DNSEXT Working Group Levon Esibov INTERNET-DRAFT Bernard Aboba Category: Standards Track Dave Thaler
<draft-ietf-dnsext-mdns-13.txt><draft-ietf-dnsext-mdns-14.txt> Microsoft 4 November 200222 March 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 (2002).(2003). All Rights Reserved. Abstract Today, with the rise of home networking, there are an increasing number of ad-hoc networks operating without a 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. Table of Contents 1. Introduction .......................................... 3 1.1 Requirements .................................... 34 1.2 Terminology ..................................... 34 2. Name resolution using LLMNR ........................... 4 2.1 Sender behavior ................................. 45 2.2 Responder behavior .............................. 5 2.3 Unicast queries ................................. 7 2.4 Addressing ...................................... 7 2.42.5 TTL ............................................. 7 2.5 No/multiple responses ........................... 72.6 Retransmissions ................................. 8 2.7 DNS TTL ......................................... 8 3. Usage model ........................................... 8 3.1 Unqualified names ............................... 9 3.2 LLMNR configuration ............................. 9 4. Sequence of events .................................... 10 5.Conflict resolution ................................... 10 5.111 4.1 Considerations for multiple interfaces .......... 12 5.213 4.2 API issues ...................................... 14 6.5. Security considerations ............................... 14 6.15.1 Scope restriction ............................... 14 6.215 5.2 Usage restriction ............................... 15 6.35.3 Cache and port separation ....................... 16 6.45.4 Authentication .................................. 16 7.6. IANA considerations ................................... 16 8.17 7. Normative References .................................. 16 9.17 8. Informative References ................................ 17 Acknowledgments .............................................. 18 Authors' Addresses ........................................... 1819 Intellectual Property Statement .............................. 19 Full Copyright Statement ..................................... 1920 1. Introduction This document discusses Link-Local Multicast Name Resolution (LLMNR), which operates on a separate port from DNS, with a distinct resolver cache, but does not change the format of DNS packets. LLMNR supports all current and future DNS formats, types and classes. However, since LLMNR only operates on the local link, it cannot be considered a substitute for DNS. 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 RCODE seterrors, as described in Section 3. LLMNR queries are sent to NXRRSET. Since IPv4and IPv6 utilize distinct configuration mechanisms, it is possible forreceived on port TBD using a dual stack hostLINKLOCAL address as specified in "Administratively Scoped IP Multicast" [RFC2365] for IPv4. The LLMNR LINKLOCAL address to be configured withused for IPv4 is 18.104.22.168. For IPv6, the "solicited name" LINKLOCAL multicast addresses are used for A/AAAA queries, and a separate multicast address TBD for all other queries. LINKLOCAL multicast addresses are used to prevent propagation of a DNS server over IPv4, while remaining unconfigured with a DNS server suitableLLMNR traffic across routers, potentially flooding the network; for use over IPv6.details, see Section 2.4. In these situations, a dual stack host will send AAAAcircumstances described in Section 2.3, LLMNR queries can also be sent to a unicast address. 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 home gateway, then the configurednetwork either has a DNS server over IPv4. However, an IPv6-only host will not be able to resolve names. Since automatic IPv6 DNS configuration mechanisms suchor the home gateway can function as [DHCPv6DNS] and [DNSDisc] are not yet widely deployed, and not alla DNS servers support IPv6, "partial configuration" may be common in the short term,proxy. By implementing DHCPv4 as well as a DNS proxy and LLMNR may prove useful in enabling linklocaldynamic DNS, home gateways can provide name resolution for the names of hosts over IPv6. However, inIPv4 on the long term,local network. For small IPv6 networks, equivalent functionality can be provided by a home gateway implementing DHCPv6 for DNS configuration, andconfiguration [DHCPv6DNS], as well as a DNS supportproxy supporting AAAA RRs and dynamic DNS, providing name resolution for the names of hosts over IPv6 will become more common soon the local network. This should be adequate as long as home gateways implementing DNS configuration also support dynamic DNS in some form. In the future, LLMNR may be defined to support greater than LINKLOCAL multicast scope. This would occur if LLMNR deployment is successful, the assumption that LLMNR usageis not needed on multiple links proves incorrect, and multicast routing becomes ubiquitous. For example, it is not clear that this assumption will typicallybe restricted tovalid in large adhoc networksnetworking scenarios. Once we have experience in which neither IPv4 nor IPv6 DNS servers are configured, situationsLLMNR deployment in terms of administrative issues, usability and impact on the network it will be possible reevaluate which DNS servers do not respond to queries, or where they respondmulticast scopes are appropriate for use with RCODE set to NXRRSET.multicast name resolution mechanisms. 