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DNSOP Working Group Paul Vixie, ISC (Ed.)
INTERNET-DRAFT Akira Kato, WIDE
<draft-ietf-dnsop-respsize-01.txt> July, 2004
DNS Response Size Issues
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of
which we are aware have been or will be disclosed, and any of which
we 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.
Copyright Notice
Copyright (C) The Internet Society (2003-2004). All Rights Reserved.
Abstract
With a mandated default minimum maximum 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.
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1 - Introduction and Overview
1.1. The DNS standard (see [RFC1035 4.2.1]) limits message size to 512
octets. Even though this limitation was due to the required minimum UDP
reassembly limit for IPv4, it is a hard DNS protocol limit and is not
implicitly relaxed by changes in transport, for example to IPv6.
1.2. The EDNS0 standard (see [RFC2671 2.3, 4.5]) permits larger
responses by mutual agreement of the requestor and responder. However,
deployment of EDNS0 cannot be expected to reach every Internet resolver
in the short or medium term. The 512 octet message size limit remains
in practical effect at this time.
1.3. Since DNS responses include a copy of the request, the space
available for response data is somewhat less than the full 512 octets.
For negative responses, there is rarely a space constraint. For
positive and delegation responses, though, every octet must be carefully
and sparingly allocated. This document specifically addresses
delegation response sizes.
2 - Delegation Details
2.1. A delegation response will include the following elements:
Header Section: fixed length (12 octets)
Question Section: original query (name, class, type)
Answer Section: (empty)
Authority Section: NS RRset (nameserver names)
Additional Section: A and AAAA RRsets (nameserver addresses)
2.2. If the total response size would exceed 512 octets, and if the data
that would not fit was in the question, answer, or authority section,
then the TC bit will be set (indicating truncation) which may cause the
requestor to retry using TCP, depending on what information was present
and what was omitted. If a retry using TCP is needed, the total cost of
the transaction is much higher.
2.3. RRsets are never sent partially, so if truncation occurs, entire
RRsets are omitted. Note that the authority section consists of a
single RRset. It is absolutely essential that truncation not occur in
the authority section.
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2.4. DNS label compression allows a domain name to be instantiated only
once per DNS message, and then referenced with a two-octet "pointer"
from other locations in that same DNS message. If all nameserver names
in a message are similar (for example, all ending in ".ROOT-
SERVERS.NET"), then more space will be available for uncompressable data
(such as nameserver addresses).
2.5. The query name can be as long as 255 characters of presentation
data, which can be up to 256 octets of network data. In this worst case
scenario, the question section will be 260 octets in size, which would
leave only 240 octets for the authority and additional sections (after
deducting 12 octets for the fixed length header.)
2.6. 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. For cost and performance
reasons, the majority of requests should be satisfied without truncation
or TCP retry.
2.7. Requestors who deliberately send large queries to force truncation
are only increasing their own costs, and cannot effectively attack the
resources of an authority server since the requestor would have to retry
using TCP to complete the attack. An attack that always used TCP would
have a lower cost.
2.8. The minimum useful number of address records is two, since with
only one address, the probability that it would refer to an unreachable
server is too high. Truncation which occurs after two address records
have been added to the additional data section is therefore less
operationally significant than truncation which occurs earlier.
2.9. The best case is no truncation. (This is because many requestors
will retry using TCP by reflex, without considering whether the omitted
data was actually necessary.)
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3 - Analysis
3.1. An instrumented protocol trace of a best case delegation response
follows. Note that 13 servers are named, and 13 addresses are given.
This query was artificially designed to exactly reach the 512 octet
limit.
