| Network Working Group | C. Huitema |
| Internet-Draft | |
| Intended status: Standards Track | D. Kaiser |
| Expires: April 30, 2017 | University of Konstanz |
| October 27, 2016 |
Privacy Extensions for DNS-SD
draft-ietf-dnssd-privacy-00.txt
DNS-SD (DNS Service Discovery) normally discloses information about both the devices offering services and the devices requesting services. This information includes host names, network parameters, and possibly a further description of the corresponding service instance. Especially when mobile devices engage in DNS Service Discovery over Multicast DNS at a public hotspot, a serious privacy problem arises.
We propose to solve this problem by a two-stage approach. In the first stage, hosts discover Private Discovery Service Instances via DNS-SD using special formats to protect their privacy. These service instances correspond to Private Discovery Servers running on peers. In the second stage, hosts directly query these Private Discovery Servers via DNS-SD over TLS. A pairwise shared secret necessary to establish these connections is only known to hosts authorized by a pairing system.
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 April 30, 2017.
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.
DNS-SD [RFC6763] enables distribution and discovery in local networks without configuration. It is very convenient for users, but it requires the public exposure of the offering and requesting identities along with information about the offered and requested services. Some of the information published by the announcements can be very revealing. These privacy issues and potential solutions are discussed in [KW14a] and [KW14b].
There are cases when nodes connected to a network want to provide or consume services without exposing their identity to the other parties connected to the same network. Consider for example a traveler wanting to upload pictures from a phone to a laptop when connected to the Wi-Fi network of an Internet cafe, or two travelers who want to share files between their laptops when waiting for their plane in an airport lounge.
We expect that these exchanges will start with a discovery procedure using DNS-SD [RFC6763]. One of the devices will publish the availability of a service, such as a picture library or a file store in our examples. The user of the other device will discover this service, and then connect to it.
When analyzing these scenarios in Section 2, we find that the DNS-SD messages leak identifying information such as the instance name, the host name or service properties. We review the design constraint of a solution in Section 3, and describe the proposed solution in Section 4.
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 [RFC2119].
DNS-Based Service Discovery (DNS-SD) is defined in [RFC6763]. It allows nodes to publish the availability of an instance of a service by inserting specific records in the DNS ([RFC1033], [RFC1034], [RFC1035]) or by publishing these records locally using multicast DNS (mDNS) [RFC6762]. Available services are described using three types of records:
In the remaining subsections, we will review the privacy issues related to publishing instance names, node names, service attributes and other data, as well as review the implications of using the discovery service as a client.
In the first phase of discovery, the client obtains all the PTR records associated with a service type in a given naming domain. Each PTR record contains a Service Instance Name defined in Section 4 of [RFC6763]:
Service Instance Name = <Instance> . <Service> . <Domain>
The <Instance> portion of the Service Instance Name is meant to convey enough information for users of discovery clients to easily select the desired service instance. Nodes that use DNS-SD over mDNS [RFC6762] in a mobile environment will rely on the specificity of the instance name to identify the desired service instance. In our example of users wanting to upload pictures to a laptop in an Internet Cafe, the list of available service instances may look like:
Alice's Images . _imageStore._tcp . local Alice's Mobile Phone . _presence._tcp . local Alice's Notebook . _presence._tcp . local Bob's Notebook . _presence._tcp . local Carol's Notebook . _presence._tcp . local
Alice will see the list on her phone and understand intuitively that she should pick the first item. The discovery will "just work".
However, DNS-SD/mDNS will reveal to anybody that Alice is currently visiting the Internet Cafe. It further discloses the fact that she uses two devices, shares an image store, and uses a chat application supporting the _presence protocol on both of her devices. She might currently chat with Bob or Carol, as they are also using a _presence supporting chat application. This information is not just available to devices actively browsing for and offering services, but to anybody passively listing to the network traffic.
