draft-ietf-dnsop-respsize-13.txt   draft-ietf-dnsop-respsize-14.txt 
Internet Engineering Task Force P. Vixie Internet Engineering Task Force P. Vixie
Internet-Draft Internet Systems Consortium Internet-Draft Internet Systems Consortium
Intended status: Informational A. Kato Intended status: Informational A. Kato
Expires: March 16, 2012 Keio University/WIDE Project Expires: November 11, 2012 Keio University/WIDE Project
September 13, 2011 May 10, 2012
DNS Referral Response Size Issues DNS Referral Response Size Issues
draft-ietf-dnsop-respsize-13 draft-ietf-dnsop-respsize-14
Abstract Abstract
With a mandated default minimum maximum UDP message size of 512 With a mandated default minimum maximum UDP message size of 512
octets, the DNS protocol presents some special problems for zones octets, the DNS protocol presents some special problems for zones
wishing to expose a moderate or high number of authority servers (NS wishing to expose a moderate or high number of authority servers (NS
RRs). This document explains the operational issues caused by, or RRs). This document explains the operational issues caused by, or
related to this response size limit, and suggests ways to optimize related to this response size limit, and suggests ways to optimize
the use of this limited space. Guidance is offered to DNS server the use of this limited space. Guidance is offered to DNS server
implementors and to DNS zone operators. implementors and to DNS zone operators.
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 16, 2012. This Internet-Draft will expire on November 11, 2012.
Copyright Notice Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
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carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
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modifications of such material outside the IETF Standards Process. modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other it for publication as an RFC or to translate it into languages other
than English. than English.
1. Introduction and Overview 1. Introduction and Overview
The original DNS standard limited UDP message size to 512 octets (see The original DNS standard limited the UDP message size to 512 octets
[RFC1035] 4.2.1). Even though this limitation was due to the (see Section 4.2.1 of [RFC1035]). Even though this limitation was
required minimum IP reassembly limit for IPv4, it became a hard DNS due to the required minimum IP reassembly limit for IPv4, it became a
protocol limit and is not implicitly relaxed by changes in a network hard DNS protocol limit and is not implicitly relaxed by changes in a
layer protocol, for example to IPv6. network layer protocol, for example to IPv6.
The EDNS (Extension Mechanisms for DNS) protocol extension starting The EDNS (Extension Mechanisms for DNS) protocol extension starting
with version 0 permits larger responses by mutual agreement of the with version 0 permits larger responses by mutual agreement of the
requester and responder (see [RFC2671] 2.3, 4.5), and it is requester and responder (see Section 4.3 and Section 6.2 of
recommended to support EDNS. The 512 octets UDP message size limit [RFC2671bis]), and it is recommended to support EDNS. The 512 octets
will remain in practical effect until virtually all DNS servers and UDP message size limit will remain in practical effect until
resolvers support EDNS. virtually all DNS servers and resolvers support EDNS.
Since DNS responses include a copy of the request, the space Since DNS responses include a copy of the request, the space
available for response data is somewhat less than the full 512 available for response data is somewhat less than the full 512
octets. Negative responses are quite small, but for positive and octets. Negative responses are quite small, but for positive and
referral responses, every octet must be carefully and sparingly referral responses, every octet must be carefully and sparingly
allocated. While the response size of positive responses is also a allocated. While the response size of positive responses is also a
concern in [RFC3226], this document specifically addresses referral concern in [RFC3226], this document specifically addresses referral
response size. response size.
While more than twelve years passed since the publication of EDNS0 While more than twelve years passed since the publication of the
document[RFC2671], approximately 65% of the clients support it as original EDNS0 document [RFC2671], approximately 65% of the clients
observed at a root name server and this fraction has not changed in support it as observed at a root name server and this fraction has
recent few years. The long tail of EDNS deployment may eventually be not changed in recent few years. The long tail of EDNS deployment
measured in decades. may eventually be measured in decades.