;; 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. 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
;; ADDITIONAL SECTION:
A.GTLD-SERVERS.NET. 86400 A 192.5.6.30 ;; @320
B.GTLD-SERVERS.NET. 86400 A 192.33.14.30 ;; @336
C.GTLD-SERVERS.NET. 86400 A 192.26.92.30 ;; @352
D.GTLD-SERVERS.NET. 86400 A 192.31.80.30 ;; @368
E.GTLD-SERVERS.NET. 86400 A 192.12.94.30 ;; @384
F.GTLD-SERVERS.NET. 86400 A 192.35.51.30 ;; @400
G.GTLD-SERVERS.NET. 86400 A 192.42.93.30 ;; @416
H.GTLD-SERVERS.NET. 86400 A 192.54.112.30 ;; @432
I.GTLD-SERVERS.NET. 86400 A 192.43.172.30 ;; @448
J.GTLD-SERVERS.NET. 86400 A 192.48.79.30 ;; @464
K.GTLD-SERVERS.NET. 86400 A 192.52.178.30 ;; @480
L.GTLD-SERVERS.NET. 86400 A 192.41.162.30 ;; @496
M.GTLD-SERVERS.NET. 86400 A 192.55.83.30 ;; @512
;; MSG SIZE sent: 80 rcvd: 512
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3.2. 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. The following output from a response simulator demonstrates these
properties:
% perl respsize.pl 13 13 0
common name, average case: msg:303 nsaddr#13 (green)
common name, worst case: msg:495 nsaddr# 1 (red)
uncommon name, average case: msg:457 nsaddr# 3 (orange)
uncommon name, worst case: msg:649(*) nsaddr# 0 (red)
% perl respsize.pl 13 13 2
common name, average case: msg:303 nsaddr#11 (orange)
common name, worst case: msg:495 nsaddr# 1 (red)
uncommon name, average case: msg:457 nsaddr# 2 (orange)
uncommon name, worst case: msg:649(*) nsaddr# 0 (red)
(Note: The response simulator program is shown in Section 5.)
Here we use the term "green" if all address records could fit, or
"orange" if two or more could fit, or "red" if fewer than two could fit.
It's clear that without a common parent for nameserver names, much space
would be lost.
We're assuming an average query name size of 64 since that is the
typical average maximum 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 more new measurements and new estimates will have to be
made.
4 - Conclusions
4.1. 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. (Note that in this case it is wise to serve the
common parent domain's zone from the same servers that are named within
it, in order to limit external dependencies when all your eggs are in a
single basket.)
4.2. Thirteen (13) seems to be the effective maximum number of
nameserver names usable traditional (non-extended) DNS, assuming a
common parent domain name, and assuming that additional-data truncation
is undesirable in the average case.
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4.3. Adding two to five IPv6 nameserver address records (AAAA RRs) to a
prototypical delegation that currently contains thirteen (13) IPv4
nameserver addresses (A RRs) for thirteen (13) nameserver names under a
common parent, would not have a significant negative operational impact
on the domain name system.
5 - Source Code
#!/usr/bin/perl -w
$asize = 2+2+2+4+2+4;
$aaaasize = 2+2+2+4+2+16;
($nns, $na, $naaaa) = @ARGV;
test("common", "average", common_name_average($nns),
$na, $naaaa);
test("common", "worst", common_name_worst($nns),
$na, $naaaa);
test("uncommon", "average", uncommon_name_average($nns),
$na, $naaaa);
test("uncommon", "worst", uncommon_name_worst($nns),
$na, $naaaa);
exit 0;
sub test { my ($namekind, $casekind, $msg, $na, $naaaa) = @_;
my $nglue = numglue($msg, $na, $naaaa);
printf "%8s name, %7s case: msg:%3d%s nsaddr#%2d (%s)\n",
$namekind, $casekind,
$msg, ($msg > 512) ? "(*)" : " ",
$nglue, ($nglue == $na + $naaaa) ? "green"
: ($nglue >= 2) ? "orange"
: "red";
}
sub pnum { my ($num, $tot) = @_;
return sprintf "%3d%s",
}
sub numglue { my ($msg, $na, $naaaa) = @_;
my $space = ($msg > 512) ? 0 : (512 - $msg);
my $num = 0;
while ($space && ($na || $naaaa )) {
if ($na) {
if ($space >= $asize) {
$space -= $asize;
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$num++;
}
$na--;
}
if ($naaaa) {
if ($space >= $aaaasize) {
$space -= $aaaasize;
$num++;
}
$naaaa--;
}
}
return $num;
}
sub msgsize { my ($qname, $nns, $nsns) = @_;
return 12 + # header
$qname+2+2 + # query
0 + # answer
$nns * (4+2+2+4+2+$nsns); # authority
}
sub average_case { my ($nns, $nsns) = @_;
return msgsize(64, $nns, $nsns);
}
sub worst_case { my ($nns, $nsns) = @_;
return msgsize(256, $nns, $nsns);
}
sub common_name_average { my ($nns) = @_;
return 15 + average_case($nns, 2);
}
sub common_name_worst { my ($nns) = @_;
return 15 + worst_case($nns, 2);
}
sub uncommon_name_average { my ($nns) = @_;
return average_case($nns, 15);
}
sub uncommon_name_worst { my ($nns) = @_;
return worst_case($nns, 15);
}
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Security Considerations
The recommendations contained in this document have no known security
implications.