The SRV records contain the DNS name of the node publishing the service. Typical implementations construct this DNS name by concatenating the "host name" of the node with the name of the local domain. The privacy implications of this practice are reviewed in [I-D.ietf-intarea-hostname-practice]. Depending on naming practices, the host name is either a strong identifier of the device, or at a minimum a partial identifier. It enables tracking of the device, and by extension of the device's owner.
The TXT record's attribute and value pairs contain information on the characteristics of the corresponding service instance. This in turn reveals information about the devices that publish services. The amount of information varies widely with the particular service and its implementation:
Combinations of attributes have more information power than specific attributes, and can potentially be used for "fingerprinting" a specific device.
Information contained in TXT records does not only breach privacy by making devices trackable, but might directly contain private information about the user. For instance the _presence service reveals the "chat status" to everyone in the same network. Users might not be aware of that.
Further, TXT records often contain version information about services allowing potential attackers to identify devices running exploit-prone versions of a certain service.
The combination of information published in DNS-SD has the potential to provide a "fingerprint" of a specific device. Such information includes:
This combination of services and attributes will often be sufficient to identify the version of the software running on a device. If a device publishes many services with rich sets of attributes, the combination may be sufficient to identify the specific device.
There is however an argument that devices providing services can be discovered by observing the local traffic, and that trying to hide the presence of the service is futile. The same argument can be extended to say that the pattern of services offered by a device allows for fingerprinting the device. This may or may not be true, since we can expect that services will be designed or updated to avoid leaking fingerprints. In any case, the design of the discovery service should avoid making a bad situation worse, and should as much as possible avoid providing new fingerprinting information.
The consumers of services engage in discovery, and in doing so reveal some information such as the list of services they are interested in and the domains in which they are looking for the services. When the clients select specific instances of services, they reveal their preference for these instances. This can be benign if the service type is very common, but it could be more problematic for sensitive services, such as for example some private messaging services.
One way to protect clients would be to somehow encrypt the requested service types. Of course, just as we noted in Section 2.4, traffic analysis can often reveal the service.
In this section, we present the design of a two-stage solution that enables private use of DNS-SD, without affecting existing users. The solution is largely based on the architecture proposed in [KW14b], which separates the general private discovery problem in three components. The first component is an offline pairing mechanism, which is performed only once per pair of users. It establishes a shared secret over an authenticated channel, allowing devices to authenticate using this secret without user interaction at any later point in time. We use the pairing system proposed in [I-D.kaiser-dnssd-pairing].
The further two components are online (in contrast to pairing they are performed anew each time joining a network) and compose the two service discovery stages, namely
In other words, the hosts first discover paired peers and then directly engage in privacy preserving service discovery.
The stages are independent with respect to means used for transmitting the necessary data. While in our extension the messages for the first stage are transmitted using IP multicast, the messages for the second stage are transmitted via unicast. One could also imagine using a Distributed Hash Table for the first stage, being completely independent of multicast.
Any private discovery solution needs to differentiate between authorized devices, which are allowed to get information about discoverable entities, and other devices, which should not be aware of the availability of private entities. The commonly used solution to this problem is establishing a "device pairing".
Device pairing has to be performed only once per pair of users. This is important for user-friendliness, as it is the only step that demands user-interaction. After this single pairing, privacy preserving service discovery works fully automatically. In this document, we leverage [I-D.kaiser-dnssd-pairing] as pairing mechanism.
The pairing yields a mutually authenticated shared secret, and optionally mutually authenticated public keys or certificates added to a local web of trust. Public key technology has many advantages, but shared secrets are typically easier to handle on small devices.
The first stage of service discovery is to check whether instances of compatible Private Discovery Services are available in the local scope. The goal of that stage is to identify devices that share a pairing with the querier, and are available locally. The service instances can be discovered using regular DNS-SD procedures, but the list of discovered services will have to be filtered so only paired devices are retained.
