Internet Engineering Task Force (IETF) Z. Hu
Request for Comments: 7858 L. Zhu
Category: Standards Track J. Heidemann
ISSN: 2070-1721 USC/ISI
May 2016 Specification for DNS over Transport Layer Security (TLS)
This document describes the use of Transport Layer Security (TLS) to
provide privacy for DNS. Encryption provided by TLS eliminates
opportunities for eavesdropping and on-path tampering with DNS
queries in the network, such as discussed in RFC 7626. In addition,
this document specifies two usage profiles for DNS over TLS and
provides advice on performance considerations to minimize overhead
from using TCP and TLS with DNS.
This document focuses on securing stub-to-recursive traffic, as per
the charter of the DPRIVE Working Group. It does not prevent future
applications of the protocol to recursive-to-authoritative traffic.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
Today, nearly all DNS queries [RFC1034] [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 recent IETF work has specified privacy considerations for DNS
Prior work has addressed some aspects of DNS security, but until
recently, there has been little work on privacy between a DNS client
and server. 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. By intention, DNSSEC does not protect request
and 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 [RFC7258].
Other work that has offered the potential to encrypt between DNS
clients and servers includes DNSCurve [DNSCurve], DNSCrypt
[DNSCRYPT-WEBSITE], Confidential DNS [CONFIDENTIAL-DNS], and IPSECA
[IPSECA]. In addition to the present specification, the DPRIVE
Working Group has also adopted a proposal for DNS over Datagram
Transport Layer Security (DTLS) [DNSoD].
This document describes using DNS over TLS on a well-known port and
also offers advice on performance considerations to minimize
overheads from using TCP and TLS with DNS.
Initiation of DNS over TLS is very straightforward. By establishing
a connection over a well-known port, clients and servers expect and
agree to negotiate a TLS session to secure the channel. Deployment
will be gradual. Not all servers will support DNS over TLS and the
well-known port might be blocked by some firewalls. Clients will be
expected to keep track of servers that support TLS and those that
don't. Clients and servers will adhere to the TLS implementation
recommendations and security considerations of [BCP195].
The protocol described here works for queries and responses between
stub clients and recursive servers. It might work equally between
recursive clients and authoritative servers, but this application of
the protocol is out of scope for the DNS PRIVate Exchange (DPRIVE)
Working Group per its current charter.
This document describes two profiles in Section 4 that provide
different levels of assurance of privacy: an opportunistic privacy
profile and an out-of-band key-pinned privacy profile. It is
expected that a future document based on [TLS-DTLS-PROFILES] will
further describe additional privacy profiles for DNS over both TLS
An earlier draft version of this document described a technique for
upgrading a DNS-over-TCP connection to a DNS-over-TLS session with,
essentially, "STARTTLS for DNS". To simplify the protocol, this
document now only uses a well-known port to specify TLS use, omitting
the upgrade approach. The upgrade approach no longer appears in this
document, which now focuses exclusively on the use of a well-known
port for DNS over TLS.
2. Key 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].
3. Establishing and Managing DNS-over-TLS Sessions
3.1. Session Initiation
By default, a DNS server that supports DNS over TLS MUST listen for
and accept TCP connections on port 853, unless it has mutual
agreement with its clients to use a port other than 853 for DNS over
TLS. In order to use a port other than 853, both clients and servers
would need a configuration option in their software.
By default, a DNS client desiring privacy from DNS over TLS from a
particular server MUST establish a TCP connection to port 853 on the
server, unless it has mutual agreement with its server to use a port
other than port 853 for DNS over TLS. Such another port MUST NOT be
port 53 but MAY be from the "first-come, first-served" port range.
This recommendation against use of port 53 for DNS over TLS is to
avoid complication in selecting use or non-use of TLS and to reduce
risk of downgrade attacks. The first data exchange on this TCP
connection MUST be the client and server initiating a TLS handshake
using the procedure described in [RFC5246].
DNS clients and servers MUST NOT use port 853 to transport cleartext
DNS messages. DNS clients MUST NOT send and DNS servers MUST NOT
respond to cleartext DNS messages on any port used for DNS over TLS
(including, for example, after a failed TLS handshake). There are
significant security issues in mixing protected and unprotected data,
and for this reason, TCP connections on a port designated by a given
server for DNS over TLS are reserved purely for encrypted
DNS clients SHOULD remember server IP addresses that don't support
DNS over TLS, including timeouts, connection refusals, and TLS
handshake failures, and not request DNS over TLS from them for a
reasonable period (such as one hour per server). DNS clients
following an out-of-band key-pinned privacy profile (Section 4.2) MAY
be more aggressive about retrying DNS-over-TLS connection failures.
