dnsop W. Kumari
Internet-Draft Google
Intended status: Standards Track Z. Yan
Expires: July 15, 2015 CNNIC
W. Hardaker
Parsons, Inc.
January 11, 2015
Returning multiple answers in a DNS response.
draft-wkumari-dnsop-multiple-responses-00
Abstract
This document (re)introduces the ability to provide multiple answers
in a DNS response.
Status of This Memo
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This Internet-Draft will expire on July 15, 2015.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Returning multiple answers . . . . . . . . . . . . . . . . . 3
5. Additional records pseudo-RR . . . . . . . . . . . . . . . . 4
6. Signalling support . . . . . . . . . . . . . . . . . . . . . 5
7. Use of Additional information . . . . . . . . . . . . . . . . 6
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
9. Security Considerations . . . . . . . . . . . . . . . . . . . 6
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
11.1. Normative References . . . . . . . . . . . . . . . . . . 7
11.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
Often the name being resolved in the DNS provides information about
why the name is being resolved, allowing the authoritative name
server operator to predict what other answers the client will soon
query for. By providing multiple answers in the response, the
authoritative name server operator can ensure that the recursive
server that the client is using has all the answers in its cache.
For example, the name server operator of Example Widgets, Inc
(example.com) knows that the example.com web page at www.example.com
contains various resources, including some images (served from
images.example.com), some Cascading Style Sheets (served from
css.example.com) and some JavaScript (data.example.com). A client
attempting to resolve www.example.com is very likely to be a web
browser, rendering the page, and so will need to also resolve all of
the other names for these other resources. Providing all of these
answers in response to a query for www.example.com allows the
recursive server to populate its cache and have all of the answers
available when the client asks for them.
Other examples where this technique is useful include SMTP (including
the mail server address when serving the MX record), SRV (providing
the target information in addition to the SRV response) and TLSA
(providing any TLSA records associated with a name).
This is purely an optimization - by providing all of other, related
answers that the client is likely to need along with the answer that
they requested, users get a better experience, iterative servers need
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to perform less queries, authoritative servers have to answer fewer
queries, etc.
1.1. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Background
The existing DNS specifications allow for additional information to
be included in the "additional" section of the DNS response, but in
order to defeat cache poisoning attacks most implementations either
ignore or don't trust additional information (other than for "glue").
For some more background, see [Ref.Bellovin], [RFC1034], [RFC2181].
Not trusting the information in the additional section was necessary
because there was no way to authenticate it. If you queried for
www.example.com and got back answers for www.invalid.com you couldn't
tell if these were actually from invalid.com or if an attacker was
trying to get bad information for invalid.com into your cache. In a
world of ubiquitous DNSSEC deployment [Ed note: By the time this
document is published, there *will* be ubiquitous DNSSEC :-) ] the
iterative server can validate the information and trust it.
3. Terminology
Additional records Additional records are records that the
authoritative nameserver has included in the Additional section.
Primary query A Primary query (or primary question) is a QNAME that
the name server operator would like to return additional answers
for.
Supporting information Supporting information is the DNSSEC RRSIGs
that prove the authenticity of the Additional records.
4. Returning multiple answers
The authoritative nameserver should include as many of the instructed
Additional records and Supporting information as will fit in the
response packet.
In order to include Additional records in a response, certain
conditions need to be met. [Ed note: Some discussion on each rule is
below]
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1. Additional records MUST only be included when the primary name is
DNSSEC secured.
2. Additional records MUST only be served over TCP connections.
This is to mitigate Denial of Service reflection attacks.[1]
3. Additional records MUST be leaf records at the same node in the
DNS tree[2]
4. The DNSSEC supporting information must be included. This is the
RRSIGs required to validate the Additional record information.
5. All of the records MUST be signed with the same DNSSEC keys.
6. The authoritative nameserver SHOULD include as many of the
additional records as will fit in the response. Each Additional
record MUST have its matching Supporting information. Additional
records MUST be inserted in the order specified in the Additional
records list.
7. Operators SHOULD only include Additional answers that they expect
a client to actually need. [3]
[Ed note 1: The above MAY be troll bait. I'm not really sure if this
is a good idea or not - moving folk towards TCP is probably a good
idea, and this is somewhat of an optional record type. Then again,
special handing (TCP only) for a record would be unusual. Additional
records could cause responses to become really large, but there are
already enough large records that can be used for reflection attacks
that we can just give up on the whole "keep responses as small as
possible" ship. ]
[Ed note 2: This is poorly worded. I mumbled about bailiwick,
subdomains, etc but nothing I could come up with was better. Also,
is this rule actually needed? I *think* it would be bad for .com
servers to be able to include Additional records for
www.foo.bar.baz.example.com, but perhaps <handwave>public-suffix-
list?! This rule also makes it easier to decide what all DNSSEC
information is required.]
