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IETF RFC 7828
Last modified on Thursday, April 7th, 2016
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Internet Engineering Task Force (IETF) P. Wouters
Request for Comments: 7828 Red Hat
Category: Standards Track J. Abley
ISSN: 2070-1721 Dyn, Inc.
S. Dickinson
Sinodun
R. Bellis
ISC
April 2016
The edns-tcp-keepalive EDNS0 Option
 Abstract
DNS messages between clients and servers may be received over either
UDP or TCP. UDP transport involves keeping less state on a busy
server, but can cause truncation and retries over TCP. Additionally,
UDP can be exploited for reflection attacks. Using TCP would reduce
retransmits and amplification. However, clients commonly use TCP
only for retries and servers typically use idle timeouts on the order
of seconds.
This document defines an EDNS0 option ("edns-tcp-keepalive") that
allows DNS servers to signal a variable idle timeout. This
signalling encourages the use of long-lived TCP connections by
allowing the state associated with TCP transport to be managed
effectively with minimal impact on the DNS transaction time.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 7828.
Wouters, et al. Standards Track PAGE 1 
RFC 7828 The edns-tcp-keepalive EDNS0 Option April 2016
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
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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 4
3. The edns-tcp-keepalive Option . . . . . . . . . . . . . . . . 5
3.1. Option Format . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Use by DNS Clients . . . . . . . . . . . . . . . . . . . 5
3.2.1. Sending Queries . . . . . . . . . . . . . . . . . . . 5
3.2.2. Receiving Responses . . . . . . . . . . . . . . . . . 6
3.3. Use by DNS Servers . . . . . . . . . . . . . . . . . . . 6
3.3.1. Receiving Queries . . . . . . . . . . . . . . . . . . 6
3.3.2. Sending Responses . . . . . . . . . . . . . . . . . . 6
3.4. TCP Session Management . . . . . . . . . . . . . . . . . 7
3.5. Non-clean Paths . . . . . . . . . . . . . . . . . . . . . 8
3.6. Anycast Considerations . . . . . . . . . . . . . . . . . 8
4. Intermediary Considerations . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
Wouters, et al. Standards Track PAGE 2
RFC 7828 The edns-tcp-keepalive EDNS0 Option April 2016
1. Introduction
DNS messages between clients and servers may be received over either
UDP or TCP [RFC 1035]. Historically, DNS clients used APIs that only
facilitated sending and receiving a single query over either UDP or
TCP. New APIs and deployment of DNSSEC validating resolvers on hosts
that in the past were using stub resolving only is increasing the DNS
client base that prefer using long-lived TCP connections. Long-lived
TCP connections can result in lower request latency than the case
where UDP transport is used and truncated responses are received.
This is because clients that retry over TCP following a truncated UDP
response typically only use the TCP session for a single (request,
response) pair, continuing with UDP transport for subsequent queries.
The use of TCP transport requires state to be retained on DNS
servers. If a server is to perform adequately with a significant
query load received over TCP, it must manage its available resources
to ensure that all established TCP sessions are well-used, and idle
connections are closed after an appropriate amount of time.
UDP transport is stateless, and hence presents a much lower resource
burden on a busy DNS server than TCP. An exchange of DNS messages
over UDP can also be completed in a single round trip between
communicating hosts, resulting in optimally short transaction times.
UDP transport is not without its risks, however.
A single-datagram exchange over UDP between two hosts can be
exploited to enable a reflection attack on a third party. Response
Rate Limiting [RRL] is designed to help mitigate such attacks against
authoritative-only servers. One feature of RRL is to let some amount
of responses "slip" through the rate limiter. These are returned
with the TC (truncation) bit set, which causes legitimate clients to
resend the same query using TCP transport.
[RFC 1035] specified a maximum DNS message size over UDP transport of
512 bytes. Deployment of DNSSEC [RFC 4033] and other protocols
subsequently increased the observed frequency at which responses
exceed this limit. EDNS0 [RFC 6891] allows DNS messages larger than
512 bytes to be exchanged over UDP, with a corresponding increased
incidence of fragmentation. Fragmentation is known to be problematic
in general, and has also been implicated in increasing the risk of
cache poisoning attacks [fragmentation-considered-poisonous].
TCP transport is less susceptible to the risks of fragmentation and
reflection attacks. However, TCP transport for DNS as currently
deployed has expensive setup overhead, compared to using UDP (when no
retry is required).
