DPRIVE T. Reddy
Internet-Draft D. Wing
Intended status: Standards Track P. Patil
Expires: May 27, 2016 Cisco
November 24, 2015
DNS over DTLS (DNSoD)
draft-ietf-dprive-dnsodtls-03
Abstract
DNS queries and responses are visible to network elements on the path
between the DNS client and its server. These queries and responses
can contain privacy-sensitive information which is valuable to
protect. An active attacker can send bogus responses causing
misdirection of the subsequent connection.
To counter passive listening and active attacks, this document
proposes the use of Datagram Transport Layer Security (DTLS) for DNS,
to protect against passive listeners and certain active attacks. As
DNS needs to remain fast, this proposal also discusses mechanisms to
reduce DTLS round trips and reduce DTLS handshake size. The proposed
mechanism runs over port 853.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 27, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Relationship to TCP Queries and to DNSSEC . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. DTLS session initiation, Polling and Discovery . . . . . . . 3
4. Performance Considerations . . . . . . . . . . . . . . . . . 4
5. Established sessions . . . . . . . . . . . . . . . . . . . . 5
6. Anycast . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Downgrade attacks . . . . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9.1. Authenticating a DNS Privacy Server . . . . . . . . . . . 8
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
11.1. Normative References . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
The Domain Name System is specified in [RFC1034] and [RFC1035]. DNS
queries and responses are normally exchanged unencrypted and are thus
vulnerable to eavesdropping. Such eavesdropping can result in an
undesired entity learning domains that a host wishes to access, thus
resulting in privacy leakage. DNS privacy problem is further
discussed in [I-D.bortzmeyer-dnsop-dns-privacy].
Active attackers have long been successful at injecting bogus
responses, causing cache poisoning and causing misdirection of the
subsequent connection (if attacking A or AAAA records). A popular
mitigation against that attack is to use ephemeral and random source
ports for DNS queries [RFC5452].
This document defines DNS over DTLS (DNSoD, pronounced "dee-enn-sod")
which provides confidential DNS communication for stub resolvers,
recursive resolvers, iterative resolvers and authoritative servers.
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The motivations for proposing DNSoD are that
o TCP suffers from network head-of-line blocking, where the loss of
a packet causes all other TCP segments to not be delivered to the
application until the lost packet is re-transmitted. DNSoD,
because it uses UDP, does not suffer from network head-of-line
blocking.
o DTLS session resumption consumes 1 round trip whereas TLS session
resumption can start only after TCP handshake is complete.
Although TCP Fast Open [RFC7413] can reduce that handshake, TCP
Fast Open is not yet available in commercially-popular operating
systems.
1.1. Relationship to TCP Queries and to DNSSEC
DNS queries can be sent over UDP or TCP. The scope of this document,
however, is only UDP. DNS over TCP could be protected with TLS, as
described in [I-D.ietf-dprive-dns-over-tls].
DNS Security Extensions (DNSSEC [RFC4033]) provides object integrity
of DNS resource records, allowing end-users (or their resolver) to
verify legitimacy of responses. However, DNSSEC does not protect
privacy of DNS requests or responses. DNSoD works in conjunction
with DNSSEC, but DNSoD does not replace the need or value of DNSSEC.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
3. DTLS session initiation, Polling and Discovery
Many modern operating systems already detect if a web proxy is
interfering with Internet communications, using proprietary
mechanisms that are out of scope of this document. After that
mechanism has run (and detected Internet connectivity is working),
the DNSoD procedure described in this document should commence. This
timing avoids delays in joining the network (and displaying an icon
indicating successful Internet connection), at the risk that those
initial DNS queries will be sent without protection afforded by
DNSoD.
DNSoD MUST run over standard UDP port 853 as defined in Section 8. A
DNS server that supports DNSoD MUST listen for and accept DTLS
packets on a designated port 853.
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The host should determine if the DNS server supports DNSoD by sending
a DTLS ClientHello message. A DNS server that does not support DNSoD
will not respond to ClientHello messages sent by the client. The
client MUST use timer values defined in Section 4.2.4.1 of [RFC6347]
for retransmission of ClientHello message and if no response is
received from the DNS server. After 15 seconds, it MUST cease
attempts to re-transmit its ClientHello. If the DNS client receives
a hard ICMP error [RFC1122], it MUST immediately cease attempts to
re-transmit its ClientHello. Thereafter, the client MAY repeat that
procedure in the event the DNS server has been upgraded to support
DNSoD, but such probing SHOULD NOT be done more frequently than every
24 hours and MUST NOT be done more frequently than every 15 minutes.
