Internet DRAFT - draft-hzhwm-start-tls-for-dns
draft-hzhwm-start-tls-for-dns
Network Working Group Z. Hu
Internet-Draft L. Zhu
Intended status: Standards Track J. Heidemann
Expires: January 5, 2015 USC/Information Sciences
Institute
A. Mankin
D. Wessels
Verisign Labs
July 4, 2014
Starting TLS over DNS
draft-hzhwm-start-tls-for-dns-01
Abstract
This document describes a technique for upgrading a DNS TCP
connection to use Transport Layer Security (TLS) over standard ports.
Encryption provided by DNS-over-TLS eliminates opportunities for
eavesdropping of DNS queries in the network. The proposed mechanism
is backwards compatible with clients and servers that are not aware
of DNS-over-TLS.
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 January 5, 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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publication of this document. Please review these documents
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described in the Simplified BSD License.
1. Introduction
Today, nearly all DNS queries ([RFC1034] and [RFC1035]) are sent
unencrypted, which makes them vulnerable to eavesdropping by an
attacker that has access to the network channel, reducing the privacy
of the querier. Recent news reports have elevated these concerns,
and ongoing efforts are beginning to identify privacy concerns about
DNS ([draft-bortzmeyer-dnsop-dns-privacy]).
Prior work has addressed some aspects of DNS security, but none
addresses privacy between a DNS client and server using standard
protocols. DNS Security Extensions (DNSSEC, [RFC4033]) provide
_response integrity_ by defining mechanisms to cryptographically sign
zones, allowing end-users (or their first-hop resolver) to verify
replies are correct. DNSSEC however does nothing to protect request
or response privacy. Traditionally, either privacy was not
considered a requirement for DNS traffic, or it was assumed that
network traffic was sufficiently private, however these perceptions
are evolving due to recent events.
More recently, DNSCurve [draft-dempsky-dnscurve] defines a method to
provide link-level confidentiality and integrity between DNS clients
and servers. However, it does so with a new cryptographic protocol
and so does not take advantage of TLS. ConfidentialDNS
[draft-wijngaards-confidentialdns] and IPSECA
[draft-osterweil-dane-ipsec] use opportunistic encryption to provide
privacy for DNS queries and responses. However, it is unclear how a
client can locate an RR specific to its first-hop resolver. Finally,
others have suggested DNS-over-TLS. Recent work suggests DNS-over-
TLS ([draft-bortzmeyer-dnsop-privacy-sol]), and the Unbound DNS
software [unbound] includes a DNS-over-TLS implementation. However,
neither defines methods to negotiate TLS use over an existing
connection; unbound instead requires DNS-over-TLS to run on a
different port.
The mechanism described in this document enables DNS clients and
servers to upgrade an existing DNS-over-TCP connection to a DNS-over-
TLS connection. It is analogous to STARTTLS [RFC2595] used in SMTP
[RFC3207], IMAP [RFC3501] and POP [RFC1939].
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This document defines only the protocol extensions necessary to
support TLS negotiation. It does not describe how DNS clients might
validate server certificates or specify trusted certificate
authorities. Solutions for certificate authentication are outside
the scope of this document.
1.1. Reserved Words
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 [RFC2119].
2. Protocol Changes
Clients and servers indicate their support for, and desire to use,
DNS-over-TLS by setting a bit in the Flags field of the EDNS0
[RFC6891] OPT meta-RR. The "TLS OK" (TO) bit is defined as the
second bit of the third and fourth bytes of the "extended RCODE and
flags" portion of the EDNS0 OPT meta-RR, immediately adjacent to the
"DNSSEC OK" (DO) bit [RFC4033]:
+0 (MSB) +1 (LSB)
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
0: | EXTENDED-RCODE | VERSION |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
2: |DO|TO| Z |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
2.1. Use by DNS Clients
2.1.1. Sending Queries
DNS clients MAY set the TO bit in queries sent using UDP transport to
signal their general ability to support DNS-over-TLS. Clients which
get no response to UDP TO=1 queries SHOULD retransmit them without
the TO bit set.
DNS clients MAY set the TO bit in the initial query sent to a server
using TCP transport to signal their desire that the TCP connection be
upgraded to TLS. DNS clients MUST NOT set the TO bit on subsequent
queries when using TCP or TLS transport (to avoid ambiguity).
Since the motivation for DNS-over-TLS is to preserve privacy, DNS
clients SHOULD use a query that reveals no private information in the
initial TO=1 query to a server. To provide a standard "dummy" query,
it is RECOMMENDED to send the initial query with RD=0,
QNAME="STARTTLS", QCLASS=CH, and QTYPE=TXT ("STARTTLS/CH/TXT")
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analogous to administrative queries already in widespread use
[RFC4892].
