| Network Working Group | C. Huitema |
| Internet-Draft | Private Octopus Inc. |
| Intended status: Informational | D. Kaiser |
| Expires: June 22, 2020 | University of Luxembourg |
| December 20, 2019 |
DNS-SD Privacy and Security Requirements
draft-ietf-dnssd-prireq-03
DNS-SD (DNS Service Discovery) normally discloses information about devices offering and 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, serious privacy problems arise. We analyze the requirements of a privacy respecting discovery service.
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DNS-SD [RFC6763] over mDNS [RFC6762] enables zero-configuration service discovery in local networks. 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. Parts of the published information can seriously breach the user's privacy. These privacy issues and potential solutions are discussed in [KW14a], [KW14b] and [K17].
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] over mDNS [RFC6762]. 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 3.2, we find that the DNS-SD messages leak identifying information such as the service instance name, the host name, or service properties.
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].
This document considers the following attacker types sorted by increasing power. All these attackers can either be passive, i.e. they just listen to network traffic they have access to, or active, i.e. they additionally can craft and send (malicious) packets.
In this section we analyse how the attackers described in the previous section might threaten the privacy of legal persons operating devices engaging in service discovery. We focus on attacks leveraging data transmitted in service discovery protocol messages.
In this section, we review common service discovery scenarios and discuss privacy threats and their privacy requirements. In all three of these common scenarios the attacker is of the type passive on-link.
Perhaps the simplest private discovery scenario involves a single client connecting to a public server through a public network. A common example would be a traveler using a publicly available printer in a business center, in an hotel, or at an airport.
( Taking notes:
( David is printing
( a document
~~~~~~~~~~~
o
___ o ___
/ \ _|___|_
| | client server |* *|
\_/ __ \_/
| / / Discovery +----------+ |
/|\ /_/ <-----------> | +----+ | /|\
/ | \__/ +--| |--+ / | \
/ | |____/ / | \
/ | / | \
/ \ / \
/ \ / \
/ \ / \
/ \ / \
/ \ / \
David adversary
In that scenario, the server is public and wants to be discovered, but the client is private. The adversary will be listening to the network traffic, trying to identify the visitors' devices and their activity. Identifying devices leads to identifying people, either just for tracking people or as a preliminary to targeted attacks.
The requirement in that scenario is that the discovery activity should not disclose the identity of the client.
The second private discovery scenario involves a private client connecting to a private server. A common example would be two people engaging in a collaborative application in a public place, such as for example an airport's lounge.
( Taking notes:
( David is meeting
( with Stuart
~~~~~~~~~~~
o
___ ___ o ___
/ \ / \ _|___|_
| | server client | | |* *|
\_/ __ __ \_/ \_/
| / / Discovery \ \ | |
/|\ /_/ <-----------> \_\ /|\ /|\
/ | \__/ \__/ | \ / | \
/ | | \ / | \
/ | | \ / | \
/ \ / \ / \
/ \ / \ / \
/ \ / \ / \
/ \ / \ / \
/ \ / \ / \
David Stuart Adversary
In that scenario, the collaborative application on one of the devices will act as a server, and the application on the other device will act as a client. The server wants to be discovered by the client, but has no desire to be discovered by anyone else. The adversary will be listening to network traffic, attempting to discover the identity of devices as in the first scenario, and also attempting to discover the patterns of traffic, as these patterns reveal the business and social interactions between the owners of the devices.
The requirement in that scenario is that the discovery activity should not disclose the identity of either the client or the server.
The third private discovery scenario involves wearable devices. A typical example would be the watch on someone's wrist connecting to the phone in their pocket.
( Taking notes:
( David' is here. His watch is
( talking to his phone
~~~~~~~~~~~
o
___ o ___
/ \ _|___|_
| | client |* *|
\_/ \_/
| _/ |
/|\ // /|\
/ | \__/ ^ / | \
/ |__ | Discovery / | \
/ |\ \ v / | \
/ \\_\ / \
/ \ server / \
/ \ / \
/ \ / \
/ \ / \
David Adversary
This third scenario is in many ways similar to the second scenario. It involves two devices, one acting as server and the other acting as client, and it leads to the same requirement of the discovery traffic not disclosing the identity of either the client or the server. The main difference is that the devices are managed by a single owner, which can lead to different methods for establishing secure relations between the devices. There is also an added emphasis on hiding the type of devices that the person wears.
In addition to tracking the identity of the owner of the devices, the adversary is interested in the characteristics of the devices, such as type, brand, and model. Identifying the type of device can lead to further attacks, from theft to device specific hacking. The combination of devices worn by the same person will also provide a "fingerprint" of the person, allowing identification.
