Internet DRAFT - draft-dickinson-dnsop-5966-bis
draft-dickinson-dnsop-5966-bis
Network Working Group J. Dickinson
Internet-Draft Sinodun Internet Technologies
Updates: 5966 (if approved) R. Bellis
Intended status: Standards Track Nominet
Expires: April 29, 2015 A. Mankin
D. Wessels
Verisign Labs
October 26, 2014
DNS Transport over TCP - Implementation Requirements
draft-dickinson-dnsop-5966-bis-00
Abstract
This document updates the requirements for the support of TCP as a
transport protocol for DNS implementations.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on April 29, 2015.
Copyright Notice
Copyright (c) 2014 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
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Transport Protocol Selection . . . . . . . . . . . . . . . . 4
5. Connection Handling . . . . . . . . . . . . . . . . . . . . . 5
6. Query Pipelining . . . . . . . . . . . . . . . . . . . . . . 6
7. Response Reordering . . . . . . . . . . . . . . . . . . . . . 6
8. TCP Fast Open . . . . . . . . . . . . . . . . . . . . . . . . 7
9. Summary of Advantages and Disadvantages to using TCP for DNS 8
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
11. Security Considerations . . . . . . . . . . . . . . . . . . . 9
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
13.1. Normative References . . . . . . . . . . . . . . . . . . 9
13.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Changes to RFC 5966 . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
Most DNS [RFC1034] transactions take place over UDP [RFC0768]. TCP
[RFC0793] is always used for full zone transfers (AXFR) and is often
used for messages whose sizes exceed the DNS protocol's original
512-byte limit.
Section 6.1.3.2 of [RFC1123] states:
DNS resolvers and recursive servers MUST support UDP, and SHOULD
support TCP, for sending (non-zone-transfer) queries.
However, some implementors have taken the text quoted above to mean
that TCP support is an optional feature of the DNS protocol.
The majority of DNS server operators already support TCP and the
default configuration for most software implementations is to support
TCP. The primary audience for this document is those implementors
whose failure to support TCP restricts interoperability and limits
deployment of new DNS features.
This document therefore updates the core DNS protocol specifications
such that support for TCP is henceforth a REQUIRED part of a full DNS
protocol implementation.
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There are several advantages and disadvantages to the increased use
of TCP as well as implementation details that need to be considered.
This document addresses these issues and updates [RFC5966], with
additional considerations and lessons learned from new research and
implementations [Connection-Oriented-DNS].
Whilst this document makes no specific requirements for operators of
DNS servers to meet, it does offer some suggestions to operators to
help ensure that support for TCP on their servers and network is
optimal. It should be noted that failure to support TCP (or the
blocking of DNS over TCP at the network layer) may result in
resolution failure and/or application-level timeouts.
2. Terminology
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. Discussion
In the absence of EDNS0 (Extension Mechanisms for DNS 0) (see below),
the normal behaviour of any DNS server needing to send a UDP response
that would exceed the 512-byte limit is for the server to truncate
the response so that it fits within that limit and then set the TC
flag in the response header. When the client receives such a
response, it takes the TC flag as an indication that it should retry
over TCP instead.
RFC 1123 also says:
... 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. Thus, resolvers and
name servers should implement TCP services as a backup to UDP
today, with the knowledge that they will require the TCP service
in the future.
Existing deployments of DNS Security (DNSSEC) [RFC4033] have shown
that truncation at the 512-byte boundary is now commonplace. For
example, a Non-Existent Domain (NXDOMAIN) (RCODE == 3) response from
a DNSSEC-signed zone using NextSECure 3 (NSEC3) [RFC5155] is almost
invariably larger than 512 bytes.
Since the original core specifications for DNS were written, the
Extension Mechanisms for DNS (EDNS0 [RFC6891]) have been introduced.
These extensions can be used to indicate that the client is prepared
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to receive UDP responses larger than 512 bytes. An EDNS0-compatible
server receiving a request from an EDNS0-compatible client may send
UDP packets up to that client's announced buffer size without
truncation.