Wouters, et al. Standards Track PAGE 3
RFC 7828 The edns-tcp-keepalive EDNS0 Option April 2016
The overhead of the three-way TCP handshake for a single DNS
transaction is substantial, increasing the transaction time for a
single (request, response) pair of DNS messages from 1x RTT to 2x
RTT. There is no such overhead for a session that is already
established; therefore, the overhead of the initial TCP handshake is
minimised when the resulting session is used to exchange multiple DNS
message pairs over a single session. The extra RTT time for session
setup can be represented as the equation (1 + N)/N, where N
represents the number of DNS message pairs that utilize the session
and the result approaches unity as N increases.
With increased deployment of DNSSEC and new RR types containing
application-specific cryptographic material, there is an increase in
the prevalence of truncated responses received over UDP with retries
over TCP. The overhead for a DNS transaction over UDP truncated due
to RRL is 3x RTT higher than the overhead imposed on the same
transaction initiated over TCP.
This document proposes a signalling mechanism between DNS clients and
servers that encourages the use of long-lived TCP connections by
allowing the state associated with TCP transport to be managed
effectively with minimal impact on the DNS transaction time.
This mechanism will be of benefit for both stub-resolver and
resolver-authoritative TCP connections. In the latter case, the
persistent nature of the TCP connection can provide improved defence
against attacks including DDoS.
The reduced overhead of this extension adds up significantly when
combined with other EDNS0 extensions, such as [CHAIN-QUERY] and
[DNS-over-TLS]. For example, the combination of these EDNS0
extensions make it possible for hosts on high-latency mobile networks
to natively and efficiently perform DNSSEC validation and encrypt
queries.
2. 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 [RFC 2119].
Wouters, et al. Standards Track PAGE 4
RFC 7828 The edns-tcp-keepalive EDNS0 Option April 2016
3. The edns-tcp-keepalive Option
This document specifies a new EDNS0 [RFC 6891] option, edns-tcp-
keepalive, which can be used by DNS clients and servers to signal a
willingness to keep an idle TCP session open to conduct future DNS
transactions, with the idle timeout being specified by the server.
This specification does not distinguish between different types of
DNS client and server in the use of this option.
[RFC 7766] defines an 'idle DNS-over-TCP session' from both the client
and server perspective. The idle timeout described here begins when
the idle condition is met per that definition and should be reset
when that condition is lifted, i.e., when a client sends a message or
when a server receives a message on an idle connection.
3.1. Option Format
The edns-tcp-keepalive option is encoded as follows:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-------------------------------+-------------------------------+
! OPTION-CODE ! OPTION-LENGTH !
+-------------------------------+-------------------------------+
| TIMEOUT !
+-------------------------------+
where:
OPTION-CODE: the EDNS0 option code assigned to edns-tcp-keepalive,
11
OPTION-LENGTH: the value 0 if the TIMEOUT is omitted, the value 2
if it is present;
TIMEOUT: an idle timeout value for the TCP connection, specified in
units of 100 milliseconds, encoded in network byte order.
3.2. Use by DNS Clients
3.2.1. Sending Queries
DNS clients MUST NOT include the edns-tcp-keepalive option in queries
sent using UDP transport.
DNS clients MAY include the edns-tcp-keepalive option in the first
query sent to a server using TCP transport to signal their desire to
keep the connection open when idle.
Wouters, et al. Standards Track PAGE 5
RFC 7828 The edns-tcp-keepalive EDNS0 Option April 2016
DNS clients MAY include the edns-tcp-keepalive option in subsequent
queries sent to a server using TCP transport to signal their
continued desire to keep the connection open when idle.
Clients MUST specify an OPTION-LENGTH of 0 and omit the TIMEOUT
value.
3.2.2. Receiving Responses
A DNS client that receives a response using UDP transport that
includes the edns-tcp-keepalive option MUST ignore the option.
A DNS client that receives a response using TCP transport that
includes the edns-tcp-keepalive option MAY keep the existing TCP
session open when it is idle. It SHOULD honour the timeout received
in that response (overriding any previous timeout) and initiate close
of the connection before the timeout expires.
A DNS client that receives a response that includes the edns-tcp-
keepalive option with a TIMEOUT value of 0 SHOULD send no more
queries on that connection and initiate closing the connection as
soon as it has received all outstanding responses.
