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Link-Local Multicast Name Resolution

Link-Local Multicast Name Resolution (LLMNR) is an informational protocol specified in RFC 4795 that enables and hosts on the same local network link to resolve hostnames to addresses using queries when conventional DNS resolution is unavailable. Published in January 2007, it reuses the DNS packet format, including support for all DNS query types, classes, and opcodes, but operates exclusively at the to avoid propagation across routers. LLMNR functions by having a querying host send UDP multicast packets to port 5355 on the link-local addresses 224.0.0.252 () or FF02::1:3 (), prompting any matching host to respond with a unicast reply containing its . Responses are authoritative only from the queried host itself, and the protocol maintains a separate name resolution cache per network interface to prevent interference with standard DNS caching. Optional features include TCP unicast queries for larger responses and support for security mechanisms like , SIG(0), , and DNSSEC to authenticate queries and responses. Primarily implemented in Microsoft Windows starting with and carried forward in later versions such as , 8, 10, and Server editions, LLMNR serves as a fallback mechanism in small or ad-hoc networks lacking a dedicated DNS . The Windows profile adheres closely to RFC 4795 but treats certain extensions, such as EDNS(0) and mandatory TCP fallback, as optional. It has also been adopted in environments through tools like systemd-resolved for similar resolution needs. Despite its utility, LLMNR's reliance on unauthenticated exposes it to spoofing attacks, where off-link or on-link adversaries can forge responses to redirect traffic, enabling man-in-the-middle exploits or denial-of-service conditions. Such vulnerabilities, including remote code execution risks from malformed queries, have prompted security bulletins and recommendations to disable LLMNR in favor of configured DNS where possible. In enterprise settings, disabling it via —under Computer Configuration > Administrative Templates > Network > DNS Client > Turn off multicast name resolution—is a common hardening step to mitigate these risks without impacting core functionality.

Overview

Definition and Purpose

Link-Local Multicast Name Resolution (LLMNR) is a protocol developed by that enables hostname-to-IP address resolution within a local network segment using UDP queries. It operates on the IPv4 224.0.0.252 and the link-local FF02::1:3, both over port 5355, and is based on the DNS packet format to support existing and future DNS message structures, types, and classes. Introduced as part of and enabled by default in subsequent Windows versions, LLMNR serves primarily as a fallback mechanism for name resolution in environments lacking a traditional DNS . The core purpose of LLMNR is to facilitate communication and device discovery in small-scale networks where conventional DNS infrastructure is unavailable or unreachable, such as in home or ad-hoc setups. By allowing hosts to broadcast queries and respond directly via , it enables automatic resolution without manual configuration or centralized authority, particularly benefiting Windows-based systems in legacy or isolated subnets. This protocol is especially useful in single-subnet environments for tasks like or printer discovery when DNS queries fail. LLMNR's scope is strictly limited to link-local operation, meaning queries and responses do not propagate beyond the local network segment or across routers, ensuring it remains confined to a single . Similar to (mDNS) in providing cross-platform zero-configuration resolution, LLMNR is optimized for Windows ecosystems as a complementary tool rather than a DNS replacement. In practice, it acts as a reliable fallback for older Windows deployments in unmanaged networks, promoting seamless connectivity without additional setup.

History and Development

Link-Local Multicast Name Resolution (LLMNR) was introduced by with the release of on January 30, 2007, as a component of broader enhancements and features aimed at simplifying local network interactions without requiring a dedicated DNS . The protocol enabled hosts to resolve names via on the local link, supporting both IPv4 and in scenarios where traditional DNS queries would fail due to the absence of a . Microsoft developed LLMNR internally to bridge gaps in Windows-based local name resolution, particularly for ad-hoc and peer-to-peer environments, while drawing from IETF discussions on multicast-based alternatives to DNS. This effort culminated in RFC 4795, an informational specification published by the IETF in January 2007 and authored by Microsoft engineers Bernard Aboba, Dave Thaler, and Levon Esibov, which outlined the protocol but did not pursue formal standardization on the standards track. Unlike fully standardized protocols, LLMNR was implemented directly in the Windows networking stack to accelerate deployment and address Microsoft-specific ecosystem needs. LLMNR was enabled by default starting with , and continued in (released in 2009) and , expanding its role in client and server environments for seamless local discovery. These versions also introduced options to disable the protocol, allowing administrators to manage it in controlled settings. distributed the Bonjour SDK for Windows developers starting around 2008 to enable mDNS support in applications for cross-platform in mixed environments, separate from LLMNR. As of 2022, announced plans to deprecate LLMNR and in favor of mDNS, though it remains enabled by default in as of November 2025. The protocol persists as enabled by default in and , but and security guidelines recommend its disablement in enterprise deployments to reduce exposure to multicast-related vulnerabilities.