3.2. TLS Handshake and Authentication
Once the DNS client succeeds in connecting via TCP on the well-known
port for DNS over TLS, it proceeds with the TLS handshake [RFC5246],
following the best practices specified in [BCP195].
The client will then authenticate the server, if required. This
document does not propose new ideas for authentication. Depending on
the privacy profile in use (Section 4), the DNS client may choose not
to require authentication of the server, or it may make use of a
trusted Subject Public Key Info (SPKI) Fingerprint pin set.
After TLS negotiation completes, the connection will be encrypted and
is now protected from eavesdropping.
3.3. Transmitting and Receiving Messages
All messages (requests and responses) in the established TLS session
MUST use the two-octet length field described in Section 4.2.2 of
[RFC1035]. For reasons of efficiency, DNS clients and servers SHOULD
pass the two-octet length field, and the message described by that
length field, to the TCP layer at the same time (e.g., in a single
"write" system call) to make it more likely that all the data will be
transmitted in a single TCP segment ([RFC7766], Section 8).
In order to minimize latency, clients SHOULD pipeline multiple
queries over a TLS session. When a DNS client sends multiple queries
to a server, it should not wait for an outstanding reply before
sending the next query ([RFC7766], Section 126.96.36.199).
Since pipelined responses can arrive out of order, clients MUST match
responses to outstanding queries on the same TLS connection using the
Message ID. If the response contains a Question Section, the client
MUST match the QNAME, QCLASS, and QTYPE fields. Failure by clients
to properly match responses to outstanding queries can have serious
consequences for interoperability ([RFC7766], Section 7).
3.4. Connection Reuse, Close, and Reestablishment
For DNS clients that use library functions such as "getaddrinfo()"
and "gethostbyname()", current implementations are known to open and
close TCP connections for each DNS query. To avoid excess TCP
connections, each with a single query, clients SHOULD reuse a single
TCP connection to the recursive resolver. Alternatively, they may
prefer to use UDP to a DNS-over-TLS-enabled caching resolver on the
same machine that then uses a system-wide TCP connection to the
In order to amortize TCP and TLS connection setup costs, clients and
servers SHOULD NOT immediately close a connection after each
response. Instead, clients and servers SHOULD reuse existing
connections for subsequent queries as long as they have sufficient
resources. In some cases, this means that clients and servers may
need to keep idle connections open for some amount of time.
Proper management of established and idle connections is important to
the healthy operation of a DNS server. An implementor of DNS over
TLS SHOULD follow best practices for DNS over TCP, as described in
[RFC7766]. Failure to do so may lead to resource exhaustion and
denial of service.
Whereas client and server implementations from the era of [RFC1035]
are known to have poor TCP connection management, this document
stipulates that successful negotiation of TLS indicates the
willingness of both parties to keep idle DNS connections open,
independent of timeouts or other recommendations for DNS over TCP
without TLS. In other words, software implementing this protocol is
assumed to support idle, persistent connections and be prepared to
manage multiple, potentially long-lived TCP connections.
This document does not make specific recommendations for timeout
values on idle connections. Clients and servers should reuse and/or
close connections depending on the level of available resources.
Timeouts may be longer during periods of low activity and shorter
during periods of high activity. Current work in this area may also
assist DNS-over-TLS clients and servers in selecting useful timeout
values [RFC7828] [TDNS].
Clients and servers that keep idle connections open MUST be robust to
termination of idle connection by either party. As with current DNS
over TCP, DNS servers MAY close the connection at any time (perhaps
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.
When reestablishing a DNS-over-TCP connection that was terminated, as
discussed in [RFC7766], TCP Fast Open [RFC7413] is of benefit.
Underlining the requirement for sending only encrypted DNS data on a
DNS-over-TLS port (Section 3.2), when using TCP Fast Open, the client
and server MUST immediately initiate or resume a TLS handshake
(cleartext DNS MUST NOT be exchanged). DNS servers SHOULD enable
fast TLS session resumption [RFC5077], and this SHOULD be used when
When closing a connection, DNS servers SHOULD use the TLS close-
notify request to shift TCP TIME-WAIT state to the clients.