This mechanism requires no additional signaling between the client
and server.
4. Performance Considerations
To reduce number of octets of the DTLS handshake, especially the size
of the certificate in the ServerHello (which can be several
kilobytes), DNS client and server can use raw public keys [RFC7250]
or Cached Information Extension [I-D.ietf-tls-cached-info]. Cached
Information Extension avoids transmitting the server's certificate
and certificate chain if the client has cached that information from
a previous TLS handshake.
Multiple DNS queries can be sent over a single DTLS session and the
DNSoD client need not wait for an outstanding reply before sending
the next query. The existing Query ID allows multiple requests and
responses to be interleaved in whatever order they can be fulfilled
by the DNS server. This means DNSoD reduces the consumption of UDP
port numbers, and because DTLS protects the communication between a
DNS client and its server, the resolver SHOULD NOT use random
ephemeral source ports (Section 9.2 of [RFC5452]) because such source
port use would incur additional, unnecessary DTLS load on the DNSoD
server. When sending multiple queries over a single DTLS session,
clients MUST take care to avoid Message ID collisions. In other
words, they MUST not re-use the DNS Message ID of an in-flight query.
It is highly advantageous to avoid server-side DTLS state and reduce
the number of new DTLS sessions on the server which can be done with
[RFC5077]. This also eliminates a round-trip for subsequent DNSoD
queries, because with [RFC5077] the DTLS session does not need to be
re-established.
Compared to normal DNS, DTLS adds at least 13 octets of header, plus
cipher and authentication overhead to every query and every response.
This reduces the size of the DNS payload that can be carried. DNS
client and server MUST support the EDNS0 option defined in [RFC6891]
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so that the DNS client can indicate to the DNS server the maximum DNS
response size it can handle without IP fragmentation. If the DNS
sever's response exceeds the EDNS0 value, the DNS server sets the TC
(truncated) bit. On receiving a response with the TC bit set, the
client establishes a DNS-over-TLS connection to the same server, and
sends a new DNS request for the same resource record
DNSoD puts an additional computational load on servers. The largest
gain for privacy is to protect the communication between the DNS
client (the end user's machine) and its caching resolver.
Implementing DNSoD on root servers is outside the scope of this
document.
5. Established sessions
In DTLS, all data is protected using the same record encoding and
mechanisms. When the mechanism described in this document is in
effect, DNS messages are encrypted using the standard DTLS record
encoding. When a user of DTLS wishes to send an DNS message, it
delivers it to the DTLS implementation as an ordinary application
data write (e.g., SSL_write()). To reduce client and server
workload, clients SHOULD re-use the DTLS session. A single DTLS
session can be used to send multiple DNS requests and receive
multiple DNS responses.
DNSoD client and server can use DTLS heartbeat [RFC6520] to verify
that the peer still has DTLS state. DTLS session is terminated by
the receipt of an authenticated message that closes the connection
(e.g., a DTLS fatal alert).
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Client Server
------ ------
ClientHello -------->
<------- HelloVerifyRequest
(contains cookie)
ClientHello -------->
(contains cookie)
(empty SessionTicket extension)
ServerHello
(empty SessionTicket extension)
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished -------->
NewSessionTicket
[ChangeCipherSpec]
<-------- Finished
DNS Request --------->
<--------- DNS Response
Message Flow for Full Handshake Issuing New Session Ticket
6. Anycast
DNS servers are often configured with anycast addresses. While the
network is stable, packets transmitted from a particular source to an
anycast address will reach the same server that has the cryptographic
context from the DNS over DTLS handshake. But when the network
configuration changes, a DNS over DTLS packet can be received by a
server that does not have the necessary cryptographic context. To
encourage the client to initiate a new DTLS handshake, DNS servers
SHOULD generate a DTLS Alert message in response to receiving a DTLS
packet for which the server does not have any cryptographic context.