While the discovery process illustrated in the scenarios in Section 2 most likely would be based on [RFC6762] as a means for making service information available, this document considers all kinds of means for making DNS-SD resource records available. These means comprise but are not limited to mDNS [RFC6762], DNS servers ([RFC1033] [RFC1034], [RFC1035]), e.g. using SRP [I-D.ietf-dnssd-srp], and multi-link [RFC7558] networks.
The discovery scenarios in Section 3.1 illustrate three separate abstract privacy requirements that vary based on the use case. These are not limited to mDNS.
In this section, we describe aspects of DNS-SD that make these requirements difficult to achieve in practice.
Client identity privacy, if not addressed properly, can be thwarted by a passive attacker (see Section 2). The type of passive attacker necessary depends on the means of making service information available. Information conveyed via multicast messages can be obtained by an on-link attacker, while unicast messages are only available to MITM attackers. Using multi-link service discovery solutions [RFC7558], external attackers have to be taken into consideration as well, e.g., when relaying multicast messages to other links.
Server identity privacy can be thwarted by a passive attacker in the same way as client identity privacy. Additionally, active attackers querying for information have to be taken into consideration as well. This is mainly relevant for unicast based discovery, where listening to discovery traffic requires a MITM attacker; however, an external active attacker might be able to learn the server identity by just querying for service information, e.g. via DNS.
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 first phase of discovery, clients obtain all 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 listening to the network traffic, i.e. a passive on-link attacker.
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 [RFC8117]. 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 both the device, and, by extension, the device's owner.
The TXT record's attribute-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.
A sometimes heard argument is that devices providing services can be identified by observing the local traffic, and that trying to hide the presence of the service is futile. However,
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 3.2.5, traffic analysis can often reveal the service.
For each of the operations described above, we must also consider security threats we are concerned about.
Can we trust the information we receive? Has it been modified in flight by an adversary? Do we trust the source of the information? Is the source of information fresh, i.e., not replayed? Freshness may or may not be required depending on whether the discovery process is meant to be online. In some cases, publishing discovery information to a shared directory or registry, rather than to each online recipient through a broadcast channel, may suffice.
Confidentiality is about restricting information access to only authorized individuals. Ideally this should only be the appropriate trusted parties, though it can be challenging to define who are "the appropriate trusted parties." In some uses cases, this may mean that only mutually authenticated and trusting clients and servers can read messages sent for one another. The "Discover" operation in particular is often used to discover new entities that the device did not previously know about. It may be tricky to work out how a device can have an established trust relationship with a new entity it has never previously communicated with.
It can be tempting to use (publicly computable) hash functions to obscure sensitive identifiers. This transforms a sensitive unique identifier such as an email address into a "scrambled" but still unique identifier. Unfortunately simple solutions may be vulnerable to offline dictionary attacks.
In any protocol where the receiver of messages has to perform cryptographic operations on those messages, there is a risk of a brute-force flooding attack causing the receiver to expend excessive amounts of CPU time and, where appliciable, battery power just processing and discarding those messages.
Also, amplification attacks have to be taken into consideration. Messages with larger payloads should only be sent as an answer to a query sent by a verified client.
Sender impersonation is an attack wherein messages such as service offers are forged by entities who do not possess the corresponding secret key material. These attacks may be used to learn the identity of a communicating party, actively or passively.
Deniability of sender activity, e.g., of broadcasting a discovery request, may be desirable or necessary in some use cases. This property ensures that eavesdroppers cannot prove senders issued a specific message destined for one or more peers.
Many modern devices, especially battery-powered devices, use power management techniques to conserve energy. One such technique is for a device to transfer information about itself to a proxy, which will act on behalf of the device for some functions, while the device itself goes to sleep to reduce power consumption. When the proxy determines that some action is required which only the device itself can perform, the proxy may have some way, such as Ethernet "Magic Packet", to wake the device.
In many cases, the device may not trust the network proxy sufficiently to share all its confidential key material with the proxy. This poses challenges for combining private discovery that relies on per-query cryptographic operations, with energy-saving techniques that rely on having (somewhat untrusted) network proxies answer queries on behalf of sleeping devices.
Creating a discovery protocol that has the desired security properties may result in a design that is not efficient. To perform the necessary operations the protocol may need to send and receive a large number of network packets. This may consume an unreasonable amount of network capacity, particularly problematic when it is a shared wireless spectrum. Further it may cause an unnecessary level of power consumption which is particularly problematic on battery devices, and may result in the discovery process being slow.
It is a difficult challenge to design a discovery protocol that has the property of obscuring the details of what it is doing from unauthorized observers, while also managing to do that efficiently.