Additional requirements and guidance for optimizing DNS over TCP are
provided by [RFC7766].
4. Usage Profiles
This protocol provides flexibility to accommodate several different
use cases. This document defines two usage profiles: (1)
opportunistic privacy and (2) out-of-band key-pinned authentication
that can be used to obtain stronger privacy guarantees if the client
has a trusted relationship with a DNS server supporting TLS.
Additional methods of authentication will be defined in a forthcoming
4.1. Opportunistic Privacy Profile
For opportunistic privacy, analogous to SMTP opportunistic security
[RFC7435], one does not require privacy, but one desires privacy when
With opportunistic privacy, a client might learn of a TLS-enabled
recursive DNS resolver from an untrusted source. One possible
example flow would be if the client used the DHCP DNS server option
[RFC3646] to discover the IP address of a TLS-enabled recursive and
then attempted DNS over TLS on port 853. With such a discovered DNS
server, the client might or might not validate the resolver. These
choices maximize availability and performance, but they leave the
client vulnerable to on-path attacks that remove privacy.
Opportunistic privacy can be used by any current client, but it only
provides privacy when there are no on-path active attackers.
4.2. Out-of-Band Key-Pinned Privacy Profile
The out-of-band key-pinned privacy profile can be used in
environments where an established trust relationship already exists
between DNS clients and servers (e.g., stub-to-recursive in
enterprise networks, actively maintained contractual service
relationships, or a client using a public DNS resolver). The result
of this profile is that the client has strong guarantees about the
privacy of its DNS data by connecting only to servers it can
authenticate. Operators of a DNS-over-TLS service in this profile
are expected to provide pins that are specific to the service being
pinned (i.e., public keys belonging directly to the end entity or to
a service-specific private certificate authority (CA)) and not to a
public key(s) of a generic public CA.
In this profile, clients authenticate servers by matching a set of
SPKI Fingerprints in an analogous manner to that described in
[RFC7469]. With this out-of-band key-pinned privacy profile, client
administrators SHOULD deploy a backup pin along with the primary pin,
for the reasons explained in [RFC7469]. A backup pin is especially
helpful in the event of a key rollover, so that a server operator
does not have to coordinate key transitions with all its clients
simultaneously. After a change of keys on the server, an updated pin
set SHOULD be distributed to all clients in some secure way in
preparation for future key rollover. The mechanism for an
out-of-band pin set update is out of scope for this document.
Such a client will only use DNS servers for which an SPKI Fingerprint
pin set has been provided. The possession of a trusted pre-deployed
pin set allows the client to detect and prevent person-in-the-middle
and downgrade attacks.
However, a configured DNS server may be temporarily unavailable when
configuring a network. For example, for clients on networks that
require authentication through web-based login, such authentication
may rely on DNS interception and spoofing. Techniques such as those
used by DNSSEC-trigger [DNSSEC-TRIGGER] MAY be used during network
configuration, with the intent to transition to the designated DNS
provider after authentication. The user MUST be alerted whenever
possible that the DNS is not private during such bootstrap.
Upon successful TLS connection and handshake, the client computes the
SPKI Fingerprints for the public keys found in the validated server's
certificate chain (or in the raw public key, if the server provides
that instead). If a computed fingerprint exactly matches one of the
configured pins, the client continues with the connection as normal.
Otherwise, the client MUST treat the SPKI validation failure as a
non-recoverable error. Appendix A provides a detailed example of how
this authentication could be performed in practice.
Implementations of this privacy profile MUST support the calculation
of a fingerprint as the SHA-256 [RFC6234] hash of the DER-encoded
ASN.1 representation of the SPKI of an X.509 certificate.
Implementations MUST support the representation of a SHA-256
fingerprint as a base64-encoded character string [RFC4648].
Additional fingerprint types MAY also be supported.
5. Performance Considerations
DNS over TLS incurs additional latency at session startup. It also
requires additional state (memory) and increased processing (CPU).
Latency: Compared to UDP, DNS over TCP requires an additional round-
trip time (RTT) of latency to establish a TCP connection. TCP
Fast Open [RFC7413] can eliminate that RTT when information exists
from prior connections. 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. Fast TLS connection resumption [RFC5077]
further reduces the setup delay and avoids the DNS server keeping
per-client session state.
