| Internet Engineering Task Force | T. Lemon |
| Internet-Draft | Nibbhaya Consulting |
| Intended status: Informational | S. Cheshire |
| Expires: January 14, 2021 | Apple Inc. |
| July 13, 2020 |
Service Registration Protocol for DNS-Based Service Discovery
draft-ietf-dnssd-srp-03
The Service Registration Protocol for DNS-Based Service Discovery uses the standard DNS Update mechanism to enable DNS-Based Service Discovery using only unicast packets. This makes it possible to deploy DNS Service Discovery without multicast, which greatly improves scalability and improves performance on networks where multicast service is not an optimal choice, particularly 802.11 (Wi‑Fi) and 802.15.4 (IoT) networks. DNS‑SD Service registration uses public keys and SIG(0) to allow services to defend their registrations against attack.
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 https://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 January 14, 2021.
Copyright (c) 2020 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 (https://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-Based Service Discovery is a component of Zero Configuration Networking [RFC6760] [ZC] [I-D.cheshire-dnssd-roadmap].
This document describes an enhancement to DNS-Based Service Discovery that allows services to automatically register their services using the DNS protocol rather than using Multicast DNS (mDNS). There is already a large installed base of DNS‑SD clients that can discover services using the DNS protocol. This extension makes it much easier to take advantage of this existing functionality.
This document is intended for three audiences: implementors of software that provides services that should be advertised using DNS‑SD, implementors of DNS servers that will be used in contexts where DNS‑SD registration is needed, and administrators of networks where DNS‑SD service is required. The document is intended to provide sufficient information to allow interoperable implementation of the registration protocol.
DNS-Based Service Discovery (DNS‑SD) allows services to advertise the fact that they provide service, and to provide the information required to access that service. Clients can then discover the set of services of a particular type that are available. They can then select a service from among those that are available and obtain the information required to use it.
The Service Registration Protocol for DNS‑SD (SRP), described in this document, provides a reasonably secure mechanism for publishing this information. Once published, these services can be readily discovered by clients using standard DNS lookups.
The DNS‑SD specification, Section 10 (“Populating the DNS with Information”), briefly discusses ways that services can publish their information in the DNS namespace. In the case of mDNS, it allows services to publish their information on the local link, using names in the ".local" namespace, which makes their services directly discoverable by peers attached to that same local link.
RFC6763 also allows clients to discover services using the DNS protocol. This can be done by having a system administrator manually configure service information in the DNS, but manually populating DNS authoritative server databases is costly and potentially error-prone, and requires a knowledgable network administrator. Consequently, although all DNS‑SD client implementations of which we are aware support DNS‑SD using DNS queries, in practice it is used much less frequently than mDNS.
The Discovery Proxy provides one way to automatically populate the DNS namespace, but is only appropriate on networks where services are easily advertised using mDNS. This document describes a solution more suitable for networks where multicast is inefficient, or where sleepy devices are common, by supporting both offering of services, and discovery of services, using unicast.
Services that implement SRP use DNS Update [RFC2136] [RFC3007] to publish service information in the DNS. Two variants exist, one for full-featured hosts, and one for devices designed for "Constrained-Node Networks" [RFC7228].
Full-featured hosts are either configured manually with a registration domain, or use the "dr._dns‑sd._udp.<domain>" query ([RFC6763] Section 11) to learn the default registration domain from the network. RFC6763 says to discover the registration domain using either ".local" or a network-supplied domain name for <domain>. Services using SRP MUST use the domain name received through the DHCPv4 Domain Name option ([RFC2132] section 3.17), if available, or the Neighbor Discovery DNS Search List option [RFC8106]. If the DNS Search List option contains more than one domain name, it MUST NOT be used. If neither option is available, the Service Registration protocol is not available on the local network.
Manual configuration of the registraton domain can be done either by querying the list of available registration zones ("r._dns‑sd._udp") and allowing the user to select one from the UI, or by any other means appropriate to the particular use case being addressed. Full-featured devices construct the names of the SRV, TXT, and PTR records describing their service(s) as subdomains of the chosen service registration domain. For these names they then discover the zone apex of the closest enclosing DNS zone using SOA queries [I-D.ietf-dnssd-push]. Having discovered the enclosing DNS zone, they query for the "_dnssd‑srp._tcp<zone>" SRV record to discover the server to which they should send DNS updates. Hosts that support SRP updates using TLS use the "_dnssd‑srp‑tls._tcp<zone>" SRV record instead.