The discovery relies on the advertisement of "proofs" by the publishers of the service. Each proof is the hash of a nonce with the key shared between the publisher and one of the paired devices. In order to reduce the overall number of messages, we use a special encoding of the instance name. Suppose that the publisher manages N pairings with the associated keys K1, K2, ... Kn. The instance name will be set to an encoding of N "proofs" of the N keys, where each proof is computed as function of the key and a nonce:
The querier can test the instance name by computing the same "proof" for each of its own keys. Suppose that the receiver manages P pairings, with the corresponding keys X1, X2, .. Xp. The receiver verification procedure will be:
for each received instance name:
retrieve nonce from instance name
for (j = 1 to P)
retrieve the key Xj of pairing number j
compute F = hash(nonce, Xj)
for (i=1 to N)
retrieve the proof Fi
if F is equal to Fi
mark the pairing number j as available
The procedure presented here requires on average O(M*N) iterations of the hash function. It also requires O(M*N^2) comparison operations, but these are less onerous than cryptographic operations. Further, when setting the nonce to a timestamp, the Fi have to be calculated only once per time interval.
The number of pairing proofs that can be encoded in a single record is limited by the maximum size of a DNS label, which is 63 bytes. Since this are characters and not pure binary values, nonce and proofs will have to be encoded using BASE64 ([RFC2045] section 6.8), resulting in at most 378 bits. The nonce should not be repeated, and the simplest way to achieve that is to set the nonce to a 32 bit timestamp value. The remaining 346 bits could encode up to 10 proofs of 32 bits each, which would be sufficient for many practical scenarios.
In practice, a 32 bit proof should be sufficient to distinguish between available devices. However, there is clearly a risk of collision. The Private Discovery Service as described here will find the available pairings, but it might also find a spurious number of "false positives". The chances of that happening are however quite small: less than 0.02% for a device managing 10 pairings and processing 10000 responses.
The Private Discovery Service discovery allows discovering a list of available paired devices, and verifying that either party knows the corresponding shared secret. At that point, the querier can engage in a series of directed discoveries.
We have considered defining an ad-hoc protocol for the private discovery service, but found that just using TLS would be much simpler. The Directed Private Discovery service is just a regular DNS-SD service, accessed over TLS, using the encapsulation of DNS over TLS defined in [RFC7858]. The main difference with simple DNS over TLS is the need for authentication.
We assume that the pairing process has provided each pair of authorized client and server with a shared secret. We can use that shared secret to provide mutual authentication of clients and servers using "Pre Shared Key" authentication, as defined in [RFC4279] and incorporated in the latest version of TLS [I-D.ietf-tls-tls13].
One difficulty is the reliance on a key identifier in the protocol. For example, in TLS 1.3 the PSK extension is defined as:
opaque psk_identity<0..2^16-1>;
struct {
select (Role) {
case client:
psk_identity identities<2..2^16-1>;
case server:
uint16 selected_identity;
}
} PreSharedKeyExtension
According to the protocol, the PSK identity is passed in clear text at the beginning of the key exchange. This is logical, since server and clients need to identify the secret that will be used to protect the connection. But if we used a static identifier for the key, adversaries could use that identifier to track server and clients. The solution is to use a time-varying identifier, constructed exactly like the "hint" described in Section 3.2, by concatenating a nonce and the hash of the nonce with the shared secret.
Our solution uses a variant of the DNS over TLS protocol [RFC7858] defined by the DNS Private Exchange working group (DPRIVE). DPRIVE is also working on an UDP variant, DNS over DTLS [I-D.ietf-dprive-dnsodtls], which would also be a candidate.
DPRIVE and Private Discovery solve however two somewhat different problems. DPRIVE is concerned with the confidentiality of DNS transactions, addressing the problems outlined in [RFC7626]. However, DPRIVE does not address the confidentiality or privacy issues with publication of services, and is not a direct solution to DNS-SD privacy:
In contrast, we propose using mutual authentication of the client and server as part of the TLS solution, to ensure that only authorized parties learn the presence of a service.
Instead of publishing their actual name in the SRV records, nodes could publish a randomized name. That is the solution argued for in [I-D.ietf-intarea-hostname-practice].
