Internet DRAFT - draft-wing-dnsop-dnsodtls
draft-wing-dnsop-dnsodtls
DNSOP Working Group T. Reddy
Internet-Draft D. Wing
Intended status: Standards Track P. Patil
Expires: January 4, 2015 Cisco
July 3, 2014
DNS over DTLS (DNSoD)
draft-wing-dnsop-dnsodtls-01
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 the default DNS port and can also run over an
alternate port.
Status of This Memo
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This Internet-Draft will expire on January 4, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Relationship to TCP Queries and to DNSSEC . . . . . . . . . . 3
3. Common problems with DNS Privacy . . . . . . . . . . . . . . 3
3.1. Firewall Blocking Ports or DNS Privacy Protocol . . . . . 3
3.2. Authenticating the DNS Privacy Server . . . . . . . . . . 4
3.3. Downgrade attacks . . . . . . . . . . . . . . . . . . . . 4
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Incremental Deployment . . . . . . . . . . . . . . . . . . . 5
6. Demultiplexing, Polling, Port Usage, and Discovery . . . . . 5
7. Performance Considerations . . . . . . . . . . . . . . . . . 6
8. Established sessions . . . . . . . . . . . . . . . . . . . . 7
9. DTLS Features and Cipher Suites . . . . . . . . . . . . . . . 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
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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.
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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.
2. 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.hzhwm-start-tls-for-dns]. Alternatively, a shim
protocol could be defined between DTLS and DNS, allowing large
responses to be sent over DTLS itself, see Section 7.
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.
3. Common problems with DNS Privacy
This section describes problems common to any DNS privacy solution.
To achieve DNS privacy an encrypted and integrity-protected channel
is needed between the client and server. This channel can be
blocked, and the client needs to react to such blockages.
3.1. Firewall Blocking Ports or DNS Privacy Protocol
When sending DNS over an encrypted channel, there are two choices:
send the encrypted traffic over the DNS ports (UDP 53, TCP 53) or
send the encrypted traffic over a different port. The encrypted
traffic is not normal DNS traffic, but rather is a cryptographic
handshake followed by encrypted payloads. There can be firewalls,
other security devices, or intercepting DNS proxies which block the
non-DNS traffic or otherwise react negatively (e.g., quarantining the
host for suspicious behavior). Alternatively, if a different port is
used for the encrypted traffic, a firewall or other security device
might block that port or otherwise react negatively.
There is no panacea, and only experiments on the Internet will
uncover which technique or combination of techniques will work best.
The authors believe a combination of techniques will be necessary, as
that has proven necessary with other protocols that desire to work on
existing networks.
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3.2. Authenticating the 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 of those DNS privacy servers.
When connecting a DNS privacy server, the server's IP address can be
converted to its hostname by doing a DNS PTR lookup, verifying that
the name matches the pre-configured list of DNS privacy servers, and
finally validating its certificate trust chain or a local list of
certificates. 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
certificate fingerprint of the DNS privacy server (i.e., a simple
whitelist of name and certificate fingerprint).
3.3. 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. An implementation will attempt to obtain DNS
privacy by contacting DNS servers on the local network (provided by
DHCP) and on the Internet, and will make those attempts in parallel
to reduce user impact. If DNS privacy cannot be successfully
negotiated for whatever reason, client can do three things:
1. refuse to send DNS queries on this network, which means the
client can not make effective use of this network, as modern
networks require DNS; or,
2. use DNS privacy with an un-authorized server, which means an
attacker could be spoofing the handshake with the DNS privacy
server; 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.
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4. 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].
5. Incremental Deployment
DNSoD can be deployed incrementally by the Internet Service Provider
or as an Internet service.
If the ISP's DNS resolver supports DNSoD, then DNS queries are
protected from passive listening and from many active attacks along
that path.
DNSoD can be offered as an Internet service, and a stub resolver or
DNS resolver can be configured to point to that DNSoD server (rather
than to the ISP-provided DNS server).
6. Demultiplexing, Polling, Port Usage, and Discovery
[Note - This section requires further discussion]
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 can run over standard UDP port 53 as defined in [RFC1035]. A
DNS client or server that does not implement this specification will
not respond to the incoming DTLS packets because they don't parse as
DNS packets (the DNS Opcode would be 15, which is undefined). A DNS
client or server that does implement this specification can
demultiplex DNS and DTLS packets by examining the third octet. For
TLS 1.2, which is what is defined by this specification, a DTLS
packet will contain 253 in the third octet, whereas a DNS packet will
never contain 253 in the third octet.
There has been some concern with sending DNSoD traffic over the same
port as normal, un-encrypted DNS traffic. The intent of this section
is to show that DNSoD could successfully be sent over port 53.