IANA Considerations
This document does not call for changes or additions to any IANA
registry.
IPR Statement
Copyright (C) The Internet Society (2003-2004). This document is
subject to the rights, licenses and restrictions contained in BCP 78,
and except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR
IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Authors' Addresses
Paul Vixie
950 Charter Street
Redwood City, CA 94063
+1 650 423 1301
vixie@isc.org
Akira Kato
University of Tokyo, Information Technology Center
2-11-16 Yayoi Bunkyo
Tokyo 113-8658, JAPAN
+81 3 5841 2750
kato@wide.ad.jp
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Network Working Group S. Woolf
Internet-Draft Internet Systems Consortium, Inc.
Expires: January 16, 2005 D. Conrad
Nominum, Inc.
July 18, 2004
Identifying an Authoritative Name `Server
draft-ietf-dnsop-serverid-02
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of
which he or she is aware have been or will be disclosed, and any of
which he or she 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 January 16, 2005.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
With the increased use of DNS anycast, load balancing, and other
mechanisms allowing more than one DNS name server to share a single
IP address, it is sometimes difficult to tell which of a pool of name
servers has answered a particular query. A standardized mechanism to
determine the identity of a name server responding to a particular
query would be useful, particularly as a diagnostic aid. Existing ad
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hoc mechanisms for addressing this concern are not adequate. This
document attempts to describe the common ad hoc solution to this
problem, including its advantages and disadvantasges, and to
characterize an improved mechanism.
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1. Introduction
With the increased use of DNS anycast, load balancing, and other
mechanisms allowing more than one DNS name server to share a single
IP address, it is sometimes difficult to tell which of a pool of name
servers has answered a particular query. A standardized mechanism to
determine the identity of a name server responding to a particular
query would be useful, particularly as a diagnostic aid.
Unfortunately, existing ad-hoc mechanisms for providing such
identification have some shortcomings, not the least of which is the
lack of prior analysis of exactly how such a mechanism should be
designed and deployed. This document describes the existing
convention used in one widely deployed implementation of the DNS
protocol and discusses requirements for an improved solution to the
problem.
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2. Rationale
Identifying which name server is responding to queries is often
useful, particularly in attempting to diagnose name server
difficulties. However, relying on the IP address of the name server
has become more problematic due the deployment of various load
balancing solutions, including the use of shared unicast addresses as
documented in [RFC3258].
An unfortunate side effect of these load balancing solutions is that
traditional methods of determining which server is responding can be
unreliable. Specifically, non-DNS methods such as ICMP ping, TCP
connections, or non-DNS UDP packets (e.g., as generated by tools such
as "traceroute"), etc., can end up going to a different server than
that which receives the DNS queries.
The widespread use of the existing convention suggests a need for a
documented, interoperable means of querying the identity of a
nameserver that may be part of an anycast or load-balancing cluster.
At the same time, however, it also has some drawbacks that argue
against standardizing it as it's been practiced so far.
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3. Existing Conventions
Recent versions of the commonly deployed Berkeley Internet Name
Domain implementation of the DNS protocol suite from the Internet
Software Consortium [BIND] support a way of identifying a particular
server via the use of a standard, if somewhat unusual, DNS query.
Specifically, a query to a late model BIND server for a TXT resource
record in class 3 (CHAOS) for the domain name "HOSTNAME.BIND." will
return a string that can be configured by the name server
administrator to provide a unique identifier for the responding
server (defaulting to the value of a gethostname() call). This
mechanism, which is an extension of the BIND convention of using
CHAOS class TXT RR queries to sub-domains of the "BIND." domain for
version information, has been copied by several name server vendors.
