DNSEXT Working Group Levon Esibov INTERNET-DRAFT Bernard Aboba Category: Standards Track Dave Thaler <draft-ietf-dnsext-mdns-33.txt> Microsoft 18 July 2004 Linklocal Multicast Name Resolution (LLMNR) By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on January 2, 2005. Copyright Notice Copyright (C) The Internet Society 2004. All rights reserved. Abstract Today, with the rise of home networking, there are an increasing number of ad-hoc networks operating without a Domain Name System (DNS) server. The goal of Link-Local Multicast Name Resolution (LLMNR) is to enable name resolution in scenarios in which conventional DNS name resolution is not possible. LLMNR supports all current and future DNS formats, types and classes, while operating on a separate port from DNS, and with a distinct resolver cache. Since LLMNR only operates on the local link, it cannot be considered a substitute for DNS. Esibov, Aboba & Thaler Standards Track [Page 1]
INTERNET-DRAFT LLMNR 18 July 2004 Table of Contents 1. Introduction .......................................... 3 1.1 Requirements .................................... 4 1.2 Terminology ..................................... 4 2. Name resolution using LLMNR ........................... 4 2.1 LLMNR packet format ............................. 6 2.2 Sender behavior ................................. 8 2.3 Responder behavior .............................. 8 2.4 Unicast queries ................................. 11 2.5 Off-link detection .............................. 11 2.6 Responder responsibilities ...................... 12 2.7 Retransmission and jitter ....................... 13 2.8 DNS TTL ......................................... 13 2.9 Use of the authority and additional sections .... 14 3. Usage model ........................................... 14 3.1 LLMNR configuration ............................. 15 4. Conflict resolution ................................... 16 4.1 Considerations for multiple interfaces .......... 18 4.2 API issues ...................................... 19 5. Security considerations ............................... 20 5.1 Scope restriction ............................... 20 5.2 Usage restriction ............................... 21 5.3 Cache and port separation ....................... 22 5.4 Authentication .................................. 22 6. IANA considerations ................................... 22 7. References ............................................ 22 7.1 Normative References ............................ 22 7.2 Informative References .......................... 23 Acknowledgments .............................................. 24 Authors' Addresses ........................................... 25 Intellectual Property Statement .............................. 25 Disclaimer of Validity ....................................... 26 Full Copyright Statement ..................................... 26 Esibov, Aboba & Thaler Standards Track [Page 2]
INTERNET-DRAFT LLMNR 18 July 2004 1. Introduction This document discusses Link Local Multicast Name Resolution (LLMNR), which utilizes the DNS packet format and supports all current and future DNS formats, types and classes. LLMNR operates on a separate port from the Domain Name System (DNS), with a distinct resolver cache. The goal of LLMNR is to enable name resolution in scenarios in which conventional DNS name resolution is not possible. These include scenarios in which hosts are not configured with the address of a DNS server, where configured DNS servers do not reply to a query, or where they respond with errors, as described in Section 2. Since LLMNR only operates on the local link, it cannot be considered a substitute for DNS. Link-scope multicast addresses are used to prevent propagation of LLMNR traffic across routers, potentially flooding the network. LLMNR queries can also be sent to a unicast address, as described in Section 2.4. Propagation of LLMNR packets on the local link is considered sufficient to enable name resolution in small networks. The assumption is that if a network has a gateway, then the network is able to provide DNS server configuration. Configuration issues are discussed in Section 3.1. In the future, it may be desirable to consider use of multicast name resolution with multicast scopes beyond the link-scope. This could occur if LLMNR deployment is successful, the need arises for multicast name resolution beyond the link-scope, or multicast routing becomes ubiquitous. For example, expanded support for multicast name resolution might be required for mobile ad-hoc networking scenarios, or where no DNS server is available that is authoritative for the names of local hosts, and can support dynamic DNS, such as in wireless hotspots. Once we have experience in LLMNR deployment in terms of administrative issues, usability and impact on the network, it will be possible to reevaluate which multicast scopes are appropriate for use with multicast name resolution. Service discovery in general, as well as discovery of DNS servers using LLMNR in particular, is outside of the scope of this document, as is name resolution over non-multicast capable media. Esibov, Aboba & Thaler Standards Track [Page 3]
INTERNET-DRAFT LLMNR 18 July 2004 1.1. Requirements In this document, several words are used to signify the requirements of the specification. 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]. 1.2. Terminology This document assumes familiarity with DNS terminology defined in [RFC1035]. Other terminology used in this document includes: Positively Resolved Responses with RCODE set to zero are referred to in this document as "positively resolved". Routable Address An address other than a Link-Local address. This includes globally routable addresses, as well as private addresses. Reachable An address is considered reachable over a link if either an ARP or neighbor discovery cache entry exists for the address on the link. Responder A host that listens to LLMNR queries, and responds to those for which it is authoritative. Sender A host that sends an LLMNR query. 2. Name resolution using LLMNR LLMNR is a peer-to-peer name resolution protocol that is not intended as a replacement for DNS. LLMNR queries are sent to and received on port 5355. IPv4 administratively scoped multicast usage is specified in "Administratively Scoped IP Multicast" [RFC2365]. The IPv4 link- scope multicast address a given responder listens to, and to which a sender sends queries, is 224.0.0.252. The IPv6 link-scope multicast address a given responder listens to, and to which a sender sends all queries, is FF02:0:0:0:0:0:1:3. Typically a host is configured as both an LLMNR sender and a responder. A host MAY be configured as a sender, but not a responder. However, a host configured as a responder MUST act as a sender to verify the uniqueness of names as described in Section 4. This document does not specify how names are chosen or configured. Esibov, Aboba & Thaler Standards Track [Page 4]