Protocol Mechanics

Name Resolution Process

When conventional DNS resolution fails for a hostname on a local network segment, a device initiates an LLMNR query by constructing a DNS-like message and sending it as a datagram to port 5355 on the appropriate : 224.0.0.252 for IPv4 or FF02::1:3 for . This query contains the target in the question section, typically requesting A (IPv4) or (IPv6) resource records. The query is disseminated via link-local , ensuring it reaches all devices on the same local link without propagating beyond routers or layer-3 boundaries, due to the and a or hop limit of 1 in the . Any device on the link that recognizes the queried name as its own responds directly to the querier with a message to 5355, including the corresponding in the answer section; multiple responders may reply if name conflicts exist, but the protocol does not enforce uniqueness at query time. Upon receiving a valid response, the querying device caches the name-to-IP mapping for a short period, with responses including a recommended of 30 seconds to minimize stale entries in dynamic environments. The querier then uses this cached information for subsequent communication until the expires or the entry is invalidated. To handle potential name s, devices perform proactive probing before claiming a , sending special conflict-detection queries with the 'C' () bit set in the header; if a conflict is detected via multiple responses, the device logs it and immediately stops using the , potentially attempting to configure a new name after the expires. If no response arrives within the LLMNR timeout period—1 second for most media or 100 milliseconds for networks—the query is considered unanswered, and the name is treated as non-existent for that attempt. In Windows implementations, unresolved LLMNR queries for single-label names may fallback to name resolution if enabled, continuing the overall host resolution sequence after DNS and LLMNR. For dual-stack environments supporting both IPv4 and , queries use distinct addresses for each version, allowing independent on IPv4 and IPv6 links. In Windows implementations, the querier sends an AAAA query first, and if no response is received within the timeout, sends an A query; a successful AAAA response suppresses the subsequent A query to prefer IPv6 addresses.

Message Types and Formats

Link-Local Multicast Name Resolution (LLMNR) employs two primary message types: queries and responses, both structured in a format compatible with the (DNS) to facilitate -to-IP address resolution on local links. A query message is initiated by a sender seeking to resolve a and consists of a header followed by a question section that specifies the target and the desired type, such as A for IPv4 addresses or for addresses. These queries are to all hosts on the link, prompting potential responders to check their local configurations for a match. The response message, sent unicast by a matching responder back to the querier, includes the header with the QR flag set to indicate a response, along with an answer section containing one or more resource records (RRs) that fulfill the query. Additional sections, such as authority and additional, may accompany the answer to provide further details like negative caching information via SOA records or supplementary RRs for efficiency. In Microsoft implementations, responders are required to handle queries for A, , PTR (for reverse lookups), and ANY types, while optionally supporting others but typically discarding unsupported ones. LLMNR message formats mirror DNS packets, featuring a fixed-size header with fields like a 16-bit transaction ID for matching queries to responses, the QR bit to distinguish queries (0) from responses (1), an opcode fixed at 0 for standard queries, and response code (RCODE) values to denote status. Counts for each section—questions (QDCOUNT), answers (ANCOUNT), name server records (NSCOUNT), and additional records (ARCOUNT)—allow variable-length payloads, with flags such as the authoritative answer (AA) bit set in positive responses and the truncation (TC) bit indicating oversized messages that require TCP fallback. Although the protocol supports all standard DNS resource record types in principle, practical usage emphasizes A, AAAA, and PTR for core address resolution, excluding specialized types like MX or SRV which are not typically resolved via LLMNR. Error responses in LLMNR utilize DNS RCODE values within the header; for instance, RCODE 3 (NXDOMAIN) signals that the queried hostname does not exist on the link, while RCODE 2 (SERVFAIL) indicates a server failure in processing the query. These are conveyed in responses to avoid unnecessary traffic, and negative responses may include an authority section with an to enable caching of the failure for a specified duration. Hostnames in LLMNR messages follow the standard DNS wire format, encoded as a sequence of labels separated by single-byte length prefixes, with optional compression pointers to reference repeated name segments for brevity. This encoding supports both single-label names (common in local networks) and fully qualified domain names, using for internationalized characters in deployments.