TLS False Start [TLS-FALSESTART] can also lead to a latency
reduction in certain situations. Implementations supporting TLS
False Start need to be aware that it imposes additional
constraints on how one uses TLS, over and above those stated in
[BCP195]. It is unsafe to use False Start if your implementation
and deployment does not adhere to these specific requirements.
See [TLS-FALSESTART] for the details of these additional
State: The use of connection-oriented TCP requires keeping
additional state at the server in both the kernel and application.
The state requirements are of particular concern on servers with
many clients, although memory-optimized TLS can add only modest
state over TCP. Smaller timeout values will reduce the number of
concurrent connections, and servers can preemptively close
connections when resource limits are exceeded.
Processing: The use of TLS encryption algorithms results in slightly
higher CPU usage. Servers can choose to refuse new DNS-over-TLS
clients if processing limits are exceeded.
Number of connections: To minimize state on DNS servers and
connection startup time, clients SHOULD minimize the creation of
new TCP connections. Use of a local DNS request aggregator (a
particular type of forwarder) allows a single active DNS-over-TLS
connection from any given client computer to its server.
Additional guidance can be found in [RFC7766].
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
[TDNS] and [RFC7766].
6. IANA Considerations
IANA has added the following value to the "Service Name and Transport
Protocol Port Number Registry" in the System Range. The registry for
that range requires IETF Review or IESG Approval [RFC6335], and such
a review was requested using the early allocation process [RFC7120]
for the well-known TCP port in this document.
IANA has reserved the same port number over UDP for the proposed DNS-
over-DTLS protocol [DNSoD].
Service Name domain-s
Port Number 853
Transport Protocol(s) TCP/UDP
Contact IETF Chair
Description DNS query-response protocol run over TLS/DTLS
Reference This document
7. Design Evolution
Earlier draft versions of this document proposed an upgrade-based
approach to establish a TLS session. The client would signal its
interest in TLS by setting a "TLS OK" bit in the Extensions
Mechanisms for DNS (EDNS(0)) flags field. A server would signal its
acceptance by responding with the TLS OK bit set.
Since we assume the client doesn't want to reveal (leak) any
information prior to securing the channel, we proposed the use of a
"dummy query" that clients could send for this purpose. The proposed
query name was STARTTLS, query type TXT, and query class CH.
The TLS OK signaling approach has both advantages and disadvantages.
One important advantage is that clients and servers could negotiate
TLS. If the server is too busy, or doesn't want to provide TLS
service to a particular client, it can respond negatively to the TLS
probe. An ancillary benefit is that servers could collect
information on adoption of DNS over TLS (via the TLS OK bit in
queries) before implementation and deployment. Another anticipated
advantage is the expectation that DNS over TLS would work over port
53. That is, no need to "waste" another port and deploy new firewall
rules on middleboxes.
However, at the same time, there was uncertainty whether or not
middleboxes would pass the TLS OK bit, given that the EDNS0 flags
field has been unchanged for many years. Another disadvantage is
that the TLS OK bit may make downgrade attacks easy and
indistinguishable from broken middleboxes. From a performance
standpoint, the upgrade-based approach had the disadvantage of
requiring 1xRTT additional latency for the dummy query.
Following this proposal, DNS over DTLS was proposed separately. DNS
over DTLS claimed it could work over port 53, but only because a non-
DTLS server interprets a DNS-over-DTLS query as a response. That is,
the non-DTLS server observes the QR flag set to 1. While this
technically works, it seems unfortunate and perhaps even undesirable.
DNS over both TLS and DTLS can benefit from a single well-known port
and avoid extra latency and misinterpreted queries as responses.
8. Security Considerations
Use of DNS over TLS is designed to address the privacy risks that
arise out of the ability to eavesdrop on DNS messages. It does not
address other security issues in DNS, and there are a number of
residual risks that may affect its success at protecting privacy:
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; please refer to the TLS RFCs for
discussion of these security issues. Clients and servers MUST
adhere to the TLS implementation recommendations and security
considerations of [BCP195]. DNS clients keeping track of servers
known to support TLS enables clients to detect downgrade attacks.