Even if EDNS deployment reached 100% of all DNS initiators and Even if EDNS deployment reached 100% of all DNS initiators and
responders there will still be cases when path MTU limitations or IP responders there will still be cases when path MTU limitations or IP
fragmentation/reassembly problems in firewalls and other middleboxes fragmentation/reassembly problems in firewalls and other middleboxes
will cause EDNS failures which leads to non-extended DNS retries. A will cause EDNS failures which leads to non-extended DNS retries. A
smaller referral response will always be better than a larger one if smaller referral response will always be better than a larger one if
the same end result can be achieved either way. 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. Delegation Details
2.1. Relevant Protocol Elements 2.1. Relevant Protocol Elements
A positive delegation response will include the following elements: A positive delegation response will include the following elements:
Header Section: fixed length (12 octets) Header Section: fixed length (12 octets)
Question Section: original query (name, class, type) Question Section: original query (name, class, type)
Answer Section: empty, or a CNAME/DNAME chain Answer Section: empty, or a CNAME/DNAME chain
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entire subtree to another domain ([RFC2672]). entire subtree to another domain ([RFC2672]).
If the total size of the UDP response exceeds 512 octets or the size 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", advertised in EDNS, and if the data that does not fit was "required",
then the TC bit will be set (indicating truncation). This will then the TC bit will be set (indicating truncation). This will
usually cause the requester to retry using TCP, depending on what usually cause the requester to retry using TCP, depending on what
information was desired and what information was omitted. For information was desired and what information was omitted. For
example, truncation in the authority section is of no interest to a example, truncation in the authority section is of no interest to a
stub resolver who only plans to consume the answer section. If 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 retry using TCP is needed, the total cost of the transaction is much
higher. See [RFC1123] 6.1.3.2 for details on the requirement that higher. See Section 6.1.3.2 of [RFC1123] for details on the
UDP be attempted before falling back to TCP. requirement that UDP be attempted before falling back to TCP.
RRsets (Resource Record Set, see [RFC2136]) are never sent partially RRsets (Resource Record Set, see [RFC2136]) are never sent partially
unless the TC bit is set to indicate truncation. When the TC bit is 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 set, the final apparent RRset in the final non-empty section must be
considered "possibly damaged" (see [RFC1035] 6.2, [RFC2181] 9). 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 With or without truncation, the glue present in the additional data
section should be considered "possibly incomplete", and requesters section should be considered "possibly incomplete", and requesters
should be prepared to re-query for any damaged or missing RRsets. should be prepared to re-query for any damaged or missing RRsets.
Note that truncation of the additional data section might not be Note that truncation of the additional data section might not be
signaled via the TC bit since additional data is often optional (see signaled via the TC bit since additional data is often optional (see
discussion in [RFC4472] B). discussion in Appendix B of [RFC4472]).
DNS label compression allows the component labels of a domain name to DNS label compression allows the component labels of a domain name to
be instantiated exactly once per DNS message, and then referenced be instantiated exactly once per DNS message, and then referenced
with a two-octet "pointer" from other locations in that same DNS with a two-octet "pointer" from other locations in that same DNS
message (see [RFC1035] 4.1.4). If all nameserver names in a message message (see Section 4.1.4 of [RFC1035]). If all nameserver names in
share a common parent (for example, all of them are in "ROOT- a message share a common parent (for example, all of them are in
SERVERS.NET." zone), then more space will be available for "ROOT-SERVERS.NET." zone), then more space will be available for
incompressible data (such as nameserver addresses). incompressible data (such as nameserver addresses).
The query name can be as long as 255 octets of network data. In this 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, worst case scenario, the question section will be 259 octets in size,
which would leave only 240 octets for the authority and additional which would leave only 240 octets for the authority and additional
sections (after deducting 12 octets for the fixed length header) in a sections (after deducting 12 octets for the fixed length header) in a
referral. referral.