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. 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 "MAY", "MUST,"MUST", "MUST NOT", "OPTIONAL", "RECOMMENDED","REQUIRED", "SHALL", "SHALL NOT", "SHOULD", and"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 1.2. Terminology Responder A host that listens to LLMNR queries, and responds to those for which it is authoritative is called "responder".authoritative. Sender A host that sends an LLMNR query. Typically a host is configured as both a sender and a responder, butresponder. However, a host may be configured as a "sender",sender, but not a "responder"responder or as a "responder"responder, but not a "sender".sender. Routable address An address other than a linklocal address. This includes site local and globally routable addresses, as well as private addresses. 2. Name resolution using LLMNR While operating on a different port with a distinct resolver cache, LLMNR makes no change to the current formatThe sequence of DNS packets.events for LLMNR queries are sentusage is as follows:  If a sender needs to and received on port TBD usingresolve a LINKLOCAL address as specified in "Administratively Scoped IP Multicast" [RFC2365]query for IPv4 anda name "host.example.com", then it sends a LLMNR query to the "solicited name"LINKLOCAL multicast addressesaddress.  A responder responds to this query only if it is authoritative for IPv6, and usingthe domain name "host.example.com". The responder sends a response to the sender via unicast addressesover UDP.  Upon the reception of the response, the sender verifies that the Hop Limit field in a few scenarios described belowIPv6 header or TTL field in Section 3. The LLMNR LINKLOCAL address to be used forIPv4 is 22.214.171.124. LINKLOCAL addresses are used to prevent propagation of LLMNR traffic across routers, potentially floodingheader (depending on the network. Propagationprotocol used) of LLMNR packets onthe local linkresponse is considered sufficientset to enable name resolution in small networks.255. The assumption is that if a network has a home gateway,sender then verifies compliance with the network either has a DNS server oraddressing requirements for IPv4, described in [IPV4Link], and IPv6, described in [RFC2373]. If these conditions are met, then the home gateway can function as a DNS proxy. By implementing DHCPv4 as well as a DNS proxysender uses and dynamic DNS, home gateways can provide name resolution forcaches the names of hosts over IPv4 onreturned response. If not, then the local network. For small IPv6 networks, equivalent functionality can be provided by a home gateway implementing DHCPv6 for DNS configuration [DHCPv6DNS], as well as a DNS proxy supporting AAAA RRssender ignores the response and dynamic DNS, providing name resolutioncontinues waiting for the namesresponse. Further details of hosts over IPv6 on the local network. This should be adequate as long as home gateways implementing DNS configuration also support dynamic DNS in some form. In the future, LLMNR may be defined to support greater than LINKLOCAL multicast scope. This would occur if LLMNR deployment is successful, the assumption that LLMNR is not needed on multiple links proves incorrect,sender and multicast routing becomes ubiquitous. For example, it is not clear that this assumption will be valid in large adhoc networking scenarios. Once we have experience in LLMNR deploymentresponder behavior are provided in terms of administrative issues, usability and impact onthe network it will be possible reevaluate which multicast scopes are appropriate for use with multicast name resolution mechanisms.sections that follow. 2.1. Sender behavior A sender sends an LLMNR query for any legal Type of resource record (e.g. A, PTR, etc.) to the LINKLOCAL multicast address. Notice that in some scenariosAn LLMNR sender MAY send requests for any name. Under conditions described belowin Section 32.3, a sender may also send a unicast query. The RD (Recursion Desired) bit MUST NOT be set. If a responder receives a query with the header containing RD set bit, the responder MUST ignore the RD bit. The IPv6 LINKLOCAL address a given responder listens to, and to which asender sends, is a link-local multicast address formed as follows: The name of the resource recordMUST anticipate receiving no replies to some LLMNR queries, in question is expressedthe event that no responders are available within the linklocal multicast scope, or in its canonical form (see [RFC2535], section 8.1), which is uncompressed with all alphabetic characters in lower case. The first label of the FQDN in the query is then hashed using the MD5 algorithm, described in [RFC1321]. The