable spam. The absence of a PTR leads to the desired behavior. Serving ads: "This host is probably in town.province." An ISP that does not provide PTR records might affect somebody else's geolocation. Accepting SSH connections: The presence of a PTR may be inferred to mean "This host has an administrator with enough clue to set up forward and reverse DNS." This is a poor inference. Log files: Many systems will record the PTR of remote hosts in their log files, to make it easier to see what network the remote host uses when reading logs later. Traceroute: The ability to identify an interface and name of any intermediate node or router is important for troubleshooting. As a general guideline, when address assignment and name are under the same authority, or when a host has a static address and name, AAAA and PTR records should exist and match. For residential users, if these four use cases are important to the ISP, the administrator will then need to consider how to provide PTR records. The best accuracy would be achieved if ISPs delegate authority along with address delegation, but residential users rarely have domain names or authoritative name servers. Dynamic DNS updates can provide accurate data, but there is no standard way to indicate to residential devices where to send updates, if the hosts support it, and if it scales. An ISP has no knowledge of its residential users' hostnames, and therefore can either provide a wildcard response or a dynamically generated response. A valid negative response (such as NXDomain) is a valid response, if the four cases above are not essential; lame delegation should be avoided.

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, and may become unneeded if other means for evaluating trustworthiness (such as positive reputations) become predominant in IPv6. Providing location information in PTR records is useful for troubleshooting, law enforcement, and geolocation services, but for the same reasons can be considered sensitive information.

Security considerations of using dynamic DNS are described in [RFC3007]. DNS Security Extensions are documented in [RFC4033]. Interactions with DNSsec are described throughout this document.

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

The author would like to thank Alain Durand, JINMEI Tatuya, David Freedman, Andrew Sullivan, Chris Griffiths, Darryl Tanner, Ed Lewis, John Brzozowski, Chris Donley, Wes George, Jason Weil, John Spence, Ted Lemon, Stephan Lagerholm, Steinar Haug, Mark Andrews, and Chris Roosenraad, Fernando Gont, John Levine, and many others who discussed and provided suggestions for this document.

There are no IANA considerations or implications that arise from this document.

[RFC1033] Lottor, M., "Domain Administrators Operators Guide", 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. [RFC2845] "Secret Key Transaction Authentication for DNS (TSIG)". [RFC2865] "Remote Authentication Dial In User Service (RADIUS)". [RFC2866] "RADIUS Accounting". [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. [RFC4033] "DNS Security Introduction and Requirements". [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. [RFC4704] Stapp, M., Volz, Y., and Y. Rekhter, "The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN) Option". [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", September 2007. [RFC4941] "Privacy Extensions for Stateless Address Autoconfiguration in IPv6". [RFC6106] "IPv6 Router Advertisement Options for DNS Configuration".

[RFC2317] Eidnes, H., de Groot, G., and P. Vixie, "Classless IN- ADDR.ARPA delegation", March 1998. [RFC4472] Durand, A., Ihren, J., and P. Savola, "Operational Considerations and Issues with IPv6 DNS", April 2006. [RFC6092] Woodyatt, J., "Recommended Simple Security Capabilities in Customer Premises Equipment (CPE) for Providing Residential IPv6 Internet Service", January 2011. [inaddr-reqd] Senie, D., "draft-ietf-dnsop-inaddr-required-07", August 2005. [rmap-consider] Senie, D. and A. Sullivan, "draft-ietf-dnsop-reverse-mapping-considerations-06", March 2008.