< draft-ietf-dnsop-nsec-aggressiveuse-05.txt   draft-ietf-dnsop-nsec-aggressiveuse-06.txt >
Network Working Group K. Fujiwara Network Working Group K. Fujiwara
Internet-Draft JPRS Internet-Draft JPRS
Updates: 4035 (if approved) A. Kato Updates: 4035 (if approved) A. Kato
Intended status: Standards Track Keio/WIDE Intended status: Standards Track Keio/WIDE
Expires: April 23, 2017 W. Kumari Expires: May 20, 2017 W. Kumari
Google Google
October 20, 2016 November 16, 2016
Aggressive use of NSEC/NSEC3 Aggressive use of NSEC/NSEC3
draft-ietf-dnsop-nsec-aggressiveuse-05 draft-ietf-dnsop-nsec-aggressiveuse-06
Abstract Abstract
The DNS relies upon caching to scale; however, the cache lookup The DNS relies upon caching to scale; however, the cache lookup
generally requires an exact match. This document specifies the use generally requires an exact match. This document specifies the use
of NSEC/NSEC3 resource records to allow DNSSEC validating resolvers of NSEC/NSEC3 resource records to allow DNSSEC validating resolvers
to generate negative answers within a range, and positive answers to generate negative answers within a range, and positive answers
from wildcards. This increases performance / decreases latency, from wildcards. This increases performance / decreases latency,
decreases resource utilization on both authoritative and recursive decreases resource utilization on both authoritative and recursive
servers, and also increases privacy. It may also help increase servers, and also increases privacy. It may also help increase
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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 April 23, 2017. This Internet-Draft will expire on May 20, 2017.
Copyright Notice Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2016 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
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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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
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3 3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
4. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Aggressive Negative Caching . . . . . . . . . . . . . . . . . 5 5. Aggressive use of Cache . . . . . . . . . . . . . . . . . . . 5
5.1. NSEC . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5.1. NSEC . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. NSEC3 . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5.2. NSEC3 . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.3. Wildcards . . . . . . . . . . . . . . . . . . . . . . . . 7 5.3. Wildcards . . . . . . . . . . . . . . . . . . . . . . . . 6
5.4. Consideration on TTL . . . . . . . . . . . . . . . . . . 7 5.4. Consideration on TTL . . . . . . . . . . . . . . . . . . 7
6. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . 7 6. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Update to RFC 4035 . . . . . . . . . . . . . . . . . . . . . 8 7. Update to RFC 4035 . . . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 9 9. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10. Implementation Status . . . . . . . . . . . . . . . . . . . . 9 10. Implementation Status . . . . . . . . . . . . . . . . . . . . 9
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
11.1. Change History . . . . . . . . . . . . . . . . . . . . . 10 11.1. Change History . . . . . . . . . . . . . . . . . . . . . 10
11.1.1. Version draft-fujiwara-dnsop-nsec-aggressiveuse-01 . 12 11.1.1. Version draft-fujiwara-dnsop-nsec-aggressiveuse-01 . 12
11.1.2. Version draft-fujiwara-dnsop-nsec-aggressiveuse-02 . 13 11.1.2. Version draft-fujiwara-dnsop-nsec-aggressiveuse-02 . 13
11.1.3. Version draft-fujiwara-dnsop-nsec-aggressiveuse-03 . 13 11.1.3. Version draft-fujiwara-dnsop-nsec-aggressiveuse-03 . 13
11.2. new section . . . . . . . . . . . . . . . . . . . . . . 13 11.2. new section . . . . . . . . . . . . . . . . . . . . . . 13
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
12.1. Normative References . . . . . . . . . . . . . . . . . . 13 12.1. Normative References . . . . . . . . . . . . . . . . . . 13
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information they have in the cache. This allows validating resolvers information they have in the cache. This allows validating resolvers
to respond with NXDOMAIN immediately if the name in question falls to respond with NXDOMAIN immediately if the name in question falls
into a range expressed by a NSEC/NSEC3 resource record already in the into a range expressed by a NSEC/NSEC3 resource record already in the
cache. It also allows the synthesis of positive answers in the cache. It also allows the synthesis of positive answers in the
presence of wildcard records. presence of wildcard records.
