# mDNS, DNS-SD, and Bonjour: Introduction The modern networking landscape is riddled with many different types of devices from Smart LEDs, to Laptops and Smartphones. In order for these devices to communicate with one another and function as they are intended, they have to be able to identify one another and advertise what their functions are in a standard way. This is where mDNS and DNS-SD come into play. In this document, we will: * Define: * `mDNS` * `DNS-SD` * `Bonjour` * `Avahi` * Explain the relationships between these protocols and services * Show practical examples of how these systems work together to facilitate different functions on modern networks. ## Definitions ### mDNS * Multicast Domain Name System (mDNS) * A `zero-configuration` (Zeroconf) networking protocol designed to allow devices on a local network to discover each other and resolve hostnames to IP addresses without needing a centralized DNS server * Enables `plug-and-play` networking with no manual setup * How It Works: * `Multicast communication`: sends multicast UDP packets to all devices on the local subnet using dedicated addresses on port 5353 * IPv4: `224.0.0.251` * This is a special-purpose multicast address reserved for mDNS by the `Internet Assigned Numbers Authority` (IANA) * All traffic to this address will be received and processed by all devices on the local network that understand mDNS * The `224.0.0.0`-`223.0.0.255` range is reserved for local subnet protocols like mDNS * This address corresponds to the MAC Address `01:00:5E:00:00:FB` * IPv4 multicast addresses map to MAC addresses in the range `01:00:5E:00:00:00`-`01:00:5E:7F:FF:FF`. * IPv6: `FF02::FB` * `Query/Response`: When an mDNS client needs to resolve a hostname, it sends an IP multicast query that asks the device having that hostname to identify itself. The target machine then multicasts a message that includes its IP adderss. All machines in that subnet can then use that information to update their mDNS caches. * `TTL and Caching`: Resource Records (RRs) include a `Time-To-Live` value, dictating how long answers are cached to reduce repeated queries. * `Domain restriction`: By default, mDNS only resolves hostnames ending with `.local` to avoid conflicts with regular DNS * This may still cause conflicts where a unicast DNS hostname ends with `.local`. * Key Use Cases: * `Service Discovery`: Devices like printers, media servers, smart TVs, and IoT devices announce their presence so other devices can find them easily * `Apple Bonjour`: Uses mDNS (and DNS-SD) to enable seamless discovery between Macs, iPhones, and third-party Devices * `Universal Adoption`: Supported widely across operating systems, including Linux (Avahi), macOS (Bonjour), and Windows (since Windows 10 v1703) * Security and Network Considerations: * `Scope`: Limited to the local broadcast domain. Does not cross routers without special relays or configurations * `Network Load`: Can Cause congestion in large networks because every query is broadcast to all nodes * `Potential Conflicts`: Can conflict with unicast DNS as mentioned above * Protocol HIstory and Standards * `Developed by`: Apple, originally proposed at IETF in 2000 * `Standardized as`: [RFC 6762](https://datatracker.ietf.org/doc/html/rfc6762) - Multicast DNS ## DNS-SD * DNS-Based Service Discovery (DNS-SD) * An extension of the Domain Name System designed to enable automatic discovery of network services on both local and wide area networks * Core Mechanism: * `Service Advertising`: Devices publish service information (such as their type, name, host and capabilities) by inserting specifig DNS resource records (RRs) into a local or global DNS zone * `Service Discovery`: Clients query for serviices of interest using a structured DNS naming convention and receive records listing all available instances of that service * `Standard Record Types Used`: * `PTR`: Points to the list of available service instance names * `SRV`: Specifies the hostname and port for a specific service instance * `TXT`: Provides metadata and details (e.g., features, configuration) about the service instance * Typical Service Naming Scheme ```bash {Instance}._{Service}._{Protocol}.