For devices designed for Constrained-Node Networks [RFC7228] some simplifications are available. Instead of being configured with (or discovering) the service registration domain, the (proposed) special-use domain name (see [RFC6761]) "default.service.arpa" is used. The details of how SRP server(s) are discovered will be specific to the constrained network, and therefore we do not suggest a specific mechanism here.
SRP clients on constrained networks are expected to receive from the network a list of SRP servers with which to register. It is the responsibility of a Constrained-Node Network supporting SRP to provide one or more SRP server addresses. It is the responsibility of the SRP server supporting a Constrained-Node Network to handle the updates appropriately. In some network environments, updates may be accepted directly into a local "default.service.arpa" zone, which has only local visibility. In other network environments, updates for names ending in "default.service.arpa" may be rewritten internally to names with broader visibility.
The reason for these different assumptions is that low-power devices that typically use Constrained-Node Networks may have very limited battery power. The series of DNS lookups required to discover an SRP server and then communicate with it will increase the power required to advertise a service; for low-power devices, the additional flexibility this provides does not justify the additional use of power. It is also fairly typical of such networks that some network service information is obtained as part of the process of joining the network, and so this can be relied upon to provide nodes with the information they need.
Networks that are not constrained networks can more complicated topologies at the Internet layer. Nodes connected to such networks can be assumed to be able to do DNSSD service registration domain discovery. Such networks are generally able to provide registration domain discovery and routing. By requiring the use of TCP, the possibility of off-network spoofing is eliminated.
We will discuss several parts to this process: how to know what to publish, how to know where to publish it (under what name), how to publish it, how to secure its publication, and how to maintain the information once published.
We refer to the DNS Update message sent by services using SRP as an SRP update. Three types of updates appear in an SRP update: Service Discovery records, Service Description records, and Host Description records.
RFC 6763 describes the details of what each of these types of updates contains and is the definitive source for information about what to publish; the reason for summarizing this here is to provide the reader with enough information about what will be published that the service registration process can be understood at a high level without first learning the full details of DNS‑SD. Also, the "Service Instance Name" is an important aspect of first-come, first-serve naming, which we describe later on in this document.
Multicast DNS uses a single namespace, ".local", which is valid on the local link. This convenience is not available for DNS‑SD using the DNS protocol: services must exist in some specific unicast namespace.
As described above, full-featured devices are responsible for knowing in what domain they should register their services. Devices made for Constrained-Node Networks register in the (proposed) special use domain name [RFC6761] "default.service.arpa", and let the SRP server handle rewriting that to a different domain if necessary.
It is possible to issue a DNS Update that does several things at once; this means that it's possible to do all the work of adding a PTR resource record to the PTR RRset on the Service Name, and creating or updating the Service Instance Name and Host Description, in a single transaction.
An SRP update takes advantage of this: it is implemented as a single DNS Update message that contains a service's Service Discovery records, Service Description records, and Host Description records.
Updates done according to this specification are somewhat different than regular DNS Updates as defined in RFC2136. The RFC2136 update process can involve many update attempts: you might first attempt to add a name if it doesn't exist; if that fails, then in a second message you might update the name if it does exist but matches certain preconditions. Because the registration protocol uses a single transaction, some of this adaptability is lost.
In order to allow updates to happen in a single transaction, SRP updates do not include update prerequisites. The requirements specified in Section 2.4.3 are implicit in the processing of SRP updates, and so there is no need for the service sending the SRP update to put in any explicit prerequisites.
DNS‑SD Service Registration is based on standard RFC2136 DNS Update, with some differences:
Traditional DNS update is secured using the TSIG protocol, which uses a secret key shared between the client (which issues the update) and the server (which authenticates it). This model does not work for automatic service registration.
The goal of securing the DNS‑SD Registration Protocol is to provide the best possible security given the constraint that service registration has to be automatic. It is possible to layer more operational security on top of what we describe here, but what we describe here is an improvement over the security of mDNS. The goal is not to provide the level of security of a network managed by a skilled operator.
First-Come First-Serve naming provides a limited degree of security: a service that registers its service using DNS‑SD Registration protocol is given ownership of a name for an extended period of time based on the key used to authenticate the DNS Update. As long as the registration service remembers the name and the key used to register that name, no other service can add or update the information associated with that. FCFS naming is used to protect both the Service Description and the Host Description.
The service generates a public/private key pair. This key pair MUST be stored in stable storage; if there is no writable stable storage on the client, the client MUST be pre-configured with a public/private key pair in read-only storage that can be used. This key pair MUST be unique to the device.