A DNS client that sent a query containing the edns-keepalive-option
but receives a response that does not contain the edns-keepalive-
option SHOULD assume the server does not support keepalive and behave
following the guidance in [RFC 7766]. This holds true even if a
previous edns-keepalive-option exchange occurred on the existing TCP
connection.
3.3. Use by DNS Servers
3.3.1. Receiving Queries
A DNS server that receives a query using UDP transport that includes
the edns-tcp-keepalive option MUST ignore the option.
A DNS server that receives a query using TCP transport that includes
the edns-tcp-keepalive option MAY modify the local idle timeout
associated with that TCP session if resources permit.
3.3.2. Sending Responses
A DNS server that receives a query sent using TCP transport that
includes an OPT RR (with or without the edns-tcp-keepalive option)
MAY include the edns-tcp-keepalive option in the response to signal
the expected idle timeout on a connection. Servers MUST specify the
TIMEOUT value that is currently associated with the TCP session. It
Wouters, et al. Standards Track PAGE 6
RFC 7828 The edns-tcp-keepalive EDNS0 Option April 2016
is reasonable for this value to change according to local resource
constraints. The DNS server SHOULD send an edns-tcp-keepalive option
with a timeout of 0 if it deems its local resources are too low to
service more TCP keepalive sessions or if it wants clients to close
currently open connections.
3.4. TCP Session Management
Both DNS clients and servers are subject to resource constraints that
will limit the extent to which TCP sessions can persist. Effective
limits for the number of active sessions that can be maintained on
individual clients and servers should be established, either as
configuration options or by interrogation of process limits imposed
by the operating system. Servers that implement edns-tcp-keepalive
should also engage in TCP connection management by recycling existing
connections when appropriate, closing connections gracefully, and
managing request queues to enable fair use.
In the event that there is greater demand for TCP sessions than can
be accommodated, servers may reduce the TIMEOUT value signalled in
successive DNS messages to minimise idle time on existing sessions.
This also allows, for example, clients with other candidate servers
to query to establish new TCP sessions with different servers in
expectation that an existing session is likely to be closed or to
fall back to UDP.
Based on TCP session resources, servers may signal a TIMEOUT value of
0 to request clients to close connections as soon as possible. This
is useful when server resources become very low or a denial-of-
service attack is detected and further maximises the shifting of
TIME_WAIT state to well-behaved clients.
However, it should be noted that RFC 6891 states:
Lack of presence of an OPT record in a request MUST be taken as an
indication that the requestor does not implement any part of this
specification and that the responder MUST NOT include an OPT
record in its response.
Since servers must be faithful to this specification even on a
persistent TCP connection, it means that (following the initial
exchange of timeouts) a server may not be presented with the
opportunity to signal a change in the idle timeout associated with a
connection if the client does not send any further requests
containing EDNS0 OPT RRs. This limitation makes persistent
connection handling via an initial idle timeout signal more
Wouters, et al. Standards Track PAGE 7
RFC 7828 The edns-tcp-keepalive EDNS0 Option April 2016
attractive than a mechanism that establishes default persistence and
then uses a connection close signal (in a similar manner to HTTP 1.1
[RFC 7230]).
If a client includes the edns-tcp-keepalive option in the first
query, it SHOULD include an EDNS0 OPT RR periodically in any further
messages it sends during the TCP session. This will increase the
chance of the client being notified should the server modify the
timeout associated with a session. The algorithm for choosing when
to do this is out of scope of this document and is left up to the
implementor and/or operator.
DNS clients and servers MAY close a TCP session at any time in order
to manage local resource constraints. The algorithm by which clients
and servers rank active TCP sessions in order to determine which to
close is not specified in this document.
3.5. Non-clean Paths
Many paths between DNS clients and servers suffer from poor hygiene,
limiting the free flow of DNS messages that include particular EDNS0
options or messages that exceed a particular size. A fallback
strategy similar to that described in [RFC 6891], Section 6.2.2 SHOULD
be employed to avoid persistent interference due to non-clean paths.
3.6. Anycast Considerations
DNS servers of various types are commonly deployed using anycast
[RFC 4786].
Changes in network topology between clients and anycast servers may
cause disruption to TCP sessions making use of edns-tcp-keepalive
more often than with TCP sessions that omit it, since the TCP
sessions are expected to be longer lived. It might be possible for
anycast servers to avoid disruption due to topology changes by making
use of TCP multipath [RFC 6824] to anchor the server side of the TCP
connection to an unambiguously unicast address.