Aggressive Negative Caching was first proposed in Section 6 of DNSSEC Aggressive Negative Caching was first proposed in Section 6 of DNSSEC
Lookaside Validation (DLV) [RFC5074] in order to find covering NSEC Lookaside Validation (DLV) [RFC5074] in order to find covering NSEC
records efficiently. records efficiently.
Section 3 of [I-D.vixie-dnsext-resimprove] "Stopping Downward Cache [RFC8020] proposes a first step to using NXDOMAIN information for
Search on NXDOMAIN" and [I-D.ietf-dnsop-nxdomain-cut] proposed more effective caching. This takes this technique further.
another approach to use NXDOMAIN information effectively.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
Many of the specialized terms used in this document are defined in Many of the specialized terms used in this document are defined in
DNS Terminology [RFC7719]. DNS Terminology [RFC7719].
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3. Problem Statement 3. Problem Statement
The DNS negative cache caches negative (non-existent) information, The DNS negative cache caches negative (non-existent) information,
and requires an exact match in most instances [RFC2308]. and requires an exact match in most instances [RFC2308].
Assume that the (DNSSEC signed) "example.com" zone contains: Assume that the (DNSSEC signed) "example.com" zone contains:
albatross.example.com IN A 192.0.2.1 albatross.example.com IN A 192.0.2.1
elephant.example.com IN A 192.0.2.2 elephant.example.com IN A 192.0.2.2
zebra.example.com IN A 192.0.2.3 zebra.example.com IN A 192.0.2.3
If a validating resolver receives a query for cat.example.com, it If a validating resolver receives a query for cat.example.com, it
contacts its resolver (which may be itself) to query the example.com contacts its resolver (which may be itself) to query the example.com
servers and will get back an NSEC record starting that there are no servers and will get back an NSEC record starting that there are no
records (alphabetically) between albatross and elephant, or an NSEC3 records (alphabetically) between albatross and elephant, or an NSEC3
record stating there is nothing between two hashed names. The record stating there is nothing between two hashed names. The
resolver then knows that cat.example.com does not exist; however, it resolver then knows that cat.example.com does not exist; however, it
does not use the fact that the proof covers a range (albatross to does not use the fact that the proof covers a range (albatross to
elephant) to suppress queries for other labels that fall within this elephant) to suppress queries for other labels that fall within this
range. This means that if the validating resolver gets a query for range. This means that if the validating resolver gets a query for
ball.example.com (or dog.example.com) it will once again go off and ball.example.com (or dog.example.com) it will once again go off and
query the example.com servers for these names. query the example.com servers for these names.
Apart from wasting bandwidth, this also wastes resources on the
recursive server (it needs to keep state for outstanding queries),
wastes resources on the authoritative server (it has to answer
additional questions), increases latency (the end user has to wait
longer than necessary to get back an NXDOMAIN answer), can be used by
attackers to cause a DoS (see additional resources), and also has
privacy implications (e.g: typos leak out further than necessary).
Now, assume that the (DNSSEC signed) "example.org" zone contains: Now, assume that the (DNSSEC signed) "example.org" zone contains:
avocado.example.org IN A 192.0.2.1 avocado.example.org IN A 192.0.2.1
*.example.org IN A 192.0.2.2 *.example.org IN A 192.0.2.2
zucchini.example.org IN A 192.0.2.3 zucchini.example.org IN A 192.0.2.3
If a query is received for leek.example.org, it contacts its resolver If a query is received for leek.example.org, it contacts its resolver
(which may be itself) to query the example.org servers and will get (which may be itself) to query the example.org servers and will get
back an NSEC record stating that there are no records back an NSEC record stating that there are no records
(alphabetically) between avocado and zucchini (or an NSEC3 record (alphabetically) between avocado and zucchini (or an NSEC3 record
stating there is nothing between two hashed names), as well as an stating there is nothing between two hashed names), as well as an
answer for leek.example.org, with the label count of the signature answer for leek.example.org, with the label count of the signature
set to two (see [RFC7129], section 5.3 for more details). set to two (see [RFC7129], section 5.3 for more details).
Apart from wasting bandwidth, this also wastes resources on the If the validating resolver gets a query for banana.example.org it
recursive server (it needs to keep state for outstanding queries), will once again go off and query the example.com servers for
wastes resources on the authoritative server (it has to answer banana.example.com (even though it already has proof that there is a
additional questions), increases latency (the end user has to wait wildcard record) - just like above, this has privacy implications,
longer than necessary to get back an NXDOMAIN answer), can be used by wastes resources, can be used to contribute to a DoS, etc.
attackers to cause a DoS (see additional resources), and also has
privacy implications (e.g: typos leak out further than necessary).