Upon receipt of an un-authenicated DTLS alert, the DTLS client
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validates the Alert is within the replay window, as usual
(Section 4.1.2.6 of [RFC6347]). It is difficult for the DTLS client
to validate the DTLS alert was generated by the DTLS server in
response to a request or was generated by an on- or off-path
attacker. Thus, upon receipt of an in-window DTLS Alert, the client
SHOULD continue re-transmitting the DTLS packet (in the event the
Alert was spoofed), and at the same time it SHOULD initiate DTLS
session resumption.
7. Downgrade attacks
Using DNS privacy with an authenticated server is most preferred, DNS
privacy with an unauthenticated server is next preferred, and plain
DNS is least preferred. This section gives a non-normative
discussion on common behaviors and choices.
An implementation MAY attempt to obtain DNS privacy by contacting DNS
servers on the local network (provided by DHCP) and on the Internet,
and make those attempts in parallel to reduce user impact. If DNS
privacy cannot be successfully negotiated for whatever reason, the
client can do three things:
1. refuse to send DNS queries on this network, which means the
client cannot make effective use of this network, as modern
networks require DNS; or,
2. use opportunistic security, as described in [RFC7435]. or,
3. send plain DNS queries on this network, which means no DNS
privacy is provided.
Heuristics can improve this situation, but only to a degree (e.g.,
previous success of DNS privacy on this network may be reason to
alert the user about failure to establish DNS privacy on this network
now). Still, the client (in cooperation with the end user) has to
decide to use the network without the protection of DNS privacy.
8. IANA Considerations
IANA is requested to add the following value to the "Service Name and
Transport Protocol Port Number Registry" registry in the System
Range. The registry for that range requires IETF Review or IESG
Approval [RFC6335] and such a review has been requested using the
Early Allocation process [RFC7120] for the well-known UDP port in
this document.
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Service Name domain-s
Transport Protocol(s) UDP/TCP
Port 853
Assignee IESG
Contact dwing@cisco.com
Description DNS query-response protocol runs over
DTLS and TLS
Reference This document
9. Security Considerations
The interaction between a DNS client and DNS server requires Datagram
Transport Layer Security (DTLS) with a ciphersuite offering
confidentiality protection and guidance given in [RFC7525] must be
followed to avoid attacks on DTLS. Once a DNSoD client has
established a security association with a particular DNS server, and
outstanding normal DNS queries with that server (if any) have been
received, the DNSoD client MUST ignore any subsequent normal DNS
responses from that server, as all subsequent responses should be
encrypted. This behavior mitigates all possible attacks described in
Measures for Making DNS More Resilient against Forged Answers
[RFC5452].
The DNS Fragment extension does not impact security of DTLS session
establishment or application data exchange. DNS Fragment provides
fragmentation and reassembly of the encrypted DNS payload.
A malicious client might attempt to perform a high number of DTLS
handshakes with a server. As the clients are not uniquely identified
by the protocol and can be obfuscated with IPv4 address sharing and
with IPv6 temporary addresses, a server needs to mitigate the impact
of such an attack. Such mitigation might involve rate limiting
handshakes from a certain subnet or more advanced DoS/DDoS techniques
beyond the scope of this paper.
9.1. Authenticating a DNS Privacy Server
DNS privacy requires encrypting the query (and response) from passive
attacks. Such encryption typically provides integrity protection as
a side-effect, which means on-path attackers cannot simply inject
bogus DNS responses. However, to provide stronger protection from
active attackers pretending to be the server, the server itself needs
to be authenticated.
To authenticate the server providing DNS privacy, the DNS client
needs to be configured with the names or IP addresses of those DNS
privacy servers. The server certificate MUST contain DNS-ID
(subjectAltName) as described in Section 4.1 of [RFC6125]. DNS names
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and IP addresses can be contained in the subjectAltName entries. The
client MUST use the rules and guidelines given in section 6 of
[RFC6125] to validate the DNS server identity.
This could be implemented by adding the certificate name to the /etc/
resolv.conf file, such as below:
nameserver 8.8.8.8
certificate google-public-dns.google.com
nameserver 208.67.220.220
certificate resolver.opendns.com
For DNS privacy servers that don't have a certificate trust chain
(e.g., because they are on a home network or a corporate network),
the configured list of DNS privacy servers can contain the Subject
Public Key Info (SPKI) fingerprint of the DNS privacy server (i.e., a
simple whitelist of name and SPKI fingerprint). The public key is
used for the same reasons HTTP pinning [RFC7469] uses the public key.