For reference, the other well-known name used by recent versions of
BIND within the CHAOS class "BIND." domain is "VERSION.BIND." A
query for a TXT RR for this name will return an administratively re-
definable string which defaults to the version of the server
responding.
3.1 Advantages
There are several valuable attributes to this mechanism, which
account for its usefulness.
1. This mechanism is within the DNS protocol itself. An
identification mechanism that relies on the DNS protocol is more
likely to be successful (although not guaranteed) in going to the
same machine as a "normal" DNS query.
2. It is simple to configure. An administrator can easily turn on
this feature and control the results of the relevant query.
3. It allows the administrator complete control of what information
is given out in the response, minimizing passive leakage of
implementation or configuration details. Such details are often
considered sensitive by infrastructure operators.
3.2 Disadvantages
At the same time, there are some forbidding drawbacks to the
VERSION.BIND mechanism that argue against standardizing it as it
currently operates.
1. It requires an additional query to correlate between the answer
to a DNS query under normal conditions and the supposed identity
of the server receiving the query. There are a number of
situations in which this simply isn't reliable.
2. It reserves an entire class in the DNS (CHAOS) for what amounts
to one zone. While CHAOS class is defined in [RFC1034] and
[RFC1035], it's not clear that supporting it solely for this
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purpose is a good use of the namespace or of implementation
effort.
3. It is implementation specific. BIND is one DNS implementation.
At the time of this writing, it is probably the most prevalent,
for authoritative servers anyway. This does not justify
standardizing on its ad hoc solution to a problem shared across
many operators and implementors.
The first of the listed disadvantages is technically the most
serious. It argues for an attempt to design a good answer to the
problem that "I need to know what nameserver is answering my
queries", not simply a convenient one.
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4. Characteristics of an Implementation Neutral Convention
The discussion above of advantages and disadvantages to the
HOSTNAME.BIND mechanism suggest some requirements for a better
solution to the server identification problem. These are summarized
here as guidelines for any effort to provide appropriate protocol
extensions:
1. The mechanism adopted MUST be in-band for the DNS protocol. That
is, it needs to allow the query for the server's identifying
information to be part of a normal, operational query. It SHOULD
also permit a separate, dedicated query for the server's
identifying information.
2. The new mechanism should not require dedicated namespaces or
other reserved values outside of the existing protocol mechanisms
for these, i.e. the OPT pseudo-RR.
3. Support for the identification functionality SHOULD be easy to
implement and easy to enable. It MUST be easy to disable and
SHOULD lend itself to access controls on who can query for it.
4. It should be possible to return a unique identifier for a server
without requiring the exposure of information that may be
non-public and considered sensitive by the operator, such as a
hostname or unicast IP address maintained for administrative
purposes.
5. The identification mechanism SHOULD NOT be
implementation-specific.
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5. IANA Considerations
This document proposes no specific IANA action. Protocol extensions,
if any, to meet the requirements described are out of scope for this
document. Should such extensions be specified and adopted by normal
IETF process, the specification will include appropriate guidance to
IANA.
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6. Security Considerations
Providing identifying information as to which server is responding
can be seen as information leakage and thus a security risk. This
motivates the suggestion above that a new mechanism for server
identification allow the administrator to disable the functionality
altogether or partially restrict availability of the data. It also
suggests that the serverid data should not be readily correlated with
a hostname or unicast IP address that may be considered private to
the nameserver operator's management infrastructure.
Propagation of protocol or service meta-data can sometimes expose the
application to denial of service or other attack. As DNS is a
critically important infrastructure service for the production
Internet, extra care needs to be taken against this risk for
designers, implementors, and operators of a new mechanism for server
identification.
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7. Acknowledgements
The technique for host identification documented here was initially
implemented by Paul Vixie of the Internet Software Consortium in the
Berkeley Internet Name Daemon package. Comments and questions on
earlier drafts were provided by Bob Halley, Brian Wellington, Andreas
Gustafsson, Ted Hardie, Chris Yarnell, Randy Bush, and members of the
ICANN Root Server System Advisory Committee. The newest draft takes
a significantly different direction from previous versions, owing to
discussion among contributors to the DNSOP working group and others,
particularly Olafur Gudmundsson, Ed Lewis, Bill Manning, Sam Weiler,
and Rob Austein.
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Intellectual Property Statement
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Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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