DNS Operations WG A. Durand Internet-Draft SUN Microsystems, Inc. Expires: September 30, 2004 J. Ihren Autonomica P. Savola CSC/FUNET Apr 2004 Operational Considerations and Issues with IPv6 DNS
draft-ietf-dnsop-ipv6-dns-issues-05.txtdraft-ietf-dnsop-ipv6-dns-issues-06.txt Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. 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-Drafts. 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 http:// www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on September 30, 2004. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This memo presents operational considerations and issues with IPv6 Domain Name System (DNS), including a summary of special IPv6 addresses, documentation of known DNS implementation misbehaviour, recommendations and considerations on how to perform DNS naming for service provisioning and for DNS resolver IPv6 support, considerations for DNS updates for both the forward and reverse trees, and miscellaneous issues. This memo is aimed to include a summary of information about IPv6 DNS considerations for those who have experience with IPv4 DNS. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 Representing IPv6 Addresses in DNS Records . . . . . . . . 3 1.2 Independence of DNS Transport and DNS Records . . . . . . 3 1.3 Avoiding IPv4/IPv6 Name Space Fragmentation . . . . . . . 4 2. DNS Considerations about Special IPv6 Addresses . . . . . . . 4 2.1 Limited-scope Addresses . . . . . . . . . . . . . . . . . 4 2.2 Temporary Addresses . . . . . . . . . . . . . . . . . . . 5 2.3 6to4 Addresses . . . . . . . . . . . . . . . . . . . . . . 5 3. Observed DNS Implementation Misbehaviour . . . . . . . . . . . 5 3.1 Misbehaviour of DNS Servers and Load-balancers . . . . . . 6 3.2 Misbehaviour of DNS Resolvers . . . . . . . . . . . . . . 6 4. Recommendations for Service Provisioning using DNS . . . . . . 6 4.1 Use of Service Names instead of Node Names . . . . . . . . 6 4.2 Separate vs the Same Service Names for IPv4 and IPv6 . . . 7 4.3 Adding the Records Only when Fully IPv6-enabled . . . . . 8 4.4 Behaviour of Additional Data in IPv4/IPv6 Environments . . 8 4.5 The Use of TTL for IPv4 and IPv6 RRs . . . . . . . . . . . 910 4.6 IPv6 Transport Guidelines for DNS Servers . . . . . . . . 1011 5. Recommendations for DNS Resolver IPv6 Support . . . . . . . . 1011 5.1 DNS Lookups May Query IPv6 Records Prematurely . . . . . . 1011 5.2 Obtaining a List of DNS Recursive Resolvers . . . . . . . 1213 5.3 IPv6 Transport Guidelines for Resolvers . . . . . . . . . 1314 6. Considerations about Forward DNS Updating . . . . . . . . . . 1314 6.1 Manual or Custom DNS Updates . . . . . . . . . . . . . . . 1314 6.2 Dynamic DNS . . . . . . . . . . . . . . . . . . . . . . . 1314 7. Considerations about Reverse DNS Updating . . . . . . . . . . 1415 7.1 Applicability of Reverse DNS . . . . . . . . . . . . . . . 1415 7.2 Manual or Custom DNS Updates . . . . . . . . . . . . . . . 1516 7.3 DDNS with Stateless Address Autoconfiguration . . . . . . 1516 7.4 DDNS with DHCP . . . . . . . . . . . . . . . . . . . . . . 1617 7.5 DDNS with Dynamic Prefix Delegation . . . . . . . . . . . 1718 8. Miscellaneous DNS Considerations . . . . . . . . . . . . . . . 1819 8.1 NAT-PT with DNS-ALG . . . . . . . . . . . . . . . . . . . 1819 8.2 Renumbering Procedures and Applications' Use of DNS . . . 1819 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 1819 10. Security Considerations . . . . . . . . . . . . . . . . . . 1819 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 1920 11.1 Normative References . . . . . . . . . . . . . . . . . . . . 1920 11.2 Informative References . . . . . . . . . . . . . . . . . . . 1920 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 2223 A. Site-local Addressing Considerations for DNS . . . . . . . . . 2324 Intellectual Property and Copyright Statements . . . . . . . . 2425 1. Introduction This memo presents operational considerations and issues with IPv6 DNS; it is meant to be an extensive summary and a list of pointers for more information about IPv6 DNS considerations for those with experience with IPv4 DNS. The purpose of this document is to give information about various issues and considerations related to DNS operations with IPv6; it is not meant to be a normative specification or standard for IPv6 DNS. The first section gives a brief overview of how IPv6 addresses and names are represented in the DNS, how transport protocols and resource records (don't) relate, and what IPv4/IPv6 name space fragmentation means and how to avoid it; all of these are described at more length in other documents. The second section summarizes the special IPv6 address types and how they relate to DNS. The third section describes observed DNS implementation misbehaviours which have a varying effect on the use of IPv6 records with DNS. The fourth section lists recommendations and considerations for provisioning services with DNS. The fifth section in turn looks at recommendations and considerations about providing IPv6 support in the resolvers. The sixth and seventh sections describe considerations with forward and reverse DNS updates, respectively. The eighth section introduces several miscellaneous IPv6 issues relating to DNS for which no better place has been found in this memo. Appendix A looks briefly at the requirements for site-local addressing. 1.1 Representing IPv6 Addresses in DNS Records In the forward zones, IPv6 addresses are represented using AAAA records. In the reverse zones, IPv6 address are represented using PTR records in the nibble format under the ip6.arpa. tree. See  for more about IPv6 DNS usage, and  or  for background information. In particular one should note that the use of A6 records in the forward tree or Bitlabels in the reverse tree is not recommended . Using DNAME records is not recommended in the reverse tree in conjunction with A6 records; the document did not mean to take a stance on any other use of DNAME records . 