Randomized host names will prevent some of the tracking. Host names are typically not visible by the users, and randomizing host names will probably not cause much usability issues.
It is important that the obfuscation of instance names is performed at the right time, and that the obfuscated names change in synchrony with other identifiers, such as MAC Addresses, IP Addresses or host names. If the randomized host name changed but the instance name remained constant, an adversary would have no difficulty linking the old and new host names. Similarly, if IP or MAC addresses changed but host names remained constant, the adversary could link the new addresses to the old ones using the published name.
The problem is handled in [I-D.ietf-intarea-hostname-practice], which recommends to pick a new random host name at the time of connecting to a new network. New instance names for the Private Discovery Services should be composed at the same time.
The proposed solution uses the following components:
These components are detailed in the following subsections.
Nodes publishing services with DNS-SD and concerned about their privacy MUST use a randomized host name. The randomized name MUST be changed when network connectivity changes, to avoid the correlation issues described in Section 3.5. The randomized host name MUST be used in the SRV records describing the service instance, and the corresponding A or AAAA records MUST be made available through DNS or MDNS, within the same scope as the PTR, SRV and TXT records used by DNS-SD.
If the link-layer address of the network connection is properly obfuscated (e.g. using MAC Address Randomization), the Randomized Host Name MAY be computed using the algorithm described in section 3.7 of [RFC7844]. If this is not possible, the randomized host name SHOULD be constructed by simply picking a 48 bit random number meeting the Randomness Requirements for Security expressed in [RFC4075], and then use the hexadecimal representation of this number as the obfuscated host name.
Nodes that want to leverage the Private Directory Service for private service discovery among peers MUST share a secret with each of these peers. Each shared secret MUST be a 256 bit randomly chosen number. We RECOMMEND using the pairing mechanism proposed in [I-D.kaiser-dnssd-pairing] to establish these secrets.
[[TODO: Should we support mutually authenticated certificates? They can also be used to initiate TLS and have several advantages, i.e. allow setting an expiry date.]]
A Private Discovery Server (PDS) is a minimal DNS server running on each host. Its task is to offer resource records corresponding to private services only to authorized peers. These peers MUST share a secret with the host (see Section 4.2). To ensure privacy of the requests, the service is only available over TLS [RFC5246], and the shared secrets are used to mutually authenticate peers and servers.
The Private Name Server SHOULD support DNS push notifications [I-D.ietf-dnssd-push], e.g. to facilitate an up-to-date contact list in a chat application without polling.
The PDS MUST only answer queries via DNS over TLS [RFC7858] and MUST use a PSK authenticated TLS handshake [RFC4279]. The client and server should negotiate a forward secure cipher suite such as DHE-PSK or ECDHE-PSK when available. The shared secret exchanged during pairing MUST be used as PSK.
When using the PSK based authentication, the "psk_identity" parameter identifying the pre-shared key MUST be composed as follows, using the conventions of TLS [RFC7858]:
struct {
uint32 gmt_unix_time;
opaque random_bytes[4];
} nonce;
long_proof = HASH(nonce | pairing_key )
proof = first 12 bytes of long_proof
psk_identity = BASE64(nonce) "." BASE64(proof)
In this formula, HASH SHOULD be the function SHA256 defined in [RFC4055]. Implementers MAY eventually replace SHA256 with a stronger algorithm, in which cases both clients and servers will have to agree on that algorithm during the pairing process. The first 32 bits of the nonce are set to the current time and date in standard UNIX 32-bit format (seconds since the midnight starting Jan 1, 1970, UTC, ignoring leap seconds) according to the client's internal clock. The next 32 bits of the nonce are set to a value generated by a secure random number generator.
In this formula, the identity is finally set to a character string, using BASE64 ([RFC2045] section 6.8). This transformation is meant to comply with the PSK identity encoding rules specified in section 5.1 of [RFC4279].
The server will check the received key identity, trying the key against the valid keys established through pairing. If one of the keys matches, the TLS connection is accepted, otherwise it is declined.