INTERNET-DRAFT LLMNR 18 July 2004 This may occur via any mechanism, including DHCPv4 [RFC2131] or DHCPv6 [RFC3315]. LLMNR usage MAY be configured manually or automatically on a per interface basis. By default, LLMNR responders SHOULD be enabled on all interfaces, at all times. Enabling LLMNR for use in situations where a DNS server has been configured will result in a change in default behavior without a simultaneous update to configuration information. Where this is considered undesirable, LLMNR SHOULD NOT be enabled by default, so that hosts will neither listen on the link- scope multicast address, nor will they send queries to that address. An LLMNR sender may send a request for any name. However, by default, LLMNR requests SHOULD be sent only when one of the following conditions are met: [1] No manual or automatic DNS configuration has been performed. If an interface has been configured with DNS server address(es), then LLMNR SHOULD NOT be used as the primary name resolution mechanism on that interface, although it MAY be used as a name resolution mechanism of last resort. [2] DNS servers do not respond. [3] DNS servers respond to a DNS query with RCODE=3 (Authoritative Name Error) or RCODE=0, and an empty answer section. A typical sequence of events for LLMNR usage is as follows: [a] DNS servers are not configured or do not respond to a DNS query, or respond with RCODE=3, or RCODE=0 and an empty answer section. [b] An LLMNR sender sends an LLMNR query to the link-scope multicast address(es) defined in Section 2, unless a unicast query is indicated. A sender SHOULD send LLMNR queries for PTR RRs via unicast, as specified in Section 2.4. [c] A responder responds to this query only if it is authoritative for the domain name in the query. A responder responds to a multicast query by sending a unicast UDP response to the sender. Unicast queries are responded to as indicated in Section 2.4. [d] Upon reception of the response, the sender processes it. Further details of sender and responder behavior are provided in the sections that follow. Esibov, Aboba & Thaler Standards Track [Page 5]
INTERNET-DRAFT LLMNR 18 July 2004 2.1. LLMNR packet format LLMNR utilizes the DNS packet format defined in [RFC1035] Section 4 for both queries and responses. LLMNR implementations SHOULD send UDP queries and responses only as large as are known to be permissible without causing fragmentation. When in doubt a maximum packet size of 512 octets SHOULD be used. LLMNR implementations MUST accept UDP queries and responses as large as permitted by the link MTU. 2.1.1. LLMNR header format LLMNR queries and responses utilize the DNS header format defined in [RFC1035] with exceptions noted below: 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | ID | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |QR| Opcode | Z|TC| Z| Z| Z| Z| Z| RCODE | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | QDCOUNT | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | ANCOUNT | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | NSCOUNT | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | ARCOUNT | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ where: ID A 16 bit identifier assigned by the program that generates any kind of query. This identifier is copied from the query to the response and can be used by the sender to match responses to outstanding queries. The ID field in a query SHOULD be set to a pseudo-random value. QR A one bit field that specifies whether this message is an LLMNR query (0), or an LLMNR response (1). OPCODE A four bit field that specifies the kind of query in this message. This value is set by the originator of a query and copied into the response. This specification defines the behavior of standard queries and responses (opcode value of zero). Future specifications may define the use of other opcodes with LLMNR. Esibov, Aboba & Thaler Standards Track [Page 6]
INTERNET-DRAFT LLMNR 18 July 2004 LLMNR senders and responders MUST support standard queries (opcode value of zero). LLMNR queries with unsupported OPCODE values MUST be silently discarded by responders. TC TrunCation - specifies that this message was truncated due to length greater than that permitted on the transmission channel. The TC bit MUST NOT be set in an LLMNR query and if set is ignored by an LLMNR responder. If the TC bit is set an LLMNR response, then the sender MAY use the response if it contains all necessary information, or the sender MAY discard the response and resend the LLMNR query over TCP using the unicast address of the responder as the destination address. See [RFC2181] and Section 2.4 of this specification for further discussion of the TC bit. Z Reserved for future use. Implementations of this specification MUST set these bits to zero in both queries and responses. If these bits are set in a LLMNR query or response, implementations of this specification MUST ignore them. Since reserved bits could conceivably be used for different purposes than in DNS, implementors are advised not to enable processing of these bits in an LLMNR implementation starting from a DNS code base. RCODE Response code -- this 4 bit field is set as part of LLMNR responses. In an LLMNR query, the RCODE MUST be zero, and is ignored by the responder. The response to a multicast LLMNR query MUST have RCODE set to zero. A sender MUST silently discard an LLMNR response with a non-zero RCODE sent in response to a multicast query. If an LLMNR responder is authoritative for the name in a multicast query, but an error is encountered, the responder SHOULD send an LLMNR response with an RCODE of zero, no RRs in the answer section, and the TC bit set. This will cause the query to be resent using TCP, and allow the inclusion of a non-zero RCODE in the response to the TCP query. Responding with the TC bit set is preferrable to not sending a response, since it enables errors to be diagnosed. Since LLMNR responders only respond to LLMNR queries for names for which they are authoritative, LLMNR responders MUST NOT respond with an RCODE of 3; instead, they should not respond at all. LLMNR implementations MUST support EDNS0 [RFC2671] and extended RCODE values. QDCOUNT An unsigned 16 bit integer specifying the number of entries in the question section. A sender MUST place only one question into the Esibov, Aboba & Thaler Standards Track [Page 7]