Packet Structure

Header Components

The Link-Local Multicast Name Resolution (LLMNR) protocol adapts the 12-byte header format from the (DNS) as defined in RFC 1035, serving as the fixed prefix for all LLMNR packets to facilitate query-response matching and basic message control on local links. This header ensures compatibility with DNS-like parsing while incorporating LLMNR-specific modifications for multicast environments, operating over port 5355 for both IPv4 and IPv6. The header begins with a 16-bit identifier (ID) field, generated pseudo-randomly by the query sender to uniquely tag each request and mitigate spoofing risks, with responders required to copy this value verbatim into the corresponding response for validation. Following the ID are the 16-bit flags, structured into two bytes: the first byte includes the 1-bit Query/Response (QR) flag (0 for queries, 1 for responses), a 4-bit Operation Code (OPCODE) set to 0 for standard queries (with non-zero values silently discarded), the 1-bit Conflict (C) flag (set in queries upon detecting multiple responses to indicate non-unique names, and in responses to signal name conflicts), the 1-bit Truncation (TC) flag (unset in queries and ignored if set, but indicates payload truncation in responses prompting TCP fallback), and the 1-bit Tentative (T) flag (set in responses for unverified names, ignored in queries). The second byte of flags contains three reserved Zero (Z) bits (must be zero and ignored if set) and a 4-bit Response Code (RCODE) field (0 in queries and ignored by responders, 0 in multicast responses, with support for extended values via EDNS0). Unlike standard DNS, LLMNR omits the Authoritative Answer (AA), Recursion Desired (RD), and Recursion Available (RA) flags, replacing them with C, T, and Z to suit link-local multicast dynamics where recursion and authority delegation are irrelevant. The header concludes with four 16-bit counters in network byte order (big-endian): Questions (QDCOUNT, must be 1 in queries and 0 in responses), Answers (ANCOUNT, 0 in queries), (NSCOUNT, 0 in queries), and Additional (ARCOUNT, counting any extra records). All multi-byte fields adhere to this big-endian convention for interoperability. LLMNR implementations must support EDNS0 extensions for larger payloads and variable-length RCODE values, but transaction signature () and SIG(0) are optional, with no mandatory use in typical deployments to maintain simplicity. These adaptations distinguish LLMNR from DNS by prioritizing , non-hierarchical without overhead, though the random ID mechanism provides basic anti-spoofing.

Resource Record Details

Resource records (RRs) in Link-Local Multicast Name Resolution (LLMNR) follow the standard (DNS) format as defined in RFC 1035, consisting of a variable-length NAME field (often a 16-bit pointer for ), a 16-bit TYPE field, a 16-bit CLASS field (set to 1 for the class, IN), a 32-bit field, a 16-bit RDLENGTH field indicating the length of the RDATA, and a variable-length RDATA field containing the record-specific data. This structure is inherited directly by LLMNR from DNS to ensure compatibility and simplicity in implementation. LLMNR supports standard DNS resource record types relevant to name resolution, including A records for 32-bit IPv4 addresses, records for 128-bit addresses, CNAME records for aliases, and PTR records for reverse-domain mappings. While the protocol accommodates all current and future DNS types and classes, implementations typically prioritize A, , and PTR for core functionality, with CNAME used for alias resolution. The field specifies the caching duration for the resource record, with a recommended default of 30 seconds for positive responses to balance freshness and network efficiency in dynamic link-local environments. For negative responses (e.g., NXDOMAIN), the is determined by the in the authority section, adhering to negative caching guidelines where all RRs in a response share the same value. RDATA encoding varies by type: A and AAAA records use binary representations of IP addresses (4 bytes for IPv4, 16 bytes for IPv6), while CNAME and PTR records employ compressed domain names, following DNS standards without additional variable-length complexities. This ensures compact payloads suitable for multicast transmission. In LLMNR messages, the Question section includes partial RRs comprising only the NAME, TYPE, and CLASS fields for the query. The Answer section contains full RRs matching the query, while the Additional section provides supplementary glue records, such as A or AAAA records accompanying a CNAME in the Answer section to aid resolution. Parsing of resource records relies on DNS name compression, where domain names are represented using 14-bit offsets pointing to prior occurrences in the message, reducing packet size and mandatory for efficient operation over links. This compression applies across all sections following the fixed header.