However, transport of UDP packets that exceed the size of the path
MTU causes IP packet fragmentation, which has been found to be
unreliable in some circumstances. Many firewalls routinely block
fragmented IP packets, and some do not implement the algorithms
necessary to reassemble fragmented packets. Worse still, some
network devices deliberately refuse to handle DNS packets containing
EDNS0 options. Other issues relating to UDP transport and packet
size are discussed in [RFC5625].
The MTU most commonly found in the core of the Internet is around
1500 bytes, and even that limit is routinely exceeded by DNSSEC-
signed responses.
The future that was anticipated in RFC 1123 has arrived, and the only
standardised UDP-based mechanism that may have resolved the packet
size issue has been found inadequate.
4. Transport Protocol Selection
All general-purpose DNS implementations MUST support both UDP and TCP
transport.
o Authoritative server implementations MUST support TCP so that they
do not limit the size of responses to what fits in a single UDP
packet.
o Recursive server (or forwarder) implementations MUST support TCP
so that they do not prevent large responses from a TCP-capable
server from reaching its TCP-capable clients.
o Stub resolver implementations (e.g., an operating system's DNS
resolution library) MUST support TCP since to do otherwise would
limit their interoperability with their own clients and with
upstream servers.
Regarding the choice of when to use UDP or TCP, Section 6.1.3.2 of
RFC 1123 also says:
... a DNS resolver or server that is sending a non-zone-transfer
query MUST send a UDP query first.
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This requirement is hereby relaxed. A resolver MAY elect to send
either TCP or UDP queries depending on local operational reasons. If
it already has an open TCP connection to the server it SHOULD reuse
this connection.
In essence, TCP SHOULD be considered as valid a transport as UDP. It
SHOULD NOT be used only for zone transfers and as a fallback.
In addition it is noted that all Recursive and Authoritative servers
MUST send responses using the same transport as the query arrived on.
In the case of TCP this MUST also be the same connection.
5. Connection Handling
One perceived disadvantage to DNS over TCP is the added connection
setup latency, generally equal to one RTT. To amortize connection
setup costs, both clients and servers SHOULD support connection reuse
by sending multiple queries and responses over a single TCP
connection.
DNS currently has no connection signaling mechanism. Clients and
servers may close a connection at any time. Clients MUST be prepared
to retry failed queries on broken connections.
Section 4.2.2 of [RFC1035] says:
If the server needs to close a dormant connection to reclaim
resources, it should wait until the connection has been idle for a
period on the order of two minutes. In particular, the server
should allow the SOA and AXFR request sequence (which begins a
refresh operation) to be made on a single connection. Since the
server would be unable to answer queries anyway, a unilateral
close or reset may be used instead of a graceful close.
Other more modern protocols (e.g., HTTP/1.1 [RFC7230]) have support
for persistent TCP connections and operational experience has shown
that long timeouts can easily cause resource exhaustion and poor
response under heavy load. Intentionally opening many connections
and leaving them dormant can trivially create a "denial-of-service"
attack.
It is therefore RECOMMENDED that the default application-level idle
period should be of the order of seconds, but no particular value is
specified. In practice, the idle period may vary dynamically, and
servers MAY allow dormant connections to remain open for longer
periods as resources permit.
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To mitigate the risk of unintentional server overload, DNS clients
MUST take care to minimize the number of concurrent TCP connections
made to any individual server. Similarly, servers MAY impose limits
on the number of concurrent TCP connections being handled for any
particular client. It is RECOMMENDED that for any given client -
server interaction there SHOULD be no more than one connection for
regular queries, one for zone transfers and one for each protocol
that is being used on top of TCP, for example, if the resolver was
using TLS. The server MUST NOT enforce these rules for a particular
client because it does not know if the client IP address belongs to a
single client or is, for example, multiple clients behind NAT.
6. Query Pipelining
Due to the use of TCP primarily for zone transfer and truncated
responses, no existing RFC discusses the idea of pipelining DNS
queries over a TCP connection.
In order to achieve performance on par with UDP, it is therefore
RECOMMENDED that DNS clients should pipeline their queries. When a
DNS client sends multiple queries to a server, it should not wait for
an outstanding reply before sending the next query. Clients should
treat TCP and UDP equivalently when considering the time at which to
send a particular query.
