Internet DRAFT - draft-wouters-edns-tcp-keepalive
draft-wouters-edns-tcp-keepalive
Network Working Group P. Wouters
Internet-Draft Red Hat
Intended status: Standards Track J. Abley
Expires: August 18, 2014 Dyn, Inc.
February 14, 2014
The edns-tcp-keepalive EDNS0 Option
draft-wouters-edns-tcp-keepalive-01
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 are currently
limited in their use of the TCP transport as RFC 5966 suggests
closing idle TCP sessions "in the order of seconds", making use of
TCP only suitable for individual queries generated as a fallback
protocol for truncated UDP answers.
This document defines an EDNS0 option ("edns-tcp-keepalive") that
allows DNS clients and servers to signal their respective readiness
to conduct multiple DNS transactions over individual TCP sessions.
This signalling facilitates a better balance of UDP and TCP transport
between individual clients and servers, reducing the impact of
problems associated with UDP transport and allowing the state
associated with TCP transport to be managed effectively with minimal
impact on the DNS transaction time.
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 18, 2014.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 4
3. The edns-tcp-keepalive Option . . . . . . . . . . . . . . . . 4
3.1. Option Format . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Use by DNS Clients . . . . . . . . . . . . . . . . . . . 5
3.2.1. Sending Queries . . . . . . . . . . . . . . . . . . . 5
3.2.2. Receiving Responses . . . . . . . . . . . . . . . . . 5
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 . . . . . . . . . . . . . . . . . . . . . 7
3.6. Anycast Considerations . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Editors' Notes . . . . . . . . . . . . . . . . . . . 9
A.1. Venue . . . . . . . . . . . . . . . . . . . . . . . . . . 9
A.2. Abridged Change History . . . . . . . . . . . . . . . . . 9
A.2.1. draft-wouters-edns-tcp-keepalive-00 . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
DNS messages between clients and servers may be received over either
UDP or TCP [RFC1035]. Historically, DNS clients used API's that only
facilitated sending and receiving a single query over either UDP or
TCP. New APIs and deployment of DNSSEC validating resolvers on hosts
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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,
since clients that have fallen back to TCP transport in response to a
truncated response typically only uses the TCP session for a single
(request, response) pair, continuing with UDP transport for
subsequent queries.
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. Mitigation
of such attacks on authoritative-only servers is possible using an
approach known as Response Rate-Limiting [RRL], an approach designed
to minimise the frequency at which legitimate responses are discarded
by truncating responses that appear to be motivated by an attacker,
forcing legitimate clients to re-query using TCP transport.
[RFC1035] specified a maximum DNS message size over UDP transport of
512 bytes. Deployment of DNSSEC [RFC4033] and other protocols
subsequently increased the observed frequency at which responses
exceed this limit. EDNS0 [RFC6891] 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.
The use of TCP transport does not suffer from the risks of
fragmentation nor reflection attacks. However, TCP transport as
currently deployed has expensive overhead.
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 1 x RTT to 2 x
RTT. There is no such overhead for a session that is already
established, however, and the overall impact of the TCP setup
handshake when the resulting session is used to exchange N DNS
message pairs over a single session, (1 + N)/N, approaches unity as N
increases.
(It should perhaps be noted that the overhead for a DNS transaction
over UDP truncacated due to RRL is 3x RTT, higher than the overhead
imposed on the same transaction initiated over TCP.)
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With increased deployment of DNSSEC and new RRtypes containing
application specific cryptographic material, there is an increase in
the prevalence of truncated responses received over UDP with fallback
to TCP.
The use of TCP transport requires considerably more 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 that those which are unlikely to be used for the
exchange of multiple DNS messages are closed promptly.
This document proposes a signalling mechanism between DNS clients and
servers that provides a means to better balance the use of UDP and
TCP transport, reducing the impact of problems associated with UDP
whilst constraining the impact of TCP on response times and server
resources to a manageable level.
The reduced overhead of this extension adds up significantly when
combined with other edns extensions, such as [CHAIN-QUERY]. The
combination of these two EDNS extensions make it possible for hosts
on high-latency mobile networks to natively perform DNSSEC
validation.
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 [RFC2119].
3. The edns-tcp-keepalive Option
This document specifies a new EDNS0 [RFC6891] option, edns-tcp-
keepalive, which can be used by DNS clients and servers to signal a
willingness to keep an (idle) TCP session open for a certain amount
of time to conduct future DNS transactions. This specification does
not distinguish between different types of DNS client and server in
the use of this option.
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 !
+-------------------------------+-------------------------------+
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| TIMEOUT |
+---------------------------------------------------------------+
where:
OPTION-CODE: the EDNS0 option code assigned to edns-tcp-keepalive,
[TBD]
OPTION-LENGTH: the value 2;
TIMEOUT: a timeout value for the TCP connection, specified in
seconds, encoded in network byte order.
3.2. Use by DNS Clients
3.2.1. Sending Queries
DNS clients MAY include the edns-tcp-keepalive option in queries sent
using UDP transport to signal their general ability to use individual
TCP sessions for multiple DNS transactions with a particular server.
