[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]
Versions: (draft-kristoff-dnsop-dns-tcp-requirements)
00 01 02 03 04
Domain Name System Operations J. Kristoff
Internet-Draft DePaul University
Updates: 1123 (if approved) D. Wessels
Intended status: Best Current Practice Verisign
Expires: November 17, 2018 May 16, 2018
DNS Transport over TCP - Operational Requirements
draft-ietf-dnsop-dns-tcp-requirements-02
Abstract
This document encourages the practice of permitting DNS messages to
be carried over TCP on the Internet. It also considers the
consequences with this form of DNS communication 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 https://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 November 17, 2018.
Copyright Notice
Copyright (c) 2018 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
(https://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
Kristoff & Wessels Expires November 17, 2018 [Page 1]
Internet-Draft DNS Transport over TCP May 2018
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Uneven Transport Usage and Preference . . . . . . . . . . 3
2.2. Waiting for Large Messages and Reliability . . . . . . . 4
2.3. EDNS0 . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.4. Fragmentation and Truncation . . . . . . . . . . . . . . 5
2.5. "Only Zone Transfers Use TCP" . . . . . . . . . . . . . . 6
3. DNS over TCP Requirements . . . . . . . . . . . . . . . . . . 6
4. Network and System Considerations . . . . . . . . . . . . . . 8
4.1. Connection Admission . . . . . . . . . . . . . . . . . . 8
4.2. Connection Management . . . . . . . . . . . . . . . . . . 9
4.3. Connection Termination . . . . . . . . . . . . . . . . . 9
5. DNS over TCP Filtering Risks . . . . . . . . . . . . . . . . 10
5.1. DNS Wedgie . . . . . . . . . . . . . . . . . . . . . . . 10
5.2. DNS Root Zone KSK Rollover . . . . . . . . . . . . . . . 11
5.3. DNS-over-TLS . . . . . . . . . . . . . . . . . . . . . . 11
6. Logging and Monitoring . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
9. Security Considerations . . . . . . . . . . . . . . . . . . . 12
10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 12
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
11.1. Normative References . . . . . . . . . . . . . . . . . . 13
11.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Standards Related to DNS Transport over TCP . . . . 17
A.1. TODO - additional, relevant RFCs . . . . . . . . . . . . 17
A.2. IETF RFC 5936 - DNS Zone Transfer Protocol (AXFR) . . . . 17
A.3. IETF RFC 6304 - AS112 Nameserver Operations . . . . . . . 17
A.4. IETF RFC 6762 - Multicast DNS . . . . . . . . . . . . . . 17
A.5. IETF RFC 6950 - Architectural Considerations on
Application Features in the DNS . . . . . . . . . . . . . 18
A.6. IETF RFC 7477 - Child-to-Parent Synchronization in DNS . 18
A.7. IETF RFC 7720 - DNS Root Name Service Protocol and
Deployment Requirements . . . . . . . . . . . . . . . . . 18
A.8. IETF RFC 7766 - DNS Transport over TCP - Implementation
Requirements . . . . . . . . . . . . . . . . . . . . . . 18
A.9. IETF RFC 7828 - The edns-tcp-keepalive EDNS0 Option . . . 18
A.10. IETF RFC 7858 - Specification for DNS over Transport
Layer Security (TLS) . . . . . . . . . . . . . . . . . . 18
A.11. IETF RFC 7873 - Domain Name System (DNS) Cookies . . . . 19
A.12. IETF RFC 7901 - CHAIN Query Requests in DNS . . . . . . . 19
A.13. IETF RFC 8027 - DNSSEC Roadblock Avoidance . . . . . . . 19
Kristoff & Wessels Expires November 17, 2018 [Page 2]
Internet-Draft DNS Transport over TCP May 2018
A.14. IETF RFC 8094 - DNS over Datagram Transport Layer
Security (DTLS) . . . . . . . . . . . . . . . . . . . . . 19
A.15. IETF RFC 8162 - Using Secure DNS to Associate
Certificates with Domain Names for S/MIME . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
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.
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
The curious state of disagreement in operational best practices and
guidance for DNS transport protocols derives from conflicting
messages operators have gotten from other operators, implementors,
and even the IETF. Sometimes these mixed signals have been explicit,
on other occasions they have suspiciously implicit. Here we
summarize our interpretation of the storied and conflicting history
that has brought us to this document.