Raw public key-based authentication mechanism defined in [RFC7250]
can be also used to authenticate the DNS server.
This could be implemented by adding the SPKI fingerprint to the /etc/
resolv.conf file, such as below (line split for Internet Draft
formatting):
nameserver 192.168.1.1
sha256 : "d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="
The only algorithm considered at this time is "sha256", i.e., the
hash algorithm SHA256 [RFC6234]; additional algorithms may be allowed
for use in this context in the future. The quoted-string is a
sequence of base 64 digits: the base64-encoded SPKI Fingerprint
[RFC4648].
10. Acknowledgements
Thanks to Phil Hedrick for his review comments on TCP and to Josh
Littlefield for pointing out DNSoD load on busy servers (most notably
root servers). The authors would like to thank Simon Josefsson,
Daniel Kahn Gillmor, Bob Harold, Ilari Liusvaara and Sara Dickinson
for discussions and comments on the design of DNSoD.
11. References
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11.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<http://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<http://www.rfc-editor.org/info/rfc4033>.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492,
DOI 10.17487/RFC4492, May 2006,
<http://www.rfc-editor.org/info/rfc4492>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<http://www.rfc-editor.org/info/rfc4648>.
[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, <http://www.rfc-editor.org/info/rfc5077>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
DOI 10.17487/RFC5288, August 2008,
<http://www.rfc-editor.org/info/rfc5288>.
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[RFC5452] Hubert, A. and R. van Mook, "Measures for Making DNS More
Resilient against Forged Answers", RFC 5452,
DOI 10.17487/RFC5452, January 2009,
<http://www.rfc-editor.org/info/rfc5452>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <http://www.rfc-editor.org/info/rfc6125>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<http://www.rfc-editor.org/info/rfc6234>.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
<http://www.rfc-editor.org/info/rfc6335>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>.
[RFC6520] Seggelmann, R., Tuexen, M., and M. Williams, "Transport
Layer Security (TLS) and Datagram Transport Layer Security
(DTLS) Heartbeat Extension", RFC 6520,
DOI 10.17487/RFC6520, February 2012,
<http://www.rfc-editor.org/info/rfc6520>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC6891, April 2013,
<http://www.rfc-editor.org/info/rfc6891>.
[RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code
Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
2014, <http://www.rfc-editor.org/info/rfc7120>.
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
December 2014, <http://www.rfc-editor.org/info/rfc7435>.
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[RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
2015, <http://www.rfc-editor.org/info/rfc7469>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <http://www.rfc-editor.org/info/rfc7525>.
11.2. Informative References
[I-D.bortzmeyer-dnsop-dns-privacy]
Bortzmeyer, S., "DNS privacy considerations", draft-
bortzmeyer-dnsop-dns-privacy-02 (work in progress), April
2014.
[I-D.ietf-dprive-dns-over-tls]
Zi, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "DNS over TLS: Initiation and Performance
Considerations", draft-ietf-dprive-dns-over-tls-01 (work
in progress), October 2015.
[I-D.ietf-tls-cached-info]
Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension", draft-ietf-tls-
cached-info-20 (work in progress), October 2015.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<http://www.rfc-editor.org/info/rfc1122>.
[RFC3749] Hollenbeck, S., "Transport Layer Security Protocol
Compression Methods", RFC 3749, DOI 10.17487/RFC3749, May
2004, <http://www.rfc-editor.org/info/rfc3749>.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<http://www.rfc-editor.org/info/rfc4821>.
[RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
"Increasing TCP's Initial Window", RFC 6928,
DOI 10.17487/RFC6928, April 2013,
<http://www.rfc-editor.org/info/rfc6928>.
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[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <http://www.rfc-editor.org/info/rfc7250>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<http://www.rfc-editor.org/info/rfc7413>.
Authors' Addresses
Tirumaleswar Reddy
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: tireddy@cisco.com
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, California 95134
USA
Email: dwing@cisco.com
Prashanth Patil
Cisco Systems, Inc.
Bangalore
India
Email: praspati@cisco.com
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