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draft-ietf-dnsop-dns-tcp-requirements
Domain Name System Operations J. Kristoff
Internet-Draft DePaul University
Intended status: Best Current Practice August 25, 2016
Expires: February 26, 2017
DNS Transport over TCP - Operational Requirements
draft-kristoff-dnsop-dns-tcp-requirements-01
Abstract
This document encourages the practice of permitting DNS messages to
be carried over TCP on the Internet. It also describes some of the
consequences of this behavior and the potential operational issues
that can arise when this best common practice is not applied.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 26, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1. Uneven Transport Usage and Preference . . . . . . . . . . 2
2.2. Waiting for Large Messages and Reliability . . . . . . . 3
2.3. EDNS0 . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. DNS over TCP Requirements . . . . . . . . . . . . . . . . . . 4
4. DNS over TCP Filtering Risks . . . . . . . . . . . . . . . . 5
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
DNS messages may be delivered using UDP or TCP communications. While
most DNS transactions are carried over UDP, some operators have been
led to believe that any DNS over TCP traffic is unwanted or
unnecessary for general DNS operation. As usage and features have
evolved, TCP transport has become increasingly important for correct
and safe operation of the Internet DNS. Reflecting modern usage, the
DNS standards were recently updated to declare support for TCP is now
a required part of the DNS implementation specifications in
[RFC7766]. This document is the formal requirements equivalent for
the operational community, encouraging operators to ensure DNS over
TCP communications support in on par with DNS over UDP
communications.
1.1. Requirements Language
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 RFC 2119 [RFC2119].
2. Background
2.1. Uneven Transport Usage and Preference
In the original suite of DNS specifications, [RFC1034] and [RFC1035]
clearly specified that DNS messages could be carried in either UDP or
TCP, but they also made clear a preference for UDP as the transport
for queries in the general case. As stated in [RFC1035]:
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"While virtual circuits can be used for any DNS activity,
datagrams are preferred for queries due to their lower overhead
and better performance."
Another early, important, and influential document, [RFC1123],
detailed the preference for UDP more explicitly:
"DNS resolvers and recursive servers MUST support UDP, and SHOULD
support TCP, for sending (non-zone-transfer) queries."
and further stipulated:
A name server MAY limit the resources it devotes to TCP queries,
but it SHOULD NOT refuse to service a TCP query just because it
would have succeeded with UDP.
Culminating in [RFC1536], DNS over TCP came to be associated
primarily with the zone transfer mechanism, while most DNS queries
and responses were seen as the dominion of UDP.
2.2. Waiting for Large Messages and Reliability
As stipulated in the original specifications, DNS messages over UDP
were restricted to a 512-byte message size. However, even while
[RFC1123] made a clear preference for UDP, it foresaw DNS over TCP
becoming more popular in the future:
"[...] it is also clear that some new DNS record types defined in
the future will contain information exceeding the 512 byte limit
that applies to UDP, and hence will require TCP.
At least two new, widely anticipated developments were set to elevate
the need for DNS over TCP transactions. The first was dynamic
updates defined in [RFC2136] and the second was the set of extensions
collectively known as DNSSEC originally specified in [RFC2541]. The
former suggested "requestors who require an accurate response code
must use TCP", while the later warned "[...] larger keys increase the
size of KEY and SIG RRs. This increases the chance of DNS UDP packet
overflow and the possible necessity for using higher overhead TCP in
responses."
Yet defying some expectations, DNS over TCP remained little used in
real traffic across the Internet. Dynamic updates saw little
deployment between autonomous networks. Around the time DNSSEC was
first defined, another new feature affecting DNS over UDP helped
solidify its dominance for message transactions.
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2.3. EDNS0
In 1999 the IETF published the Extension Mechanisms for DNS (EDNS0)
in [RFC2671]. This document standardized a way for communicating DNS
nodes to perform rudimentary capabilities negotiation. One such
capability written into the base specification and present in every
ENDS0 compatible message is the value of the maximum UDP payload size
the sender can support. This unsigned 16-bit field specifies in
bytes the maximum DNS MTU. In practice, typical values are a subset
of the 512 to 4096 byte range. EDNS0 was rapidly and widely deployed
over the next several years and numerous surveys have shown many
systems currently support larger UDP MTUs [CASTRO2010], [NETALYZR]
with EDNS0.
The natural effect of EDNS0 deployment meant large DNS messages would
be less reliant on TCP than they might otherwise have been. While a
nonneglible population of DNS systems lack EDNS0 or may still fall
back to TCP for some transactions, DNS over TCP transactions remain a
very small fraction of overall DNS traffic [VERISIGN]. Nevertheless,
some average increase in DNS message size, the continued development
of new DNS features and a denial of service mitigation technique (see
Section 7) have suggested that DNS over TCP transactions are as
important to the correct and safe operation of the Internet DNS as
ever, if not more so. Furthermore, there has been serious research
that has suggested connection-oriented DNS transactions may provide
security and privacy advantages over UDP transport [TDNS].
Therefore, it might be desirable for network operators to avoid
artificially inhibiting the potential utility and advances in the DNS
such as these.
3. DNS over TCP Requirements
Even while many in the DNS community expect DNS over TCP transactions
to occur without interference, in practice there has been a long held
belief by some operators, particularly for security-related reasons,
to the contrary [CHES94], [DJBDNS]. A popular meme has also held the
imagination of some that DNS over TCP is only ever used for zone
transfers and is generally unnecessary otherwise, with filtering all
DNS over TCP traffic even described as a best practice. Arguably any
exposed Internet service poses some risk, but these beliefs are often
invalid. DNS over TCP filtering is considered harmful in the general
case. DNS resolver and server operators MUST provide DNS service
over both UDP and TCP transports. Likewise, network operators MUST
allow DNS service over both UDP and TCP transports.
