Network Working Group C. Huitema
Internet-Draft Private Octopus Inc.
Intended status: Standards Track D. Kaiser
Expires: September 11, 2017 University of Konstanz
March 10, 2017

Privacy Extensions for DNS-SD


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.

Status of This Memo

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This Internet-Draft will expire on September 11, 2017.

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Table of Contents

1. Introduction

DNS-SD [RFC6763] over mDNS [RFC6762] enables configurationless 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. 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] 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 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.

While we focus on a mDNS-based distribution of the DNS-SD resource records, our solution is agnostic about the distribution method and also works with other distribution methods, e.g. the classical hierarchical DNS.

1.1. Requirements

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].

2. Privacy Implications of DNS-SD

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:

PTR Record:
Associates a service type in the domain with an "instance" name of this service type.
SRV Record:
Provides the node name, port number, priority and weight associated with the service instance, in conformance with [RFC2782].
TXT Record:
Provides a set of attribute-value pairs describing specific properties of the service instance.

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.

2.1. Privacy Implication of Publishing Service Instance Names

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.

2.2. Privacy Implication of Publishing Node Names

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 the device, and by extension of the device's owner.

2.3. Privacy Implication of Publishing Service Attributes

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.

2.4. Device Fingerprinting

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.

2.5. Privacy Implication of Discovering Services

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.

3. Design of the Private DNS-SD Discovery 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.ietf-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.

3.1. Device Pairing

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.ietf-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.

3.2. Discovery of the Private Discovery Service

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.

3.2.1. Obfuscated Instance Names

The instance names for the Private Discovery Service are obfuscated, so that authorized peers can associate the instance with its publisher, but unauthorized peers can only observe what looks like a random name. To achieve this, the names are composed as the concatenation of a nonce and a proof, which is composed by hashing the nonce with a pairing key:

   PrivateInstanceName = <nonce>|<proof>
   proof = hash(<nonce>|<key>)

The publisher will publish as many instances as it has established pairings.

The discovering party that looks for instances of the service will receive lists of advertisements from nodes present on the network. For each advertisement, it will parse the instance name, and then, for each available pairing key, compares the proof to the hash of the nonce concatenated with this pairing key. If there is no match, it discards the instance name. If there is a match, it has discovered a peer.

3.2.2. Using a Predictable Nonce

Assume that there are N nodes on the local scope, and that each node has on average M pairings. Each node will publish on average M records, and the node engaging in discovery may have to process on average N*M instance names. The discovering node will have to compute on average M potential hashes for each nonce. The number of hash computations would scale as O(N*M*M), which means that it could cause a significant drain of resource in large networks.

In order to minimize the amount of computing resource, we suggest that the nonce be derived from the current time, for example set to a representation of the current time rounded to some period. With this convention, receivers can predict the nonces that will appear in the published instances. They will only need to compute O(M) hashes, instead of O(N*M*M).

The publishers will have to create new records at the end of each rounding period. If the rounding period is set too short, they will have to repeat that very often, which is inefficient. On the other hand, if the rounding period is too long, the system may be exposed to replay attacks. We propose to set a value of about 5 minutes, which seems to be a reasonable compromise.

Unix defines a 32 bit time stamp as the number of seconds elapsed since January 1st, 1970 not counting leap seconds. The most significant 24 bits of this 32 bit number represent the number of 256 seconds intervals since the epoch. 256 seconds correspond to 4 minutes and 16 seconds, which is close enough to our design goal of 5 minutes. We will thus use this 24 bit number as nonce, represented as 3 octets.

Publishers will need to compute O(M) hashes at most once per time stamp interval. If records can be created "on the fly", publishers will only need to perform that computation upon receipt of the first query during a given interval, and cache the computed results for the remainder of the interval. There are however scenarios in which records have to be produced in advance, for example when records are published within a scope defined by a domain name and managed by a "classic" DNS server. In such scenarios, publishers will need to perform the computations and publication exactly once per time stamp interval.

3.2.3. Using a Short Proof

Devices will have to publish as many instance names as they have peers. The instance names will have to be represented via a text string, which means that the binary concatenation of nonce and proof will have to be encoded using a binary-to-text conversion such as BASE64 ([RFC2045] section 6.8) or BASE32 ([RFC4648] section 6).

Using long proofs, such as the full output of SHA256 [RFC4055], would generate fairly long instance names: 48 characters using BASE64, or 56 using BASE56. These long names would inflate the network traffic required when discovering the privacy service. They would also limit the number of DNS-SD PTR records that could be packed in a single 1500 octet sized packet, to 23 or fewer with BASE64, or 20 or fewer with BASE32.