1.2 Independence of DNS Transport and DNS Records DNS has been designed to present a single, globally unique name space . This property should be maintained, as described here and in Section 1.3. In DNS, the IP version used to transport the queries and responses is independent of the records being queried: AAAA records can be queried over IPv4, and A records over IPv6. The DNS servers must not make any assumptions about what data to return for Answer and Authority sections. However, there is some debate whether the addresses in Additional section could be selected or filtered using hints obtained from which transport was being used; this has some obvious problems because in many cases the transport protocol does not correlate with the requests, and because a "bad" answer is in a way worse than no answer at all (consider the case where the client is led to believe that a name received in the additional record does not have any AAAA records to begin with). As stated in : The IP protocol version used for querying resource records is independent of the protocol version of the resource records; e.g., IPv4 transport can be used to query IPv6 records and vice versa. 1.3 Avoiding IPv4/IPv6 Name Space Fragmentation To avoid the DNS name space from fragmenting into parts where some parts of DNS are only visible using IPv4 (or IPv6) transport, the recommendation is to always keep at least one authoritative server IPv4-enabled, and to ensure that recursive DNS servers support IPv4. See DNS IPv6 transport guidelines  for more information. 2. DNS Considerations about Special IPv6 Addresses There are a couple of IPv6 address types which are somewhat special; these are considered here. 2.1 Limited-scope Addresses The IPv6 addressing architecture  includes two kinds of local-use addresses: link-local (fe80::/10) and site-local (fec0::/10). The site-local addresses are being deprecated , and are only discussed in Appendix A. Link-local addresses should never be published in DNS (whether in forward or reverse tree), because they have only local (to the connected link) significance . 2.2 Temporary Addresses Temporary addresses defined in RFC3041  (sometimes called "privacy addresses") use a random number as the interface identifier. Publishing DNS records relating to such addresses would defeat the purpose of the mechanism and is not recommended. If absolutely necessary, a mapping could be made to some non-identifiable name, as described in . 2.3 6to4 Addresses 6to4  specifies an automatic tunneling mechanism which maps a public IPv4 address V4ADDR to an IPv6 prefix 2002:V4ADDR::/48. Providing reverse DNS delegation path for such addresses is a challenge. Note that it does not seem feasible to provide reverse DNS with the other automatic tunneling mechanism, Teredo ; this is because the IPv6 address is based on the IPv4 address and UDP port of the current NAT mapping which is likely to be relatively short-lived. If the reverse DNS population would be desirable (see Section 7.1 for applicability), there are a number of ways to tackle the delegation path problem , some more applicable than the others. The main proposal  has been to allocate 188.8.131.52.ip6.arpa. to Regional Internet Registries (RIRs) and let them do subdelegations in accordance to the delegations of the respective IPv4 address space. This has a major practical drawback: those ISPs and IPv4 address space holders where 6to4 is being used do not, in general, provide any IPv6 services -- as otherwise, most people would not have to use 6to4 to begin with -- and it is improbable that the reverse delegation chain would be completed either. In most cases, creating such delegation chains might just lead to latencies caused by lookups for (almost always) non-existent DNS records. Another proposal  aims to design an autonomous reverse-delegation system that anyone being capable of communicating using a specific 6to4 address would be able to set up a reverse delegation to the corresponding 6to4 prefix. This could be deployed by e.g., RIRs. This is a more practical solution, but may have some scalability concerns. 3. Observed DNS Implementation Misbehaviour Several classes of misbehaviour in DNS servers, load-balancers and resolvers have been observed. Most of these are rather generic, not only applicable to IPv6 -- but in some cases, the consequences of this misbehaviour are extremely severe in IPv6 environments and deserve to be mentioned. 3.1 Misbehaviour of DNS Servers and Load-balancers There are several classes of misbehaviour in certain DNS servers and load-balancers which have been noticed and documented : some implementations silently drop queries for unimplemented DNS records types, or provide wrong answers to such queries (instead of a proper negative reply). While typically these issues are not limited to AAAA records, the problems are aggravated by the fact that AAAA records are being queried instead of (mainly) A records. The problems are serious because when looking up a DNS name, typical getaddrinfo() implementations, with AF_UNSPEC hint given, first try to query the AAAA records of the name, and after receiving a response, query the A records. This is done in a serial fashion -- if the first query is never responded to (instead of properly returning a negative answer), significant timeouts will occur. In consequence, this is an enormous problem for IPv6 deployments, and in some cases, IPv6 support in the software has even been disabled due to these problems. The solution is to fix or retire those misbehaving implementations, but that is likely not going to be effective. There are some possible ways to mitigate the problem, e.g. by performing the lookups somewhat in parallel and reducing the timeout as long as at least one answer has been received; but such methods remain to be investigated; slightly more on this is included in Section 5. 3.2 Misbehaviour of DNS Resolvers Several classes of misbehaviour have also been noticed in DNS resolvers . However, these do not seem to directly impair IPv6 use, and are only referred to for completeness. 4. Recommendations for Service Provisioning using DNS When names are added in the DNS to facilitate a service, there are several general guidelines to consider to be able to do it as smoothly as possible. 