Internet Engineering Task Force (IETF) Z. Hu Request for Comments: 7858 L. Zhu Category: Standards Track J. Heidemann ISSN: 2070-1721 USC/ISI A. Mankin Independent D. Wessels Verisign Labs P. Hoffman ICANN 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.
Copyright (c) 2016 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 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.
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 [RFC7626].
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.
Hu, et al. Standards Track [Page 3]
RFC 7858 DNS over TLS May 2016
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 and DTLS.
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.
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
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,
Hu, et al. Standards Track [Page 4]
RFC 7858 DNS over TLS May 2016
and for this reason, TCP connections on a port designated by a given server for DNS over TLS are reserved purely for encrypted communications.
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.
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.
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 22.214.171.124).
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).
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 recursive resolver.
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.
Hu, et al. Standards Track [Page 6]
RFC 7858 DNS over TLS May 2016
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 reestablishing connections.
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].
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 document [TLS-DTLS-PROFILES].
For opportunistic privacy, analogous to SMTP opportunistic security [RFC7435], one does not require privacy, but one desires privacy when possible.
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.
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
Hu, et al. Standards Track [Page 7]
RFC 7858 DNS over TLS May 2016
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.
Hu, et al. Standards Track [Page 8]
RFC 7858 DNS over TLS May 2016
Implementations MUST support the representation of a SHA-256 fingerprint as a base64-encoded character string [RFC4648]. Additional fingerprint types MAY also be supported.
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 constraints.
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].
Hu, et al. Standards Track [Page 9]
RFC 7858 DNS over TLS May 2016
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].
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 Assignee IESG Contact IETF Chair Description DNS query-response protocol run over TLS/DTLS Reference This document
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.
Hu, et al. Standards Track [Page 10]
RFC 7858 DNS over TLS May 2016
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.
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 query.
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
Hu, et al. Standards Track [Page 11]
RFC 7858 DNS over TLS May 2016
discard cached information about server capabilities advertised in cleartext.
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 future.
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.
[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, <https://www.rfc-editor.org/info/bcp195>.
[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>.
[TDNS] Zhu, L., Hu, Z., Heidemann, J., Wessels, D., Mankin, A., and N. Somaiya, "Connection-Oriented DNS to Improve Privacy and Security", 2015 IEEE Symposium on Security and Privacy (SP), DOI 10.1109/SP.2015.18, <http://dx.doi.org/10.1109/SP.2015.18>.
[TLS-DTLS-PROFILES] Dickinson, S., Gillmor, D., and T. Reddy, "Authentication and (D)TLS Profile for DNS-over-TLS and DNS-over-DTLS", Work in Progress, draft-ietf-dprive-dtls-and-tls- profiles-01, March 2016.
[TLS-FALSESTART] Langley, A., Modadugu, N., and B. Moeller, "Transport Layer Security (TLS) False Start", Work in Progress, draft-ietf-tls-falsestart-02, May 2016.
Hu, et al. Standards Track [Page 15]
RFC 7858 DNS over TLS May 2016
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 (two pins).
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 certificate A.
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.
Hu, et al. Standards Track [Page 16]
RFC 7858 DNS over TLS May 2016
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 idea.
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 D08PC75599.
The below individuals contributed significantly to the document:
Sara Dickinson Sinodun Internet Technologies Magdalen Centre Oxford Science Park Oxford OX4 4GA United Kingdom