DNSEXT Working Group Levon Esibov INTERNET-DRAFT Bernard Aboba Category: Standards Track Dave Thaler <draft-ietf-dnsext-mdns-26.txt> Microsoft 11 December 2003 Linklocal Multicast Name Resolution (LLMNR) This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. 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. Copyright Notice Copyright (C) The Internet Society (2003). 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. In order to allow name resolution in such environments, Link-Local Multicast Name Resolution (LLMNR) is proposed. 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. The goal of LLMNR is to enable name resolution in scenarios in which conventional DNS name resolution is not possible. 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 11 December 2003 Table of Contents 1. Introduction .......................................... 3 1.1 Requirements .................................... 4 1.2 Terminology ..................................... 4 2. Name resolution using LLMNR ........................... 4 2.1 Sender behavior ................................. 5 2.2 Responder behavior .............................. 6 2.3 Unicast queries ................................. 7 2.4 Addressing ...................................... 8 2.5 Off-link detection .............................. 8 2.6 Retransmissions ................................. 9 2.7 DNS TTL ......................................... 10 2.8 Use of the authority and additional sections .... 10 3. Usage model ........................................... 11 3.1 Responder responsibility ....................... 11 3.2 LLMNR configuration ............................. 12 4. Conflict resolution ................................... 13 4.1 Considerations for multiple interfaces .......... 15 4.2 API issues ...................................... 16 5. Security considerations ............................... 16 5.1 Scope restriction ............................... 17 5.2 Usage restriction ............................... 18 5.3 Cache and port separation ....................... 18 5.4 Authentication .................................. 19 6. IANA considerations ................................... 19 7. References ............................................ 19 7.1 Normative References ............................ 19 7.2 Informative References .......................... 20 Acknowledgments .............................................. 21 Authors' Addresses ........................................... 21 Intellectual Property Statement .............................. 22 Full Copyright Statement ..................................... 22 Esibov, Aboba & Thaler Standards Track [Page 2]
INTERNET-DRAFT LLMNR 11 December 2003 1. Introduction This document discusses Link Local Multicast Name Resolution (LLMNR), which utilizes the DNS packet format for both requests and responses, 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. LLMNR queries are sent to and received on port TBD. Link-scope multicast addresses are used to prevent propagation of LLMNR traffic across routers, potentially flooding the network; for details, see Section 2.4. LLMNR queries can also be sent to a unicast address, as described in Section 2.3. 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 3.2. 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 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 11 December 2003 1.1. Requirements In this document, several words are used to signify the requirements of the specification. These words are often capitalized. 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: Owner A host is said to be the owner of a Resource Record (RR) if it is configured to respond to an LLMNR query for that RR. Routable address An address other than a Link-Local address. This includes globally routable addresses, as well as private addresses. 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. This document does not specify how names are chosen or configured. This may occur via any mechanism, including DHCPv4 [RFC2131] or DHCPv6 [RFC3315]. 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. 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. 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 Esibov, Aboba & Thaler Standards Track [Page 4]
INTERNET-DRAFT LLMNR 11 December 2003 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: [1] 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. [2] An LLMNR sender sends an LLMNR query to the link-scope multicast address(es) defined in Section 2.4, unless a unicast query is indicated. A sender SHOULD send LLMNR queries for PTR RRs via unicast, as specified in Section 2.3. [3] 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 2.3. [4] Upon the reception of the response, the sender performs the checks described in Section 2.5. If these conditions are met, then the sender uses and caches the returned response. If not, then the sender ignores the response and continues waiting for the response. Further details of sender and responder behavior are provided in the sections that follow. 2.1. Sender behavior An LLMNR query is composed in exactly the same manner and with the same packet format as a DNS query as specified in [RFC1035]. The RD (Recursion Desired) bit MUST NOT be set in a query. 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 2.3, 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 Esibov, Aboba & Thaler Standards Track [Page 5]