[Ed note 3: This is not enforceable. ]
5. Additional records pseudo-RR
To allow the authoritative nameserver operator to configure what
additional records to serve when it receives a query to a label, we
introduce the Additional pseudo Resource Record (RR). This is a
pseudo-record as it provides instruction to the authoritative
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nameserver, and does not appear on the wire. [Ed note: I had
originally considered a comment, or some sort of format where we
listed additional records under the primary one, but we a: wanted it
to survive zone transfers, and b: not trip up zone file parsers. ]
The format of the Additional pseudo-RR is:
label ADD "label,typel; label,type; label,type; ..."
For example, if the operator of example.com would like to also return
A record answers for images.example.com, css.example.com and both an
A and AAAA for data.example.com when queried for www.example.com he
would enter:
www ADD "images,A; css,A; data,A; data,AAA;"
The entries in the ADD list are ordered. An authoritative nameserver
MUST attempt to insert the records in the order listed when filling
the response packet. This is to allow the operator to express a
preference in case all the records to not fit. The TTL of the
records added to the Additional section are the same as if queried
directly.
In some cases the operator might not really know what all additional
records clients need. For example, the owner of www.example.com may
have outsourced his DNS operations to a third party. The DNS
operator may be able to mine their query logs, and see that, in a
large majority of cases, a recursive server asks for foo.example.com
and then very soon after asks for bar.example.com, and so may decide
to optimize this by opportunistically returning bar when queried for
foo. This functionality could also be included in the authoritative
name server software itself, but discussions of these re outside the
scope of this document.
6. Signalling support
Iterative nameservers that support Additional records signal this by
setting the Z bit (bit 25 of the DNS header).
[RFC5395] Section 2.1 says:
There have been ancient DNS implementations for which the Z bit
being on in a query meant that only a response from the primary
server for a zone is acceptable. It is believed that current
DNS implementations ignore this bit.
Assigning a meaning to the Z bit requires an IETF Standards Action.
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[ Ed note: Hey, was worth a try :-) I'm fine with an EDNS0 bit
instead... ]
7. Use of Additional information
When receiving Additional information, an iterative server follows
certain rules:
1. Additional records MUST be validated before being used.
2. Additional records SHOULD be annotated in the cache as having
been received as Additional records.
3. Additional records SHOULD have lower priority in the cache than
answers received because they were requested. This is to help
evict Additional records from the cache first, and help stop
cache filling attacks.
4. Iterative servers MAY choose to ignore Additional records for any
reason, including CPU or cache space concerns, phase of the moon,
etc. It may choose to only accept all, some or none of the
Additional records.
8. IANA Considerations
This document contains no IANA considerations.Template: Fill this in!
9. Security Considerations
Additional records will make DNS responses even larger than they are
currently, leading to more large records that can be used for DNS
reflection attacks. We mitigate this by only serving these over TCP.
A malicious authorative server could include a large number of
Additional records (and associated DNSSEC information) and attempt to
DoS the recursive by making it do lots of DNSSEC validation. I don't
view this as a very serious threat (CPU for validation is cheap
compared to bandwith), but we mitigate this by allowing the iterative
to ignore Additional records whenever it wants.
By requiring the ALL of the Additional records are signed, and all
necessary DNSSEC information for validation be included we avoid
cache poisoning (I hope :-))
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10. Acknowledgements
The authors wish to thank some folk.
11. References
11.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC5395] Eastlake, D., "Domain Name System (DNS) IANA
Considerations", RFC 5395, November 2008.
[Ref.Bellovin]
Bellovin, S., "Using the Domain Name System for System
Break-Ins", 1995, <https://www.cs.columbia.edu/~smb/
papers/dnshack.pdf>.
11.2. Informative References
[I-D.ietf-sidr-iana-objects]
Manderson, T., Vegoda, L., and S. Kent, "RPKI Objects
issued by IANA", draft-ietf-sidr-iana-objects-03 (work in
progress), May 2011.
Appendix A. Changes / Author Notes.
[RFC Editor: Please remove this section before publication ]
From -00 to -01.
o Nothing changed in the template!
Authors' Addresses
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Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: warren@kumari.net
Zhiwei Yan
CNNIC
No.4 South 4th Street, Zhongguancun
Beijing 100190
P. R. China
Email: yanzhiwei@cnnic.cn
Wes Hardaker
Parsons, Inc.
P.O. Box 382
Davis, CA 95617
US
Email: ietf@hardakers.net
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