After sending the initial TO=1 query using TCP transport, DNS clients
MUST wait for the initial response before sending any subsequent
queries over the same TCP connection.
2.1.2. Receiving Responses
A DNS client that receives a response using UDP transport that has
the TO bit set MUST handle that response as usual. It MAY record the
server's support for DNS-over-TLS and use that information as part of
its server selection algorithm in the case where multiple servers are
available to service a particular query.
A DNS client that receives a response to its initial query using TCP
transport that has the TO bit set MUST immediately initiate a TLS
handshake using the procedure described in [RFC5246].
A DNS client that receives a response to its initial query using TCP
transport that has the TO bit clear MUST not initiate a TLS handshake
and SHOULD utilize the existing TCP connection for subsequent
queries. DNS clients SHOULD remember server IP addresses that don't
support DNS-over-TLS (including TLS handshake failures) and SHOULD
NOT request DNS-over-TLS from them for reasonable period. (We
suggest 1 hour, or when the client discovers a new resolver.)
2.2. Use by DNS Servers
2.2.1. Receiving Queries
A DNS server receiving a query over UDP MUST ignore the TO bit.
A DNS server receiving a query over an existing TLS connection MUST
ignore the TO bit.
A DNS server receiving an initial query over TCP that has the TO bit
set MAY inform the client it is willing to establish a TLS session,
as described in the next section.
A DNS server receiving subsequent queries over TCP MUST ignore the TO
bit. (A client wishing to start TLS after the initial query MUST
open a new TCP connection to do so.)
2.2.2. Sending Responses
A DNS server sending a response over UDP SHOULD set the TO bit to
indicate its general support for DNS-over-TLS, as long as it is
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willing and able to support a TLS connection with the particular
client.
A DNS server receiving an initial query over TCP that has the TO bit
set MAY set the TO bit in its response. The server MUST then proceed
with the TLS handshake protocol.
A DNS server receiving a "dummy" STARTTLS/CH/TXT query over TCP MUST
respond with RCODE=0 and a TXT RR in the Answer section. Contents of
the TXT RR are strictly informative (for humans) and MUST NOT be
interpreted by the client software. Recommended TXT RDATA values are
"STARTTLS" or "NO_TLS".
2.3. Established Sessions
After TLS negotiation completes, the connection will be encrypted and
is now protected from eavesdropping and normal DNS queries SHOULD
take place.
Both clients and servers SHOULD follow existing DNS-over-TCP timeout
rules, which are often implementation- and situation-dependent. In
the absence of any other advice, the RECOMMENDED timeout values are
30 seconds for recursive name servers, 60 seconds for clients of
recursive name servers, 10 seconds for authoritative name servers,
and 20 seconds for clients of authoritative name servers. Current
work in this area may assist DNS-over-TLS clients and servers select
useful timeout values [draft-wouters-edns-tcp-keepalive] [tdns].
As with current DNS-over-TCP, DNS servers MAY close the connection at
any time (e.g., due to resource constraints). As with current DNS-
over-TCP, clients MUST handle abrupt closes and be prepared to
reestablish connections and/or retry queries. DNS servers SHOULD use
the TLS close-notify request to shift TCP TIME-WAIT state to the
clients.
DNS servers SHOULD enable fast TLS session resumption [RFC5077] to
avoid keeping per-client session state.
2.4. Downgrade Attacks and Middleboxes
Middleboxes [RFC3234] may be present in some networks and have been
known to interfere with normal DNS resolution and create problems for
DNS-over-TLS. Remarkably, downgrade attacks can affect plaintext
protocols that utilize "STARTTLS" signaling in a similar way. A DNS
client attempting DNS-over-TLS through a middlebox, or in the
presence of a downgrade attack, could have one of the following
outcomes (as discussed in prior RFCs [RFC3207]):
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1. The DNS client sends a TO=1 query and receives a TO=0 response.
In this case there is no upgrade to TLS and DNS resolution occurs
normally, without encryption.
2. The DNS client sends a TO=1 query and receives a TO=1 response,
but the TLS handshake fails because the server's certificate
cannot be authenticated. In this case the client SHOULD close
the established connection and fall back to unencrypted DNS for a
reasonable period (as discussed in Section 2.1.2).
3. The DNS client sends a TO=1 query and receives a TO=1 response,
but the middlebox does not understand the TLS negotiation.