4. Intermediary Considerations
It is RECOMMENDED that DNS intermediaries that terminate TCP
connections implement edns-tcp-keepalive. An intermediary that does
not implement edns-tcp-keepalive but sits between a client and server
that both support edns-tcp-keepalive might close idle connections
unnecessarily.
Wouters, et al. Standards Track PAGE 8
RFC 7828 The edns-tcp-keepalive EDNS0 Option April 2016
5. Security Considerations
The edns-tcp-keepalive option can potentially be abused to request
large numbers of long-lived sessions in a quick burst. When a DNS
server detects abusive behaviour, it SHOULD immediately close the TCP
connection and free the resources used.
Servers could choose to monitor client behaviour with respect to the
edns-tcp-keepalive option to build up profiles of clients that do not
honour the specified timeout.
Readers are advised to familiarise themselves with the security
considerations outlined in [RFC 7766]
6. IANA Considerations
IANA has assigned an EDNS0 option code for the edns-tcp-keepalive
option from the "DNS EDNS0 Option Codes (OPT)" registry as follows:
+-------+--------------------+----------+-----------+
| Value | Name | Status | Reference |
+-------+--------------------+----------+-----------+
| 11 | edns-tcp-keepalive | Standard | RFC 7828 |
+-------+--------------------+----------+-----------+
7. References
7.1. Normative References
[RFC 1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC 1035,
November 1987, <http://www.rfc-editor.org/info/RFC 1035>.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC 4033, March 2005,
<http://www.rfc-editor.org/info/RFC 4033>.
[RFC 4786] Abley, J. and K. Lindqvist, "Operation of Anycast
Services", BCP 126, RFC 4786, DOI 10.17487/RFC 4786,
December 2006, <http://www.rfc-editor.org/info/RFC 4786>.
Wouters, et al. Standards Track PAGE 9
RFC 7828 The edns-tcp-keepalive EDNS0 Option April 2016
[RFC 6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC 6891, April 2013,
<http://www.rfc-editor.org/info/RFC 6891>.
[RFC 7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC 7230, June 2014,
<http://www.rfc-editor.org/info/RFC 7230>.
[RFC 7766] Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
D. Wessels, "DNS Transport over TCP - Implementation
Requirements", RFC 7766, DOI 10.17487/RFC 7766, March 2016,
<http://www.rfc-editor.org/info/RFC 7766>.
7.2. Informative References
[CHAIN-QUERY]
Wouters, P., "Chain Query requests in DNS", Work in
Progress, draft-ietf-dnsop-edns-chain-query-07, February
2016.
[DNS-over-TLS]
Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over TLS", Work in
Progress, draft-ietf-dprive-dns-over-tls-09, March 2016.
[fragmentation-considered-poisonous]
Herzberg, A. and H. Shulman, "Fragmentation Considered
Poisonous", arXiv: 1205.4011, May 2012,
<http://arxiv.org/abs/1205.4011>.
[RFC 6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, DOI 10.17487/RFC 6824, January 2013,
<http://www.rfc-editor.org/info/RFC 6824>.
[RRL] Vixie, P. and V. Schryver, "DNS Response Rate Limiting
(DNS RRL)", ISC-TN 2012-1-Draft1, April 2012,
<https://ftp.isc.org/isc/pubs/tn/isc-tn-2012-1.txt>.
Wouters, et al. Standards Track PAGE 10
RFC 7828 The edns-tcp-keepalive EDNS0 Option April 2016
Acknowledgements
The authors acknowledge the contributions of Jinmei TATUYA and Mark
Andrews. Thanks to Duane Wessels for detailed review and the many
others who contributed to the mailing list discussion.
Authors' Addresses
Paul Wouters
Red Hat
Email: pwouters@redhat.com
Joe Abley
Dyn, Inc.
103-186 Albert Street
London, ON N6A 1M1
Canada
Phone: +1 519 670 9327
Email: jabley@dyn.com
Sara Dickinson
Sinodun Internet Technologies
Magdalen Centre
Oxford Science Park
Oxford OX4 4GA
United Kingdom
Email: sara@sinodun.com
URI: http://sinodun.com
Ray Bellis
Internet Systems Consortium, Inc
950 Charter Street
Redwood City, CA 94063
United States
Phone: +1 650 423 1200
Email: ray@isc.org
URI: http://www.isc.org
Wouters, et al. Standards Track PAGE 11
RFC TOTAL SIZE: 24282 bytes
PUBLICATION DATE: Thursday, April 7th, 2016
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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