INTERNET-DRAFT LLMNR 18 July 2004 question section of an LLMNR query. LLMNR responders MUST silently discard LLMNR queries with QDCOUNT not equal to one. LLMNR senders MUST silently discard LLMNR responses with QDCOUNT not equal to one. ANCOUNT An unsigned 16 bit integer specifying the number of resource records in the answer section. LLMNR responders MUST silently discard LLMNR queries with ANCOUNT not equal to zero. NSCOUNT An unsigned 16 bit integer specifying the number of name server resource records in the authority records section. Authority record section processing is described in Section 2.9. ARCOUNT An unsigned 16 bit integer specifying the number of resource records in the additional records section. Additional record section processing is described in Section 2.9. 2.2. Sender behavior A sender may send an LLMNR query for any legal resource record type (e.g. A, AAAA, SRV, etc.) to the link-scope multicast address. As described in Section 2.4, a sender may also send a unicast query. Sections 2 and 3 describe the circumstances in which LLMNR queries may be sent. The sender MUST anticipate receiving no replies to some LLMNR queries, in the event that no responders are available within the link-scope or in the event no positive non-null responses exist for the transmitted query. If no positive response is received, a resolver treats it as a response that no records of the specified type and class exist for the specified name (it is treated the same as a response with RCODE=0 and an empty answer section). Since the responder may order the RRs in the response so as to indicate preference, the sender SHOULD preserve ordering in the response to the querying application. 2.3. Responder behavior An LLMNR response MUST be sent to the sender via unicast. Upon configuring an IP address responders typically will synthesize corresponding A, AAAA and PTR RRs so as to be able to respond to LLMNR queries for these RRs. An SOA RR is synthesized only when a Esibov, Aboba & Thaler Standards Track [Page 8]
INTERNET-DRAFT LLMNR 18 July 2004 responder has another RR as well; the SOA RR MUST NOT be the only RR that a responder has. However, in general whether RRs are manually or automatically created is an implementation decision. For example, a host configured to have computer name "host1" and to be a member of the "example.com" domain, and with IPv4 address 10.1.1.1 and IPv6 address 2001:0DB8::1:2:3:FF:FE:4:5:6 might be authoritative for the following records: host1. IN A 10.1.1.1 IN AAAA 2001:0DB8::1:2:3:FF:FE:4:5:6 host1.example.com. IN A 10.1.1.1 IN AAAA 2001:0DB8::1:2:3:FF:FE:4:5:6 1.1.1.10.in-addr.arpa. IN PTR host1. IN PTR host1.example.com. 6.0.5.0.4.0.E.F.F.F.3.0.2.0.1.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa IN PTR host1. IN PTR host1.example.com An LLMNR responder might be further manually configured with the name of a local mail server with an MX RR included in the "host1." and "host1.example.com." records. In responding to queries: [a] Responders MUST listen on UDP port 5355 on the link-scope multicast address(es) defined in Section 2, and on UDP and TCP port 5355 on the unicast address(es) that could be set as the source address(es) when the responder responds to the LLMNR query. [b] Responders MUST direct responses to the port from which the query was sent. When queries are received via TCP this is an inherent part of the transport protocol. For queries received by UDP the responder MUST take note of the source port and use that as the destination port in the response. Responses SHOULD always be sent from the port to which they were directed. [c] Responders MUST respond to LLMNR queries for names and addresses they are authoritative for. This applies to both forward and reverse lookups. [d] Responders MUST NOT respond to LLMNR queries for names they are not authoritative for. Esibov, Aboba & Thaler Standards Track [Page 9]
INTERNET-DRAFT LLMNR 18 July 2004 [e] Responders MUST NOT respond using cached data. [f] If a DNS server is running on a host that supports LLMNR, the DNS server MUST respond to LLMNR queries only for the RRSets relating to the host on which the server is running, but MUST NOT respond for other records for which the server is authoritative. DNS servers also MUST NOT send LLMNR queries in order to resolve DNS queries. [g] If a responder is authoritative for a name, it MAY respond with RCODE=0 and an empty answer section, if the type of query does not match a RR that the responder has. As an example, a host configured to respond to LLMNR queries for the name "foo.example.com." is authoritative for the name "foo.example.com.". On receiving an LLMNR query for an A RR with the name "foo.example.com." the host authoritatively responds with A RR(s) that contain IP address(es) in the RDATA of the resource record. If the responder has a AAAA RR, but no A RR, and an A RR query is received, the responder would respond with RCODE=0 and an empty answer section. In conventional DNS terminology a DNS server authoritative for a zone is authoritative for all the domain names under the zone apex except for the branches delegated into separate zones. Contrary to conventional DNS terminology, an LLMNR responder is authoritative only for the zone apex. For example the host "foo.example.com." is not authoritative for the name "child.foo.example.com." unless the host is configured with multiple names, including "foo.example.com." and "child.foo.example.com.". As a result, "foo.example.com." cannot reply to an LLMNR query for "child.foo.example.com." with RCODE=3 (authoritative name error). The purpose of limiting the name authority scope of a responder is to prevent complications that could be caused by coexistence of two or more hosts with the names representing child and parent (or grandparent) nodes in the DNS tree, for example, "foo.example.com." and "child.foo.example.com.". In this example (unless this limitation is introduced) an LLMNR query for an A resource record for the name "child.foo.example.com." would result in two authoritative responses: RCODE=3 (authoritative name error) received from "foo.example.com.", and a requested A record - from "child.foo.example.com.". To prevent this ambiguity, LLMNR enabled hosts could perform a dynamic update of the parent (or grandparent) zone with a delegation to a child zone. In this example a host "child.foo.example.com." would send a dynamic update for the NS and glue A record to "foo.example.com.", but this approach Esibov, Aboba & Thaler Standards Track [Page 10]
