Internet Engineering Task Force P. Vixie
Internet-Draft Internet Systems Consortium
Intended status: Informational A. Kato
Expires: November 11, 2012 Keio University/WIDE Project
May 10, 2012
DNS Referral Response Size Issues
draft-ietf-dnsop-respsize-14
Abstract
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.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on November 11, 2012.
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1. Introduction and Overview
The original DNS standard limited the UDP message size to 512 octets
(see Section 4.2.1 of [RFC1035]). 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 Section 4.3 and Section 6.2 of
[RFC2671bis]), 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], this document specifically addresses referral
response size.
While more than twelve years passed since the publication of the
original EDNS0 document [RFC2671], approximately 65% of the clients
support it as observed at a root name server and this fraction has
not changed in recent few years. The long tail of EDNS deployment
may eventually be measured in decades.
Even if EDNS deployment reached 100% of all DNS initiators and
responders there will still be cases when path MTU limitations or IP
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fragmentation/reassembly problems in firewalls and other middleboxes
will cause EDNS failures which leads to non-extended DNS retries. A
smaller referral response will always be better than a larger one if
the same end result can be achieved either way. See [RFC5625],
[SSAC035], and Section 6.2.6 of [RFC2671bis] for details.
2. Delegation Details
2.1. Relevant Protocol Elements
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 ([RFC1034]) while DNAME maps an
entire subtree to another domain ([RFC2672]).
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 Section 6.1.3.2 of [RFC1123] for details on the
requirement that UDP be attempted before falling back to TCP.
RRsets (Resource Record Set, see [RFC2136]) 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 Section 6.2 of [RFC1035] and
Section 9 of [RFC2181]).
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 Appendix B of [RFC4472]).
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
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message (see Section 4.1.4 of [RFC1035]). 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 Section 2.1 of [RFC2308] for
more information about the format of negative responses.
The minimum useful number of name servers is two, for redundancy (see
Section 4.1 of [RFC1034]). 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 [RFC3258] 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].
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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 Section 9 of [RFC2181]). 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
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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 of the version of IP used
in the query because the DNS server might just see a query from an
intermediate server rather than the query from the original client.
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.
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;; flags: qr rd; QUERY: 1, ANS: 0, AUTH: 13, ADDIT: 13
;; QUERY SECTION:
;; [23456789.123456789.123456789.\
123456789.123456789.123456789.com A IN] ;; @80
;; AUTHORITY SECTION:
com. 172800 NS E.GTLD-SERVERS.NET. ;; @112
com. 172800 NS F.GTLD-SERVERS.NET. ;; @128
com. 172800 NS G.GTLD-SERVERS.NET. ;; @144
com. 172800 NS H.GTLD-SERVERS.NET. ;; @160
com. 172800 NS I.GTLD-SERVERS.NET. ;; @176
com. 172800 NS J.GTLD-SERVERS.NET. ;; @192
com. 172800 NS K.GTLD-SERVERS.NET. ;; @208
com. 172800 NS L.GTLD-SERVERS.NET. ;; @224
com. 172800 NS M.GTLD-SERVERS.NET. ;; @240
com. 172800 NS A.GTLD-SERVERS.NET. ;; @256
com. 172800 NS B.GTLD-SERVERS.NET. ;; @272
com. 172800 NS C.GTLD-SERVERS.NET. ;; @288
com. 172800 NS D.GTLD-SERVERS.NET. ;; @304
;; ADDITIONAL SECTION:
A.GTLD-SERVERS.NET. 172800 A 192.5.6.30 ;; @320
B.GTLD-SERVERS.NET. 172800 A 192.33.14.30 ;; @336
C.GTLD-SERVERS.NET. 172800 A 192.26.92.30 ;; @352
D.GTLD-SERVERS.NET. 172800 A 192.31.80.30 ;; @368
E.GTLD-SERVERS.NET. 172800 A 192.12.94.30 ;; @384
F.GTLD-SERVERS.NET. 172800 A 192.35.51.30 ;; @400
G.GTLD-SERVERS.NET. 172800 A 192.42.93.30 ;; @416
H.GTLD-SERVERS.NET. 172800 A 192.54.112.30 ;; @432
I.GTLD-SERVERS.NET. 172800 A 192.43.172.30 ;; @448
J.GTLD-SERVERS.NET. 172800 A 192.48.79.30 ;; @464
K.GTLD-SERVERS.NET. 172800 A 192.52.178.30 ;; @480
L.GTLD-SERVERS.NET. 172800 A 192.41.162.30 ;; @496
M.GTLD-SERVERS.NET. 172800 A 192.55.83.30 ;; @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 (written in perl [PERL]) shown in
Figure 2 and Figure 3 demonstrate these properties.
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% perl respsize.pl a.dns.br b.dns.br c.dns.br d.dns.br
a.dns.br requires 10 bytes
b.dns.br requires 4 bytes
c.dns.br requires 4 bytes
d.dns.br 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 respsize.pl ns-ext.isc.org ns.psg.com ns.ripe.net ns.eu.int
ns-ext.isc.org requires 16 bytes
ns.psg.com requires 12 bytes
ns.ripe.net requires 13 bytes
ns.eu.int 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 that
results in larger query names be deployed significantly in advance of
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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 Section 2 of [RFC1996] 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 that 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.
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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, Alfred
Hoenes, Alexander Mayrhofer, and Ray Bellis for their valuable
comments and suggestions.
This work was supported by the US National Science Foundation
(research grant SCI-0427144) and DNS-OARC.
8. References
8.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
8.2. Informative References
[PERL] Wall, L., Christiansen, T., and J. Orwant, "Programming
Perl, 3rd ed.", ISBN 0-596-00027-8, July 2000.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone
Changes (DNS NOTIFY)", RFC 1996, August 1996.
[RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
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"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, April 1997.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, March 1998.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
[RFC2671bis]
Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS0)", draft-ietf-dnsext-rfc2671bis-edns0-08 ,
February 2012.
[RFC2672] Crawford, M., "Non-Terminal DNS Name Redirection",
RFC 2672, August 1999.
[RFC3226] Gudmundsson, O., "DNSSEC and IPv6 A6 aware server/resolver
message size requirements", RFC 3226, December 2001.
[RFC3258] Hardie, T., "Distributing Authoritative Name Servers via
Shared Unicast Addresses", RFC 3258, April 2002.
[RFC3901] Durand, A. and J. Ihren, "DNS IPv6 Transport Operational
Guidelines", BCP 91, RFC 3901, September 2004.
[RFC4472] Durand, A., Ihren, J., and P. Savola, "Operational
Considerations and Issues with IPv6 DNS", RFC 4472,
April 2006.
[RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines",
BCP 152, RFC 5625, August 2009.
[SSAC035] Bellis, R. and L. Phifer, "Test Report: DNSSEC Impact on
Broadband Routers and Firewalls", SSAC 035,
September 2008.
Appendix A. The response simulator program
#!/usr/bin/perl
#
# SYNOPSIS
# respsize.pl [ -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
#
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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;
$n_ns++;
$len = server_name_len($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), $n_ns);
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$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
US
Phone: +1 650 423 1300
Email: vixie@isc.org
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Akira Kato
Keio University/WIDE Project
Graduate School of Media Design, 4-1-1 Hiyoshi
Kohoku, Yokohama 223-8526
JP
Phone: +81 45 564 2490
Email: kato@wide.ad.jp
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