Comparisons and Interoperability

Relation to

Link-Local Multicast Name Resolution (LLMNR) shares several core principles with (mDNS), as both protocols facilitate zero-configuration name resolution on local networks without relying on a centralized DNS . They employ multicast transmissions to query and respond to resolutions within the same link-local scope, leveraging DNS-compatible packet formats to ensure compatibility with existing DNS infrastructure. This design allows devices to discover peers automatically, promoting seamless local communication in environments lacking traditional DNS support. Despite these parallels, LLMNR and mDNS diverge in key technical specifications. LLMNR utilizes distinct multicast addresses—224.0.0.252 for IPv4 and FF02::1:3 for —and operates exclusively on / port 5355. In contrast, mDNS employs 224.0.0.251 for IPv4 and FF02::FB for , running on port 5353. LLMNR was developed primarily for Windows ecosystems, serving as a fallback for single-label resolution, whereas mDNS is a cross-platform standard implemented in Apple's , Linux's Avahi, and other systems for broader compatibility. Interoperability between LLMNR and mDNS is limited due to their incompatible addressing and port usage, though Windows systems support both protocols concurrently. Starting with , introduced native mDNS support, allowing resolution of .local domains alongside LLMNR's handling of unqualified names. Earlier, Windows users could enable mDNS via Apple's add-on for integration with non-Windows devices. However, enabling both can result in redundant queries on mixed networks, potentially leading to multiple responses for the same if devices respond via either protocol. A notable distinction lies in service discovery capabilities: mDNS extends beyond basic hostname resolution through DNS Service Discovery (DNS-SD), incorporating SRV and TXT resource records to advertise and locate services such as printers or file shares. LLMNR, while supporting standard DNS record types like A and AAAA, lacks these specialized extensions, limiting it to simpler host-to-IP mappings without advanced service enumeration. In terms of adoption, mDNS has become the preferred protocol in heterogeneous environments since around 2010, owing to its standardization and wider ecosystem support, while LLMNR functions as a Microsoft-specific fallback in Windows-centric setups. has signaled a shift toward mDNS, with plans to phase out LLMNR in future Windows releases to streamline local resolution, as announced in 2022; as of 2025, LLMNR remains enabled by default.

Relation to NetBIOS

NetBIOS, or Network Basic Input/Output System, is a legacy protocol for local network name resolution that operates using broadcast messages over UDP port 137 to resolve 15-character NetBIOS names to IPv4 addresses within a subnet. Link-Local Multicast Name Resolution (LLMNR) evolved as a successor and complement to NetBIOS, introduced in 2007 via RFC 4795 to support name resolution in IPv6 environments where traditional DNS is unavailable, adopting DNS packet formats and semantics to handle longer hostnames up to 255 characters per label. In Windows operating systems, name resolution for single-label names follows a fallback chain starting with DNS queries using appended connection-specific suffixes; if unsuccessful, the system queries LLMNR next, followed by NetBIOS broadcast if enabled, and then Windows Internet Name Service (WINS) if configured. This order is configurable through registry settings, such as under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters, allowing administrators to prioritize or disable components like NetBIOS over TCP/IP. Key differences include LLMNR's use of multicast addresses (224.0.0.252 for and FF02::1:3 for ) for queries, enabling more targeted local link communication, compared to NetBIOS's reliance on full broadcasts to 255.255.255.255, which can lead to or broadcast storms in larger subnets. LLMNR permits unicast responses directly to the querying host for efficiency, whereas NetBIOS supports group names alongside unique names, allowing multiple hosts to register under shared identifiers but complicating . Although over TCP/IP has been deprecated in favor of DNS-based resolution in modern Windows versions since , it continues to coexist for with legacy applications and systems. LLMNR is preferred in small, flat networks for its simplicity and lack of need for centralized servers like WINS. Both protocols ultimately resolve names to addresses for local communication, but LLMNR circumvents limitations such as the 255-name registration cap per host (due to 255 possible suffixes) and vulnerability to broadcast storms by leveraging DNS-compatible structures without requiring broadcast flooding.