INTERNET-DRAFT LLMNR 18 July 2004 significantly complicates implementation of LLMNR and would not be acceptable for lightweight hosts. 2.4. Unicast queries and responses Unicast queries SHOULD be sent when: [a] A sender repeats a query after it received a response with the TC bit set to the previous LLMNR multicast query, or [b] The sender queries for a PTR RR of a fully formed IP address within the "in-addr.arpa" or "ip6.arpa" zones. Unicast LLMNR queries MUST be done using TCP and the responses MUST be sent using the same TCP connection as the query. Senders MUST support sending TCP queries, and responders MUST support listening for TCP queries. If the sender of a TCP query receives a response to that query not using TCP, the response MUST be silently discarded. Unicast UDP queries MUST be silently discarded. If TCP connection setup cannot be completed in order to send a unicast TCP query, this is treated as a response that no records of the specified type and class exist for the specified name (it is treated the same as a response with RCODE=0 and an empty answer section). 2.5. "Off link" detection For IPv4, an "on link" address is defined as a link-local address [IPv4Link] or an address whose prefix belongs to a subnet on the local link. For IPv6 [RFC2460] an "on link" address is either a link-local address, defined in [RFC2373], or an address whose prefix belongs to a subnet on the local link. A sender MUST select a source address for LLMNR queries that is "on link". The destination address of an LLMNR query MUST be a link- scope multicast address or an "on link" unicast address. A responder MUST select a source address for responses that is "on link". The destination address of an LLMNR response MUST be an "on link" unicast address. On receiving an LLMNR query, the responder MUST check whether it was sent to a LLMNR multicast addresses defined in Section 2. If it was sent to another multicast address, then the query MUST be silently discarded. Esibov, Aboba & Thaler Standards Track [Page 11]
INTERNET-DRAFT LLMNR 18 July 2004 Section 2.4 discusses use of TCP for LLMNR queries and responses. In composing an LLMNR query using TCP, the sender MUST set the Hop Limit field in the IPv6 header and the TTL field in the IPv4 header of the response to one (1). The responder SHOULD set the TTL or Hop Limit settings on the TCP listen socket to one (1) so that SYN-ACK packets will have TTL (IPv4) or Hop Limit (IPv6) set to one (1). This prevents an incoming connection from off-link since the sender will not receive a SYN-ACK from the responder. For UDP queries and responses the Hop Limit field in the IPv6 header, and the TTL field in the IPV4 header MAY be set to any value. However, it is RECOMMENDED that the value 255 be used for compatibility with Apple Rendezvous. Implementation note: In the sockets API for IPv4 [POSIX], the IP_TTL and IP_MULTICAST_TTL socket options are used to set the TTL of outgoing unicast and multicast packets. The IP_RECVTTL socket option is available on some platforms to retrieve the IPv4 TTL of received packets with recvmsg(). [RFC2292] specifies similar options for setting and retrieving the IPv6 Hop Limit. 2.6. Responder responsibilities It is the responsibility of the responder to ensure that RRs returned in LLMNR responses MUST only include values that are valid on the local interface, such as IPv4 or IPv6 addresses valid on the local link or names defended using the mechanism described in Section 4. In particular: [a] If a link-scope IPv6 address is returned in a AAAA RR, that address MUST be valid on the local link over which LLMNR is used. [b] If an IPv4 address is returned, it MUST be reachable through the link over which LLMNR is used. [c] If a name is returned (for example in a CNAME, MX or SRV RR), the name MUST be resolvable on the local link over which LLMNR is used. Routable addresses MUST be included first in the response, if available. This encourages use of routable address(es) for establishment of new connections. Esibov, Aboba & Thaler Standards Track [Page 12]
INTERNET-DRAFT LLMNR 18 July 2004 2.7. Retransmission and jitter An LLMNR sender uses the timeout interval LLMNR_TIMEOUT to determine when to retransmit an LLMNR query and how long to collect responses to an LLMNR query. If an LLMNR query sent over UDP is not resolved within LLMNR_TIMEOUT, then a sender MAY repeat the transmission of the query in order to assure that it was received by a host capable of responding to it. Retransmission of UDP queries SHOULD NOT be attempted more than 3 times. Where LLMNR queries are sent using TCP, retransmission is handled by the transport layer. Because an LLMNR sender cannot know in advance if a query sent using multicast will receive no response, one response, or more than one response, the sender SHOULD wait for LLMNR_TIMEOUT in order to collect all possible responses, rather than considering the multicast query answered after the first response is received. A unicast query sender considers the query answered after the first response is received, so that it only waits for LLMNR_TIMEOUT if no response has been received. An LLMNR sender SHOULD dynamically compute the value of LLMNR_TIMEOUT for each transmission. It is suggested that the computation of LLMNR_TIMEOUT be based on the response times for earlier LLMNR queries sent on the same interface. For example, the algorithms described in RFC 2988 [RFC2988] (including exponential backoff) compute an RTO, which is used as the value of LLMNR_TIMEOUT. Smaller values MAY be used for the initial RTO (discussed in Section 2 of [RFC2988], paragraph 2.1), the minimum RTO (discussed in Section 2 of [RFC2988], paragraph 2.4), and the maximum RTO (discussed in Section 2 of [RFC2988], paragraph 2.5). Recommended values are an initial RTO of 1 second, a minimum RTO of 200ms, and a maximum RTO of 5 seconds. In order to avoid synchronization, the transmission of each LLMNR query and response SHOULD delayed by a time randomly selected from the interval 0 to 100 ms. This delay MAY be avoided by responders responding with RRs which they have previously determined to be UNIQUE (see Section 4 for details). 