Internet Engineering Task ForceP. Vixie
Internet-DraftInternet Systems Consortium
Intended status: InformationalA. Kato
Expires: August 27, 2008The University of Tokyo/WIDE
 February 24, 2008

DNS Referral Response Size Issues

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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.

1.  Introduction

1.1.  Introduction and Overview

The DNS standard limits UDP message size to 512 octets (see [RFC1035] (Mockapetris, P., “Domain names - implementation and specification,” November 1987.) 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 protocol extension starting with version 0 permits larger responses by mutual agreement of the requester and responder (see [RFC2671] (Vixie, P., “Extension Mechanisms for DNS (EDNS0),” August 1999.) 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 [RFC3226] (Gudmundsson, O., “DNSSEC and IPv6 A6 aware server/resolver message size requirements,” December 2001.), this document specifically addresses referral response size.

2.  Delegation Details

2.1.  Relevant Protocol Elements

A 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)

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 [RFC1123] (Braden, R., “Requirements for Internet Hosts - Application and Support,” October 1989.) for details on the requirement that UDP be attempted before falling back to TCP.

RRsets 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 [RFC1035] (Mockapetris, P., “Domain names - implementation and specification,” November 1987.) 6.2, [RFC2181] (Elz, R. and R. Bush, “Clarifications to the DNS Specification,” July 1997.) 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 [RFC4472] (Durand, A., Ihren, J., and P. Savola, “Operational Considerations and Issues with IPv6 DNS,” April 2006.) 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 [RFC1035] (Mockapetris, P., “Domain names - implementation and specification,” November 1987.) 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.

2.2.  Advice to Zone Owners

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 [RFC2308] (Andrews, M., “Negative Caching of DNS Queries (DNS NCACHE),” March 1998.) 2.1 for more information about the format of negative responses.

The minimum useful number of name servers is two, for redundancy (see [RFC1034] (Mockapetris, P., “Domain names - concepts and facilities,” November 1987.) 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. It is important to ensure that a zone has servers reachable by all IP protocol in common use (e.g., IPv4 and IPv6).

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 [RFC3258] (Hardie, T., “Distributing Authoritative Name Servers via Shared Unicast Addresses,” April 2002.) 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 [RFC3901] (Durand, A. and J. Ihren, “DNS IPv6 Transport Operational Guidelines,” September 2004.).

2.3.  Advice to Server Implementors

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 [RFC2181] (Elz, R. and R. Bush, “Clarifications to the DNS Specification,” July 1997.) 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.


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;


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);


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" is not able to fill 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.

3.  Analysis

An instrumented protocol trace of a best case delegation response is shown in Figure 1. Note that 13 servers are named, and 13 addresses are given. This query was artificially designed to exactly reach the 512 octets limit.

   ;; flags: qr rd; QUERY: 1, ANS: 0, AUTH: 13, ADDIT: 13
   ;;  [23456789.123456789.123456789.\ A IN]        ;; @80

   com.                 86400 NS  E.GTLD-SERVERS.NET.  ;; @112
   com.                 86400 NS  F.GTLD-SERVERS.NET.  ;; @128
   com.                 86400 NS  G.GTLD-SERVERS.NET.  ;; @144
   com.                 86400 NS  H.GTLD-SERVERS.NET.  ;; @160
   com.                 86400 NS  I.GTLD-SERVERS.NET.  ;; @176
   com.                 86400 NS  J.GTLD-SERVERS.NET.  ;; @192
   com.                 86400 NS  K.GTLD-SERVERS.NET.  ;; @208
   com.                 86400 NS  L.GTLD-SERVERS.NET.  ;; @224
   com.                 86400 NS  M.GTLD-SERVERS.NET.  ;; @240
   com.                 86400 NS  A.GTLD-SERVERS.NET.  ;; @256
   com.                 86400 NS  B.GTLD-SERVERS.NET.  ;; @272
   com.                 86400 NS  C.GTLD-SERVERS.NET.  ;; @288
   com.                 86400 NS  D.GTLD-SERVERS.NET.  ;; @304

   A.GTLD-SERVERS.NET.  86400 A           ;; @320
   B.GTLD-SERVERS.NET.  86400 A         ;; @336
   C.GTLD-SERVERS.NET.  86400 A         ;; @352
   D.GTLD-SERVERS.NET.  86400 A         ;; @368
   E.GTLD-SERVERS.NET.  86400 A         ;; @384
   F.GTLD-SERVERS.NET.  86400 A         ;; @400
   G.GTLD-SERVERS.NET.  86400 A         ;; @416
   H.GTLD-SERVERS.NET.  86400 A        ;; @432
   I.GTLD-SERVERS.NET.  86400 A        ;; @448
   J.GTLD-SERVERS.NET.  86400 A         ;; @464
   K.GTLD-SERVERS.NET.  86400 A        ;; @480
   L.GTLD-SERVERS.NET.  86400 A        ;; @496
   M.GTLD-SERVERS.NET.  86400 A         ;; @512

   ;; MSG SIZE  sent: 80  rcvd: 512

 Figure 1 

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 Appendix A (The response simulator program) (written in perl [PERL] (Wall, L., Christiansen, T., and J. Orwant, “Programming Perl, 3rd ed.,” July 2000.)) shown in Figure 2 and Figure 3 demonstrate these properties.