4.1 Use of Service Names instead of Node Names When a node includes multiple services, one should keep them logically separate in the DNS. This can be done by the use of service names instead of node names (or, "hostnames"). This operational technique is not specific to IPv6, but required to understand the considerations described in Section 4.2 and Section 4.3. For example, assume a node named "pobox.example.com" provides both SMTP and IMAP service. Instead of configuring the MX records to point at "pobox.example.com", and configuring the mail clients to look up the mail via IMAP from "pobox.example.com", one should use e.g. "smtp.example.com" for SMTP (for both message submission and mail relaying between SMTP servers) and "imap.example.com" for IMAP. Note that in the specific case of SMTP relaying, the server itself must typically also be configured to know all its names to ensure loops do not occur. DNS can provide a layer of indirection between service names and where the service actually is, and using which addresses. This is a good practice with IPv4 as well, because it provides more flexibility and enables easier migration of services from one host to another. A specific reason why this is relevant for IPv6 is that the different services may have a different level of IPv6 support -- that is, one node providing multiple services might want to enable just one service to be IPv6-visible while keeping some others as IPv4-only. Using service names enables more flexibility with different IP versions as well. 4.2 Separate vs the Same Service Names for IPv4 and IPv6 The service naming can be achieved in basically two ways: when a service is named "service.example.com" for IPv4, the IPv6-enabled service could be either added to "service.example.com", or added separately to a sub-domain, like, "service.ipv6.example.com". Both methods have different characteristics. Using a sub-domain allows for easier service piloting, minimizing the disturbance to the "regular" users of IPv4 service; however, the service would not be used without explicitly asking for it (or, within a restricted network, modifying the DNS search path) -- so it will not actually be used that much. Using the same service name is the "long-term" solution, but may degrade performance for those clients whose IPv6 performance is lower than IPv4, or does not work as well (see the next subsection for more). In most cases, it makes sense to pilot or test a service using separate service names, and move to the use of the same name when confident enough that the service level will not degrade for the users unaware of IPv6. 4.3 Adding the Records Only when Fully IPv6-enabled The recommendation is that AAAA records for a service should not be added to the DNS until all of following are true: 1. The address is assigned to the interface on the node. 2. The address is configured on the interface. 3. The interface is on a link which is connected to the IPv6 infrastructure. In addition, if the AAAA record is added for the node, instead of service as recommended, all the services of the node should be IPv6-enabled prior to adding the resource record. For example, if an IPv6 node is isolated from an IPv6 perspective (e.g., it is not connected to IPv6 Internet) constraint #3 would mean that it should not have an address in the DNS. Consider the case of two dual-stack nodes, which both have IPv6 enabled, but the server does not have (global) IPv6 connectivity. As the client looks up the server's name, only A records are returned (if the recommendations above are followed), and no IPv6 communication, which would have been unsuccessful, is even attempted. The issues are not always so black-and-white. Usually it's important if the service offered using both protocols is of roughly equal quality, using the appropriate metrics for the service (e.g., latency, throughput, low packet loss, general reliability, etc.) -- this is typically very important especially for interactive or real-time services. In many cases, the quality of IPv6 connectivity is not yet equal to that of IPv4, at least globally -- this has to be taken into consideration when enabling services . 4.4 Behaviour of Additional Data in IPv4/IPv6 Environments Consider thesthe case where the query name is so long, the number of the additional records (originated from "glue") is so high, or for other reasons that the entire response must either be truncatedwould not fit in a single UDP packet. In some cases, the responder truncates the response with the TC bit being set (leading to a retry with TCP) or some ofTCP), in order for the additional data removed fromquerier to get the reply.entire response later. However, note that if too much additional information that is not strictly necessary would be added, one should remove unnecessary information instead of setting TC bit for this "courtesy" information . Further,Also notice that there are two kinds of additional data: 1. "critical" additional data; this must be included (all the possible RRsets) in all scenarios, and 2. "courtesy" additional data; this could be sent in full, with only a few RRsets, or with no RRsets, and can be fetched separately as well, but which could lead to non-optimal results. Meanwhile, resource record sets (RRsets) are never "broken up", so if a name has 4 A records and 5 AAAA records, you can either return all 9, all 4 A records, all 5 AAAA records or nothing. Notice that for the "critical" additional data getting all the RRsets can be critical. An example of the "courtesy" additional data is A/AAAA records in conjunction of MX records as shown in the next section; an example of the "critical" additional data is shown below (where getting both the A and AAAA RRsets is critical): child.example.com. IN NS ns.child.example.com. ns.child.example.com. IN A 192.0.2.1 ns.child.example.com. IN AAAA 2001:db8::1 In the case of too much additional data,data (whether courtesy or critical), it might be tempting to not return the AAAA records if the transport for DNS query was IPv4, or not return the A records, if the transport was IPv6. However, this breaks the model of independence of DNS transport and resource records, as noted in Section 1.2. This temptation would have significant problems in multiple areas. Remember that often the end-node, which will be using the records, is not the same one as the node requesting them from the authoritative DNS server (or even a caching resolver). So, whichever version the