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This memo is in full 13 conformance with all provisions of Section 10 of RFC2026 15 Internet-Drafts are draft documents valid for a maximum of six months 16 and may be updated, replaced, or obsoleted by other documents at any 17 time. It is inappropriate to use Internet- Drafts as reference 18 material or to cite them other than as "work in progress." 20 The list of current Internet-Drafts can be accessed at 21 http://www.ietf.org/1id-abstracts.html 23 The list of Internet-Draft Shadow Directories can be accessed at 24 http://www.ietf.org/shadow.html 26 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 27 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 28 document are to be interpreted as described in [2119]. 30 Abstract 32 This memo provides guidelines and best common practice to operate DNS 33 in a mixed world of IPv4 and IPv6 transport. 35 1. Terminology 37 The phrase "IPv4 name server" indicates a name server available over 38 IPv4 transport. It does not imply anything about what DNS data is 39 served. Likewise, "IPv6 name server" indicates a name server 40 available over IPv6 transport. 42 2. Introduction to the problem of name space fragmentation: 43 following the referral chain 45 The caching resolver that tries to lookup a name starts out at the 46 root, and follows referrals until it is referred to a nameserver that 47 is authoritative for the name. If somewhere down the chain of 48 referrals it is referred to a nameserver that is only accessible over 49 a type of transport that is unavailable, a traditional nameserver is 50 unable to finish the task. 52 When the Internet moves from IPv4 to a mixture of IPv4 and IPv6 it is 53 only a matter of time until this starts to happen and the complete 54 DNS hierarchy starts to fragment into a graph where authoritative 55 nameservers for certain nodes are only accessible over a certain 56 transport. What is feared is that a node using only a particular 57 version of IP, querying information about another node using the same 58 version of IP can not do it because, somewhere in the chain of 59 servers accessed during the resolution process, one or more of them 60 will only be accessible with the other version of IP. 62 With all DNS data only available over IPv4 transport everything is 63 simple. IPv4 resolvers can use the intended mechanism of following 64 referrals from the root and down while IPv6 resolvers have to work 65 through a "translator", i.e. they have to use a second name server on 66 a so-called "dual stack" host as a "forwarder" since they cannot 67 access the DNS data directly. 69 With all DNS data only available over IPv6 transport everything would 70 be equally simple, with the exception of old legacy IPv4 name servers 71 having to switch to a forwarding configuration. 73 However, the second situation will not arise in a foreseeable time. 74 Instead, it is expected that the transition will be from IPv4 only to 75 a mixture of IPv4 and IPv6, with DNS data of theoretically three 76 categories depending on whether it is available only over IPv4 77 transport, only over IPv6 or both. 79 The latter is the best situation, and a major question is how to 80 ensure that it as quickly as possible becomes the norm. However, 81 while it is obvious that some DNS data will only be available over v4 82 transport for a long time it is also obvious that it is important to 83 avoid fragmenting the name space available to IPv4 only hosts. I.e. 84 during transition it is not acceptable to break the name space that 85 we presently have available for IPv4-only hosts. 87 3. Policy based avoidance of name space fragmentation. 89 Today there are only a few DNS "zones" on the public Internet that 90 are available over IPv6 transport, and they can mostly be regarded 91 as "experimental". However, as soon as there is a root name server 92 available over IPv6 transport it is reasonable to expect that it will 93 become more common to have zones served by IPv6 servers over time. 95 Having those zones served only by IPv6-only name server would not be 96 a good development, since this will fragment the previously 97 unfragmented IPv4 name space and there are strong reasons to find a 98 mechanism to avoid it. 100 The RECOMMENDED approach to maintain name space continuity is to use 101 administrative policies. 103 4. DNS IPv6 transport RECOMMENDED guidelines: 105 In order to preserve name space continuity, the following administrative 106 policies are RECOMMENDED: 107 - every recursive DNS server SHOULD be either IPv4-only or dual 108 stack, 109 - every single DNS zone SHOULD be served by at least one IPv4 110 reachable DNS server. 112 This rules out IPv6-only DNS servers performing full recursion and 113 DNS zones served only by IPv6-only DNS servers. This approach could 114 be revisited if/when translation techniques between IPv4 and IPv6 115 were to be widely deployed. 117 In order to enforce the second point, the zone validation process 118 SHOULD ensure that there is at least one IPv4 address record 119 available for the name servers of any child delegations within the 120 zone. 122 5. Security considerations 124 Being a critical piece of the Internet infrastructure, the DNS is a 125 potential value target and thus should be protected. Great care 126 should be taken not to weaken the security of DNS while introducing 127 IPv6 operation. 129 The RECOMMENDED guidelines are compatible with the operation of 130 DNSsec and do not introduce any new security issues. 132 6. Author addresses 134 Alain Durand 135 SUN Microsystems, Inc 136 17 Network circle UMPK17-202 137 Menlo Park, CA, 94025 138 USA 139 Mail: Alain.Durand@sun.com 141 Johan Ihren 142 Autonomica 143 Bellmansgatan 30 144 SE-118 47 Stockholm, Sweden 145 Mail: johani@autonomica.se 147 7. References 149 [2119] Bradner, S., "Key Words for Use in RFCs to Indicate 150 Requirement Levels", BCP 14, RFC 2119, March 1997. 152 8. Full Copyright Statement 154 "Copyright (C) The Internet Society (2001). All Rights Reserved. 156 This document and translations of it may be copied and furnished to 157 others, and derivative works that comment on or otherwise explain it 158 or assist in its implementation may be prepared, copied, published 159 and distributed, in whole or in part, without restriction of any 160 kind, provided that the above copyright notice and this paragraph are 161 included on all such copies and derivative works. However, this 162 document itself may not be modified in any way, such as by removing 163 the copyright notice or references to the Internet Society or other 164 Internet organizations, except as needed for the purpose of 165 developing Internet standards in which case the procedures for 166 copyrights defined in the Internet Standards process must be 167 followed, or as required to translate it into languages other than 168 English. 170 The limited permissions granted above are perpetual and will not be 171 revoked by the Internet Society or its successors or assigns. 173 This document and the information contained herein is provided on an 174 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING 175 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING 176 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION 177 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF 178 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.