DNS Transport over TCP - Operational Requirements
draft-ietf-dnsop-dns-tcp-requirements-00
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Duane Wessels
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Domain Name System Operations J. Kristoff
Internet-Draft DePaul University
Intended status: Best Current Practice D. Wessels
Expires: December 22, 2017 Verisign
June 20, 2017
DNS Transport over TCP - Operational Requirements
draft-ietf-dnsop-dns-tcp-requirements-00
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 upheld.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on December 22, 2017.
Copyright Notice
Copyright (c) 2017 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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Uneven Transport Usage and Preference . . . . . . . . . . 3
2.2. Waiting for Large Messages and Reliability . . . . . . . 3
2.3. EDNS0 . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.4. "Only Zone Tranfers Use TCP" . . . . . . . . . . . . . . 5
3. DNS over TCP Requirements . . . . . . . . . . . . . . . . . . 5
4. Network and System Considerations . . . . . . . . . . . . . . 6
5. DNS over TCP Filtering Risks . . . . . . . . . . . . . . . . 6
5.1. DNS Wedgie . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. DNS Root Zone KSK Rollover . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Standards Related to DNS Transport over TCP . . . . 10
A.1. TODO - additional, relevant RFCs . . . . . . . . . . . . 10
A.2. IETF RFC 7477 - Child-to-Parent Synchronization in DNS . 11
A.3. IETF RFC 7766 - DNS Transport over TCP - Implementation
Requirements . . . . . . . . . . . . . . . . . . . . . . 11
A.4. IETF RFC 7828 - The edns-tcp-keepalive EDNS0 Option . . . 11
A.5. IETF RFC 7873 - Domain Name System (DNS) Cookies . . . . 11
A.6. IETF RFC 7901 - CHAIN Query Requests in DNS . . . . . . . 11
A.7. IETF RFC 8027 - DNSSEC Roadblock Avoidance . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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 is on par with DNS over UDP
communications.
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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]:
"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
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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.
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 8) 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]. In fact,
[RFC7858], a Standards Track document is just this sort of
specification. Therefore, it might be desirable for network
operators to avoid artificially inhibiting the potential utility and
advances in the DNS such as these.
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2.4. "Only Zone Tranfers Use TCP"
There are many in the DNS community who configure DNS over TCP
services and expect DNS over TCP transactions to occur without
interference. However there has also been a long held belief by some
operators, particularly for security-related reasons, that DNS over
TCP services should be purposely limited or not provided at all
[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.
The position on restricting DNS over TCP had some justification given
that historic implementations of DNS nameservers provided very little
in the way of TCP connection management (for example see
Section 6.1.2 of [RFC7766] for more details). However modern
standards and implementations are moving to align with the more
sophisticated TCP management techniques employed by, for example,
HTTP(S) servers and load balancers.
3. DNS over TCP Requirements
Section 6.1.3.2 in [RFC1123] is updated: All general-purpose DNS
servers MUST be able to service both UDP and TCP queries.
o Authoritative servers MUST service TCP queries so that they do not
limit the size of responses to what fits in a single UDP packet.
o Recursive servers (or forwarders) MUST service TCP queries so that
they do not prevent large responses from a TCP-capable server from
reaching its TCP-capable clients.
Regarding the choice of limiting the resources a server devotes to
queries, Section 6.1.3.2 in [RFC1123] also says:
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.
This requirement is hereby updated: A name server MAY limit the the
resources it devotes to queries, but it MUST NOT refuse to service a
query just because it would have succeeded with another transport
protocol.
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. It must be acknowledged
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that DNS over TCP service can pose operational challenges that are
not present when running DNS over UDP alone. However, it is the aim
of this document to argue that the potential damage incurred by
prohibiting DNS over TCP service is more detrimental to the continued
utility and success of the DNS than when its usage is allowed.
4. Network and System Considerations
TODO: refer to IETF RFC 7766 connection handling discussion, various
TCP hardening documents, network operator protocol and traffic best
practices, etc.
5. DNS over TCP Filtering Risks
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
successfully 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.
This section enumerates some of the known risks we know about at the
time of this writing when networks filter DNS over TCP.
5.1. DNS Wedgie
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, not only will full answer be unavailable, but the
resolver will incur the full extent of TCP retransmissions and time
outs. This situation might place extreme strain on resolver
resources. If the nunber and frequency of these truncated answers
are sufficiently high, we refer to the steady-state of lost resources
as a result a "DNS" wedgie". A DNS wedgie is often not easily or
completely mitigated by the affected DNS resolver operator.
5.2. DNS Root Zone KSK Rollover
Recent plans for a new root zone DNSSEC KSK have highlighted a
potential problem in retrieving the keys.[LEWIS] Some packets in the
KSK rollover process will be larger than 1280 bytes, the IPv6 minimum
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MTU for links carrying IPv6 traffic.[RFC2460] While studies have
shown that problems due to fragment filtering or an inability to
generate and receive these larger messages are negible, any DNS
server that is unable to receive large DNS over UDP messages or
perform DNS over TCP may experience severe disruption of DNS service
if performing DNSSEC validation.
6. 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 a teaching
colleague, JPL, who perhaps unknowingly encouraged the initial
research into the differences of what the community has historically
said and did. Thanks to all the NANOG 63 attendees who provided
feedback to an early talk on this subject.