Middleboxes SHOULD clear TO in replies if they are not prepared
to pass through TLS negotiation. Clients SHOULD retry DNS
without TO set if negotiation fails, and then retry with TLS
after a reasonable period (see Section 2.1.2).
4. The DNS client sends a TO=1 query but receives no response at
all. The middlebox might be silently dropping the query due to
the presence of the TO bit, when it should, in fact, ignore and
pass through unknown flag bits [RFC6891]. The client SHOULD fall
back to normal (unencrypted) DNS for a reasonable period (as
discussed in Section 2.1.2).
In general, clients that attempt TLS and fail can either fall back on
unencrypted DNS, or wait and retry later, depending on their privacy
requirements. If the problem of middleboxes and threat of downgrade
attacks is too serious, the IETF might consider allocating a
dedicated port for DNS-over-TLS [RFC6335].
3. Performance Considerations
DNS-over-TLS incurs additional latency at session startup. It also
requires additional state (memory) increased processing (CPU).
1. Latency: Compared to UDP, DNS-over-TCP requires an additional
round-trip-time (RTT) of latency to establish the connection.
The TLS handshake adds another two RTTs of latency. Clients and
servers should support connection keepalive (reuse) and out-of-
order processing to amortize connection setup costs. Moreover,
TLS connection resumption can further reduce the setup delay.
2. State: The use of connection-oriented TCP requires keeping
additional state in both kernels and applications. TLS has
marginal increases in state over TCP alone. The state
requirements are of particular concerns on servers with many
clients. Smaller timeout values will reduce the number of
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concurrent connections, and servers can preemptively close
connections when resources limits are exceeded.
3. Processing: Use of TLS encryption algorithms results in slightly
higher CPU usage. Servers can choose to refuse new DNS-over-TCP
clients if processing limits are exceeded.
A full performance evaluation is outside the scope of this
specification. A more detailed analysis of the performance
implications of DNS-over-TLS (and DNS-over-TCP) is discussed in a
technical report [tdns].
4. IANA Considerations
This document defines a new bit ("TO") in the Flags field of the
EDNS0 OPT meta-RR. At the time of approval of this draft in the
standards track, as per the IANA Considerations of RFC 6891, IANA is
requested to reserve the second leftmost bit of the flags as the TO
bit, immediately adjacent to the DNSSEC DO bit, as shown in
Section 2.
5. Security Considerations
The goal of this proposal is to address the security risks that arise
because DNS queries may be eavesdropped upon, as described above.
There are a number of residual risks that may impact this goal.
1. There are known attacks on TLS, such as person-in-the-middle and
protocol downgrade. These are general attacks on TLS and not
specific to DNS-over-TLS; we refer to the TLS RFCs for discussion
of these security issues.
2. Any protocol interactions prior to the TLS handshake are
performed in the clear and can be modified by a man-in-the-middle
attacker. For this reason, clients MAY discard cached
information about server capabilities advertised prior to the
start of the TLS handshake.
3. As with other uses of STARTTLS-upgrade to TLS, the mechanism
specified here is susceptible to downgrade attacks, where a
person-in-the-middle prevents a successful TLS upgrade. Keeping
track of servers known to support TLS (i.e., "pinning") enables
clients to detect downgrade attacks. For servers with no
connection history, clients may choose to refuse non-TLS DNS, or
they may continue without TLS, depending on their privacy
requirements.
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4. This document does not propose new ideas for certificate
authentication for TLS in the context of DNS. Several external
methods are possible, although each has weaknesses. The current
Certificate Authority infrastructure [RFC5280] is used by HTTP/
TLS [RFC2818]. With many trusted CAs, this approach has
recognized weaknesses [CA_Compromise]. Some work is underway to
partially address these concerns (for example, with certificate
pinning [certificate_pinning], but more work is needed. DANE
[RFC6698] provides mechanisms to root certificate trust with
DNSSEC. That use here must be carefully evaluated to address
potential issues in trust recursion. For stub-to-recursive
resolver use, certificate authentication is sometimes either easy
or nearly impossible. If the recursive resolver is manually
configured, its certificate can be authenticated when it is
configured. If the recursive resolver is automatically
configured (such as with DHCP [RFC2131]), it could use DHCP
authentication mechanisms [RFC3118]).
Ongoing discussion of opportunistic TLS (connections without CA
validation, [draft-hoffman-uta-opportunistic-tls]) may be relevant to
DNS-over-TLS.
6. Acknowledgments
We would like to thank Stephane Bortzmeyer, Brian Haberman, Paul
Hoffman, Kim-Minh Kaplan, Bill Manning, George Michaelson, Eric
Osterweil and Glen Wiley for reviewing this Internet-draft, and to
Nikita Somaiya for early work on this idea.