requestor ("the middleman") uses makes no difference to the ultimate user of the records. This might result in e.g., inappropriately returning A records to an IPv6-only node, going through a translation, or opening up another IP-level session (e.g., a PDP context ). The problem of too much additional data seems to be an operational one: the zone administrator entering too many records which will be returned either truncated or impartialmissing some RRsets to the users. A protocol fix for this is using EDNS0  to signal the capacity for larger UDP packet sizes, pushing up the relevant threshold. TheFurther, DNS server implementations should rather omit courtesy additional data completely rather than including only some RRsets. An operational fix for this is having the DNS server implementations return a warning when the administrators create the zones which would result in too much additional data being returned. Additionally, to avoid the case where an application would not get an address at all due to non-critical additional data being omitted, the applications should be able to query the specific records of the desired protocol, not just rely on getting all the required RRsets in the additional section. 4.5 The Use of TTL for IPv4 and IPv6 RRs In the previous section, we discussed a danger with queries, potentially leading to omitting records informationRRsets from the additional section.section; this could happen to both critical and "courtesy" additional data. This section describesdiscusses another problem with the latter, leading to omitting recordsRRsets in cached data, highlighted in the IPv4/IPv6 environment. The behaviour of DNS caching when different TTL values are used for different recordsRRsets of the same name requires explicit discussion. For example, let's consider a part of a zone: example.com. 300 IN MX foo.example.com. foo.example.com. 300 IN A 192.0.2.1 foo.example.com. 100 IN AAAA 2001:db8::1 When a caching resolver asks for the MX record of example.com, it gets back "foo.example.com". It may also get back either one or both of the A and AAAA records in the additional section. So, there are three cases about returning records for the MX in the additional section: 1. We get back no A or AAAA records:RRsets: this is the simplest case, because then we have to query which information is required explicitly, guaranteeing that we get all the information we're interested in. 2. We get back all the records:RRsets: this is an optimization as there is no need to perform more queries, causing lower latency. However, it is impossible to guarantee that in fact we would always get back all the records (the only way to ensure that is to send a AAAA query for the name after getting the cached reply); however, one could try to work in the direction to try to ensure it as far as possible. 3. We only get back A or AAAA recordsRRsets even if both existed: this is indistinguishable from the previous case, and problematic as described in the nextprevious section. So,As the third case was considered in the previous section, we assume we get back both A and AAAA records of foo.example.com, or the stub resolver explicitly asks, in two separate queries, both A and AAAA records. After 100 seconds, the AAAA record is removed from the cachecache(s) because its TTL expired. It would be useful for the cachecaching resolvers to re-query the AAAA record ordiscard the A record when the shorter TTL (in this case, for the AAAA record) expires; this would avoid the situation where there would be a window of 200 seconds when incomplete information is returned from the cache. However, this is not mandated or mentioned by the specification(s). To simplify the situation, it is recommendedmight help to use the same TTL for all the recordsresource record sets referring to the same name.name, unless there is a particular reason for not doing so. However, there are some scenarios (e.g., when renumbering IPv6 but keeping IPv4 intact) where a different strategy is preferable. Thus, applications that use the response should not rely on a particular TTL configuration. For example, even if an application gets a response that only has the A record in the example described above, it should not assume there is no AAAA record for "foo.example.com". Instead, the application should try to fetch the missing records by itself if it needs the record. 4.6 IPv6 Transport Guidelines for DNS Servers As described in Section 1.3 and , there should continue to be at least one authoritative IPv4 DNS server for every zone, even if the zone has only IPv6 records. (Note that obviously, having more servers with robust connectivity would be preferable, but this is the minimum recommendation; also see .) 5. Recommendations for DNS Resolver IPv6 Support When IPv6 is enabled on a node, there are several things to consider to ensure that the process is as smooth as possible. 5.1 DNS Lookups May Query IPv6 Records Prematurely The system library that implements the getaddrinfo() function for looking up names is a critical piece when considering the robustness of enabling IPv6; it may come in basically three flavours: 1. The system library does not know whether IPv6 has been enabled in the kernel of the operating system: it may start looking up AAAA records with getaddrinfo() and AF_UNSPEC hint when the system is upgraded to a system library version which supports IPv6. 2. The system library might start to perform IPv6 queries with getaddrinfo() only when IPv6 has been enabled in the kernel. However, this does not guarantee that there exists any useful IPv6 connectivity (e.g., the node could be isolated from the other IPv6 networks, only having link-local addresses). 