The following individuals provided an array of feedback to help
improve this document: Sara Dickinson, Bob Harold, Tatuya Jinmei, and
Paul Hoffman. The authors are indebted to their contributions. Any
remaining errors or imperfections are the sole responsbility of the
document authors.
7. IANA Considerations
This memo includes no request to IANA.
8. 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.
DNS over TCP is not unlike many other Internet TCP services. TCP
threats and many mitigation strategies have been well documented in a
series of documents such as [RFC4953], [RFC4987], [RFC5927], and
[RFC5961].
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9. References
9.1. 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>.
9.2. 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>.
[LEWIS] Lewis, E., "2017 DNSSEC KSK Rollover", RIPE 74 Budapest,
Hungary, May 2017.
[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>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <http://www.rfc-editor.org/info/rfc2460>.
[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>.
[RFC7477] Hardaker, W., "Child-to-Parent Synchronization in DNS",
RFC 7477, DOI 10.17487/RFC7477, March 2015,
<http://www.rfc-editor.org/info/rfc7477>.
[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>.
[RFC7828] Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
edns-tcp-keepalive EDNS0 Option", RFC 7828,
DOI 10.17487/RFC7828, April 2016,
<http://www.rfc-editor.org/info/rfc7828>.
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[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <http://www.rfc-editor.org/info/rfc7858>.
[RFC7873] Eastlake 3rd, D. and M. Andrews, "Domain Name System (DNS)
Cookies", RFC 7873, DOI 10.17487/RFC7873, May 2016,
<http://www.rfc-editor.org/info/rfc7873>.
[RFC7901] Wouters, P., "CHAIN Query Requests in DNS", RFC 7901,
DOI 10.17487/RFC7901, June 2016,
<http://www.rfc-editor.org/info/rfc7901>.
[RFC8027] Hardaker, W., Gudmundsson, O., and S. Krishnaswamy,
"DNSSEC Roadblock Avoidance", BCP 207, RFC 8027,
DOI 10.17487/RFC8027, November 2016,
<http://www.rfc-editor.org/info/rfc8027>.
[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.
[VERISIGN]
Thomas, M. and D. Wessels, "An Analysis of TCP Traffic in
Root Server DITL Data", DNS-OARC 2014 Fall Workshop Los
Angeles, 2014.
Appendix A. Standards Related to DNS Transport over TCP
This section enumerates all known IETF RFC documents that are
currently of status standard, informational, best common practice or
experimental and either implicitly or explicitly make assumptions or
statements about the use of TCP as a transport for the DNS germane to
this document.
A.1. TODO - additional, relevant RFCs
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A.2. IETF RFC 7477 - Child-to-Parent Synchronization in DNS
This standards track document [RFC7477] specifies a RRType and
protocol to signal and synchronize NS, A, and AAAA resource record
changes from a child to parent zone. Since this protocol may require
multiple requests and responses, it recommends utilizing DNS over TCP
to ensure the conversation takes place between a consistent pair of
end nodes.
A.3. IETF RFC 7766 - DNS Transport over TCP - Implementation
Requirements
The standards track document [RFC7766] might be considered the direct
ancestor of this operational requirements document. The
implementation requirements document codifies mandatory support for
DNS over TCP in compliant DNS software.
A.4. IETF RFC 7828 - The edns-tcp-keepalive EDNS0 Option
This standards track document [RFC7828] defines an EDNS0 option to
negotiate an idle timeout value for long-lived DNS over TCP
connections. Consequently, this document is only applicable and
relevant to DNS over TCP sessions and between implementations that
support this option.
A.5. IETF RFC 7873 - Domain Name System (DNS) Cookies
This standards track document [RFC7873] describes an EDNS0 option to
provide additional protection against query and answer forgery. This
specification mentions DNS over TCP as a reasonable fallback
mechanism when DNS Cookies are not available. The specification does
make mention of DNS over TCP processing in two specific situations.
In one, when a server receives only a client cookie in a request, the
server should consider whether the request arrived over TCP and if
so, it should consider accepting TCP as sufficient to authenticate
the request and respond accordingly. In another, when a client
receives a BADCOOKIE reply using a fresh server cookie, the client
should retry using TCP as the transport.
A.6. IETF RFC 7901 - CHAIN Query Requests in DNS
This experimental specification [RFC7901] describes an EDNS0 option
that can be used by a security-aware validating resolver to request
and obtain a complete DNSSEC validation path for any single query.
This document requires the use of DNS over TCP or a source IP address
verified transport mechanism such as EDNS-COOKIE.[RFC7873]
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A.7. IETF RFC 8027 - DNSSEC Roadblock Avoidance
This document [RFC8027] details observed problems with DNSSEC
deployment and mitigation techniques. Network traffic blocking and
restrictions, including DNS over TCP messages, are highlighted as one
reason for DNSSEC deployment issues. While this document suggests
these sorts of problems are due to "non-compliant infrastructure" and
is of type BCP, the scope of the document is limited to detection and
mitigation techniques to avoid so-called DNSSEC roadblocks.
Authors' Addresses
John Kristoff
DePaul University
Chicago, IL 60604
US
Phone: +1 312 493 0305
Email: jtk@depaul.edu
URI: https://aharp.iorc.depaul.edu
Duane Wessels
Verisign
12061 Bluemont Way
Reston, VA 20190
US
Phone: +1 703 948 3200
Email: dwessels@verisign.com
URI: http://verisigninc.com
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