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."
Kristoff & Wessels Expires November 17, 2018 [Page 3]
Internet-Draft DNS Transport over TCP May 2018
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
In the original specifications, the maximum DNS over UDP message size
was enshrined at 512 bytes. However, even while [RFC1123] made a
clear preference for UDP, it foresaw DNS over TCP becoming more
popular in the future to overcome this limitation:
"[...] 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 helped solidify UDP's transport
dominance for message transactions.
2.3. EDNS0
In 1999 the IETF published the Extension Mechanisms for DNS (EDNS0)
in [RFC2671] (superseded in 2013 by an update in [RFC6891]). This
document standardized a way for communicating DNS nodes to perform
Kristoff & Wessels Expires November 17, 2018 [Page 4]
Internet-Draft DNS Transport over TCP May 2018
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
(possibly fragmented) DNS message size a node is capable of
receiving. In practice, typical values are a subset of the 512 to
4096 byte range. EDNS0 became 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 DNS messages larger than
512 bytes would be less reliant on TCP than they might otherwise have
been. While a non-negligible 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].
2.4. Fragmentation and Truncation
Although EDNS0 provides a way for endpoints to signal support for DNS
messages exceeding 512 bytes, the realities of a diverse and
inconsistently deployed Internet may result in some large messages
being unable to reach their destination. Any IP datagram whose size
exceeds the MTU of a link it transits will be fragmented and then
reassembled by the receiving host. Unfortunately, it is not uncommon
for middleboxes and firewalls to block IP fragments. If one or more
fragments do not arrive, the application does not receive the message
and the request times out.
For IPv4-connected hosts, the de-facto MTU is often the Ethernet
payload size of 1500 bytes. This means that the largest unfragmented
UDP DNS message that can be sent over IPv4 is likely 1472 bytes. For
IPv6, the situation is a little more complicated. First, IPv6
headers are 40 bytes (versus 20 without option in IPv4). Second, it
seems as though some people have mis-interpreted IPv6's required
minimum MTU of 1280 as a required maximum. Third, fragmentation in
IPv6 can only be done by the host originating the datagram. The need
to fragment is conveyed in an ICMPv6 "packet too big" message. The
originating host indicates a fragmented datagram with IPv6 extension
headers. Unfortunately, it is quite common for both ICMPv6 and IPv6
extension headers to be blocked by middleboxes. According to
[HUSTON] some 35% of IPv6-capable recursive resolvers are unable to
receive a fragmented IPv6 packet.
The practical consequence of all this is that DNS requestors must be
prepared to retry queries with different EDNS0 maximum message size
values. Administrators of BIND are likely to be familiar with seeing
Kristoff & Wessels Expires November 17, 2018 [Page 5]
Internet-Draft DNS Transport over TCP May 2018
"success resolving ... after reducing the advertised EDNS0 UDP packet
size to 512 octets" messages in their system logs.
Often, reducing the EDNS0 UDP packet size leads to a successful
response. That is, the necessary data fits within the smaller
message size. However, when the data does not fit, the server sets
the truncated flag in its response, indicating the client should
retry over TCP to receive the whole response. This is undesirable
from the client's point of view because it adds more latency, and
potentially undesirable from the server's point of view due to the
increased resource requirements of TCP.
The issues around fragmentation, truncation, and TCP are driving
certain implementation and policy decisions in the DNS. Notably,
Cloudflare implemented what it calls "DNSSEC black lies" [CLOUDFLARE]
and uses ECDSA algorithms, such that their signed responses fit
easily in 512 bytes. The KSK Rollover design team [DESIGNTEAM] spent
a lot of time thinking and worrying about response sizes. There is
growing sentiment in the DNSSEC community that RSA key sizes beyond
2048-bits are impractical and that critical infrastructure zones
should transition to elliptic curve algorithms to keep response sizes
manageable.
2.5. "Only Zone Transfers Use TCP"
Today, the majority of the DNS community expects, or at least has a
desire, to see 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
An average increase in DNS message size, the continued development of
new DNS features and a denial of service mitigation technique (see
Section 9) have suggested that DNS over TCP transactions are as
Kristoff & Wessels Expires November 17, 2018 [Page 6]
Internet-Draft DNS Transport over TCP May 2018
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, we now believe it is undesirable for
network operators to artificially inhibit the potential utility and
advances in the DNS such as these.