2.8. DNS TTL The responder should use a pre-configured TTL value in the records returned an LLMNR response. A default value of 30 seconds is RECOMMENDED. In highly dynamic environments (such as mobile ad-hoc networks), the TTL value may need to be reduced. Esibov, Aboba & Thaler Standards Track [Page 13]
INTERNET-DRAFT LLMNR 18 July 2004 Due to the TTL minimalization necessary when caching an RRset, all TTLs in an RRset MUST be set to the same value. 2.9. Use of the authority and additional sections Unlike the DNS, LLMNR is a peer-to-peer protocol and does not have a concept of delegation. In LLMNR, the NS resource record type may be stored and queried for like any other type, but it has no special delegation semantics as it does in the DNS. Responders MAY have NS records associated with the names for which they are authoritative, but they SHOULD NOT include these NS records in the authority sections of responses. Responders SHOULD insert an SOA record into the authority section of a negative response, to facilitate negative caching as specified in [RFC2308]. The owner name of this SOA record MUST be equal to the query name. Responders SHOULD NOT perform DNS additional section processing, except as required for EDNS0 and DNSSEC. Senders MUST NOT cache RRs from the authority or additional section of a response as answers, though they may be used for other purposes such as negative caching. 3. Usage model Since LLMNR is a secondary name resolution mechanism, its usage is in part determined by the behavior of DNS implementations. This document does not specify any changes to DNS resolver behavior, such as searchlist processing or retransmission/failover policy. However, robust DNS resolver implementations are more likely to avoid unnecessary LLMNR queries. As noted in [DNSPerf], even when DNS servers are configured, a significant fraction of DNS queries do not receive a response, or result in negative responses due to missing inverse mappings or NS records that point to nonexistent or inappropriate hosts. This has the potential to result in a large number of unnecessary LLMNR queries. [RFC1536] describes common DNS implementation errors and fixes. If the proposed fixes are implemented, unnecessary LLMNR queries will be reduced substantially, and so implementation of [RFC1536] is recommended. For example, [RFC1536] Section 1 describes issues with retransmission and recommends implementation of a retransmission policy based on Esibov, Aboba & Thaler Standards Track [Page 14]
INTERNET-DRAFT LLMNR 18 July 2004 round trip estimates, with exponential backoff. [RFC1536] Section 4 describes issues with failover, and recommends that resolvers try another server when they don't receive a response to a query. These policies are likely to avoid unnecessary LLMNR queries. [RFC1536] Section 3 describes zero answer bugs, which if addressed will also reduce unnecessary LLMNR queries. [RFC1536] Section 6 describes name error bugs and recommended searchlist processing that will reduce unnecessary RCODE=3 (authoritative name) errors, thereby also reducing unnecessary LLMNR queries. 3.1. LLMNR configuration Since IPv4 and IPv6 utilize distinct configuration mechanisms, it is possible for a dual stack host to be configured with the address of a DNS server over IPv4, while remaining unconfigured with a DNS server suitable for use over IPv6. In these situations, a dual stack host will send AAAA queries to the configured DNS server over IPv4. However, an IPv6-only host unconfigured with a DNS server suitable for use over IPv6 will be unable to resolve names using DNS. Automatic IPv6 DNS configuration mechanisms (such as [RFC3315] and [DNSDisc]) are not yet widely deployed, and not all DNS servers support IPv6. Therefore lack of IPv6 DNS configuration may be a common problem in the short term, and LLMNR may prove useful in enabling linklocal name resolution over IPv6. Where a DHCPv4 server is available but not a DHCPv6 server [RFC3315], IPv6-only hosts may not be configured with a DNS server. Where there is no DNS server authoritative for the name of a host or the authoritative DNS server does not support dynamic client update over IPv6 or DHCPv6-based dynamic update, then an IPv6-only host will not be able to do DNS dynamic update, and other hosts will not be able to resolve its name. For example, if the configured DNS server responds to AAAA RR queries sent over IPv4 or IPv6 with an authoritative name error (RCODE=3), then it will not be possible to resolve the names of IPv6-only hosts. In this situation, LLMNR over IPv6 can be used for local name resolution. Similarly, if a DHCPv4 server is available providing DNS server configuration, and DNS server(s) exist which are authoritative for the A RRs of local hosts and support either dynamic client update over IPv4 or DHCPv4-based dynamic update, then the names of local Esibov, Aboba & Thaler Standards Track [Page 15]