INTERNET-DRAFT LLMNR 11 December 2003 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). 2.2. Responder behavior A response to an LLMNR query is composed in exactly the same manner and with the same packet format as a response to a DNS query as specified in [RFC1035]. 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. However, in general whether RRs are manually or automatically created is an implementation decision. Responders MUST NOT respond using cached data, and the AA (Authoritative Answer) bit MUST be set. The response MUST be sent to the sender via unicast. If a responder receives a query with the header containing RD set bit, the responder MUST ignore the RD bit. A responder MUST listen on UDP port TBD on the link-scope multicast address(es) defined in Section 2.4 and on UDP and TCP port TBD on the unicast address(es) that could be set as the source address(es) when the responder responds to the LLMNR query. Responders MUST NOT respond to LLMNR queries for names they are not authoritative for. Responders SHOULD respond to LLMNR queries for names and addresses they are authoritative for. This applies to both forward and reverse lookups. A response to an LLMNR query MUST have RCODE set to zero. Responses with RCODE set to zero are referred to in this document as "positively resolved". LLMNR responders may respond only to queries which they can resolve positively. 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 owned by the responder. 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 owns 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. 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 Esibov, Aboba & Thaler Standards Track [Page 6]
INTERNET-DRAFT LLMNR 11 December 2003 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 they receive from DNS clients. In conventional DNS terminology a DNS server authoritative for a zone is authoritative for all the domain names under the zone root except for the branches delegated into separate zones. Contrary to conventional DNS terminology, an LLMNR responder is authoritative only for the zone root. 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 significantly complicates implementation of LLMNR and would not be acceptable for lightweight hosts. 2.3. 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. If a TC (truncation) bit is set in the response, then the sender MAY use the response if it contains all necessary information, or the sender MAY discard the response and resend the query over TCP using the unicast address of the responder. The RA (Recursion Available) bit in the Esibov, Aboba & Thaler Standards Track [Page 7]
INTERNET-DRAFT LLMNR 11 December 2003 header of the response MUST NOT be set. If the RA bit is set in the response header, the sender MUST ignore the RA bit. Unicast LLMNR queries SHOULD be sent using TCP. Responses to TCP unicast LLMNR queries MUST be sent using TCP, using the same connection as the query. If the sender of a TCP query receives a response not using TCP, the response MUST be silently discarded. Unicast UDP queries MAY be responded to with a UDP response containing an empty answer section and the TC bit set, so as to require the sender to resend the query using TCP. Senders MUST support sending TCP queries, and Responders MUST support listening for TCP queries. 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. If an ICMP "Time Exceeded" message is received in response to a unicast UDP query, or 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). The UDP sender receiving an ICMP "Time Exceeded" message SHOULD verify that the ICMP error payload contains a valid LLMNR query packet, which matches a query that is currently in progress, so as to guard against a potential Denial of Service (DoS) attack. If a match cannot be made, then the sender relies on the retransmission and timeout behavior described in Section 2.6. 2.4. Addressing 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 TBD. The IPv6 link-scope multicast address a given responder listens to, and to which a sender sends all queries, is TBD. 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. Esibov, Aboba & Thaler Standards Track [Page 8]
INTERNET-DRAFT LLMNR 11 December 2003 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 2.4. If it was sent to another multicast address, then the query MUST be silently discarded. A sender SHOULD prefer RRs including reachable addresses where RRs involving both reachable and unreachable addresses are returned in response to a query. In composing LLMNR queries, the sender MUST set the Hop Limit field in the IPv6 header and the TTL field in IPv4 header of the response to one (1). Even 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. Therefore setting the IPv6 Hop Limit or IPv4 TTL field to one provides an additional precaution against leakage of LLMNR queries. In composing a response to an LLMNR query, the responder MUST set the Hop Limit field in the IPv6 header and the TTL field in IPv4 header of the response to one (1). This is done so as to prevent the use of LLMNR for denial of service attacks across the Internet. 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. Retransmissions In order to avoid synchronization, LLMNR queries and responses are delayed by a time randomly selected from the interval 0 to 200 ms. If an LLMNR query sent over UDP is not resolved within the timeout interval (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. Esibov, Aboba & Thaler Standards Track [Page 9]
INTERNET-DRAFT LLMNR 11 December 2003 Since a multicast query sender cannot know beforehand whether it will receive no response, one response, or more than one response, it 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. LLMNR implementations SHOULD dynamically compute the timeout value (LLMNR_TIMEOUT). It is suggested that this be based on the last response received for a query, on a per-interface basis. For example, the algorithms described in [RFC2988] (including exponential backoff) may be used to estimate RTO, which when combined with jittering, 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 20 seconds. 2.7. DNS TTL The responder should use a pre-configured TTL value in the records returned in the LLMNR query response. A default value of 0 is recommended in highly dynamic environments (such as mobile ad-hoc networks). In less dynamic environments, LLMNR traffic can be reduced by setting the TTL to a higher value. Due to the TTL minimalization necessary when caching an RRset, all TTLs in an RRset MUST be set to the same value. 2.8. 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 own 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. Esibov, Aboba & Thaler Standards Track [Page 10]