DNS servers (especially recursive) SHOULD expect to receive pipelined
queries. The server should process TCP queries in parallel, just as
it would for UDP. The handling of responses to pipelined queries is
covered in the following section.
When pipelining queries over TCP it is very easy to send more DNS
queries than there are DNS Message ID's. Implementations MUST take
care to check their list of outstanding DNS Message ID's before
sending a new query over an existing TCP connection. This is
especially important if the server could be performing out-of-order
processing. In addition, when sending multiple queries over TCP it
is very easy for a name server to overwhelm its own network
interface. Implementations MUST take care to manage buffer sizes or
to throttle writes to the network interface.
7. Response Reordering
RFC 1035 is ambiguous on the question of whether TCP responses may be
reordered -- the only relevant text is in Section 4.2.1, which
relates to UDP:
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Queries or their responses may be reordered by the network, or by
processing in name servers, so resolvers should not depend on them
being returned in order.
For the avoidance of future doubt, this requirement is clarified.
Authoritative servers and recursive resolvers are RECOMMENDED to
support the sending of responses in parallel and/or out-of-order,
regardless of the transport protocol in use. Stub and recursive
resolvers MUST be able to process responses that arrive in a
different order to that in which the requests were sent, regardless
of the transport protocol in use.
In order to achieve performance on par with UDP, recursive resolvers
SHOULD process TCP queries in parallel and return individual
responses as soon as they are available, possibly out-of-order.
Since responses may arrive out-of-order, clients must take care to
match responses to outstanding queries, using the ID field, port
number, query name/type/class, and any other relevant protocol
features.
8. TCP Fast Open
This section is non-normative.
TCP fastopen [I-D.ietf-tcpm-fastopen] (TFO) allows data to be carried
in the SYN packet. It also saves up to one RTT compared to standard
TCP.
TFO mitigates the security vulnerabilities inherent in sending data
in the SYN, especially on a system like DNS where amplification
attacks are possible, by use of a server-supplied cookie. TFO
clients request a server cookie in the initial SYN packet at the
start of a new connection. The server returns a cookie in its SYN-
ACK. The client caches the cookie and reuses it when opening
subsequent connections to the same server.
The cookie is stored by the client's TCP stack (kernel) and persists
if either the client or server processes are restarted. TFO also
falls back to a regular TCP handshake gracefully.
Adding support for this to existing name server implementations is
relatively easy, but does require source code modifications. On the
client, the call to connect() is replaced with a TFO aware version of
sendmsg() or sendto(). On the server, TFO must be switched into
server mode by changing the kernel parameter (net.ipv4.tcp_fastopen
on Linux) to enable the server bit (Set the integer value to 2
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(server only) or 3 (client and server)) and setting a socket option
between the bind() and listen() calls.
DNS services taking advantage of IP anycast [RFC4786] may need to
take additional steps when enabling TFO. From
[I-D.ietf-tcpm-fastopen]:
Servers that accept connection requests to the same server IP
address should use the same key such that they generate identical
Fast Open Cookies for a particular client IP address. Otherwise a
client may get different cookies across connections; its Fast Open
attempts would fall back to regular 3WHS.
9. Summary of Advantages and Disadvantages to using TCP for DNS
The TCP handshake generally prevents address spoofing and, therefore,
the reflection/amplification attacks which plague UDP.
TCP does not suffer from UDP's issues with fragmentation.
Middleboxes are known to block IP fragments, leading to timeouts and
forcing client implementations to "hunt" for EDNS0 reply size values
supported by the network path. Additionally, fragmentation may lead
to cache poisoning [fragmentation-considered-poisonous].
TCP setup costs an additional RTT compared to UDP queries. Setup
costs can be amortized by reusing connections, pipelining queries,
and enabling TCP Fast Open.
TCP imposes additional state-keeping requirements on clients and
servers. The use of TCP Fast Open reduces the cost of closing and
re-opening TCP connections.