INTERNET-DRAFT LLMNR 18 July 2004 IPv4 hosts can be resolved over IPv4 without LLMNR. However, if no DNS server is authoritative for the names of local hosts, or the authoritative DNS server(s) do not support dynamic update, then LLMNR enables linklocal name resolution over IPv4. Where DHCPv4 or DHCPv6 is implemented, DHCP options can be used to configure LLMNR on an interface. The LLMNR Enable Option, described in [LLMNREnable], can be used to explicitly enable or disable use of LLMNR on an interface. The LLMNR Enable Option does not determine whether or in which order DNS itself is used for name resolution. The order in which various name resolution mechanisms should be used can be specified using the Name Service Search Option (NSSO) for DHCP [RFC2937], using the LLMNR Enable Option code carried in the NSSO data. It is possible that DNS configuration mechanisms will go in and out of service. In these circumstances, it is possible for hosts within an administrative domain to be inconsistent in their DNS configuration. For example, where DHCP is used for configuring DNS servers, one or more DHCP servers can fail. As a result, hosts configured prior to the outage will be configured with a DNS server, while hosts configured after the outage will not. Alternatively, it is possible for the DNS configuration mechanism to continue functioning while configured DNS servers fail. Unless unconfigured hosts periodically retry configuration, an outage in the DNS configuration mechanism will result in hosts continuing to use LLMNR even once the outage is repaired. Since LLMNR only enables linklocal name resolution, this represents an unnecessary degradation in capabilities. As a result, it is recommended that hosts without a configured DNS server periodically attempt to obtain DNS configuration. For example, where DHCP is used for DNS configuration, [RFC2131] recommends a maximum retry interval of 64 seconds. In the absence of other guidance, a default retry interval of one (1) minute is RECOMMENDED. 4. Conflict resolution The sender MUST anticipate receiving multiple replies to the same LLMNR query, in the event that several LLMNR enabled computers receive the query and respond with valid answers. When this occurs, the responses may first be concatenated, and then treated in the same manner that multiple RRs received from the same DNS server would; the sender perceives no inherent conflict in the receipt of multiple responses. Esibov, Aboba & Thaler Standards Track [Page 16]
INTERNET-DRAFT LLMNR 18 July 2004 There are some scenarios when multiple responders MAY respond to the same query. There are other scenarios when only one responder MAY respond to a query. Resource records for which the latter queries are submitted are referred as UNIQUE throughout this document. The uniqueness of a resource record depends on a nature of the name in the query and type of the query. For example it is expected that: - multiple hosts may respond to a query for an SRV type record - multiple hosts may respond to a query for an A or AAAA type record for a cluster name (assigned to multiple hosts in the cluster) - only a single host may respond to a query for an A or AAAA type record for a name. Every responder that responds to an LLMNR query AND includes a UNIQUE record in the response: [1] MUST verify that there is no other host within the scope of the LLMNR query propagation that can return a resource record for the same name, type and class. [2] MUST NOT include a UNIQUE resource record in the response without having verified its uniqueness. Where a host is configured to issue LLMNR queries on more than one interface, each interface should have its own independent LLMNR cache. For each UNIQUE resource record in a given interface's configuration, the host MUST verify resource record uniqueness on that interface. To accomplish this, the host MUST send an LLMNR query for each UNIQUE resource record. By default, a host SHOULD be configured to behave as though all RRs are UNIQUE. Uniqueness verification is carried out when the host: - starts up or is rebooted - wakes from sleep (if the network interface was inactive during sleep) - is configured to respond to the LLMNR queries on an interface enabled for transmission and reception of IP traffic - is configured to respond to the LLMNR queries using additional UNIQUE resource records - detects that an interface is connected and is usable (e.g. an IEEE 802 hardware link-state change indicating that a cable was attached or completion of authentication (and if needed, association) with a wireless base station or adhoc network When a host that has a UNIQUE record receives an LLMNR query for that record, the host MUST respond. After the client receives a response, Esibov, Aboba & Thaler Standards Track [Page 17]
INTERNET-DRAFT LLMNR 18 July 2004 it MUST check whether the response arrived on an interface different from the one on which the query was sent. If the response arrives on a different interface, the client can use the UNIQUE resource record in response to LLMNR queries. If not, then it MUST NOT use the UNIQUE resource record in response to LLMNR queries. The name conflict detection mechanism doesn't prevent name conflicts when previously partitioned segments are connected by a bridge. In order to minimize the chance of conflicts in such a situation, it is recommended that steps be taken to ensure name uniqueness. For example, the name could be chosen randomly from a large pool of potential names, or the name could be assigned via a process designed to guarantee uniqueness. When name conflicts are detected, they SHOULD be logged. To detect duplicate use of a name, an administrator can use a name resolution utility which employs LLMNR and lists both responses and responders. This would allow an administrator to diagnose behavior and potentially to intervene and reconfigure LLMNR responders who should not be configured to respond to the same name. 4.1. Considerations for Multiple Interfaces A multi-homed host may elect to configure LLMNR on only one of its active interfaces. In many situations this will be adequate. However, should a host need to configure LLMNR on more than one of its active interfaces, there are some additional precautions it MUST take. Implementers who are not planning to support LLMNR on multiple interfaces simultaneously may skip this section. A multi-homed host checks the uniqueness of UNIQUE records as described in Section 4. The situation is illustrated in figure 1. ---------- ---------- | | | | [A] [myhost] [myhost] Figure 1. Link-scope name conflict In this situation, the multi-homed myhost will probe for, and defend, its host name on both interfaces. A conflict will be detected on one interface, but not the other. The multi-homed myhost will not be able to respond with a host RR for "myhost" on the interface on the right (see Figure 1). The multi-homed host may, however, be configured to use the "myhost" name on the interface on the left. Since names are only unique per-link, hosts on different links could be using the same name. If an LLMNR client sends requests over Esibov, Aboba & Thaler Standards Track [Page 18]