One of the challenges implicit in the preceding discussions is that whenever we discuss "trusted entities" versus "untrusted entities", there needs to be some way that trust is initially established, to convert an "untrusted entity" into a "trusted entity".
One way to establish trust between two entities is to trust a third party to make that determination for us. For example, the X.509 certificates used by TLS and HTTPS web browsing are based on the model of trusting a third party to tell us whom to trust. There are some difficulties in using this model for establishing trust for service discovery uses. If we want to print our tax returns or medical documents on "our" printer, then we need to know which printer on the network we can trust be be "our" printer. All of the printers we discover on the network may be legitimate printers made by legitimate printer manufacturers, but not all of them are "our" printer. A third-party certificate authority cannot tell us which one of the printers is ours.
Another common way to establish a trust relationship is Trust On First Use (TOFU), as used by ssh. The first usage is a Leap Of Faith, but after that public keys are exchanged and at least we can confirm that subsequent communications are with the same entity. In today's world, where there may be attackers present even at that first use, it would be preferable to be able to establish a trust relationship without requiring an initial Leap Of Faith.
Techniques now exist for securely establishing a trust relationship without requiring an initial Leap Of Faith. Trust can be established securely using a short passphrase or PIN with cryptographic algorithms such as Secure Remote Password (SRP) or a Password Authenticated Key Exchange like J-PAKE using a Schnorr Non-interactive Zero-Knowledge Proof.
Such techniques require a user to enter the correct passphrase or PIN in order for the cryptographic algorithms to establish working communication. This avoids the human tendency to simply press the "OK" button when asked if they want to do something on their electronic device. It removes the human fallibility element from the equation, and avoids the human users inadvertently sabotaging their own security.
Without these techniques, users who try to print their tax return on a printer they've never used before will be tempted to just go ahead if the name looks right. With these techniques they'll be prompted to enter a pairing PIN, and *cannot* ignore that warning. They can’t just press an “OK” button. They have to walk to the printer and read the displayed PIN and enter it. And if the intended printer is not displaying a pairing PIN, or is displaying a different pairing PIN, that means the user may be being spoofed, and the connection will not succeed, and the failure will not reveal any secret information to the attacker. As much as the human desires to "just give me an OK button to make it print", and the attacker desires them to click that OK button, too, the cryptographic algorithms do not give the user the ability to opt out of the security, and consequently do not give the attacker any way to persuade the user to opt out of the security protections.
Trust establishment may depend on external, and optionally online, parties. Systems which have such a dependency may be attacked by interfering with communication to external dependencies. Where possible, such dependencies should be minimized. Local trust models are best for secure initialization in the presence of active attackers.
Given the considerations discussed in the previous sections, we state requirements for privacy preserving DNS-SD in the following subsections.
Defining a solution according to these requirements will lead to a solution that does not transmit privacy violating DNS-SD messages and further does not open pathways to new attacks against the operation of DNS-SD.
However, while this document gives advice on which privacy protecting mechanisms should be used on deeper layer network protocols and on how to actually connect to services in a privacy preserving way, stating corresponding requirements is out of the scope of this document. To mitigate attacks against privacy on lower layers, both servers and clients must use privacy options available at lower layers, and for example avoid publishing static IPv4 or IPv6 addresses, or static IEEE 802 MAC addresses. For services advertised on a single network link, link local IP addresses should be used; see [RFC3927] and [RFC4291] for IPv4 and IPv6, respectively. Static servers advertising services globally via DNS can hide their IP addresses from unauthorized clients using the split mode topology shown in [I-D.ietf-tls-esni]. Hiding static MAC addresses can be achieved via MAC address randomization (see [RFC7844]).
For all three scenarios described in Section 3.1, client privacy is a requirement. Client privacy, as a requirement, can be subdivided into:
Listing and resolving services via DNS-SD, clients typically disclose their interest in specific services types and specific instances of these types, respectively.
Privacy solutions fulfilling these requirements must be resilient to fingerprinting attacks (see Section 3.2.5) that could be used for breaching these requirements.
Servers like the "printer" discussed in scenario 1 are public, but the servers discussed in scenarios 2 and 3 are by essence private. Private servers have server privacy as a requirement, which can be subdivided into:
Offering services via DNS-SD, servers typically disclose their hostnames (SRV, A/AAAA), instance names of offered services (PRT, SRV), and information about services (TXT). Heeding these requirements protects a server's privacy on the DNS-SD level.
In order to be secure and feasible, a DNS-SD privacy extension must also heed the following security and operational requirements.
All scenarios require:
This draft does not require any IANA action.
This draft incorporates many contributions from Stuart Cheshire and Chris Wood. Thanks to Florian Adamsky for extensive review and suggestions on the organization of the threat model.