4. Background 4. Background
DNSSEC [RFC4035] and [RFC5155] both provide "authenticated denial of DNSSEC [RFC4035] and [RFC5155] both provide "authenticated denial of
existence"; this is a cryptographic proof that the queried for name existence"; this is a cryptographic proof that the queried for name
does not exist, accomplished by providing a (DNSSEC secured) record does not exist, accomplished by providing a (DNSSEC secured) record
containing the names which appear alphabetically before and after the containing the names which appear alphabetically before and after the
queried for name. In the first example above, if the (DNSSEC queried for name. In the first example above, if the (DNSSEC
validating) recursive server were to query for dog.example.com it validating) recursive server were to query for dog.example.com it
would receive a (signed) NSEC record stating that there are no labels would receive a (signed) NSEC record stating that there are no labels
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We believe this recommendation can be relaxed because, in the absense We believe this recommendation can be relaxed because, in the absense
of this technique, a lookup for the exact name could have come in of this technique, a lookup for the exact name could have come in
during this interval, and so a negative answer could already be during this interval, and so a negative answer could already be
cached (see [RFC2308] for more background). This means that zone cached (see [RFC2308] for more background). This means that zone
operators should have no expectation that an added name would work operators should have no expectation that an added name would work
immediately. With DNSSEC and Aggressive NSEC, the TTL of the NSEC immediately. With DNSSEC and Aggressive NSEC, the TTL of the NSEC
record is the authoritative statement of how quickly a name can start record is the authoritative statement of how quickly a name can start
working within a zone. working within a zone.
5. Aggressive Negative Caching 5. Aggressive use of Cache
Section 4.5 of [RFC4035] says that "In theory, a resolver could use Section 4.5 of [RFC4035] says that "In theory, a resolver could use
wildcards or NSEC RRs to generate positive and negative responses wildcards or NSEC RRs to generate positive and negative responses
(respectively) until the TTL or signatures on the records in question (respectively) until the TTL or signatures on the records in question
expire. However, it seems prudent for resolvers to avoid blocking expire. However, it seems prudent for resolvers to avoid blocking
new authoritative data or synthesizing new data on their own. new authoritative data or synthesizing new data on their own.
Resolvers that follow this recommendation will have a more consistent
view of the namespace".
This document relaxes this this restriction, as follows: Resolvers that follow this recommendation will have a more consistent
view of the namespace". This document relaxes this this restriction,
+--------------------------------------------------------------+ see Section 7 for more detail.
| Once the records are validated, DNSSEC enabled validating |
| resolvers MAY use wildcards and NSEC/NSEC3 resource records |
| to generate positive and negative responses until the |
| effective TTLs or signatures for those records expire. |
+--------------------------------------------------------------+
If the negative cache of the validating resolver has sufficient If the negative cache of the validating resolver has sufficient
information to validate the query, the resolver SHOULD use NSEC, information to validate the query, the resolver SHOULD use NSEC,
NSEC3 and wildcard records aggressively. Otherwise, it MUST fall NSEC3 and wildcard records aggressively. Otherwise, it MUST fall
back to send the query to the authoritative DNS servers. back to send the query to the authoritative DNS servers.
It is recommended that resolvers that implement Aggressive Negative It is recommended that resolvers that implement Aggressive Negative
Caching provide a configuration switch to disable the feature. Caching provide a configuration switch to disable the feature.
Separate configuration switches may be implemented for the aggressive Separate configuration switches may be implemented for the aggressive
use of NSEC, NSEC3 and wildcard records, and it is recommended to use of NSEC, NSEC3 and wildcard records, and it is recommended to
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validating resolvers limit the maximum effective TTL value of validating resolvers limit the maximum effective TTL value of
negative responses (NSEC/NSEC3 RRs) to this same value. negative responses (NSEC/NSEC3 RRs) to this same value.
Section 5 of [RFC2308]also states that a negative cache entry TTL is Section 5 of [RFC2308]also states that a negative cache entry TTL is
taken from the minimum of the SOA.MINIMUM field and SOA's TTL. This taken from the minimum of the SOA.MINIMUM field and SOA's TTL. This
can be less than the TTL of an NSEC or NSEC3 record, since their TTL can be less than the TTL of an NSEC or NSEC3 record, since their TTL
is equal to the SOA.MINIMUM field (see [RFC4035]section 2.3 and is equal to the SOA.MINIMUM field (see [RFC4035]section 2.3 and
[RFC5155] section 3.) [RFC5155] section 3.)