TODO: I think the text below needs some work/discussion because 7766
already updated 1123 in a very similar way except that 7766 speaks of
"implement" and this one speaks of "service". 1123 speaks of
"support" and doesn't distinguish between implement/service.
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.
Filtering of DNS over TCP 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
that DNS over TCP service can pose operational challenges that are
not present when running DNS over UDP alone, and vice-versa.
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.
Kristoff & Wessels Expires November 17, 2018 [Page 7]
Internet-Draft DNS Transport over TCP May 2018
4. Network and System Considerations
This section describes measures that systems and applications can
take to optimize performance over TCP and to protect themselves from
TCP-based resource exhaustion and attacks.
4.1. Connection Admission
The SYN flooding attack is a denial-of-service method affecting hosts
that run TCP server processes [RFC4987]. This attack can be very
effective if not mitigated. One of the most effective mitigation
techniques is SYN cookies, which allows the server to avoid
allocating any state until the successful completion of the three-way
handshake.
Services not intended for use by the public Internet, such as most
recursive name servers, SHOULD be protected with access controls.
Ideally these controls are placed in the network, well before before
any unwanted TCP packets can reach the DNS server host or
application. If this is not possible, the controls can be placed in
the application itself. In some situations (e.g. attacks) it may be
necessary to deploy access controls for DNS services that should
otherwise be globally reachable.
The FreeBSD operating system has an "accept filter" feature that
postpones delivery of TCP connections to applications until a
complete, valid request has been received. The dns_accf(9) filter
ensures that a valid DNS message is received. If not, the bogus
connection never reaches the application. Applications must be coded
and configured to make use of this filter.
Per [RFC7766], applications and administrators are advised to
remember that TCP MAY be used before sending any UDP queries.
Networks and applications MUST NOT be configured to refuse TCP
queries that were not preceded by a UDP query.
TCP Fast Open [RFC7413] (TFO) allows TCP clients to shorten the
handshake for subsequent connections to the same server. TFO saves
one round-trip time in the connection setup. DNS servers SHOULD
enable TFO when possible. Furthermore, DNS servers clustered behind
a single service address (e.g., anycast or load-balancing), SHOULD
use the same TFO server key on all instances.
DNS clients SHOULD also enable TFO when possible. Currently, on some
operating systems it is not implemented or disabled by default.
[WIKIPEDIA_TFO] describes applications and operating systems that
support TFO.
Kristoff & Wessels Expires November 17, 2018 [Page 8]
Internet-Draft DNS Transport over TCP May 2018
4.2. Connection Management
Since host memory for TCP state is a finite resource, DNS servers
MUST actively manage their connections. Applications that do not
actively manage their connections can encounter resource exhaustion
leading to denial of service. For DNS, as in other protocols, there
is a tradeoff between keeping connections open for potential future
use and the need to free up resources for new connections that will
arrive.
DNS server software SHOULD provide a configurable limit on the total
number of established TCP connections. If the limit is reached, the
application is expected to either close existing (idle) connections
or refuse new connections. Operators SHOULD ensure the limit is
configured appropriately for their particular situation.
DNS server software MAY provide a configurable limit on the number of
established connections per source IP address or subnet. This can be
used to ensure that a single or small set of users can not consume
all TCP resources and deny service to other users. Operators SHOULD
ensure this limit is configured appropriately, based on their number
of diversity of users.
DNS server software SHOULD provide a configurable timeout for idle
TCP connections. For very busy name servers this might be set to a
low value, such as a few seconds. For less busy servers it might be
set to a higher value, such as tens of seconds. DNS clients and
servers SHOULD signal their timeout values using the edns-tcp-
keepalive option [RFC7828].
DNS server software MAY provide a configurable limit on the number of
transactions per TCP connection. This document does not offer advice
on particular values for such a limit.
Similarly, DNS server software MAY provide a configurable limit on
the total duration of a TCP connection. This document does not offer
advice on particular values for such a limit.
Since clients may not be aware of server-imposed limits, clients
utilizing TCP for DNS need to always be prepared to re-establish
connections or otherwise retry outstanding queries.