For servers with no connection history and no apparent support
for TLS, depending on their privacy profile and privacy
requirements, clients may choose to (a) try another server when
available, (b) continue without TLS, or (c) refuse to forward the
2. Middleboxes [RFC3234] are present in some networks and have been
known to interfere with normal DNS resolution. Use of a
designated port for DNS over TLS should avoid such interference.
In general, clients that attempt TLS and fail can either fall
back on unencrypted DNS or wait and retry later, depending on
their privacy profile and privacy requirements.
3. Any DNS protocol interactions performed in the clear can be
modified by a person-in-the-middle attacker. For example,
unencrypted queries and responses might take place over port 53
between a client and server. For this reason, clients MAY
discard cached information about server capabilities advertised
4. This document does not, itself, specify ideas to resist known
traffic analysis or side-channel leaks. Even with encrypted
messages, a well-positioned party may be able to glean certain
details from an analysis of message timings and sizes. Clients
and servers may consider the use of a padding method to address
privacy leakage due to message sizes [RFC7830]. Since traffic
analysis can be based on many kinds of patterns and many kinds of
classifiers, simple padding schemes alone might not be sufficient
to mitigate such an attack. Padding will, however, form a part
of more complex mitigations for traffic-analysis attacks that are
likely to be developed over time. Implementors who can offer
flexibility in terms of how padding can be used may be in a
better position to enable such mitigations to be deployed in the
As noted earlier, DNSSEC and DNS over TLS are independent and fully
compatible protocols, each solving different problems. The use of
one does not diminish the need nor the usefulness of the other.
9.1. Normative References
[BCP195] 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, May 2015,
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
[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,
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
Appendix A. Out-of-Band Key-Pinned Privacy Profile Example
This section presents an example of how the out-of-band key-pinned
privacy profile could work in practice based on a minimal pin set
A DNS client system is configured with an out-of-band key-pinned
privacy profile from a network service, using a pin set containing
two pins. Represented in HTTP Public Key Pinning (HPKP) [RFC7469]
style, the pins are:
The client also configures the IP addresses of its expected DNS
server: perhaps 192.0.2.3 and 2001:db8::2:4.
The client connects to one of these addresses on TCP port 853 and
begins the TLS handshake: negotiation of TLS 1.2 with a Diffie-
Hellman key exchange. The server sends a certificate message with a
list of three certificates (A, B, and C) and signs the
ServerKeyExchange message correctly with the public key found in
The client now takes the SHA-256 digest of the SPKI in cert A and
compares it against both pins in the pin set. If either pin matches,
the verification is successful; the client continues with the TLS
connection and can make its first DNS query.
If neither pin matches the SPKI of cert A, the client verifies that
cert A is actually issued by cert B. If it is, it takes the SHA-256
digest of the SPKI in cert B and compares it against both pins in the
pin set. If either pin matches, the verification is successful.
Otherwise, it verifies that B was issued by C and then compares the
pins against the digest of C's SPKI.
If none of the SPKIs in the cryptographically valid chain of certs
match any pin in the pin set, the client closes the connection with
an error and marks the IP address as failed.
The authors would like to thank Stephane Bortzmeyer, John Dickinson,
Brian Haberman, Christian Huitema, Shumon Huque, Simon Joseffson,
Kim-Minh Kaplan, Simon Kelley, Warren Kumari, John Levine, Ilari
Liusvaara, Bill Manning, George Michaelson, Eric Osterweil, Jinmei
Tatuya, Tim Wicinski, and Glen Wiley for reviewing this
specification. They also thank Nikita Somaiya for early work on this
Work by Zi Hu, Liang Zhu, and John Heidemann on this document is
partially sponsored by the U.S. Dept. of Homeland Security (DHS)
Science and Technology Directorate, Homeland Security Advanced
Research Projects Agency (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
The below individuals contributed significantly to the document:
Sinodun Internet Technologies
Oxford Science Park
Oxford OX4 4GA
Daniel Kahn Gillmor
125 Broad Street, 18th Floor
New York, NY 10004
USC/Information Sciences Institute
4676 Admiralty Way, Suite 1133
Marina del Rey, CA 90292
USC/Information Sciences Institute
4676 Admiralty Way, Suite 1133
Marina del Rey, CA 90292
USC/Information Sciences Institute
4676 Admiralty Way, Suite 1001
Marina del Rey, CA 90292
12061 Bluemont Way
Reston, VA 20190