INTERNET-DRAFT LLMNR 11 December 2003 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 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. 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: [1] 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 Esibov, Aboba & Thaler Standards Track [Page 11]
INTERNET-DRAFT LLMNR 11 December 2003 used. [2] If an IPv4 address is returned, it MUST be reachable through the link over which LLMNR is used. [3] If a name is returned (for example in a CNAME, MX or SRV RR), the name MUST be valid on the local interface. Routable addresses MUST be included first in the response, if available. This encourages use of routable address(es) for establishment of new connections. 3.2. 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 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 Esibov, Aboba & Thaler Standards Track [Page 12]
INTERNET-DRAFT LLMNR 11 December 2003 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 for DHCP [RFC2937]. 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. 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. 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 Esibov, Aboba & Thaler Standards Track [Page 13]
INTERNET-DRAFT LLMNR 11 December 2003 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, as described in Section 2.6. 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 When a host that owns a UNIQUE record receives an LLMNR query for that record, the host MUST respond. After the client receives a response, 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 Esibov, Aboba & Thaler Standards Track [Page 14]
INTERNET-DRAFT LLMNR 11 December 2003 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 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. Esibov, Aboba & Thaler Standards Track [Page 15]
INTERNET-DRAFT LLMNR 11 December 2003 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 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. Esibov, Aboba & Thaler Standards Track [Page 16]
INTERNET-DRAFT LLMNR 11 December 2003 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 queries sent to a link-scope multicast address, as well as setting the TTL (IPv4) or Hop Limit (IPv6) fields to one (1) on both queries and responses. A TTL of one (1) was chosen so as to limit the likelihood that LLMNR can be used to launch denial of service attacks. For example, were the TTL of an LLMNR Response to be set to a value larger than one (1), an attacker could send a large volume of queries from a spoofed source address, causing an off-link target to be deluged with responses. Utilizing a TTL of one (1) in LLMNR responses ensures that they will not be forwarded off-link. 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 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. There also are scenarios such as public "hotspots" where attackers can Esibov, Aboba & Thaler Standards Track [Page 17]
INTERNET-DRAFT LLMNR 11 December 2003 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, eliminating the benefits of cache separation. As a result, LLMNR is only used as a name resolution mechanism of last resort. Note: enabling LLMNR for use in situations where a DNS server has been configured will result in upgraded hosts changing their 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. 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 Esibov, Aboba & Thaler Standards Track [Page 18]
INTERNET-DRAFT LLMNR 11 December 2003 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 does not require use of DNSSEC, 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 does not create any new name spaces for IANA administration. LLMNR requires allocation of a port TBD for both TCP and UDP. Assignment of the same port for both transports is requested. LLMNR requires allocation of a link-scope multicast IPv4 address as well as a link-scope multicast IPv6 address TBD. 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. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 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. Esibov, Aboba & Thaler Standards Track [Page 19]
INTERNET-DRAFT LLMNR 11 December 2003 [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. [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. [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-10.txt, October 2003. Esibov, Aboba & Thaler Standards Track [Page 20]
INTERNET-DRAFT LLMNR 11 December 2003 [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. 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 Esibov, Aboba & Thaler Standards Track [Page 21]
INTERNET-DRAFT LLMNR 11 December 2003 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. Full Copyright Statement Copyright (C) The Internet Society (2003). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS 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. Esibov, Aboba & Thaler Standards Track [Page 22]
INTERNET-DRAFT LLMNR 11 December 2003 Open Issues Open issues with this specification are tracked on the following web site: http://www.drizzle.com/~aboba/DNSEXT/llmnrissues.html Expiration Date This memo is filed as <draft-ietf-dnsext-mdns-26.txt>, and expires June 22, 2004. Esibov, Aboba & Thaler Standards Track [Page 23]