4.3. Connection Termination
In general, it is preferable for clients to initiate the close of a
TCP connection. The TCP peer that initiates a connection close
retains the socket in the TIME_WAIT state for some amount of time,
possibly a few minutes. On a busy server, the accumulation of many
Kristoff & Wessels Expires November 17, 2018 [Page 9]
Internet-Draft DNS Transport over TCP May 2018
sockets in TIME_WAIT can cause performance problems or even denial of
service.
On systems where large numbers of sockets in TIME_WAIT are observed,
it may be beneficial to tune the local TCP parameters. For example,
the Linux kernel provides a number of "sysctl" parameters related to
TIME_WAIT, such as net.ipv4.tcp_fin_timeout, net.ipv4.tcp_tw_recycle,
and net.ipv4.tcp_tw_reuse. In extreme cases, implementors and
operators of very busy servers may find it necessary to utilize the
SO_LINGER socket option ([Stevens] Section 7.5) with a value of zero
so that the server doesn't accumulate TIME_WAIT sockets.
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.
For example, [RFC7901] describes a latency-avoiding technique that
sends extra data in DNS responses. This makes responses larger and
potentially increases the risk of DDoS reflection attacks. The
specification mandates the use of TCP or DNS Cookies ([RFC7873]).
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
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 number 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.
Kristoff & Wessels Expires November 17, 2018 [Page 10]
Internet-Draft DNS Transport over TCP May 2018
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 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 negligible, 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.
TODO: Is this "overcome by events" now? We've had 1414 byte DNSKEY
responses at the three ZSK rollover periods since KSK-2017 became
published in the root zone.
5.3. DNS-over-TLS
DNS messages may be sent over TLS to provide privacy between stubs
and recursive resolvers. [RFC7858] is a standards track document
describing how this works. Although it utilizes TCP port 853 instead
of port 53, this document applies equally well to DNS-over-TLS.
Note, however, DNS-over-TLS is currently only defined between stubs
and recursives.
The use of TLS places even strong operational burdens on DNS clients
and servers. Cryptographic functions for authentication and
encryption require additional processing. Unoptimized connection
setup takes two additional round-trips compared to TCP, but can be
reduced with Fast TLS connection resumption [RFC5077] and TLS False
Start [RFC7918].
6. Logging and Monitoring
Developers of applications that log or monitor DNS are advised to not
ignore TCP because it is rarely used or because it is hard to
process. Operators are advised to ensure that their monitoring and
logging applications properly capture DNS-over-TCP messages.
Otherwise, attacks, exfiltration attempts, and normal traffic may go
undetected.
DNS messages over TCP are in no way guaranteed to arrive in single
segments. In fact, a clever attacker may attempt to hide certain
messages by forcing them over very small TCP segments. Applications
that capture network packets (e.g., with libpcap) should be prepared
to implement and perform full TCP segment reassembly. dnscap
[dnscap] is an open-source example of a DNS logging program that
implements TCP reassembly.
Kristoff & Wessels Expires November 17, 2018 [Page 11]
Internet-Draft DNS Transport over TCP May 2018
Developers should also keep in mind connection reuse, pipelining, and
out-of-order responses when building and testing DNS monitoring
applications.
7. Acknowledgments
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 responsibility of the
document authors.
8. IANA Considerations
This memo includes no request to IANA.
9. 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].
10. Privacy Considerations
TODO: Does this document warrant privacy considerations?
Kristoff & Wessels Expires November 17, 2018 [Page 12]
Internet-Draft DNS Transport over TCP May 2018
11. References
11.1. 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,
<https://www.rfc-editor.org/info/rfc2119>.
11.2. 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.
[CLOUDFLARE]
Grant, D., "Economical With The Truth: Making DNSSEC
Answers Cheap", June 2016,
<https://blog.cloudflare.com/black-lies/>.
[DESIGNTEAM]
Design Team Report, "Root Zone KSK Rollover Plan",
December 2015, <https://www.iana.org/reports/2016/
root-ksk-rollover-design-20160307.pdf>.
[DJBDNS] D.J. Bernstein, "When are TCP queries sent?", 2002,
<https://cr.yp.to/djbdns/tcp.html#why>.
[dnscap] DNS-OARC, "DNSCAP", May 2018,
<https://www.dns-oarc.net/tools/dnscap>.
[HUSTON] Huston, G., "Dealing with IPv6 fragmentation in the DNS",
August 2017, <https://blog.apnic.net/2017/08/22/
dealing-ipv6-fragmentation-dns/>.