3. The system library might implement a toggle which would apply some heuristics to the "IPv6-readiness" of the node before starting to perform queries; for example, it could check whether only link-local IPv6 address(es) exists, or if at least one global IPv6 address exists. First, let us consider generic implications of unnecessary queries for AAAA records: when looking up all the records in the DNS, AAAA records are typically tried first, and then A records. These are done in serial, and the A query is not performed until a response is received to the AAAA query. Considering the misbehaviour of DNS servers and load-balancers, as described in Section 3.1, the look-up delay for AAAA may incur additional unnecessary latency, and introduce a component of unreliability. One option here could be to do the queries partially in parallel; for example, if the final response to the AAAA query is not received in 0.5 seconds, start performing the A query while waiting for the result (immediate parallelism might be unoptimal without information sharing between the look-up threads, as that would probably lead to duplicate non-cached delegation chain lookups). An additional concern is the address selection, which may, in some circumstances, prefer AAAA records over A records, even when the node does not have any IPv6 connectivity . In some cases, the implementation may attempt to connect or send a datagram on a physical link , incurring very long protocol timeouts, instead of quickly failing back to IPv4. Now, we can consider the issues specific to each of the three possibilities: In the first case, the node performs a number of completely useless DNS lookups as it will not be able to use the returned AAAA records anyway. (The only exception is where the application desires to know what's in the DNS, but not use the result for communication.) One should be able to disable these unnecessary queries, for both latency and reliability reasons. However, as IPv6 has not been enabled, the connections to IPv6 addresses fail immediately, and if the application is programmed properly, the application can fall gracefully back to IPv4 . The second case is similar to the first, except it happens to a smaller set of nodes when IPv6 has been enabled but connectivity has not been provided yet; similar considerations apply, with the exception that IPv6 records, when returned, will be actually tried first which may typically lead to long timeouts. The third case is a bit more complex: optimizing away the DNS lookups with only link-locals is probably safe (but may be desirable with different lookup services which getaddrinfo() may support), as the link-locals are typically automatically generated when IPv6 is enabled, and do not indicate any form of IPv6 connectivity. That is, performing DNS lookups only when a non-link-local address has been configured on any interface could be beneficial -- this would be an indication that either the address has been configured either from a router advertisement, DHCPv6 , or manually. Each would indicate at least some form of IPv6 connectivity, even though there would not be guarantees of it. These issues should be analyzed at more depth, and the fixes found consensus on, perhaps in a separate document. 5.2 Obtaining a List of DNS Recursive Resolvers In scenarios where DHCPv6 is available, a host can discover a list of DNS recursive resolvers through DHCPv6 "DNS Recursive Name Server" option . This option can be passed to a host through a subset of DHCPv6 . The IETF is considering the development of alternative mechanisms for obtaining the list of DNS recursive name servers when DHCPv6 is unavailable or inappropriate. No decision about taking on this development work has been reached as of this writing (April 2004). In scenarios where DHCPv6 is unavailable or inappropriate, mechanisms under consideration for development of dnsop WG include the use of well-known addresses , the use of Router Advertisements to convey the information . Note that even though IPv6 DNS resolver discovery is a recommended procedure, it is not required for dual-stack nodes in dual-stack networks as IPv6 DNS records can be queried over IPv4 as well as IPv6. Obviously, nodes which are meant to function without manual configuration in IPv6-only networks must implement DNS resolver discovery function. 5.3 IPv6 Transport Guidelines for Resolvers As described in Section 1.3 and , the recursive resolvers should be IPv4-only or dual-stack to be able to reach any IPv4-only DNS server. Note that this requirement is also fulfilled by an IPv6-only stub resolver pointing to a dual-stack recursive DNS resolver. 6. Considerations about Forward DNS Updating While the topic how to enable updating the forward DNS, i.e., the mapping from names to the correct new addresses, is not specific to IPv6, it bears thinking about especially due to adding Stateless Address Autoconfiguration  to the mix. Typically forward DNS updates are more manageable than doing them in the reverse DNS, because the updater can, typically, be assumed to "own" a certain DNS name -- and we can create a form of security relationship with the DNS name and the node allowed to update it to point to a new address. A more complex form of DNS updates -- adding a whole new name into a DNS zone, instead of updating an existing name -- is considered out of scope for this memo. Adding a new name in the forward zone is a problem which is still being explored with IPv4, and IPv6 does not seem to add much new in that area. 6.1 Manual or Custom DNS Updates The DNS mappings can be maintained by hand, in a semi-automatic fashion or by running non-standardized protocols. These are not considered at more length in this memo. 6.2 Dynamic DNS Dynamic DNS updates (DDNS)  is a standardized mechanism for dynamically updating the DNS. It works equally well with stateless address autoconfiguration (SLAAC), DHCPv6 or manual address configuration. The only (minor) twist is that with SLAAC, the DNS server cannot tie the authentication of the user to the IP address, and stronger mechanisms must be used . As relying on IP addresses for Dynamic DNS is rather insecure at best, stronger authentication should always be used; however, this requires that the authorization keying will be explicitly configured using unspecified operational methods. Note that with DHCP it is also possible that the DHCP server updates the DNS, not the host. The host might only indicate in the DHCP exchange which hostname it would prefer, and the DHCP server would make the appropriate updates. Nonetheless, while this makes setting up a secure channel between the updater and the DNS server easier, it does not help much with "content" security, i.e., whether the hostname was acceptable -- if the DNS server does not include