Shorter proofs lead to shorter messages, which is more efficient as long as we do not encounter too many collisions. A collision will happen if the proof computed by the publisher using one key matches a proof computed by a receiver using another key. If a receiver mistakenly believes that a proof fits one of its peers, it will attempt to connect to the service as explained in section Section 4.5 but in the absence of the proper pairwise shared key, the connection will fail. This will not create an actual error, but the probability of such events should be kept low.

The following table provides the probability that a discovery agent maintaining 100 pairings will observe a collision after receiving 100000 advertisement records. It also provides the number of characters required for the encoding of the corresponding instance name in BASE64 or BASE32, assuming 24 bit nonces.

Proof Collisions BASE64 BASE32
24 5.96046% 8 16
32 0.02328% 11 16
40 0.00009% 12 16
48 3.6E-09 12 16
56 1.4E-11 15 16

The table shows that for a proof, 24 bits would be too short. 32 bits might be long enough, but the BASE64 encoding requires padding if the input is not an even multiple of 24 bits, and BASE32 requires padding if the input is not a multiple of 40 bits. Given that, the desirable proof lengths are thus 48 bits if using BASE64, or 56 bits if using BASE32. The resulting instance name will be either 12 characters long with BASE64, allowing 54 advertisements in an 1500 byte mDNS message, or 16 characters long with BASE32, allowing 47 advertisements per message.

In the specification section, we will assume BASE64, and 48 bit proofs composed of the first 6 bytes of a SHA256 hash.

3.2.4. Direct Queries

The preceding sections assume that the discovery is performed using the classic DNS-SD process, in which a query for all available "instance names" of a service provides a list of PTR records. The discoverer will then select the instance names that correspond to its peers, and request the SRV and TXT records corresponding to the service instance, and then obtain the relevant A or AAAA records. This is generally required in DNS-SD because the instance names are not known in advance, but for the Private Discovery Service the instance names can be predicted, and a more efficient Direct Query method can be used.

At a given time, the node engaged in discovery can predict the nonce that its peer will use, since that nonce is composed by rounding the current time. The node can also compute the proofs that its peers might use, since it knows the nonce and the keys. The node can thus build a list of instance names, and directly query the SRV records corresponding to these names. If peers are present, they will answer directly.

This "direct query" process will result in fewer network messages than the regular DNS-SD query process in some circumstances, depending on the number of peers per node and the number of nodes publishing the presence discovery service in the desired scope.

When using mDNS, it is possible to pack multiple queries in a single broadcast message. Using name compression and 12 characters per instance name, it is possible to pack 70 queries in a 1500 octet mDNS multicast message. It is also possible to request unicast replies to the queries, resulting in significant efficiency gains in wireless networks.

3.3. Private Discovery Service

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 "proof" described in Section 3.2, by concatenating a nonce and the hash of the nonce with the shared secret.

3.3.1. A Note on Private DNS Services

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.

3.4. Randomized Host Names

Instead of publishing their actual name in the SRV records, nodes could publish a randomized name. That is the solution argued for in [RFC8117].

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.

3.5. Timing of Obfuscation and Randomization

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 [RFC8117], 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.

4. Private Discovery Service Specification

The proposed solution uses the following components:

These components are detailed in the following subsections.

4.1. Host Name Randomization

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.

4.2. Device Pairing

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.ietf-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.]]

4.3. Private Discovery Server

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.

4.3.1. Establishing TLS Connections

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. To guarantee interoperability, implementations of the Private Name Server MUST support TLS_PSK_WITH_AES_256_GCM_SHA384.

When using the PSK based authentication, the "psk_identity" parameter identifying the pre-shared key MUST be identical to the "Instance Identifier" defined in Section 4.4, i.e. 24 bit nonce and 48 bit proof encoded in BASE64 as 12 character string. The server will use the pairing key associated with this instance identifier.

4.4. Publishing Private Discovery Service Instances

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._tcp".

Each published instance describes one server and one pairing. In the case where a node manages more than one pairing, it should publish as many instances as necessary to advertise all available pairings.

Each instance name is composed as follows:

   pick a 24 bit nonce, set to the 24 most
   significant bits of the 32 bit Unix GMT time.

   compute a 48 bit proof:
      proof = first 48 bits of HASH(<nonce>|<pairing key>)

   set the 72 bit binary identifier as the concatenation
   of nonce and proof

   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 24 bit nonce and 48 bit proof result in a 72 bit string. The BASE64 conversion is 12 characters long per [RFC6763].