Work by Zi Hu, Liang Zhu, and John Heidemann in this paper is
partially sponsored by the U.S. Dept. of Homeland Security (DHS)
Science and Technology Directorate, HSARPA, Cyber Security Division,
BAA 11-01-RIKA and Air Force Research Laboratory, Information
Directorate under agreement number FA8750-12-2-0344, and contract
number D08PC75599.
7. References
7.1. 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.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.
7.2. Informative References
[CA_Compromise]
Infosec Island Admin, "CA Compromise", January 2012, <http
://www.infosecisland.com/blogview/
19782-Web-Authentication-A-Broken-Trust-with-No-Easy-
Fix.html>.
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP",
RFC 2595, June 1999.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP
Messages", RFC 3118, June 2001.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
[RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
Issues", RFC 3234, February 2002.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 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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[RFC4892] Woolf, S. and D. Conrad, "Requirements for a Mechanism
Identifying a Name Server Instance", RFC 4892, June 2007.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[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, August 2011.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012.
[certificate_pinning]
OWASP, "Certificate and Public Key Pinning", <https://
www.owasp.org/index.php/
Certificate_and_Public_Key_Pinning>.
[draft-bortzmeyer-dnsop-dns-privacy]
Bortzmeyer, S., "DNS Privacy issues",
draft-bortzmeyer-dnsop-dns-privacy-01 (work in progress),
November 2013, <http://tools.ietf.org/html/
draft-bortzmeyer-dnsop-dns-privacy-01>.
[draft-bortzmeyer-dnsop-privacy-sol]
Bortzmeyer, S., "Solutions to DNS privacy issues",
draft-bortzmeyer-dnsop-privacy-sol-00 (work in progress),
December 2013, <http://tools.ietf.org/html/
draft-bortzmeyer-dnsop-privacy-sol-00>.
[draft-dempsky-dnscurve]
Dempsky, M., "DNSCurve", draft-dempsky-dnscurve-01 (work
in progress), August 2010,
<http://tools.ietf.org/html/draft-dempsky-dnscurve-01>.
[draft-hoffman-uta-opportunistic-tls]
Hoffman, P., "Opportunistic Encryption Using TLS",
draft-hoffman-uta-opportunistic-tls-00 (work in progress),
February 2014, <http://tools.ietf.org/html/
draft-hoffman-uta-opportunistic-tls-00>.
[draft-osterweil-dane-ipsec]
Osterweil, E., Wiley, G., Mitchell, D., and A. Newton,
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"Opportunistic Encryption with DANE Semantics and IPsec:
IPSECA", draft-osterweil-dane-ipsec-00 (work in progress),
February 2014,
<http://tools.ietf.org/html/
draft-osterweil-dane-ipsec-00>.
[draft-wijngaards-confidentialdns]
Wijngaards, W., "Confidential DNS",
draft-wijngaards-dnsop-confidentialdns-00 (work in
progress), November 2013, <http://tools.ietf.org/html/
draft-wijngaards-dnsop-confidentialdns-00>.
[draft-wouters-edns-tcp-keepalive]
Wouters, P. and J. Abley, "The edns-tcp-keepalive EDNS0
Option", draft-wouters-edns-tcp-keepalive-00 (work in
progress), October 2013, <http://tools.ietf.org/html/
draft-wouters-edns-tcp-keepalive-00>.
[tdns] Zhu, L., Hu, Z., Heidemann, J., Wessels, D., Mankin, A.,
and N. Somaiya, "T-DNS: Connection-Oriented DNS to Improve
Privacy and Security", Technical report ISI-TR-688,
February 2014, <Technical report, ISI-TR-688,
ftp://ftp.isi.edu/isi-pubs/tr-688.pdf>.
[unbound] NLnet Labs, Verisign labs, "Unbound", December 2013,
<http://unbound.net/>.
Authors' Addresses
Zi Hu
USC/Information Sciences Institute
4676 Admiralty Way, Suite 1133
Marina del Rey, CA 90292
USA
Phone: +1 213 587-1057
Email: zihu@usc.edu
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Liang Zhu
USC/Information Sciences Institute
4676 Admiralty Way, Suite 1133
Marina del Rey, CA 90292
USA
Phone: +1 310 448-8323
Email: liangzhu@usc.edu
John Heidemann
USC/Information Sciences Institute
4676 Admiralty Way, Suite 1001
Marina del Rey, CA 90292
USA
Phone: +1 310 822-1511
Email: johnh@isi.edu
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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