policies, they must be included in the DHCP server (e.g., a regular host should not be able to state that its name is "www.example.com"). DHCP-initiated DDNS updates have been extensively described in ,  and . The nodes must somehow be configured with the information about the servers where they will attempt to update their addresses, sufficient security material for authenticating themselves to the server, and the hostname they will be updating. Unless otherwise configured, the first could be obtained by looking up the authoritative name servers for the hostname; the second must be configured explicitly unless one chooses to trust the IP address-based authentication (not a good idea); and lastly, the nodename is typically pre-configured somehow on the node, e.g. at install time. Care should be observed when updating the addresses not to use longer TTLs for addresses than are preferred lifetimes for the autoconfigured addresses, so that if the node is renumbered in a managed fashion, the amount of stale DNS information is kept to the minimum. That is, if the preferred lifetime of an address expires, the TTL of the record needs be modified unless it was already done before the expiration. For better flexibility, the DNS TTL should be much shorter (e.g., a half or a third) than the lifetime of an address; that way, the node can start lowering the DNS TTL if it seems like the address has not been renewed/refreshed in a while. Some discussion on how an administrator could manage the DNS TTL is included in ; this could be applied to (smart) hosts as well. 7. Considerations about Reverse DNS Updating Updating the reverse DNS zone may be difficult because of the split authority over an address. However, first we have to consider the applicability of reverse DNS in the first place. 7.1 Applicability of Reverse DNS Today, some applications use reverse DNS to either look up some hints about the topological information associated with an address (e.g. resolving web server access logs), or as a weak form of a security check, to get a feel whether the user's network administrator has "authorized" the use of the address (on the premises that adding a reverse record for an address would signal some form of authorization). One additional, maybe slightly more useful usage is ensuring the reverse and forward DNS contents match and correspond to a configured name or domain. As a security check, it is typically accompanied by other mechanisms, such as a user/password login; the main purpose of the DNS check is to weed out the majority of unauthorized users, and if someone managed to bypass the checks, he would still need to authenticate "properly". It is not clear whether it makes sense to require or recommend that reverse DNS records be updated. In many cases, it would just make more sense to use proper mechanisms for security (or topological information lookup) in the first place. At minimum, the applications which use it as a generic authorization (in the sense that a record exists at all) should be modified as soon as possible to avoid such lookups completely. The applicability is discussed at more length in . 7.2 Manual or Custom DNS Updates Reverse DNS can of course be updated using manual or custom methods. These are not further described here, except for one special case. One way to deploy reverse DNS would be to use wildcard records, for example, by configuring one name for a subnet (/64) or a site (/48). As a concrete example, a site (or the site's ISP) could configure the reverses of the prefix 2001:db8:f00::/48 to point to one name using a wildcard record like "*.0.0.f.0.8.b.d.0.1.0.0.2.ip6.arpa. IN PTR site.example.com." Naturally, such a name could not be verified from the forward DNS, but would at least provide some form of "topological information" or "weak authorization" if that is really considered to be useful. Note that this is not actually updating the DNS as such, as the whole point is to avoid DNS updates completely by manually configuring a generic name. 7.3 DDNS with Stateless Address Autoconfiguration Dynamic DNS with SLAAC simpler than forward DNS updates in some regard, while being more difficult in another. The address space administrator decides whether the hosts are trusted to update their reverse DNS records or not. If they are, a simple address-based authorization is typically sufficient (i.e., check that the DNS update is done from the same IP address as the record being updated); stronger security can also be used . If they aren't allowed to update the reverses, no update can occur. Address-based authorization is simpler with reverse DNS (as there is a connection between the record and the address) than with forward DNS. However, when stronger form of security is used, forward DNS updates are simpler to manage because the host knows the record it's updating, and can be assumed to have an association with the domain. Note that the user may roam to different networks, and does not necessarily have any association with the owner of that address space -- so, assuming stronger form of authorization for reverse DNS updates than an address association is generally unfeasible. Moreover, the reverse zones must be cleaned up by an unspecified janitorial process: the node does not typically know a priori that it will be disconnected, and cannot send a DNS update using the correct source address to remove a record. A problem with defining the clean-up process is that it is difficult to ensure that a specific IP address and the corresponding record are no longer being used. Considering the huge address space, and the unlikelihood of collision within 64 bits of the interface identifiers, a process which would remove the record after no traffic has been seen from a node in a long period of time (e.g., a month or year) might be one possible approach. To insert or update the record, the node must discover the DNS server to send the update to somehow, similar to as discussed in Section 6.2. One way to automate this is looking up the DNS server authoritative (e.g., through SOA record) for the IP address being updated, but the security material (unless the IP address-based authorization is trusted) must also be established by some other means. 