A resolver that supports aggressive use of NSEC and NSEC3 should A resolver that supports aggressive use of NSEC and NSEC3 should
reduce the TTL of NSEC and NSEC3 records to match the TTL of the SOA reduce the TTL of NSEC and NSEC3 records to match the SOA.MINIMUM
record in the authority section of a negative response, if the SOA field in the authority section of a negative response, if SOA.MINIMUM
TTL is smaller. is smaller.
6. Benefits 6. Benefits
The techniques described in this document provide a number of The techniques described in this document provide a number of
benefits, including (in no specific order): benefits, including (in no specific order):
Reduced latency: By answering directly from cache, validating Reduced latency: By answering directly from cache, validating
resolvers can immediately inform clients that the name they are resolvers can immediately inform clients that the name they are
looking for does not exist, improving the user experience. looking for does not exist, improving the user experience.
Decreased recursive server load: By answering negative queries from Decreased recursive server load: By answering queries from the cache
the cache, validating servers avoid having to send a query and by synthesizing answers, validating servers avoid having to send a
wait for a response. In addition to decreasing the bandwidth query and wait for a response. In addition to decreasing the
used, it also means that the server does not need to allocate and bandwidth used, it also means that the server does not need to
maintain state, thereby decreasing memory and CPU load. allocate and maintain state, thereby decreasing memory and CPU
load.
Decreased authorative server load: Because recursive servers can Decreased authorative server load: Because recursive servers can
answer (negative) queries without asking the authoritative server, answer queries without asking the authoritative server, the
the authoritative servers receive fewer queries. This decreases authoritative servers receive fewer queries. This decreases the
the authoritative server bandwidth, queries per second and CPU authoritative server bandwidth, queries per second and CPU
utilization. utilization.
The scale of the benefit depends upon multiple factors, including the The scale of the benefit depends upon multiple factors, including the
query distribution. For example, at the time of this writing, around query distribution. For example, at the time of this writing, around
65% of queries to Root Name servers result in NXDOMAIN responses (see 65% of queries to Root Name servers result in NXDOMAIN responses (see
statistics from [root-servers.org]); this technique will eliminate a statistics from [root-servers.org]); this technique will eliminate a
sizable quantity of these. sizable quantity of these.
The technique described in this document may also mitigate so-called The technique described in this document may also mitigate so-called
"random QNAME attacks", in which attackers send many queries for "random QNAME attacks", in which attackers send many queries for
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it is inappropriate for inclusion in a published RFC." ] it is inappropriate for inclusion in a published RFC." ]
Unbound currently implements aggressive negative caching, as does Unbound currently implements aggressive negative caching, as does
Google Public DNS. Google Public DNS.
11. Acknowledgments 11. Acknowledgments
The authors gratefully acknowledge DLV [RFC5074] author Samuel Weiler The authors gratefully acknowledge DLV [RFC5074] author Samuel Weiler
and the Unbound developers. and the Unbound developers.
The authors would like to specifically thank Stephane Bortzmeyer, The authors would like to specifically thank Stephane Bortzmeyer (for
Tony Finch, Tatuya JINMEI for extensive review and comments, and also standing next to and helping edit), Tony Finch, Tatuya JINMEI for
Mark Andrews, Casey Deccio, Alexander Dupuy, Olafur Gudmundsson, Bob extensive review and comments, and also Mark Andrews, Casey Deccio,
Harold, Shumon Huque, John Levine, Pieter Lexis and Matthijs Mekking Alexander Dupuy, Olafur Gudmundsson, Bob Harold, Shumon Huque, John
(who even sent pull requests!). Mark Andrews also provided the text Levine, Pieter Lexis and Matthijs Mekking (who even sent pull
requests!). Mark Andrews also provided the text
(https://www.ietf.org/mail-archive/web/dnsop/current/msg18332.html) (https://www.ietf.org/mail-archive/web/dnsop/current/msg18332.html)
which we made into Appendix B which we made into Appendix B.
11.1. Change History 11.1. Change History
RFC Editor: Please remove this section prior to publication. RFC Editor: Please remove this section prior to publication.