Further analysis and testing on the Internet may be valuable to
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determine if multiplexing on port 53, using a separate port, or some
fallback between a separate port and port 53 brings the most success.
After performing the above steps, 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, because they are not valid
DNS requests (specifically, the DNS Opcode is invalid). 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. 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.
7. 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), we should consider using plain public keys
[I-D.ietf-tls-oob-pubkey]. Considering that to authorize a certain
DNS server the client already needs explicit configuration of the DNS
servers it trusts, maybe the public key configuration problem is
really no worse than the configuration problem of those whitelisted
certificates?
Multiple DNS queries can be sent over a single DNSoD security
association. The existing QueryID 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 the
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.
It is highly advantageous to avoid server-side DTLS state and reduce
the number of new DTLS security associations on the server which can
be done with [RFC5077]. This also eliminates a round-trip for
subsequent DNSoD queries, because with [RFC5077] the DTLS security
association does not need to be re-established. Note: with the shim
(described below) perhaps we could send the query and the restore
server-side state in the ClientHello packet.
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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.
Certain DNS responses are large (e.g., many AAAA records, TXT, SRV)
and don't fit into a single UDP packet, causing a partial response
with the truncation (TC) bit set. The client is then expected to
repeat the query over TCP, which causes additional name resolution
delay. We have considered two ideas, one that reduces the need to
switch to TCP and another that eliminates the need to switch to TCP:
o Path MTU can be determined using Packetization Layer Path MTU
Discovery [RFC4821] using DTLS heartbeat. [RFC4821] does not rely
on ICMP or ICMPv6, and would not affect DNS state or
responsiveness on the client or server. However, it would be
additional chattiness.
o To avoid IP fragmentation, DTLS handshake messages incorporate
their own fragment offset and fragment length. We might utilize a
similar mechanism in a shim layer between DTLS and DNS, so that
large DNS messages could be carried without causing IP
fragmentation.
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. Because of
the load imposed, and because of the infrequency of queries to root
servers means the DTLS overhead is unlikely to be amoritized over the
DNS queries sent over that DTLS connection, implementing DNSoD on
root servers is NOT RECOMMENDED.
8. 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()). A single DTLS session can be used to
receive multiple DNS requests and generate DNS multiple responses.
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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
9. DTLS Features and Cipher Suites
To improve interoperability, the set of DTLS features and cipher
suites is restricted. The DTLS implementation MUST disable
compression. DTLS compression can lead to the exposure of
information that would not otherwise be revealed [RFC3749]. Generic
compression is unnecessary since DNS provides compression features
itself. DNS over DTLS MUST only be used with cipher suites that have
ephemeral key exchange, such as the ephemeral Diffie-Hellman (DHE)
[RFC5246] or the elliptic curve variant (ECDHE) [RFC4492]. Ephemeral
key exchange MUST have a minimum size of 2048 bits for DHE or
security level of 128 bits for ECDHE. Authenticated Encryption with
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Additional Data (AEAD) modes, such as the Galois Counter Model (GCM)
mode for AES [RFC5288] are acceptable.
10. IANA Considerations
If demultiplexing DTLS and DNS (using the third octet, Section 6) is
useful, we should reserve DNS Opcode 15 to ensure DNS always has a 0
bit where DTLS always has a 1 bit.
11. Security Considerations
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 inside DNSoD. This behavior mitigates all (?)
attacks described in Measures for Making DNS More Resilient against
Forged Answers [RFC5452].
Security considerations discussed in DTLS [RFC6347] also apply to
this document.
12. 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).
13. References
13.1. Normative References
[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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", RFC 3596,
October 2003.
[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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[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, May 2006.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, January 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
August 2008.
[RFC5452] Hubert, A. and R. van Mook, "Measures for Making DNS More
Resilient against Forged Answers", RFC 5452, January 2009.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.
13.2. 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.hzhwm-start-tls-for-dns]
Zi, Z., Zhu, L., Heidemann, J., Mankin, A., and D.
Wessels, "Starting TLS over DNS", draft-hzhwm-start-tls-
for-dns-00 (work in progress), February 2014.
[I-D.ietf-tls-oob-pubkey]
Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S., and
T. Kivinen, "Using Raw Public Keys in Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", draft-ietf-tls-oob-pubkey-11 (work in progress),
January 2014.
[RFC3749] Hollenbeck, S., "Transport Layer Security Protocol
Compression Methods", RFC 3749, May 2004.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
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[RFC4892] Woolf, S. and D. Conrad, "Requirements for a Mechanism
Identifying a Name Server Instance", RFC 4892, June 2007.
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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