7.4 DDNS with DHCP With DHCPv4, the reverse DNS name is typically already inserted to the DNS that reflects to the name (e.g., "dhcp-67.example.com"). One can assume similar practice may become commonplace with DHCPv6 as well; all such mappings would be pre-configured, and would require no updating. If a more explicit control is required, similar considerations as with SLAAC apply, except for the fact that typically one must update a reverse DNS record instead of inserting one (if an address assignment policy that reassigns disused addresses is adopted) and updating a record seems like a slightly more difficult thing to secure. However, it is yet uncertain how DHCPv6 is going to be used for address assignment. Note that when using DHCP, either the host or the DHCP server could perform the DNS updates; see the implications in Section 6.2. If disused addresses were to be reassigned, host-based DDNS reverse updates would need policy considerations for DNS record modification, as noted above. On the other hand, if disused address were not to be assigned, host-based DNS reverse updates would have similar considerations as SLAAC in Section 7.3. Server-based updates have similar properties except that the janitorial process could be integrated with DHCP address assignment. 7.5 DDNS with Dynamic Prefix Delegation In cases where a prefix, instead of an address, is being used and updated, one should consider what is the location of the server where DDNS updates are made. That is, where the DNS server is located: 1. At the same organization as the prefix delegator. 2. At the site where the prefixes are delegated to. In this case, the authority of the DNS reverse zone corresponding to the delegated prefix is also delegated to the site. 3. Elsewhere; this implies a relationship between the site and where DNS server is located, and such a relationship should be rather straightforward to secure as well. Like in the previous case, the authority of the DNS reverse zone is also delegated. In the first case, managing the reverse DNS (delegation) is simpler as the DNS server and the prefix delegator are in the same administrative domain (as there is no need to delegate anything at all); alternatively, the prefix delegator might forgo DDNS reverse capability altogether, and use e.g., wildcard records (as described in Section 7.2). In the other cases, it can be slighly more difficult, particularly as the site will have to configure the DNS server to be authoritative for the delegated reverse zone, implying automatic configuration of the DNS server -- as the prefix may be dynamic. Managing the DDNS reverse updates is typically simple in the second case, as the updated server is located at the local site, and arguably IP address-based authentication could be sufficient (or if not, setting up security relationships would be simpler). As there is an explicit (security) relationship between the parties in the third case, setting up the security relationships to allow reverse DDNS updates should be rather straightforward as well. In the first case, however, setting up and managing such relationships might be a lot more difficult. 8. Miscellaneous DNS Considerations This section describes miscellaneous considerations about DNS which seem related to IPv6, for which no better place has been found in this document. 8.1 NAT-PT with DNS-ALG NAT-PT  DNS-ALG is a critical component (unless something replacing that functionality is specified) which mangles A records to look like AAAA records to the IPv6-only nodes. Numerous problems have been identified with DNS-ALG . 8.2 Renumbering Procedures and Applications' Use of DNS One of the most difficult problems of systematic IP address renumbering procedures  is that an application which looks up a DNS name disregards information such as TTL, and uses the result obtained from DNS as long as it happens to be stored in the memory of the application. For applications which run for a long time, this could be days, weeks or even months; some applications may be clever enough to organize the data structures and functions in such a manner that look-ups get refreshed now and then. While the issue appears to have a clear solution, "fix the applications", practically this is not reasonable immediate advice; the TTL information is not typically available in the APIs and libraries (so, the advice becomes "fix the applications, APIs and libraries"), and a lot more analysis is needed on how to practically go about to achieve the ultimate goal of avoiding using the names longer than expected. 9. Acknowledgements Some recommendations (Section 4.3, Section 5.1) about IPv6 service provisioning were moved here from  by Erik Nordmark and Bob Gilligan. Havard Eidnes and Michael Patton provided useful feedback and improvements. Scott Rose, Rob Austein, Masataka Ohta, and Mark Andrews helped in clarifying the issues regarding additional data and the use of TTL. Jefsey Morfin, Ralph Droms, Peter Koch, Jinmei Tatuya, and Iljitsch van Beijnum provided useful feedback during the WG last call. 10. Security Considerations This document reviews the operational procedures for IPv6 DNS operations and does not have security considerations in itself. However, it is worth noting that in particular with Dynamic DNS Updates, security models based on the source address validation are very weak and cannot be recommended. On the other hand, it should be noted that setting up an authorization mechanism (e.g., a shared secret, or public-private keys) between a node and the DNS server has to be done manually, and may require quite a bit of time and expertise. To re-emphasize which was already stated, reverse DNS checks provide very weak security at best, and the only (questionable) security-related use for them may be in conjunction with other mechanisms when authenticating a user. 11. References 11.1 Normative References  Thomson, S., Huitema, C., Ksinant, V. and M. Souissi, "DNS Extensions to Support IP Version 6", RFC 3596, October 2003.  Bush, R., Durand, A., Fink, B., Gudmundsson, O. and T. Hain, "Representing Internet Protocol version 6 (IPv6) Addresses in the Domain Name System (DNS)", RFC 3363, August 2002.  Durand, A. and J. Ihren, "DNS IPv6 transport operational guidelines", draft-ietf-dnsop-ipv6-transport-guidelines-02 (work in progress), March 2004. 