Nodes that provide the Private Discovery Service SHOULD advertise their availability by publishing instances of the service through DNS-SD.
The DNS-SD service type for the Private Discovery Service is "_pds._tls".
Each published instance describes one server and up to 10 pairings. In the case where a node manages more than 10 pairings, it should publish as many instances as necessary to advertise all available pairings.
Each instance name is composed as follows:
pick a 32 bit nonce, e.g. using the Unix GMT time.
set the binary identifier to the nonce.
for each of up to 10 pairings
hint = first 32 bits of HASH(<nonce>|<pairing key>)
concatenate the hint to the binary identifier
set instance-ID = BASE64(binary identifier)
In this formula, HASH SHOULD be the function SHA256 defined in [RFC4055], and BASE64 is defined in section 6.8 of [RFC2045]. The concatenation of a 32 bit nonce and up to 10 pairing hints result a bit string at most 352 bit long. The BASE64 conversion will produce a string that is up to 59 characters long, which fits within the 63 characters limit defined in [RFC6763].
Nodes that wish to discover Private Discovery Service Instances will issue a DNS-SD discovery request for the service type. These request will return a series of PTR records, providing the names of the instances present in the scope.
The querier SHOULD examine each instance to see whether it hints at one of its available pairings, according to the following conceptual algorithm:
for each received instance name:
convert the instance name to binary using BASE64
if the conversion fails,
discard the instance.
if the binary instance length is a not multiple of 32 bits,
discard the instance.
nonce = first 32 bits of binary.
for each 32 bit hint after the nonce
for each available pairing
retrieve the key Xj of pairing number j
compute F = hash(nonce, Xj)
if F is equal to the 32 bit hint
mark the pairing number j as available
Once a pairing has been marked available, the querier SHOULD try connecting to the corresponding instance, using the selected key. The connection is likely to succeed, but it MAY fail for a variety of reasons. One of these reasons is the probabilistic nature of the hint, which entails a small chance of "false positive" match. This will occur if the hash of the nonce with two different keys produces the same result. In that case, the TLS connection will fail with an authentication error or a decryption error.
Once instances of the Private Discovery Service have been discovered, peers can establish TLS connections and send DNS requests over these connections, as specified in DNS-SD.
This document specifies a method to protect the privacy of service publishing nodes. This is especially useful when operating in a public space. Hiding the identity of the publishing nodes prevents some forms of "targeting" of high value nodes. However, adversaries can attempt various attacks to break the anonymity of the service, or to deny it. A list of these attacks and their mitigations are described in the following sections.
There are a variety of attacks against pairing systems, which may result in compromised pairing secrets. If an adversary manages to acquire a compromised key, the adversary will be able to perform private service discovery according to Section 4.5. This will allow tracking of the service. The adversary will also be able to discover which private services are available for the compromised pairing.
Attacks on pairing systems are detailed in [I-D.kaiser-dnssd-pairing].
The algorithm described in Section 4.5 scales as O(M*N), where M is the number of pairings per node and N is the number of nodes in the local scope. Adversaries can attack this service by publishing "fake" instances, effectively increasing the number N in that scaling equation.
Similar attacks can be mounted against DNS-SD: creating fake instances will generally increase the noise in the system and make discovery less usable. Private Discovery Service discovery SHOULD use the same mitigations as DNS-SD.
The attack is amplified because the clients need to compute proofs for all the nonces presented in Private Discovery Service Instance names. One possible mitigation would be to require that such nonces correspond to rounded timestamps. If we assume that timestamps must not be too old, there will be a finite number of valid rounded timestamps at any time. Even if there are many instances present, they would all pick their nonces from this small number of rounded timestamps, and a smart client could make sure that proofs are only computed once per valid time stamp.
Adversaries can record the service instance names published by Private Discovery Service instances, and replay them later in different contexts. Peers engaging in discovery can be misled into believing that a paired server is present. They will attempt to connect to the absent peer, and in doing so will disclose their presence in a monitored scope.