INTERNET-DRAFT LLMNR 18 July 2004 multiple interfaces, and receives replies from more than one, the result returned to the client is defined by the implementation. The situation is illustrated in figure 2. ---------- ---------- | | | | [A] [myhost] [A] Figure 2. Off-segment name conflict If host myhost is configured to use LLMNR on both interfaces, it will send LLMNR queries on both interfaces. When host myhost sends a query for the host RR for name "A" it will receive a response from hosts on both interfaces. Host myhost cannot distinguish between the situation shown in Figure 2, and that shown in Figure 3 where no conflict exists. [A] | | ----- ----- | | [myhost] Figure 3. Multiple paths to same host This illustrates that the proposed name conflict resolution mechanism does not support detection or resolution of conflicts between hosts on different links. This problem can also occur with unicast DNS when a multi-homed host is connected to two different networks with separated name spaces. It is not the intent of this document to address the issue of uniqueness of names within DNS. 4.2. API issues [RFC2553] provides an API which can partially solve the name ambiguity problem for applications written to use this API, since the sockaddr_in6 structure exposes the scope within which each scoped address exists, and this structure can be used for both IPv4 (using v4-mapped IPv6 addresses) and IPv6 addresses. Following the example in Figure 2, an application on 'myhost' issues the request getaddrinfo("A", ...) with ai_family=AF_INET6 and ai_flags=AI_ALL|AI_V4MAPPED. LLMNR requests will be sent from both interfaces and the resolver library will return a list containing multiple addrinfo structures, each with an associated sockaddr_in6 structure. This list will thus contain the IPv4 and IPv6 addresses Esibov, Aboba & Thaler Standards Track [Page 19]
INTERNET-DRAFT LLMNR 18 July 2004 of both hosts responding to the name 'A'. Link-local addresses will have a sin6_scope_id value that disambiguates which interface is used to reach the address. Of course, to the application, Figures 2 and 3 are still indistinguishable, but this API allows the application to communicate successfully with any address in the list. 5. Security Considerations LLMNR is by nature a peer-to-peer name resolution protocol. It is therefore inherently more vulnerable than DNS, since existing DNS security mechanisms are difficult to apply to LLMNR. While tools exist to alllow an attacker to spoof a response to a DNS query, spoofing a response to an LLMNR query is easier since the query is sent to a link-scope multicast address, where every host on the logical link will be made aware of it. In order to address the security vulnerabilities, the following mechanisms are contemplated: [1] Scope restrictions. [2] Usage restrictions. [3] Cache and port separation. [4] Authentication. These techniques are described in the following sections. 5.1. Scope restriction With LLMNR it is possible that hosts will allocate conflicting names for a period of time, or that attackers will attempt to deny service to other hosts by allocating the same name. Such attacks also allow hosts to receive packets destined for other hosts. Since LLMNR is typically deployed in situations where no trust model can be assumed, it is likely that LLMNR queries and responses will be unauthenticated. In the absence of authentication, LLMNR reduces the exposure to such threats by utilizing UDP queries sent to a link- scope multicast address, as well as setting the TTL (IPv4) or Hop Limit (IPv6) fields to one (1) on TCP queries and responses. Using a TTL of one (1) to set up a TCP connection in order to send a unicast LLMNR query reduces the likelihood of both denial of service attacks and spoofed responses. Checking that an LLMNR query is sent to a link-scope multicast address should prevent spoofing of multicast queries by off-link attackers. While this limits the ability of off-link attackers to spoof LLMNR queries and responses, it does not eliminate it. For example, it is Esibov, Aboba & Thaler Standards Track [Page 20]
INTERNET-DRAFT LLMNR 18 July 2004 possible for an attacker to spoof a response to a frequent query (such as an A or AAAA query for a popular Internet host), and by using a TTL or Hop Limit field larger than one (1), for the forged response to reach the LLMNR sender. When LLMNR queries are sent to a link-scope multicast address, it is possible that some routers may not properly implement link-scope multicast, or that link-scope multicast addresses may leak into the multicast routing system. Setting the IPv6 Hop Limit or IPv4 TTL field to a value larger than one in an LLMNR UDP response may enable denial of service attacks across the Internet. However, since LLMNR responders only respond to queries for which they are authoritative, and LLMNR does not provide wildcard query support, it is believed that this threat is minimal. There also are scenarios such as public "hotspots" where attackers can be present on the same link. These threats are most serious in wireless networks such as 802.11, since attackers on a wired network will require physical access to the home network, while wireless attackers may reside outside the home. Link-layer security can be of assistance against these threats if it is available. 5.2. Usage restriction As noted in Sections 2 and 3, LLMNR is intended for usage in a limited set of scenarios. If an LLMNR query is sent whenever a DNS server does not respond in a timely way, then an attacker can poison the LLMNR cache by responding to the query with incorrect information. To some extent, these vulnerabilities exist today, since DNS response spoofing tools are available that can allow an attacker to respond to a query more quickly than a distant DNS server. Since LLMNR queries are sent and responded to on the local-link, an attacker will need to respond more quickly to provide its own response prior to arrival of the response from a legitimate responder. If an LLMNR query is sent for an off-link host, spoofing a response in a timely way is not difficult, since a legitimate response will never be received. The vulnerability is more serious if LLMNR is given higher priority than DNS among the enabled name resolution mechanisms. In such a configuration, a denial of service attack on the DNS server would not be necessary in order to poison the LLMNR cache, since LLMNR queries would be sent even when the DNS server is available. In addition, the LLMNR cache, once poisoned, would take precedence over the DNS cache, Esibov, Aboba & Thaler Standards Track [Page 21]