   % perl requires 10 bytes requires 4 bytes requires 4 bytes requires 4 bytes
   # of NS: 4
   For maximum size query (255 byte):
       only A is considered:        # of A is 4 (green)
       A and AAAA are considered:   # of A+AAAA is 3 (yellow)
       preferred-glue A is assumed: # of A is 4, # of AAAA is 3 (yellow)
   For average size query (64 byte):
       only A is considered:        # of A is 4 (green)
       A and AAAA are considered:   # of A+AAAA is 4 (green)
       preferred-glue A is assumed: # of A is 4, # of AAAA is 4 (green)

 Figure 2 

   % perl requires 16 bytes requires 12 bytes requires 13 bytes requires 11 bytes
   # of NS: 4
   For maximum size query (255 byte):
       only A is considered:        # of A is 4 (green)
       A and AAAA are considered:   # of A+AAAA is 3 (yellow)
       preferred-glue A is assumed: # of A is 4, # of AAAA is 2 (yellow)
   For average size query (64 byte):
       only A is considered:        # of A is 4 (green)
       A and AAAA are considered:   # of A+AAAA is 4 (green)
       preferred-glue A is assumed: # of A is 4, # of AAAA is 4 (green)

 Figure 3 

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.

4.  Conclusions

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 [RFC1996] (Vixie, P., “A Mechanism for Prompt Notification of Zone Changes (DNS NOTIFY),” August 1996.) 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.

If a zone is served by thirteen (13) name servers having a common parent name (such as ?.ROOT-SERVERS.NET.) and each such name server has a single address record in some protocol family (e.g., an A RR), then all thirteen name servers or any subset thereof could have address records in a second protocol family by adding a second address record (e.g., an AAAA RR) without reducing the reachability of the zone thus served.

5.  Security Considerations

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

6.  IANA Considerations

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

7.  Acknowledgement

The authors thank Peter Koch, Rob Austein, Joe Abley, Mark Andrews, Kenji Rikitake, Stephane Bortzmeyer, Olafur Gudmundsson, and Alfred Hoenes for their valuable comments and suggestions.

This work was supported by the US National Science Foundation (research grant SCI-0427144) and DNS-OARC.

8. Normative References

[PERL] Wall, L., Christiansen, T., and J. Orwant, “Programming Perl, 3rd ed.,” ISBN 0-596-00027-8, July 2000.
[RFC1034] Mockapetris, P., “Domain names - concepts and facilities,” STD 13, RFC 1034, November 1987 (TXT).
[RFC1035] Mockapetris, P., “Domain names - implementation and specification,” STD 13, RFC 1035, November 1987 (TXT).
[RFC1123] Braden, R., “Requirements for Internet Hosts - Application and Support,” STD 3, RFC 1123, October 1989 (TXT).
[RFC1996] Vixie, P., “A Mechanism for Prompt Notification of Zone Changes (DNS NOTIFY),” RFC 1996, August 1996 (TXT).
[RFC2181] Elz, R. and R. Bush, “Clarifications to the DNS Specification,” RFC 2181, July 1997 (TXT, HTML, XML).
[RFC2308] Andrews, M., “Negative Caching of DNS Queries (DNS NCACHE),” RFC 2308, March 1998 (TXT, HTML, XML).
[RFC2671] Vixie, P., “Extension Mechanisms for DNS (EDNS0),” RFC 2671, August 1999 (TXT).
[RFC3226] Gudmundsson, O., “DNSSEC and IPv6 A6 aware server/resolver message size requirements,” RFC 3226, December 2001 (TXT).
[RFC3258] Hardie, T., “Distributing Authoritative Name Servers via Shared Unicast Addresses,” RFC 3258, April 2002 (TXT).
[RFC3901] Durand, A. and J. Ihren, “DNS IPv6 Transport Operational Guidelines,” BCP 91, RFC 3901, September 2004 (TXT).
[RFC4472] Durand, A., Ihren, J., and P. Savola, “Operational Considerations and Issues with IPv6 DNS,” RFC 4472, April 2006 (TXT).

Appendix A.  The response simulator program

# [ -z zone ] fqdn_ns1 fqdn_ns2 ...
#        if all queries are assumed to have a same zone suffix,
#     such as "jp" in JP TLD servers, specify it in -z option
use strict;
use Getopt::Std;

my ($sz_msg) = (512);
my ($sz_header, $sz_ptr, $sz_rr_a, $sz_rr_aaaa) = (12, 2, 16, 28);
my ($sz_type, $sz_class, $sz_ttl, $sz_rdlen) = (2, 2, 4, 2);
my (%namedb, $name, $nssect, %opts, $optz);
my $n_ns = 0;

getopt('z', %opts);
if (defined($opts{'z'})) {
    server_name_len($opts{'z'}); # just register it

foreach $name (@ARGV) {
    my $len;
    $len = server_name_len($name);
    print "$name requires $len bytes\n";
    $nssect += $sz_ptr + $sz_type + $sz_class + $sz_ttl
            +  $sz_rdlen + $len;
print "# of NS: $n_ns\n";
arsect(255, $nssect, $n_ns, "maximum");
arsect(64, $nssect, $n_ns, "average");

sub server_name_len {
    my ($name) = @_;
    my (@labels, $len, $n, $suffix);

    $name =~ tr/A-Z/a-z/;
    @labels = split(/\./, $name);
    $len = length(join('.', @labels)) + 2;
    for ($n = 0; $#labels >= 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;

Authors' Addresses

  Paul Vixie
  Internet Systems Consortium
  950 Charter Street
  Redwood City, CA 94063
Phone:  +1 650 423 1300
  Akira Kato
  The University of Tokyo/WIDE Project
  Information Technology Center, 2-11-16 Yayoi
  Bunkyo, Tokyo 113-8658
Phone:  +81 3 5841 2750

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