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4. DNS over TCP Filtering Risks
Networks that filter DNS over TCP may inadvertently cause problems
for third party resolvers as experienced by [TOYAMA]. If for
instance a resolver receives a truncated answer from a server, but if
when the resolver resends the query using TCP and the TCP response
never arrives, the resolver will incur the full extent of TCP
retransmissions and time outs.
Networks that filter DNS over TCP risk losing access to significant
or important pieces of the DNS name space. For a variety of reasons
a DNS answer may require a DNS over TCP query. This may include
large message sizes, lack of EDNS0 support, DDoS mitigation
techniques, or perhaps some future capability that is as yet
unforeseen will also demand TCP transport.
Even if any or all particular answers have consistently been returned
successfuly with UDP in the past, this continued behavior cannot be
guaranteed when DNS messages are exchanged between autonomous
systems. Therefore, filtering of DNS over TCP is considered harmful
and contrary to the safe and successful operation of the Internet.
5. Acknowledgements
This document was initially motivated by feedback from students who
pointed out that they were hearing contradictory information about
filtering DNS over TCP messages. Thanks in particular to my teaching
colleague, JPL, who perhaps unknowingly encouraged the initial
research into the differences of what the IETF community has
historically said and did. Thanks to all the NANOG 63 attendees who
provided feedback to an early talk on this subject.
6. IANA Considerations
This memo includes no request to IANA.
7. Security Considerations
Ironically, returning truncated DNS over UDP answers in order to
induce a client query to switch to DNS over TCP has become a common
response to source address spoofed, DNS denial-of-service attacks
[RRL]. Historically, operators have been wary of TCP-based attacks,
but in recent years, UDP-based flooding attacks have proven to be the
most common protocol attack on the DNS. Nevertheless, a high rate of
short-lived DNS transactions over TCP may pose challenges. While
many operators have provided DNS over TCP service for many years
without duress, past experience is no guarantee of future success.
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DNS over TCP is not unlike many other Internet TCP services. TCP
threats and many mitigation strategies have been well documented in
series of documents such as [RFC4953], [RFC4987], [RFC5927], and
[RFC5961].
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
8.2. Informative References
[CASTRO2010]
Castro, S., Zhang, M., John, W., Wessels, D., and k.
claffy, "Understanding and preparing for DNS evolution",
2010.
[CHES94] Cheswick, W. and S. Bellovin, "Firewalls and Internet
Security: Repelling the Wily Hacker", 1994.
[DJBDNS] D.J. Bernstein, "When are TCP queries sent?", 2002,
<https://cr.yp.to/djbdns/tcp.html#why>.
[NETALYZR]
Kreibich, C., Weaver, N., Nechaev, B., and V. Paxson,
"Netalyzr: Illuminating The Edge Network", 2010.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<http://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123,
DOI 10.17487/RFC1123, October 1989,
<http://www.rfc-editor.org/info/rfc1123>.
[RFC1536] Kumar, A., Postel, J., Neuman, C., Danzig, P., and S.
Miller, "Common DNS Implementation Errors and Suggested
Fixes", RFC 1536, DOI 10.17487/RFC1536, October 1993,
<http://www.rfc-editor.org/info/rfc1536>.
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[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, DOI 10.17487/RFC2136, April 1997,
<http://www.rfc-editor.org/info/rfc2136>.
[RFC2541] Eastlake 3rd, D., "DNS Security Operational
Considerations", RFC 2541, DOI 10.17487/RFC2541, March
1999, <http://www.rfc-editor.org/info/rfc2541>.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, DOI 10.17487/RFC2671, August 1999,
<http://www.rfc-editor.org/info/rfc2671>.
[RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks",
RFC 4953, DOI 10.17487/RFC4953, July 2007,
<http://www.rfc-editor.org/info/rfc4953>.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
<http://www.rfc-editor.org/info/rfc4987>.
[RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927,
DOI 10.17487/RFC5927, July 2010,
<http://www.rfc-editor.org/info/rfc5927>.
[RFC5961] Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
Robustness to Blind In-Window Attacks", RFC 5961,
DOI 10.17487/RFC5961, August 2010,
<http://www.rfc-editor.org/info/rfc5961>.
[RFC7766] Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
D. Wessels, "DNS Transport over TCP - Implementation
Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,
<http://www.rfc-editor.org/info/rfc7766>.
[RRL] Vixie, P. and V. Schryver, "DNS Response Rate Limiting
(DNS RRL)", ISC-TN 2012-1 Draft1, April 2012.
[TDNS] Zhu, L., Heidemann, J., Wessels, D., Mankin, A., and N.
Somaiya, "Connection-oriented DNS to Improve Privacy and
Security", 2015.
[TOYAMA] Toyama, K., Ishibashi, K., Ishino, M., Yoshimura, C., and
K. Fujiwara, "DNS Anomalies and Their Impacts on DNS Cache
Servers", NANOG 32 Reston, VA USA, 2004.
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[VERISIGN]
Thomas, M. and D. Wessels, "An Analysis of TCP Traffic in
Root Server DITL Data", DNS-OARC 2014 Fall Workshop Los
Angeles, 2014.
Author's Address
John Kristoff
DePaul University
Chicago, IL
US
Phone: +1 312 493 0305
Email: jtk@depaul.edu
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