-05 to -06:
o Moved some dangling text around - when the examples were added
some text added in the wrong place.
o There were some bits which mentioned "negative" in the title.
o We had the cut-and-paste of what changed in 4035 twice.
-04 to -05: -04 to -05:
o Bob pointed out that I did a stupid - when I added the wildcard to o Bob pointed out that I did a stupid - when I added the wildcard to
'example.com' I made the example wrong / confusing. I have 'example.com' I made the example wrong / confusing. I have
attempted to fix this by adding a second example zone attempted to fix this by adding a second example zone
(example.org) with the wildcard instead. (example.org) with the wildcard instead.
o More helpful changes (in a pull request, thanks!) from Matthijs o More helpful changes (in a pull request, thanks!) from Matthijs
o Included Mark Andrew's useful explanation of how to tell ENT from o Included Mark Andrew's useful explanation of how to tell ENT from
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answers. answers.
* Reworded much of the Wildcard text. * Reworded much of the Wildcard text.
o Incorporated pull request from Tony Finch (thanks!): o Incorporated pull request from Tony Finch (thanks!):
https://github.com/wkumari/draft-ietf-dnsop-nsec-aggressiveuse/ https://github.com/wkumari/draft-ietf-dnsop-nsec-aggressiveuse/
pull/1 pull/1
o More fixups from Tony (including text): https://www.ietf.org/mail- o More fixups from Tony (including text): https://www.ietf.org/mail-
archive/web/dnsop/current/msg18271.html. This included much archive/web/dnsop/current/msg18271.html. This included much
clearer text on TTL, refernces to the NSEC / NSEC3 RFCs (instead clearer text on TTL, references to the NSEC / NSEC3 RFCs (instead
of my clumsy summary), good text on replays, etc. of my clumsy summary), good text on replays, etc.
o Converted the "zone file" to a figure to make it more readable. o Converted the "zone file" to a figure to make it more readable.
o Text from Tim W: "If a validating resolver receives a query for o Text from Tim W: "If a validating resolver receives a query for
cat.example.com, it contacts its resolver (which may be itself) to cat.example.com, it contacts its resolver (which may be itself) to
query..." - which satisfies Jinmei's concern (which I was too query..." - which satisfies Jinmei's concern (which I was too
dense to grock). dense to grock).
o Fixup of the "validation required" in security considerations. o Fixup of the "validation required" in security considerations.
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Bortzmeyer, S. and S. Huque, "NXDOMAIN really means there Bortzmeyer, S. and S. Huque, "NXDOMAIN really means there
is nothing underneath", draft-ietf-dnsop-nxdomain-cut-03 is nothing underneath", draft-ietf-dnsop-nxdomain-cut-03
(work in progress), May 2016. (work in progress), May 2016.
[I-D.vixie-dnsext-resimprove] [I-D.vixie-dnsext-resimprove]
Vixie, P., Joffe, R., and F. Neves, "Improvements to DNS Vixie, P., Joffe, R., and F. Neves, "Improvements to DNS
Resolvers for Resiliency, Robustness, and Responsiveness", Resolvers for Resiliency, Robustness, and Responsiveness",
draft-vixie-dnsext-resimprove-00 (work in progress), June draft-vixie-dnsext-resimprove-00 (work in progress), June
2010. 2010.
[RFC8020] Bortzmeyer, S. and S. Huque, "NXDOMAIN: There Really Is
Nothing Underneath", RFC 8020, DOI 10.17487/RFC8020,
November 2016, <http://www.rfc-editor.org/info/rfc8020>.
[root-servers.org] [root-servers.org]
IANA, "Root Server Technical Operations Assn", IANA, "Root Server Technical Operations Assn",
<http://www.root-servers.org/>. <http://www.root-servers.org/>.
Appendix A. Detailed implementation notes Appendix A. Detailed implementation notes
o Previously, cached negative responses were indexed by QNAME, o Previously, cached negative responses were indexed by QNAME,
QCLASS, QTYPE, and the setting of the CD bit (see RFC 4035, QCLASS, QTYPE, and the setting of the CD bit (see RFC 4035,
Section 4.7), and only queries matching the index key would be Section 4.7), and only queries matching the index key would be
answered from the cache. With aggressive negative caching, the answered from the cache. With aggressive negative caching, the
 End of changes. 22 change blocks. 
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