DNS clients MAY include the edns-tcp-keepalive option in the first
query sent to a server using TCP transport to signal their desire
that that specific TCP session be used for multiple DNS transactions.
Clients MAY specify a TIMEOUT value that is representative of the
minimum expected time an individual TCP session should remain
established for it to be used by multiple DNS transactions.
In the case where there are multiple candidate servers available to
service a particular transaction, clients MAY include data associated
with all servers when computing a TIMEOUT value to be signalled to
any one of those servers.
If the client has insufficient data to be able to provide a
meaningful estimate, the TIMEOUT value MUST be set to zero.
3.2.2. Receiving Responses
A DNS client that receives a response using UDP transport that
includes the edns-tcp-keepalive option MAY record the presence of the
option and the associated TIMEOUT value, and use that information as
part of its server selection algorithm in the case where multiple
candidate servers are available to service a particular query.
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A DNS client that receives a response using TCP transport that
includes the edns-tcp-keepalive option MAY keep the existing TCP
session open.
A DNS client that receives a response that includes the edns-tcp-
keepalive option with a TIMEOUT value of 0 is allowed to keep the TCP
connection open indefinately.
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 MAY record the presence of the option
for statistical purposes, but should not otherwise modify its usual
behaviour in sending a response.
A DNS server that receives a query using TCP transport that includes
the edns-tcp-keepalive option SHOULD extend the timeout normally
associated with TCP sessions if resources permit, using the TIMEOUT
value supplied by the client as a guide.
3.3.2. Sending Responses
DNS servers MAY include the edns-tcp-keepalive option in responses
sent using UDP transport to signal their general ability to use
individual TCP sessions for multiple DNS transactions with a
particular server. The TIMEOUT value should be indicative of what a
client might expect if it was to open a TCP session with the server
and receive a response with the edns-tcp-keepalive option present.
The DNS server MAY omit including the edns-tcp-keepalive option if it
is running too low on resources to service more TCP keepalive
sessions.
DNS servers MAY include the edns-tcp-keepalive option in responses
sent using TCP transport to signal their ability to use that specific
session to exchange multiple DNS transactions. Servers MUST specify
the TIMEOUT value that is currently associated with the TCP session.
It is reasonable for this value to change according to local resource
constraints. The DNS server MAY omit including the edns-tcp-
keepalive option if it deems its local resources are too low to
service more TCP keepalive sessions.
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3.4. TCP Session Management
Both DNS clients and servers are subject to resource constraints
which 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.
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 avoid abrupt termination of a session.
This 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.
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 [RFC6891] 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
[RFC4786].
Successive DNS transactions between a client and server using UDP
transport may involve responses generated by different anycast nodes,
and the use of anycast in the implementation of a DNS server is
effectively undetectable by the client. The edns-tcp-keepalive
option SHOULD NOT be included in responses using UDP transport from
servers provisioned using anycast unless all anycast server nodes are
capable of processing the edns-tcp-keepalive option.
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. Anycast servers MAY make
use of TCP multipath [RFC6824] to anchor the server side of the TCP
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connection to an unambiguously-unicast address in order to avoid
disruption due to topology changes.
4. Security Considerations
The edns-tcp-keep-alive option can potentially be abused to request
large numbers of sessions in a quick burst. When a Nameserver
detects abusive behaviour, it SHOULD immediately close the TCP
connection and free all buffers used.
This section needs more work. As usual.
5. IANA Considerations
The IANA is directed to assign an EDNS0 option code for the edns-tcp-
keepalive option from the DNS EDNS0 Option Codes (OPT) registry as
follows:
+-------+--------------------+----------+-----------------+
| Value | Name | Status | Reference |
+-------+--------------------+----------+-----------------+
| [TBA] | edns-tcp-keepalive | Optional | [This document] |
+-------+--------------------+----------+-----------------+
6. Acknowledgements
The authors acknowledge the contributions of Ray Bellis, Jinmei
TATUYA and Mark Andrews.
7. References
7.1. Normative References
[CHAIN-QUERY]
Wouters, P., "chain Query requests in DNS", draft-wouters-
edns-chain-query (work in progress), February 2014.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, March 2005.
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[RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast
Services", BCP 126, RFC 4786, December 2006.
[RFC5966] Bellis, R., "DNS Transport over TCP - Implementation
Requirements", RFC 5966, August 2010.
[RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, January 2013.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.
7.2. Informative References
[RRL] Vixie, P. and V. Schryver, "DNS Response Rate Limiting
(DNS RRL)", ISC-TN 2012-1-Draft1, April 2012.
Appendix A. Editors' Notes
A.1. Venue
An appropriate venue for discussion of this document is
dnsext@ietf.org.
A.2. Abridged Change History
A.2.1. draft-wouters-edns-tcp-keepalive-00
Initial draft.
Authors' Addresses
Paul Wouters
Red Hat
Email: pwouters@redhat.com
Joe Abley
Dyn, Inc.
470 Moore Street
London, ON N6C 2C2
Canada
Phone: +1 519 670 9327
Email: jabley@dyn.com
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