Security and Limitations

Known Vulnerabilities

Link-Local Multicast Name Resolution (LLMNR) is susceptible to spoofing attacks due to its lack of mechanisms, enabling adversaries to forge responses to queries on port 5355 and redirect traffic to malicious systems, facilitating man-in-the-middle (MITM) interceptions. This vulnerability arises from the protocol's design, which relies on the first valid response without verifying the source, allowing attackers on the same local network segment to impersonate legitimate hosts. Poisoning risks in LLMNR stem from the acceptance of unsolicited responses, which can overwrite local name caches and direct subsequent traffic to attacker-controlled endpoints, particularly in shared or unsegmented networks where source validation is absent. Attackers exploit this by monitoring for failed DNS resolutions and injecting false mappings, often leading to the capture of authentication credentials such as NTLMv2 hashes during subsequent connections. As of 2025, LLMNR poisoning remains one of the most common vulnerabilities identified in network penetration tests. The protocol's multicast nature exposes networks to denial-of-service (DoS) potential, as attackers can flood the link-local scope with excessive queries or responses, consuming bandwidth and CPU resources on responding hosts without any built-in rate limiting in the specification. This amplification occurs because all devices on the subnet process the multicast traffic, potentially disrupting legitimate name resolutions. LLMNR broadcasts inherently reveal active hostnames and network topology during queries, providing reconnaissance opportunities for attackers to map local segments and identify targets for lateral movement without additional tools. By passively listening to multicast traffic, adversaries gain insights into host presence and naming conventions, aiding further exploitation in Windows environments. Historical incidents highlight LLMNR's exploitation, notably through tools like Responder, first publicly detailed in 2013, which automates poisoning to steal credentials in Windows domains by responding to LLMNR and NBT-NS queries. This tool has been widely used in penetration testing and real-world attacks since its release, demonstrating the protocol's role in credential theft campaigns during the . In environments, LLMNR retains the same core vulnerabilities as its IPv4 counterpart, operating over the ff02::1:3 on port 5355, where spoofing and poisoning can similarly redirect traffic despite the larger . Coexistence with IPv4 or transition mechanisms may introduce additional risks if scopes overlap, but the primary threats remain unauthenticated responses enabling MITM across dual-stack networks.

Mitigation Strategies

One primary mitigation strategy for addressing vulnerabilities in Link-Local Multicast Name Resolution (LLMNR) involves disabling the protocol entirely, particularly in enterprise environments where it is not essential. In Windows systems, this can be achieved through Group Policy by navigating to Computer Configuration > Administrative Templates > Network > DNS Client and setting "Turn off Multicast Name Resolution" to Enabled, which prevents LLMNR queries from being sent or responded to on all network adapters. Alternatively, the registry key HKLM\SOFTWARE\Policies\Microsoft\Windows NT\DNSClient can be modified by creating a DWORD value named EnableMulticast and setting it to 0, effectively disabling the feature domain-wide via Active Directory Group Policy Objects (GPOs). In 2022, Microsoft announced plans to phase out LLMNR and NetBIOS name resolution in favor of Multicast DNS (mDNS), though as of 2025, LLMNR remains enabled by default in Windows. This approach is recommended for organizations to reduce the attack surface without impacting core DNS functionality, as LLMNR serves only as a fallback mechanism. Network segmentation further limits the scope of potential LLMNR abuse by isolating multicast traffic. Deploying VLANs confines LLMNR queries to specific broadcast domains, preventing lateral movement across the . Additionally, firewalls should block and port 5355 at network boundaries to restrict LLMNR traffic from traversing subnets or reaching the , thereby containing any spoofed responses within segmented areas. To detect anomalous LLMNR activity, enterprises can implement intrusion detection and prevention systems (IDS/IPS). Tools like Snort can be configured with rules to monitor for unusual patterns, such as excessive queries or spoofed responses on 5355, alerting administrators to potential attempts. Network-based complements this by logging traffic for forensic analysis. Adopting alternatives to LLMNR enhances resolution security. Enabling (mDNS) provides similar local discovery but should be paired with strict controls, while deploying full DNS servers ensures queries to authoritative sources. For proxy configurations, using secure Web Proxy Auto-Discovery (WPAD) via mitigates related risks. Hardening measures include enforcing unicast-only communication through host-based firewalls, which block outbound multicast traffic on port 5355. Although LLMNR does not natively support DNSSEC, implementing DNSSEC on primary DNS infrastructure protects against related in hybrid setups. Best practices emphasize proactive management: regularly audit GPO application and registry settings in to verify LLMNR disablement across endpoints, using tools like scripts or compliance scanners. In mixed environments with legacy systems, educate administrators on LLMNR risks to prioritize segmentation and monitoring.

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