INTERNET-DRAFT LLMNR 18 July 2004 eliminating the benefits of cache separation. As a result, LLMNR is only used as a name resolution mechanism of last resort. 5.3. Cache and port separation In order to prevent responses to LLMNR queries from polluting the DNS cache, LLMNR implementations MUST use a distinct, isolated cache for LLMNR on each interface. The use of separate caches is most effective when LLMNR is used as a name resolution mechanism of last resort, since this minimizes the opportunities for poisoning the LLMNR cache, and decreases reliance on it. LLMNR operates on a separate port from DNS, reducing the likelihood that a DNS server will unintentionally respond to an LLMNR query. 5.4. Authentication LLMNR implementations may not support DNSSEC or TSIG, and as a result, responses to LLMNR queries may be unauthenticated. If authentication is desired, and a pre-arranged security configuration is possible, then IPsec ESP with a null-transform MAY be used to authenticate LLMNR responses. In a small network without a certificate authority, this can be most easily accomplished through configuration of a group pre-shared key for trusted hosts. 6. IANA Considerations This specification creates one new name space: the reserved bits in the LLMNR header. These are allocated by IETF Consensus, in accordance with BCP 26 [RFC2434]. LLMNR requires allocation of port 5355 for both TCP and UDP. LLMNR requires allocation of link-scope multicast IPv4 address 224.0.0.252, as well as link-scope multicast IPv6 address FF02:0:0:0:0:0:1:3. 7. References 7.1. Normative References [RFC1035] Mockapetris, P., "Domain Names - Implementation and Specification", RFC 1035, November 1987. [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992. Esibov, Aboba & Thaler Standards Track [Page 22]
INTERNET-DRAFT LLMNR 18 July 2004 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS Specification", RFC 2181, July 1997. [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)", RFC 2308, March 1998. [RFC2365] Meyer, D., "Administratively Scoped IP Multicast", BCP 23, RFC 2365, July 1998. [RFC2373] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 2373, July 1998. [RFC2434] Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [RFC2535] Eastlake, D., "Domain Name System Security Extensions", RFC 2535, March 1999. [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671, August 1999. [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission Timer", RFC 2988, November 2000. 7.2. Informative References [RFC1536] Kumar, A., et. al., "DNS Implementation Errors and Suggested Fixes", RFC 1536, October 1993. [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997. [RFC2136] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic Updates in the Domain Name System (DNS UPDATE)", RFC 2136, April 1997. [RFC2292] Stevens, W. and M. Thomas, "Advanced Sockets API for IPv6", RFC 2292, February 1998. [RFC2553] Gilligan, R., Thomson, S., Bound, J. and W. Stevens, "Basic Socket Interface Extensions for IPv6", RFC 2553, March 1999. Esibov, Aboba & Thaler Standards Track [Page 23]
INTERNET-DRAFT LLMNR 18 July 2004 [RFC2937] Smith, C., "The Name Service Search Option for DHCP", RFC 2937, September 2000. [RFC3315] Droms, R., et al., "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003. [DNSPerf] Jung, J., et al., "DNS Performance and the Effectiveness of Caching", IEEE/ACM Transactions on Networking, Volume 10, Number 5, pp. 589, October 2002. [DNSDisc] Durand, A., Hagino, I. and D. Thaler, "Well known site local unicast addresses to communicate with recursive DNS servers", Internet draft (work in progress), draft-ietf-ipv6-dns- discovery-07.txt, October 2002. [IPV4Link] Cheshire, S., Aboba, B. and E. Guttman, "Dynamic Configuration of IPv4 Link-Local Addresses", Internet draft (work in progress), draft-ietf-zeroconf-ipv4-linklocal-15.txt, May 2004. [POSIX] IEEE Std. 1003.1-2001 Standard for Information Technology -- Portable Operating System Interface (POSIX). Open Group Technical Standard: Base Specifications, Issue 6, December 2001. ISO/IEC 9945:2002. http://www.opengroup.org/austin [LLMNREnable] Guttman, E., "DHCP LLMNR Enable Option", Internet draft (work in progress), draft-guttman-mdns-enable-02.txt, April 2002. [NodeInfo] Crawford, M., "IPv6 Node Information Queries", Internet draft (work in progress), draft-ietf-ipn-gwg-icmp-name- lookups-09.txt, May 2002. Acknowledgments This work builds upon original work done on multicast DNS by Bill Manning and Bill Woodcock. Bill Manning's work was funded under DARPA grant #F30602-99-1-0523. The authors gratefully acknowledge their contribution to the current specification. Constructive input has also been received from Mark Andrews, Stuart Cheshire, Randy Bush, Robert Elz, Rob Austein, James Gilroy, Olafur Gudmundsson, Erik Guttman, Myron Hattig, Thomas Narten, Christian Huitema, Erik Nordmark, Sander Van-Valkenburg, Tomohide Nagashima, Brian Zill, Keith Moore and Markku Savela. Esibov, Aboba & Thaler Standards Track [Page 24]
INTERNET-DRAFT LLMNR 18 July 2004 Authors' Addresses Levon Esibov Microsoft Corporation One Microsoft Way Redmond, WA 98052 EMail: levone@microsoft.com Bernard Aboba Microsoft Corporation One Microsoft Way Redmond, WA 98052 Phone: +1 425 706 6605 EMail: bernarda@microsoft.com Dave Thaler Microsoft Corporation One Microsoft Way Redmond, WA 98052 Phone: +1 425 703 8835 EMail: dthaler@microsoft.com Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. Esibov, Aboba & Thaler Standards Track [Page 25]
INTERNET-DRAFT LLMNR 18 July 2004 Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Open Issues Open issues with this specification are tracked on the following web site: http://www.drizzle.com/~aboba/DNSEXT/llmnrissues.html Esibov, Aboba & Thaler Standards Track [Page 26]