{Domain} ``` > * `Instance`: Unique name for a particular device's service (e.g., OfficePrinter) > * `Service`: Type of service (e.g., http, ssh, ipp) > * `Protocol`: Transport protocol, usually `tcp` or `udp` > * `Domain`: DNS domain or `.local` for mDNS Example: ```bash OfficePrinter._ipp._tcp.local ``` > * Identifies an IPP printer called OfficePrinter on the local subnet * DNS-SD Protocol Flow 1. `Service Browsing`: Client sends a DNS PTR query for a particular service type and protocol (e.g., `_ipp._tcp.local` to retrieve all available printers 2. `Service Instance Selection`: Returns a list of instance names (e.g., `Printer1._ipp._tcp.local`, `Printer2._ipp._tcp.local`, etc.) 3. `Service Resolution`: Client queries for SRV and TXT records for the chosen instance to retrieve connection details and capabilities 4. `Host Address Resolution`: Client resolves the hostname to obtain the IPv4/IPv6 address for connecting to the service * Typical Use Cases * `Home and Office Networks`: Auto-disvocery of printers (AirPrint), media servers, smart devices, file-sharing endpoints * `IoT`: Enables smart home devices to find and interact with each other seamlessly * `Enterprise Networks`: Cross-subnet service discovery (with unicast DNS support) for printers, application endpoints, databases, and more * `Microservices and Cloud`: Locating dynamic service endpoints in distributed systems * Example of DNS-SD Discovery > Suppose a client wants to find all HTTP servers on a local network: > 1. `Query PTR`: `_http._tcp.local` -> List of HTTP service instances > 2. `SRV/TXT Lookup`: For each instance, retrieve `SRV` (host/port) and `TXT` (metadata) records > 3. `A/AAAA Lookup`: Resolve the host to an IPv4/IPv6 address * Security and Privacy Considerations * `Local-only by Default`: When operated over mDNS, traffic is confined to a single subnet/link, limiting exposure * `Wide-Area Support`: If using unicast DNS (for large-scale or WAN deployments), administrator ocntrol of which services are advertised and discovered is crucial * `Potential Risks`: Unrestricted advertising can reveal network topology or device information. * Mitigation includes DNSSEC, DNS over TLS/HTTPS, ACLs, and service registration protocols (such as the new SRP - Service Registration Protocol - for authenticated registration over unicast DNS) * Protocol Standards and Interoperability * `Main Standard`: [RFC 6763](https://datatracker.ietf.org/doc/html/rfc6763) - DNS-Based Service Discovery * `Key Improvements`: [RFC 8882](https://datatracker.ietf.org/doc/html/rfc8882) & [RFC 9665](https://datatracker.ietf.org/doc/html/rfc9665) - DNS-SD Privacy and Security Requirements & Service Registration for DNS-SD :::info :bulb: `Bonjour` is natively supported on Apple Platforms (iOS and MacOS) and Applications (i.e. iTunes on Windows) ::: :::info :bulb: `avahi` can be installed on Linux Systems to observe and interface with services that use DNS-SD ::: ## Bonjour * `Bonjour`: Apple's implementation of zero-configuration networking, which enable automatic discovery of devices and services (like printers, file shares and media devices) on a local network without manual configuration * Core Mechanism 1. `Hostname Resolution`: Bonjour uses mDNS to resolve hostnames (i.e. `MyPrinter.local`) to IP addresses in the local network without needing a DHCP-configured DNS server 2. `Service Discovery`: Bonjour uses DNS-SD to let devices advertise and discover services (e.g, printers, shared folders, media servers) using service-type queries (like `_ipp._tcp.local` for network printers) * Mechanism Example * When you open Finder on a Mac and click `Network`, `Bonjour` scans for advertised file share services using mDNS and DNS-SD * If a compatible printer (`_ipp._tcp._local`) is on the same subnet, your macOS device can discover and use it, no manual configuration needed * Service Types & Naming * Service Record Syntax: `{instance name}._{service type}._{protocol}.local` * Service Examples: * `_afpovertcp._tcp.local` - Apple File Protocol * `_airplay._tcp.local` - Airplay Devices * `_ipp._tcp.local` - IPP network Printers * `_ssh._tcp.local` - SSH servers * Components * `mDNS`: Hostname Resolution (i.e. `MyPrinter.local`) * `DNS-SD`: Advertising and Discovering Services * `Responder Daemon`: Announces Device and Service Availability to the localhost (mDNSResponder) * `Client Libraries`: APIs for Apps/Devices to use Bonjour Features * Platform Support * `Apple`: Included in macOS, iOS, iPadOS, tvOS and built into many Apple apps using [`Network.Framework`](https://developer.apple.com/documentation/network) * `Windows`: Bundled with apps like iTunes and Adobe Creative Suite :::info :abacus: For more detail on `Network.Framework` review the Resource, [Apple Network.framework Developer Guide](https://developer.apple.com/documentation/network)