[LEWIS] Lewis, E., "2017 DNSSEC KSK Rollover", RIPE 74 Budapest,
Hungary, May 2017, <https://ripe74.ripe.net/
presentations/25-RIPE74-lewis-submission.pdf>.
[NETALYZR]
Kreibich, C., Weaver, N., Nechaev, B., and V. Paxson,
"Netalyzr: Illuminating The Edge Network", 2010.
Kristoff & Wessels Expires November 17, 2018 [Page 13]
Internet-Draft DNS Transport over TCP May 2018
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://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,
<https://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,
<https://www.rfc-editor.org/info/rfc1536>.
[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,
<https://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, <https://www.rfc-editor.org/info/rfc2460>.
[RFC2541] Eastlake 3rd, D., "DNS Security Operational
Considerations", RFC 2541, DOI 10.17487/RFC2541, March
1999, <https://www.rfc-editor.org/info/rfc2541>.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, DOI 10.17487/RFC2671, August 1999,
<https://www.rfc-editor.org/info/rfc2671>.
[RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks",
RFC 4953, DOI 10.17487/RFC4953, July 2007,
<https://www.rfc-editor.org/info/rfc4953>.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
<https://www.rfc-editor.org/info/rfc4987>.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <https://www.rfc-editor.org/info/rfc5077>.
Kristoff & Wessels Expires November 17, 2018 [Page 14]
Internet-Draft DNS Transport over TCP May 2018
[RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927,
DOI 10.17487/RFC5927, July 2010,
<https://www.rfc-editor.org/info/rfc5927>.
[RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
(AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
<https://www.rfc-editor.org/info/rfc5936>.
[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,
<https://www.rfc-editor.org/info/rfc5961>.
[RFC6304] Abley, J. and W. Maton, "AS112 Nameserver Operations",
RFC 6304, DOI 10.17487/RFC6304, July 2011,
<https://www.rfc-editor.org/info/rfc6304>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6762>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC6891, April 2013,
<https://www.rfc-editor.org/info/rfc6891>.
[RFC6950] Peterson, J., Kolkman, O., Tschofenig, H., and B. Aboba,
"Architectural Considerations on Application Features in
the DNS", RFC 6950, DOI 10.17487/RFC6950, October 2013,
<https://www.rfc-editor.org/info/rfc6950>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>.
[RFC7477] Hardaker, W., "Child-to-Parent Synchronization in DNS",
RFC 7477, DOI 10.17487/RFC7477, March 2015,
<https://www.rfc-editor.org/info/rfc7477>.
[RFC7720] Blanchet, M. and L-J. Liman, "DNS Root Name Service
Protocol and Deployment Requirements", BCP 40, RFC 7720,
DOI 10.17487/RFC7720, December 2015,
<https://www.rfc-editor.org/info/rfc7720>.
[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,
<https://www.rfc-editor.org/info/rfc7766>.
Kristoff & Wessels Expires November 17, 2018 [Page 15]
Internet-Draft DNS Transport over TCP May 2018
[RFC7828] Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
edns-tcp-keepalive EDNS0 Option", RFC 7828,
DOI 10.17487/RFC7828, April 2016,
<https://www.rfc-editor.org/info/rfc7828>.
[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, <https://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,
<https://www.rfc-editor.org/info/rfc7873>.
[RFC7901] Wouters, P., "CHAIN Query Requests in DNS", RFC 7901,
DOI 10.17487/RFC7901, June 2016,
<https://www.rfc-editor.org/info/rfc7901>.
[RFC7918] Langley, A., Modadugu, N., and B. Moeller, "Transport
Layer Security (TLS) False Start", RFC 7918,
DOI 10.17487/RFC7918, August 2016,
<https://www.rfc-editor.org/info/rfc7918>.
[RFC8027] Hardaker, W., Gudmundsson, O., and S. Krishnaswamy,
"DNSSEC Roadblock Avoidance", BCP 207, RFC 8027,
DOI 10.17487/RFC8027, November 2016,
<https://www.rfc-editor.org/info/rfc8027>.
[RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
Transport Layer Security (DTLS)", RFC 8094,
DOI 10.17487/RFC8094, February 2017,
<https://www.rfc-editor.org/info/rfc8094>.