2.2. Advice to Zone Owners 2.2. Advice to Zone Owners
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zone owner, since they will know what names actually exist and can 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 measure which ones are queried for most often. Note that if the zone
contains any wildcards, it is possible for maximum length queries to contains any wildcards, it is possible for maximum length queries to
require positive responses, but that it is reasonable to expect require positive responses, but that it is reasonable to expect
truncation and TCP retry in that case. For cost and performance truncation and TCP retry in that case. For cost and performance
reasons, the majority of requests should be satisfied without reasons, the majority of requests should be satisfied without
truncation or TCP retry. truncation or TCP retry.
Some queries to non-existing names can be large, but this is not a Some queries to non-existing names can be large, but this is not a
problem because negative responses need not contain any answer, problem because negative responses need not contain any answer,
authority or additional records. See [RFC2308] 2.1 for more authority or additional records. See Section 2.1 of [RFC2308] for
information about the format of negative responses. more information about the format of negative responses.
The minimum useful number of name servers is two, for redundancy (see The minimum useful number of name servers is two, for redundancy (see
[RFC1034] 4.1). A zone's name servers should be reachable by all IP Section 4.1 of [RFC1034]). A zone's name servers should be reachable
protocols versions (e.g., IPv4 and IPv6) in common use. As long as by all IP protocols versions (e.g., IPv4 and IPv6) in common use. As
the servers are well managed, the server serving IPv6 might be long as the servers are well managed, the server serving IPv6 might
different from the server serving IPv4 sharing the same server name. be different from the server serving IPv4 sharing the same server
name.
The best case is no truncation at all. This is because many The best case is no truncation at all. This is because many
requesters will retry using TCP immediately, or will automatically requesters will retry using TCP immediately, or will automatically
requery for RRsets that are possibly truncated, without considering requery for RRsets that are possibly truncated, without considering
whether the omitted data was actually necessary. whether the omitted data was actually necessary.
Anycasting [RFC3258] is a useful tool for performance and reliability Anycasting [RFC3258] is a useful tool for performance and reliability
without increasing the size of referral responses. without increasing the size of referral responses.
While it is irrelevant to the response size issue, all zones have to While it is irrelevant to the response size issue, all zones have to
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While DNS distinguishes between necessary and optional resource While DNS distinguishes between necessary and optional resource
records, this distinction is according to protocol elements necessary records, this distinction is according to protocol elements necessary
to signify facts, and takes no official notice of protocol content to signify facts, and takes no official notice of protocol content
necessary to ensure correct operation. For example, a nameserver necessary to ensure correct operation. For example, a nameserver
name that is in or below the zone cut being described by a delegation 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 is "necessary content", since there is no way to reach that zone
unless the parent zone's delegation includes "glue records" unless the parent zone's delegation includes "glue records"
describing that name server's addresses. describing that name server's addresses.
Recall that the TC bit is only set when a required RRset can not be Recall that the TC bit is only set when a required RRset can not be
included in its entirety (see [RFC2181] 9). Even when some of the included in its entirety (see Section 9 of [RFC2181]). Even when
RRsets to be included in the additional section don't fit in the some of the RRsets to be included in the additional section don't fit
response size, the TC bit isn't set. These RRsets may be important in the response size, the TC bit isn't set. These RRsets may be
for a referral. Some DNS implementations try to resolve these important for a referral. Some DNS implementations try to resolve
missing glue records separately which will introduce extra queries these missing glue records separately which will introduce extra
and extra time to resolve a given name. queries and extra time to resolve a given name.
A delegation response should prioritize glue records as follows. A delegation response should prioritize glue records as follows.
first: first:
All glue RRsets for one name server whose name is in or below the All glue RRsets for one name server whose name is in or below the
zone being delegated, or which has multiple address RRsets zone being delegated, or which has multiple address RRsets
(currently A and AAAA), or preferably both; (currently A and AAAA), or preferably both;
second: second:
Alternate between adding all glue RRsets for any name servers Alternate between adding all glue RRsets for any name servers
whose names are in or below the zone being delegated, and all whose names are in or below the zone being delegated, and all
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If any "necessary" content cannot be fit in the response, then it is 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, 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 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 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 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 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 outlying cases, only EDNS or TCP will be large enough to contain that
data. data.