11.2 Informative References  Bush, R., "Delegation of IP6.ARPA", BCP 49, RFC 3152, August 2001.  Austein, R., "Tradeoffs in Domain Name System (DNS) Support for Internet Protocol version 6 (IPv6)", RFC 3364, August 2002.  Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6) Addressing Architecture", RFC 3513, April 2003.  Internet Architecture Board, "IAB Technical Comment on the Unique DNS Root", RFC 2826, May 2000.  Huitema, C. and B. Carpenter, "Deprecating Site Local Addresses", draft-ietf-ipv6-deprecate-site-local-03 (work in progress), March 2004.  Hazel, P., "IP Addresses that should never appear in the public DNS", draft-ietf-dnsop-dontpublish-unreachable-03 (work in progress), February 2002.  Narten, T. and R. Draves, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 3041, January 2001.  Carpenter, B. and K. Moore, "Connection of IPv6 Domains via IPv4 Clouds", RFC 3056, February 2001.  Huitema, C., "Teredo: Tunneling IPv6 over UDP through NATs", draft-huitema-v6ops-teredo-01 (work in progress), February 2004.  Moore, K., "6to4 and DNS", draft-moore-6to4-dns-03 (work in progress), October 2002.  Bush, R. and J. Damas, "Delegation of 184.108.40.206.ip6.arpa", draft-ymbk-6to4-arpa-delegation-00 (work in progress), February 2003.  Huston, G., "6to4 Reverse DNS", draft-huston-6to4-reverse-dns-02 (work in progress), April 2004.  Morishita, Y. and T. Jinmei, "Common Misbehavior against DNS Queries for IPv6 Addresses", draft-ietf-dnsop-misbehavior-against-aaaa-01 (work in progress), April 2004.  Larson, M. and P. Barber, "Observed DNS Resolution Misbehavior", draft-ietf-dnsop-bad-dns-res-01 (work in progress), June 2003.  Savola, P., "Moving from 6bone to IPv6 Internet", draft-savola-v6ops-6bone-mess-01 (work in progress), November 2002.  Elz, R. and R. Bush, "Clarifications to the DNS Specification", RFC 2181, July 1997.  Wiljakka, J., "Analysis on IPv6 Transition in 3GPP Networks", draft-ietf-v6ops-3gpp-analysis-09 (work in progress), March 2004.  Elz, R., Bush, R., Bradner, S. and M. Patton, "Selection and Operation of Secondary DNS Servers", BCP 16, RFC 2182, July 1997.  Roy, S., "Dual Stack IPv6 on by Default", draft-ietf-v6ops-v6onbydefault-01 (work in progress), February 2004.  Roy, S., "IPv6 Neighbor Discovery On-Link Assumption Considered Harmful", draft-ietf-v6ops-onlinkassumption-01 (work in progress), March 2004.  Shin, M., "Application Aspects of IPv6 Transition", draft-ietf-v6ops-application-transition-02 (work in progress), March 2004.  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.  Ohta, M., "Preconfigured DNS Server Addresses", draft-ohta-preconfigured-dns-01 (work in progress), February 2004.  Jeong, J., "IPv6 DNS Discovery based on Router Advertisement", draft-jeong-dnsop-ipv6-dns-discovery-01 (work in progress), February 2004.  Droms, R., "Stateless Dynamic Host Configuration Protocol (DHCP) Service for IPv6", RFC 3736, April 2004.  Droms, R., "DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646, December 2003.  Thomson, S. and T. Narten, "IPv6 Stateless Address Autoconfiguration", RFC 2462, December 1998.  Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic Updates in the Domain Name System (DNS UPDATE)", RFC 2136, April 1997.  Wellington, B., "Secure Domain Name System (DNS) Dynamic Update", RFC 3007, November 2000.  Stapp, M., "Resolution of DNS Name Conflicts Among DHCP Clients", draft-ietf-dhc-ddns-resolution-06 (work in progress), October 2003.  Stapp, M. and Y. Rekhter, "The DHCP Client FQDN Option", draft-ietf-dhc-fqdn-option-06 (work in progress), October 2003.  Stapp, M., Lemon, T. and A. Gustafsson, "A DNS RR for encoding DHCP information (DHCID RR)", draft-ietf-dnsext-dhcid-rr-07 (work in progress), October 2003.  Tsirtsis, G. and P. Srisuresh, "Network Address Translation - Protocol Translation (NAT-PT)", RFC 2766, February 2000.  Baker, F., Lear, E. and R. Droms, "Procedures for Renumbering an IPv6 Network without a Flag Day", draft-baker-ipv6-renumber-procedure-01draft-ietf-v6ops-renumbering-procedure-00 (work in progress), October 2003.February 2004.  Senie, D., "Requiring DNS IN-ADDR Mapping", draft-ietf-dnsop-inaddr-required-03draft-ietf-dnsop-inaddr-required-05 (work in progress), March 2002.April 2004.  Durand, A., "Issues with NAT-PT DNS ALG in RFC2766", draft-durand-v6ops-natpt-dns-alg-issues-00 (work in progress), February 2003.  Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671, August 1999.  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms for IPv6 Hosts and Routers", draft-ietf-v6ops-mech-v2-02 (work in progress), February 2004.  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast Addresses", draft-ietf-ipv6-unique-local-addr-03 (work in progress), February 2004. Authors' Addresses Alain Durand SUN Microsystems, Inc. 17 Network circle UMPL17-202 Menlo Park, CA 94025 USA EMail: Alain.Durand@sun.com Johan Ihren Autonomica Bellmansgatan 30 SE-118 47 Stockholm Sweden EMail: firstname.lastname@example.org Pekka Savola CSC/FUNET Espoo Finland EMail: email@example.com Appendix A. Site-local Addressing Considerations for DNS As site-local addressing is being deprecated, the considerations for site-local addressing are discussed briefly here. Unique local addressing format  has been proposed as a replacement, but being work-in-progress, it is not considered further. The interactions with DNS come in two flavors: forward and reverse DNS. To actually use site-local addresses within a site, this implies the deployment of a "split-faced" or a fragmented DNS name space, for the zones internal to the site, and the outsiders' view to it. The procedures to achieve this are not elaborated here. The implication is that site-local addresses must not be published in the public DNS. To faciliate reverse DNS (if desired) with site-local addresses, the stub resolvers must look for DNS information from the local DNS servers, not e.g. starting from the root servers, so that the site-local information may be provided locally. Note that the experience of private addresses in IPv4 has shown that the root servers get loaded for requests for private address lookups in any case. 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