[RFC8162] Hoffman, P. and J. Schlyter, "Using Secure DNS to
Associate Certificates with Domain Names for S/MIME",
RFC 8162, DOI 10.17487/RFC8162, May 2017,
<https://www.rfc-editor.org/info/rfc8162>.
[RRL] Vixie, P. and V. Schryver, "DNS Response Rate Limiting
(DNS RRL)", ISC-TN 2012-1 Draft1, April 2012.
[Stevens] Stevens, W., Fenner, B., and A. Rudoff, "UNIX Network
Programming Volume 1, Third Edition: The Sockets
Networking API", November 2003.
[TDNS] Zhu, L., Heidemann, J., Wessels, D., Mankin, A., and N.
Somaiya, "Connection-oriented DNS to Improve Privacy and
Security", 2015.
Kristoff & Wessels Expires November 17, 2018 [Page 16]
Internet-Draft DNS Transport over TCP May 2018
[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.
[WIKIPEDIA_TFO]
Wikipedia, "TCP Fast Open", May 2018,
<https://en.wikipedia.org/wiki/TCP_Fast_Open>.
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
A.2. IETF RFC 5936 - DNS Zone Transfer Protocol (AXFR)
The [RFC5936] standards track document provides a detailed
specification for the zone transfer protocol, as originally outlined
in the early DNS standards. AXFR operation is limited to TCP and not
specified for UDP. This document discusses TCP usage at length.
A.3. IETF RFC 6304 - AS112 Nameserver Operations
[RFC6304] is an informational document enumerating the requirements
for operation of AS112 project DNS servers. New AS112 nodes are
tested for their ability to provide service on both UDP and TCP
transports, with the implication that TCP service is an expected part
of normal operations.
A.4. IETF RFC 6762 - Multicast DNS
This standards track document [RFC6762] the TC bit is deemed to have
essentially the same meaning as described in the original DNS
specifications. That is, if a response with the TCP bit set is
receiver "[...] the querier SHOULD reissue its query using TCP in
order to receive the larger response."
Kristoff & Wessels Expires November 17, 2018 [Page 17]
Internet-Draft DNS Transport over TCP May 2018
A.5. IETF RFC 6950 - Architectural Considerations on Application
Features in the DNS
An informational document [RFC6950] that draws attention to large
data in the DNS. TCP is referenced in the context as a common
fallback mechnanism and counter to some spoofing attacks.
A.6. 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.7. IETF RFC 7720 - DNS Root Name Service Protocol and Deployment
Requirements
This best current practice[RFC7720] declares root name service "MUST
support UDP [RFC768] and TCP [RFC793] transport of DNS queries and
responses."
A.8. 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.9. 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.10. IETF RFC 7858 - Specification for DNS over Transport Layer
Security (TLS)
This standards track document [RFC7858] defines a method for putting
DNS messages into a TCP-based encrypted channel using TLS. This
specification is noteworthy for explicitly targetting the stub-to-
recursive traffic, but does not preclude its application from
recursive-to-authoritative traffic.
Kristoff & Wessels Expires November 17, 2018 [Page 18]
Internet-Draft DNS Transport over TCP May 2018
A.11. 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.12. 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]
A.13. 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.
A.14. IETF RFC 8094 - DNS over Datagram Transport Layer Security (DTLS)
This experimental specification [RFC8094] details a protocol that
uses a datagram transport (UDP), but stipulates that "DNS clients and
servers that implement DNS over DTLS MUST also implement DNS over TLS
in order to provide privacy for clients that desire Strict Privacy
[...]". This requirement implies DNS over TCP must be supported in
case the message size is larger than the path MTU.
A.15. IETF RFC 8162 - Using Secure DNS to Associate Certificates with
Domain Names for S/MIME
This experimental specification [RFC8162] describes a technique to
authenticate user X.509 certificates in an S/MIME system via the DNS.
The document points out that the new experimental resource record
types are expected to carry large payloads, resulting in the
Kristoff & Wessels Expires November 17, 2018 [Page 19]
Internet-Draft DNS Transport over TCP May 2018
suggestion that "applications SHOULD use TCP -- not UDP -- to perform
queries for the SMIMEA resource record."
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
Kristoff & Wessels Expires November 17, 2018 [Page 20]
Html markup produced by rfcmarkup 1.129c, available from
https://tools.ietf.org/tools/rfcmarkup/