The glue record order should be independent to the version of IP used The glue record order should be independent of the version of IP used
in the query because the DNS server just see a query from an in the query because the DNS server might just see a query from an
intermediate server rather than the query from the original client. intermediate server rather than the query from the original client.
3. Analysis 3. Analysis
An instrumented protocol trace of a best case delegation response is An instrumented protocol trace of a best case delegation response is
shown in Figure 1. Note that 13 servers are named, and 13 addresses shown in Figure 1. Note that 13 servers are named, and 13 addresses
are given. This query was artificially designed to exactly reach the are given. This query was artificially designed to exactly reach the
512 octets limit. 512 octets limit.
;; flags: qr rd; QUERY: 1, ANS: 0, AUTH: 13, ADDIT: 13 ;; flags: qr rd; QUERY: 1, ANS: 0, AUTH: 13, ADDIT: 13
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Here we use the term "green" if all address records could fit, or 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, "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 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 common parent for nameserver names, much space would be lost. For
these examples we use an average/common name size of 15 octets, these examples we use an average/common name size of 15 octets,
befitting our assumption of "GTLD-SERVERS.NET." as our common parent befitting our assumption of "GTLD-SERVERS.NET." as our common parent
name. name.
We're assuming a medium query name size of 64 since that is the 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 typical size seen in trace data at the time of this writing. If
Internationalized Domain Name (IDN) or any other technology which Internationalized Domain Name (IDN) or any other technology that
results in larger query names be deployed significantly in advance of results in larger query names be deployed significantly in advance of
EDNS, then new measurements and new estimates will have to be made. EDNS, then new measurements and new estimates will have to be made.
4. Conclusions 4. Conclusions
The current practice of giving all nameserver names a common parent The current practice of giving all nameserver names a common parent
(such as "GTLD-SERVERS.NET." or "ROOT-SERVERS.NET.") saves space in (such as "GTLD-SERVERS.NET." or "ROOT-SERVERS.NET.") saves space in
DNS responses and allows for more nameservers to be enumerated than DNS responses and allows for more nameservers to be enumerated than
would otherwise be possible, since the common parent domain name only would otherwise be possible, since the common parent domain name only
appears once in a DNS message and is referred to via "compression appears once in a DNS message and is referred to via "compression
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If all nameserver names for a zone share a common parent, then it is 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 operationally advisable to make all servers for the zone thus served
also be authoritative for the zone of that common parent. For also be authoritative for the zone of that common parent. For
example, the root name servers (?.ROOT-SERVERS.NET.) can answer example, the root name servers (?.ROOT-SERVERS.NET.) can answer
authoritatively for the ROOT-SERVERS.NET. zone. This is to ensure authoritatively for the ROOT-SERVERS.NET. zone. This is to ensure
that the zone's servers always have the zone's nameservers' glue that the zone's servers always have the zone's nameservers' glue
available when delegating, and will be able to respond with answers available when delegating, and will be able to respond with answers
rather than referrals if a requester who wants that glue comes back 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 asking for it. In this case the name server will likely be a
"stealth server" -- authoritative but unadvertised in the glue zone's "stealth server" -- authoritative but unadvertised in the glue zone's
NS RRset. See [RFC1996] 2 for more information about stealth NS RRset. See Section 2 of [RFC1996] for more information about
servers. stealth servers.
Thirteen (13) is the effective maximum number of nameserver names Thirteen (13) is the effective maximum number of nameserver names
usable with traditional (non-extended) DNS, assuming a common parent usable with traditional (non-extended) DNS, assuming a common parent
domain name, and given that implicit referral response truncation is domain name, and given that implicit referral response truncation is
undesirable in the average case. undesirable in the average case.
More than one address record in a protocol family per server is More than one address record in a protocol family per server is
inadvisable since the necessary glue RRsets (A or AAAA) are inadvisable since the necessary glue RRsets (A or AAAA) are
atomically indivisible, and will be larger than a single resource atomically indivisible, and will be larger than a single resource
record. Larger RRsets are more likely to lead to or encounter record. Larger RRsets are more likely to lead to or encounter
truncation. truncation.