The binary instance identifiers defined in Section 4.4 start with 32 bits encoding the "UNIX" time. In order to protect against replay attacks, clients MAY verify that this time is reasonably recent.
[[TODO: Should we somehow encode the scope in the identifier? Having both scope and time would really mitigate that attack.]]
The Private Discovery Service is only available through a mutually authenticated TLS connection, which provides state-of-the-art protection mechanisms. However, adversaries can mount a denial of service attack against the service. In the absence of shared secrets, the connections will fail, but the servers will expend some CPU cycles defending against them.
To mitigate such attacks, nodes SHOULD restrict the range of network addresses from which they accept connections, matching the expected scope of the service.
This mitigation will not prevent denial of service attacks performed by locally connected adversaries; but protecting against local denial of service attacks is generally very difficult. For example, local attackers can also attack mDNS and DNS-SD by generating a large number of multicast requests.
Adversaries may record the PSK Key Identifiers used in successful connections to a private discovery service. They could attempt to replay them later against nodes advertising the private service at other times or at other locations. If the PSK Identifier is still valid, the server will accept the TLS connection, and in doing so will reveal being the same server observed at a previous time or location.
The PSK identifiers defined in Section 4.3.1 start with 32 bits encoding the "UNIX" time. In order to mitigate replay attacks, servers SHOULD verify that this time is reasonably recent, and fail the connection if it is too old, or if it occurs too far in the future.
The processing of timestamps is however affected by the accuracy of computer clocks. If the check is too strict, reasonable connections could fail. To further mitigate replay attacks, servers MAY record the list of valid PSK identifiers received in a recent past, and fail connections if one of these identifiers is replayed.
This draft does not require any IANA action. (Or does it? What about the _pds tag?)
This draft results from initial discussions with Dave Thaler, and encouragements from the DNS-SD working group members.
| [I-D.ietf-dnssd-push] | Pusateri, T. and S. Cheshire, "DNS Push Notifications", Internet-Draft draft-ietf-dnssd-push-08, July 2016. |
| [I-D.ietf-dprive-dnsodtls] | Reddy, T., Wing, D. and P. Patil, "Specification for DNS over Datagram Transport Layer Security (DTLS)", Internet-Draft draft-ietf-dprive-dnsodtls-12, September 2016. |
| [I-D.ietf-intarea-hostname-practice] | Huitema, C., Thaler, D. and R. Winter, "Current Hostname Practice Considered Harmful", Internet-Draft draft-ietf-intarea-hostname-practice-03, July 2016. |
| [I-D.ietf-tls-tls13] | Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", Internet-Draft draft-ietf-tls-tls13-18, October 2016. |
| [I-D.kaiser-dnssd-pairing] | Huitema, C. and D. Kaiser, "Device Pairing Using Short Authentication Strings", Internet-Draft draft-kaiser-dnssd-pairing-00, September 2016. |
| [KW14a] | Kaiser, D. and M. Waldvogel, "Adding Privacy to Multicast DNS Service Discovery", DOI 10.1109/TrustCom.2014.107, 2014. |
| [KW14b] | Kaiser, D. and M. Waldvogel, "Efficient Privacy Preserving Multicast DNS Service Discovery", DOI 10.1109/HPCC.2014.141, 2014. |
| [RFC1033] | Lottor, M., "Domain Administrators Operations Guide", RFC 1033, DOI 10.17487/RFC1033, November 1987. |
| [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. |
| [RFC2782] | Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, DOI 10.17487/RFC2782, February 2000. |
| [RFC6762] | Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, DOI 10.17487/RFC6762, February 2013. |
| [RFC7626] | Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626, DOI 10.17487/RFC7626, August 2015. |
| [RFC7844] | Huitema, C., Mrugalski, T. and S. Krishnan, "Anonymity Profiles for DHCP Clients", RFC 7844, DOI 10.17487/RFC7844, May 2016. |
| [RFC7858] | Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D. and P. Hoffman, "Specification for DNS over Transport Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 2016. |