When sending DNS updates, the service includes a KEY record containing the public portion of the key in each Host Description update and each Service Description update. Each KEY record MUST contain the same public key. The update is signed using SIG(0), using the private key that corresponds to the public key in the KEY record. The lifetimes of the records in the update is set using the EDNS(0) Update Lease option [I-D.sekar-dns-ul].
The KEY record in Service Description updates MAY be omitted for brevity; if it is omitted, the SRP server MUST behave as if the same KEY record that is given for the Host Description is also given for each Service Description for which no KEY record is provided. Omitted KEY records are not used when computing the SIG(0) signature.
The lifetime of the DNS‑SD PTR, SRV, A, AAAA and TXT records uses the LEASE field of the Update Lease option, and is typically set to two hours. This means that if a device is disconnected from the network, it does not appear in the user interfaces of devices looking for services of that type for too long.
The lifetime of the KEY records is set using the KEY-LEASE field of the Update Lease Option, and should be set to a much longer time, typically 14 days. The result of this is that even though a device may be temporarily unplugged, disappearing from the network for a few days, it makes a claim on its name that lasts much longer.
This means that even if a device is unplugged from the network for a few days, and its services are not available for that time, no other device can come along and claim its name the moment it disappears from the network. In the event that a device is unplugged from the network and permanently discarded, then its name is eventually cleaned up and made available for re-use.
To remove a service registration, the client retransmits its most recent update with an Update Lease option that has a LEASE value of zero. If the registration is to be permanently removed, KEY-LEASE should also be zero. Otherwise, it should have the same value it had previously; this holds the name in reserve for when the client is once again able to provide the service.
SRP clients are normally expected to remove all service instances when removing a host. However, in some cases a client may not have retained sufficient state to know that some service instance is pointing to a host that it is removing. Nevertheless, removing the host can be assumed to mean that all service instances pointing to it are no longer valid. Therefore, SRP servers MAY remove all service instances pointing to a host when a host is removed, even if the client doesn't remove them explicitly.
The SRP server first validates that the DNS Update is a syntactically and semantically valid DNS Update according to the rules specified in RFC2136.
SRP Updates consist of a set of Instructions that together add one or more services. Each instruction consists either of a single add, or a delete followed by an add. When an instruction contains a delete and an add, the delete MUST precede the add.
The SRP server checks each Instruction in the SRP update to see that it is either a Service Discovery update, a Service Description update, or a Host Description update. Order matters in DNS updates. Specifically, deletes must precede adds for records that the deletes would affect; otherwise the add will have no effect. This is the only ordering constraint; aside from this constraint, updates may appear in whatever order is convenient when constructing the update.
Because the SRP update is a DNS update, it MUST contain a single question that indicates the zone to be updated. Every delete and update in an SRP update MUST be within the zone that is specified for the SRP Update.
An Instruction is a Service Discovery Instruction if it contains
An Instruction is a Service Description Instruction if, for the appropriate Service Instance Name, it contains
An Instruction is a Host Description Instruction if, for the appropriate hostname, it contains
An SRP Update MUST include at least one Service Discovery Instruction, at least one Service Description Instruction, and exactly one Host Description Instruction. A DNS Update that does not is not an SRP update. A DNS Update that contains any other adds, any other deletes, or any prerequisites, is not an SRP update. Such messages should either be processed as regular RFC2136 updates, including access control checks and constraint checks, if supported, or else rejected with RCODE=REFUSED.
Note that if the definitions of each of these update types are followed carefully, this means that many things that look very much like SRP updates nevertheless are not. For example, a DNS update that contains an RRset Add to a Service Name and an RRset Add to a Service Instance Name, where the Service Name does not reference the Service Instance Name, is not a valid SRP update message, but may be a valid RFC2136 update.
Assuming that a DNS Update message has been validated with these conditions and is a valid SRP Update, the server checks that the name in the Host Description Instruction exists. If so, then the server checks to see if the KEY record on that name is the same as the KEY record in the Host Description Instruction. The server performs the same check for the KEY records in any Service Description Instrructions. For KEY records that were omitted from Service Description Instructions, the KEY from the Host Description Instruction is used. If any existing KEY record corresponding to a KEY record in the SRP Update does not match the KEY same record in the SRP Update (whether provided or taken from the Host Description Instruction), then the server MUST reject the SRP Update with the YXDOMAIN RCODE.
Otherwise, the server validates the SRP Update using SIG(0) on the public key in the KEY record of the Host Description update. If the validation fails, the server MUST reject the SRP Update with the REFUSED RCODE. Otherwise, the SRP update is considered valid and authentic, and is processed according to the method described in RFC2136.