Long-lived TCP connections to anycast servers may be disrupted due to
routing changes. Clients utilizing TCP for DNS must always be
prepared to re-establish connections or otherwise retry outstanding
queries. It may also possible for TCP Multipath [RFC6824] to allow a
server to hand a connection over from the anycast address to a
unicast address.
There are many "Middleboxes" in use today that interfere with TCP
over port 53 [RFC5625]. This document does not propose any
solutions, other than to make it absolutely clear that TCP is a valid
transport for DNS and must be supported by all implementations.
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10. IANA Considerations
This memo includes no request to IANA.
11. Security Considerations
Some DNS server operators have expressed concern that wider use of
DNS over TCP will expose them to a higher risk of denial-of-service
(DoS) attacks.
Although there is a higher risk of such attacks against TCP-enabled
servers, techniques for the mitigation of DoS attacks at the network
level have improved substantially since DNS was first designed.
Readers are advised to familiarise themselves with [CPNI-TCP].
Operators of recursive servers should ensure that they only accept
connections from expected clients, and do not accept them from
unknown sources. In the case of UDP traffic, this will help protect
against reflector attacks [RFC5358] and in the case of TCP traffic it
will prevent an unknown client from exhausting the server's limits on
the number of concurrent connections.
12. Acknowledgements
The authors would like to thank Liang Zhu, Zi Hu, and John Heidemann
for extensive DNS-over-TCP discussions and code; and Lucie Guiraud
and Danny McPherson for reviewing early versions of this document.
We would also like to thank all those who contributed to RFC 5966.
13. References
13.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
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[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.
[RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast
Services", BCP 126, RFC 4786, December 2006.
[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, March 2008.
[RFC5358] Damas, J. and F. Neves, "Preventing Use of Recursive
Nameservers in Reflector Attacks", BCP 140, RFC 5358,
October 2008.
[RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines", BCP
152, RFC 5625, August 2009.
[RFC5966] Bellis, R., "DNS Transport over TCP - Implementation
Requirements", RFC 5966, August 2010.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.
[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Message Syntax and Routing", RFC 7230, June
2014.
13.2. Informative References
[CPNI-TCP]
CPNI, "Security Assessment of the Transmission Control
Protocol (TCP)", 2009, <http://www.cpni.gov.uk/Docs/
tn-03-09-security-assessment-TCP.pdf>.
[Connection-Oriented-DNS]
Zhu, L., Hu, Z., Heidemann, J., Wessels, D., Mankin, A.,
and N. Somaiya, "T-DNS: Connection-Oriented DNS to Improve
Privacy and Security (extended)", <http://www.isi.edu/
publications/trpublic/files/tr-693.pdf>.
[I-D.ietf-tcpm-fastopen]
Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", draft-ietf-tcpm-fastopen-09 (work in
progress), July 2014.
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[RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, January 2013.
[fragmentation-considered-poisonous]
Herzberg, A. and H. Shulman, "Fragmentation Considered
Poisonous", May 2012, <http://arxiv.org/abs/1205.4011>.
Appendix A. Changes to RFC 5966
This document differs from RFC 5966 in four additions:
1. DNS implementations are recommended not only to support TCP but
to support it on an equal footing with UDP
2. DNS implementations are recommended to support reuse of TCP
connections
3. DNS implementations are recommended to support pipelining and out
of order processing of the query stream
4. A non-normative discussion of use of TCP Fast Open is added
Authors' Addresses
John Dickinson
Sinodun Internet Technologies
Magdalen Centre
Oxford Science Park
Oxford OX4 4GA
UK
Email: jad@sinodun.com
URI: http://sinodun.com
Ray Bellis
Nominet
Edmund Halley Road
Oxford OX4 4DQ
UK
Phone: +44 1865 332211
Email: ray.bellis@nominet.org.uk
URI: http://www.nominet.org.uk/
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Allison Mankin
Verisign Labs
12061 Bluemont Way
Reston, VA 20190
Phone: +1 703 948-3200
Email: amankin@verisign.com
Duane Wessels
Verisign Labs
12061 Bluemont Way
Reston, VA 20190
Phone: +1 703 948-3200
Email: dwessels@verisign.com
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