4.5. Discovering Private Discovery Service Instances

Nodes that wish to discover Private Discovery Service Instances SHOULD issue a DNS-SD discovery request for the service type "_pds._tcp". They MAY, as an alternative, use the Direct Discovery procedure defined in Section 4.6. If nodes send a DNS-SD discovery request, they will receive in response 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 corresponds to 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 not multiple 72 bits,
         discard the instance.

      nonce = first 24 bits of binary.

      if nonce does not match the first 24 bits of the current
      time plus or minus 1 minute, discard the instance.   
      for each available pairing
         retrieve the key Xj of pairing number j
         compute F = first 48 bits of hash(nonce, Xj)
         if F is equal to the last 48 bits of
         the binary instance ID
            mark the pairing number j as available

The check of the current time is meant to mitigate replay attacks, while not mandating a time synchronization precision better than one minute.

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.

4.6. Direct Discovery of Private Discovery Service Instances

Nodes that wish to discover Private Discovery Service Instances MAY use the following Direct Discovery procedure instead of the regular DNS-SD Discovery explained in Section 4.5.

To perform Direct Discovery, nodes should compose a list of Private Discovery Service Instances Names. There will be one name for each pairing available to the node. The Instance ID for each name will be composed of a nonce and a proof, using the algorithm specified in Section 4.4.

The querier will issue SRV record queries for each of these names. The queries will only succeed if the corresponding instance is present, in which case a pairing is discovered. After that, the querier SHOULD try connecting to the corresponding instance, as explained in Section 4.4.

4.7. Using the Private Discovery Service

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.

5. Security Considerations

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.

5.1. Attacks Against the Pairing System

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.ietf-dnssd-pairing].

5.2. Denial of Discovery of the Private Discovery Service

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 could be amplified if the clients needed to compute proofs for all the nonces presented in Private Discovery Service Instance names. This is mitigated by the specification of nonces as rounded time stamps in Section 4.5. 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 will make sure that proofs are only computed once per valid time stamp.

5.3. Replay Attacks Against Discovery of the Private Discovery Service

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 24 bits encoding the most significant bits of the "UNIX" time. In order to protect against replay attacks, clients SHOULD verify that this time is reasonably recent, as specified in Section 4.5.

[[TODO: Should we somehow encode the scope in the identifier? Having both scope and time would really mitigate that attack. For example, one could add a local IPv4 or IPv6 prefix in the nonce. However, this won't work in networks behind NAT. It would also increase the size of the instance ID.]]

5.4. Denial of Private Discovery Service

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.

5.5. Replay Attacks against the Private Discovery Service

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 the 24 most significant bits of 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.

6. IANA Considerations

This draft does not require any IANA action. (Or does it? What about the _pds tag?)

7. Acknowledgments

This draft results from initial discussions with Dave Thaler, and encouragements from the DNS-SD working group members.

8. References

8.1. Normative References

[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC4055] Schaad, J., Kaliski, B. and R. Housley, "Additional Algorithms and Identifiers for RSA Cryptography for use in the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 4055, DOI 10.17487/RFC4055, June 2005.
[RFC4075] Kalusivalingam, V., "Simple Network Time Protocol (SNTP) Configuration Option for DHCPv6", RFC 4075, DOI 10.17487/RFC4075, May 2005.
[RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)", RFC 4279, DOI 10.17487/RFC4279, December 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013.

8.2. Informative References

[I-D.ietf-dnssd-pairing] Huitema, C. and D. Kaiser, "Device Pairing Using Short Authentication Strings", Internet-Draft draft-ietf-dnssd-pairing-01, March 2017.
[I-D.ietf-dnssd-push] Pusateri, T. and S. Cheshire, "DNS Push Notifications", Internet-Draft draft-ietf-dnssd-push-09, October 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-15, December 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.
[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.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006.
[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.
[RFC8117] Huitema, C., Thaler, D. and R. Winter, "Current Hostname Practice Considered Harmful", RFC 8117, DOI 10.17487/RFC8117, March 2017.

Authors' Addresses

Christian Huitema Private Octopus Inc. Friday Harbor, WA 98250 U.S.A. EMail: URI:
Daniel Kaiser University of Konstanz Konstanz, 78457 Germany EMail: