Internet Systems Consortium

950 Charter Street Redwood City CA 94063 US +1 650 423 1300 paul@vix.com

Keio University/WIDE Project

Graduate School of Media Design, 4-1-1 Hiyoshi Kohoku Yokohama 223-8526 JP +81 3 5418 6419 kato@wide.ad.jp

General Internet Engineering Task Force DNS delegation packet size AAAA record With a mandated default minimum maximum UDP message size of 512 octets, the DNS protocol presents some special problems for zones wishing to expose a moderate or high number of authority servers (NS RRs). This document explains the operational issues caused by, or related to this response size limit, and suggests ways to optimize the use of this limited space. Guidance is offered to DNS server implementors and to DNS zone operators.

The DNS standard limits UDP message size to 512 octets (see 4.2.1). Even though this limitation was due to the required minimum IP reassembly limit for IPv4, it became a hard DNS protocol limit and is not implicitly relaxed by changes in a network layer protocol, for example to IPv6. The EDNS (Extension Mechanisms for DNS) protocol extension starting with version 0 permits larger responses by mutual agreement of the requester and responder (see 2.3, 4.5), and it is recommended to support EDNS. The 512 octets UDP message size limit will remain in practical effect until virtually all DNS servers and resolvers support EDNS. Since DNS responses include a copy of the request, the space available for response data is somewhat less than the full 512 octets. Negative responses are quite small, but for positive and referral responses, every octet must be carefully and sparingly allocated. While the response size of positive responses is also a concern in , this document specifically addresses referral response size.

A positive delegation response will include the following elements: Header Section: fixed length (12 octets) Question Section: original query (name, class, type) Answer Section: empty, or a CNAME/DNAME chain Authority Section: NS RRset (nameserver names) Additional Section: A and AAAA RRsets (nameserver addresses) Note: CNAME defines a canonical name () while DNAME maps an entire subtree to another domain (). If the total size of the UDP response exceeds 512 octets or the size advertised in EDNS, and if the data that does not fit was "required", then the TC bit will be set (indicating truncation). This will usually cause the requester to retry using TCP, depending on what information was desired and what information was omitted. For example, truncation in the authority section is of no interest to a stub resolver who only plans to consume the answer section. If a retry using TCP is needed, the total cost of the transaction is much higher. See 6.1.3.2 for details on the requirement that UDP be attempted before falling back to TCP. RRsets (Resource Record Set, see ) are never sent partially unless the TC bit is set to indicate truncation. When the TC bit is set, the final apparent RRset in the final non-empty section must be considered "possibly damaged" (see 6.2, 9). With or without truncation, the glue present in the additional data section should be considered "possibly incomplete", and requesters should be prepared to re-query for any damaged or missing RRsets. Note that truncation of the additional data section might not be signaled via the TC bit since additional data is often optional (see discussion in B). DNS label compression allows the component labels of a domain name to be instantiated exactly once per DNS message, and then referenced with a two-octet "pointer" from other locations in that same DNS message (see 4.1.4). If all nameserver names in a message share a common parent (for example, all of them are in "ROOT-SERVERS.NET." zone), then more space will be available for incompressible data (such as nameserver addresses). The query name can be as long as 255 octets of network data. In this worst case scenario, the question section will be 259 octets in size, which would leave only 240 octets for the authority and additional sections (after deducting 12 octets for the fixed length header) in a referral.

Average and maximum question section sizes can be predicted by the zone owner, since they will know what names actually exist and can measure which ones are queried for most often. Note that if the zone contains any wildcards, it is possible for maximum length queries to require positive responses, but that it is reasonable to expect truncation and TCP retry in that case. For cost and performance reasons, the majority of requests should be satisfied without truncation or TCP retry. Some queries to non-existing names can be large, but this is not a problem because negative responses need not contain any answer, authority or additional records. See 2.1 for more information about the format of negative responses. The minimum useful number of name servers is two, for redundancy (see 4.1). A zone's name servers should be reachable by all IP protocols versions (e.g., IPv4 and IPv6) in common use. As long as the servers are well managed, the server serving IPv6 might be different from the server serving IPv4 sharing the same server name. The best case is no truncation at all. This is because many requesters will retry using TCP immediately, or will automatically requery for RRsets that are possibly truncated, without considering whether the omitted data was actually necessary. Anycasting is a useful tool for performance and reliability without increasing the size of referral responses. While it is irrelevant to the response size issue, all zones have to be served via IPv4 as well to avoid name space fragmentation .

Each NS RR for a zone will add 12 fixed octets (name, type, class, ttl, and rdlen) plus 2 to 255 variable octets (for the NSDNAME). Each A RR will require 16 octets, and each AAAA RR will require 28 octets. While DNS distinguishes between necessary and optional resource records, this distinction is according to protocol elements necessary to signify facts, and takes no official notice of protocol content necessary to ensure correct operation. For example, a nameserver name that is in or below the zone cut being described by a delegation is "necessary content", since there is no way to reach that zone unless the parent zone's delegation includes "glue records" describing that name server's addresses. Recall that the TC bit is only set when a required RRset can not be included in its entirety (see 9). Even when some of the RRsets to be included in the additional section don't fit in the response size, the TC bit isn't set. These RRsets may be important for a referral. Some DNS implementations try to resolve these missing glue records separately which will introduce extra queries and extra time to resolve a given name. A delegation response should prioritize glue records as follows. first: All glue RRsets for one name server whose name is in or below the zone being delegated, or which has multiple address RRsets (currently A and AAAA), or preferably both; second: Alternate between adding all glue RRsets for any name servers whose names are in or below the zone being delegated, and all glue RRsets for any name servers who have multiple address RRsets (currently A and AAAA); thence: All other glue RRsets, in any order. Whenever there are multiple candidates for a position in this priority scheme, one should be chosen on a round-robin or fully random basis. The goal of this priority scheme is to offer "necessary" glue first to fill into the response if possible. If any "necessary" content cannot be fit in the response, then it is advisable that the TC bit be set in order to force a TCP retry, rather than have the zone be unreachable. Note that a parent server's proper response to a query for in-child glue or below-child glue is a referral rather than an answer, and that this referral must be able to contain the in-child or below-child glue, and that in outlying cases, only EDNS or TCP will be large enough to contain that data. The glue record order should be independent to the version of IP used in the query because the DNS server just see a query from an intermediate server rather than the query from the original client.

An instrumented protocol trace of a best case delegation response is shown in . Note that 13 servers are named, and 13 addresses are given. This query was artificially designed to exactly reach the 512 octets limit.

For longer query names, the number of address records supplied will be lower. Furthermore, it is only by using a common parent name (which is "GTLD-SERVERS.NET." in this example) that all 13 addresses are able to fit, due to the use of DNS compression pointers in the last 12 occurrences of the parent domain name. The outputs from the response simulator in (written in perl ) shown in and demonstrate these properties.

Here we use the term "green" if all address records could fit, or "yellow" if two or more could fit, or "orange" if only one could fit, or "red" if no address record could fit. It's clear that without a common parent for nameserver names, much space would be lost. For these examples we use an average/common name size of 15 octets, befitting our assumption of "GTLD-SERVERS.NET." as our common parent name. We're assuming a medium query name size of 64 since that is the typical size seen in trace data at the time of this writing. If Internationalized Domain Name (IDN) or any other technology which results in larger query names be deployed significantly in advance of EDNS, then new measurements and new estimates will have to be made.

The current practice of giving all nameserver names a common parent (such as "GTLD-SERVERS.NET." or "ROOT-SERVERS.NET.") saves space in DNS responses and allows for more nameservers to be enumerated than would otherwise be possible, since the common parent domain name only appears once in a DNS message and is referred to via "compression pointers" thereafter. If all nameserver names for a zone share a common parent, then it is operationally advisable to make all servers for the zone thus served also be authoritative for the zone of that common parent. For example, the root name servers (?.ROOT-SERVERS.NET.) can answer authoritatively for the ROOT-SERVERS.NET. zone. This is to ensure that the zone's servers always have the zone's nameservers' glue available when delegating, and will be able to respond with answers rather than referrals if a requester who wants that glue comes back asking for it. In this case the name server will likely be a "stealth server" -- authoritative but unadvertised in the glue zone's NS RRset. See 2 for more information about stealth servers. Thirteen (13) is the effective maximum number of nameserver names usable with traditional (non-extended) DNS, assuming a common parent domain name, and given that implicit referral response truncation is undesirable in the average case. More than one address record in a protocol family per server is inadvisable since the necessary glue RRsets (A or AAAA) are atomically indivisible, and will be larger than a single resource record. Larger RRsets are more likely to lead to or encounter truncation. More than one address record across protocol families is less likely to lead to or encounter truncation, partly because multiprotocol clients, which are required to handle larger RRsets such as AAAA RRs, are more likely to speak EDNS which can use a larger UDP response size limit, and partly because the resource records (A and AAAA) are in different RRsets and are therefore divisible from each other. Name server names which are at or below the zone they serve are more sensitive to referral response truncation, and glue records for them should be considered "more important" than other glue records, in the assembly of referral responses.

The recommendations contained in this document have no known security implications.

This document does not call for changes or additions to any IANA registry.

The authors thank Peter Koch, Rob Austein, Joe Abley, Mark Andrews, Kenji Rikitake, Stephane Bortzmeyer, Olafur Gudmundsson, Alfred Hoenes, and Alexander Mayrhofer for their valuable comments and suggestions. This work was supported by the US National Science Foundation (research grant SCI-0427144) and DNS-OARC.

= 0; $n++, shift @labels) { $suffix = join('.', @labels); return length($name) - length($suffix) + $sz_ptr if (defined($namedb{$suffix})); $namedb{$suffix} = 1; } return $len; } sub arsect { my ($sz_query, $nssect, $n_ns, $cond) = @_; my ($space, $n_a, $n_a_aaaa, $n_p_aaaa, $ansect); $ansect = $sz_query + $sz_type + $sz_class; $space = $sz_msg - $sz_header - $ansect - $nssect; $n_a = atmost(int($space / $sz_rr_a), $n_ns); $n_a_aaaa = atmost(int($space / ($sz_rr_a + $sz_rr_aaaa)), $n_ns); $n_p_aaaa = atmost(int(($space - $sz_rr_a * $n_ns) / $sz_rr_aaaa), $n_ns); printf "For %s size query (%d byte):\n", $cond, $sz_query; printf " only A is considered: "; printf "# of A is %d (%s)\n", $n_a, &judge($n_a, $n_ns); printf " A and AAAA are considered: "; printf "# of A+AAAA is %d (%s)\n", $n_a_aaaa, &judge($n_a_aaaa, $n_ns); printf " preferred-glue A is assumed: "; printf "# of A is %d, # of AAAA is %d (%s)\n", $n_a, $n_p_aaaa, &judge($n_p_aaaa, $n_ns); } sub judge { my ($n, $n_ns) = @_; return "green" if ($n >= $n_ns); return "yellow" if ($n >= 2); return "orange" if ($n == 1); return "red"; } sub atmost { my ($a, $b) = @_; return 0 if ($a < 0); return $b if ($a > $b); return $a; } ]]>