More than one address record across protocol families is less likely More than one address record across protocol families is less likely
to lead to or encounter truncation, partly because multiprotocol to lead to or encounter truncation, partly because multiprotocol
clients, which are required to handle larger RRsets such as AAAA RRs, 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 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 size limit, and partly because the resource records (A and AAAA) are
in different RRsets and are therefore divisible from each other. in different RRsets and are therefore divisible from each other.
Name server names which are at or below the zone they serve are more Name server names that are at or below the zone they serve are more
sensitive to referral response truncation, and glue records for them sensitive to referral response truncation, and glue records for them
should be considered "more important" than other glue records, in the should be considered "more important" than other glue records, in the
assembly of referral responses. assembly of referral responses.
5. Security Considerations 5. Security Considerations
The recommendations contained in this document have no known security The recommendations contained in this document have no known security
implications. implications.
6. IANA Considerations 6. IANA Considerations
This document does not call for changes or additions to any IANA This document does not call for changes or additions to any IANA
registry. registry.
7. Acknowledgement 7. Acknowledgement
The authors thank Peter Koch, Rob Austein, Joe Abley, Mark Andrews, The authors thank Peter Koch, Rob Austein, Joe Abley, Mark Andrews,
Kenji Rikitake, Stephane Bortzmeyer, Olafur Gudmundsson, Alfred Kenji Rikitake, Stephane Bortzmeyer, Olafur Gudmundsson, Alfred
Hoenes, and Alexander Mayrhofer for their valuable comments and Hoenes, Alexander Mayrhofer, and Ray Bellis for their valuable
suggestions. comments and suggestions.
This work was supported by the US National Science Foundation This work was supported by the US National Science Foundation
(research grant SCI-0427144) and DNS-OARC. (research grant SCI-0427144) and DNS-OARC.
8. References 8. References
8.1. Normative References 8.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987. STD 13, RFC 1034, November 1987.
skipping to change at page 11, line 13 skipping to change at page 11, line 13
[RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, [RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)", "Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, April 1997. RFC 2136, April 1997.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, March 1998. NCACHE)", RFC 2308, March 1998.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999. 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", [RFC2672] Crawford, M., "Non-Terminal DNS Name Redirection",
RFC 2672, August 1999. RFC 2672, August 1999.
[RFC3226] Gudmundsson, O., "DNSSEC and IPv6 A6 aware server/resolver [RFC3226] Gudmundsson, O., "DNSSEC and IPv6 A6 aware server/resolver
message size requirements", RFC 3226, December 2001. message size requirements", RFC 3226, December 2001.
[RFC3258] Hardie, T., "Distributing Authoritative Name Servers via [RFC3258] Hardie, T., "Distributing Authoritative Name Servers via
Shared Unicast Addresses", RFC 3258, April 2002. Shared Unicast Addresses", RFC 3258, April 2002.
[RFC3901] Durand, A. and J. Ihren, "DNS IPv6 Transport Operational [RFC3901] Durand, A. and J. Ihren, "DNS IPv6 Transport Operational
Guidelines", BCP 91, RFC 3901, September 2004. Guidelines", BCP 91, RFC 3901, September 2004.
[RFC4472] Durand, A., Ihren, J., and P. Savola, "Operational [RFC4472] Durand, A., Ihren, J., and P. Savola, "Operational
Considerations and Issues with IPv6 DNS", RFC 4472, Considerations and Issues with IPv6 DNS", RFC 4472,
April 2006. 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 Appendix A. The response simulator program
#!/usr/bin/perl #!/usr/bin/perl
# #
# SYNOPSIS # SYNOPSIS
# respsize.pl [ -z zone ] fqdn_ns1 fqdn_ns2 ... # respsize.pl [ -z zone ] fqdn_ns1 fqdn_ns2 ...
# if all queries are assumed to have a same zone suffix, # if all queries are assumed to have a same zone suffix,
# such as "jp" in JP TLD servers, specify it in -z option # such as "jp" in JP TLD servers, specify it in -z option
# #
use strict; use strict;
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