first 32 bits of the resultant 128-bit hash is then appended to the prefix FF02:0:0:0:0:2::/96 to yield the 128-bit "solicited name multicast address". (Note: this procedure is intended to bethe same as that specified in section 3 of "IPv6 Node Information Queries" [NodeInfo]). A responderevent that listens for queriesno positive non-null responses exist for multiple names will necessarily listen to multiple of these solicited name multicast addresses. Ifthe LLMNR querytransmitted query. If no positive response is not resolved during a limited amount of time (LLMNR_TIMEOUT), thenreceived, a sender MAY repeat the transmission ofresolver treats it as a query in order to assure themselvesresponse that the query has been received by a host capableno records of responding tothe query. The default value for LLMNR_TIMEOUT is 1 second. Since a sender cannot know beforehand whether it will receive no response, one response, or more than one response to a query, it SHOULD waitspecified type and class for LLMNR_TIMEOUT in order to collect all possible responses, rather than consideringthe query answered afterspecified name exist (that is, it is treated the firstsame as a response is received. Repetition MUST NOT be attempted more than 3 timeswith RCODE=0 and SHOULD NOT be repeated more often than once per second to reduce unnecessary network traffic. The delay between attempts should be randomized so as to avoid synchronization effects.an empty answer section). 2.2. Responder behavior A responder listens on port TBD on the LINKLOCAL addressmulticast address(es) and on the unicast address(es) that could be set as the source address(es) when the responder responds to the LLMNR query. The host configured as a "responder"responder MUST act as a sender by using LLMNR dynamic update requeststo verify the uniqueness of names as described in Section 5.4. 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. As an example, assume thata computer "host.example.com." configured to respond to the LLMNR queries is authoritative for the domainname "host.example.com.". On receiving aan LLMNR AA/AAAA resource record query for the name "host.example.com." the host authoritatively responds with AA/AAAA record(s) that contain IP address(es) in the RDATA of the resource record. 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. For example, if the host has a AAAA RR, but no A RR, and an A RR query is received, the host 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 owned by the host on which the server is running, but MUST NOT respond for other records for which the server is authoritative. 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 "host.example.com." is not authoritative for the name "child.host.example.com." unless the host is configured with multiple names, including "host.example.com." and "child.host.example.com.". As a result, "host" cannot reply to a query for "child" with NXDOMAIN. 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, "host.example.com." and "child.host.example.com.". In this example (unless this limitation is introduced) aan LLMNR query for an A record for the name "child.host.example.com." would result in two authoritative responses: a name error received from "host.example.com.", and a requested A record - from "child.host.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.host.example.com." would send a dynamic update for the NS and glue A record to "host.example.com.", but this approach significantly complicates implementation of LLMNR and would not be acceptable for lightweight hosts. A response to a LLMNR query is composed in exactly the same manner as a response to the unicast DNS query as specified in [RFC1035]. Responders MUST neverNOT respond using cached data, and the AA (Authoritative Answer) bit MUST be set. The response is sent to the sender via unicast. 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. 2.3. Unicast queries A sender MUST NOT send a unicast LLMNR query except when: a. A sender repeats a query after it received a response to the previous LLMNR query with the TC bit set, or b. The sender's LLMNR cache contains an NS resource record that enables the sender to send a query directly to the hosts authoritative for the name in the query. 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 or using EDNS0 with larger window using the unicast address of the responder. The RA (Recursion Available) bit in the header of the response MUST NOT be set. Even ifIf the RA bit is set in the response header, the sender MUST ignore it. 126.96.36.199. Addressing ForThe IPv4 LINKLOCAL addressing, section 2.4 of "Dynamic Configuration of IPv4 Link-Local Addresses" [IPV4Link] lays out the rules with respect to sourcemulticast address selection, TTL settings,a given responder listens to, and acceptable source/destination address combinations. IPv6to which a sender sends all queries, is described in [RFC2460];188.8.131.52. The IPv6 LINKLOCAL addressing is described in [RFC2373]. LLMNR queriesmulticast address a given responder listens to, and responses MUSTto which a sender sends A/AAAA queries, is formed as follows: The name of the resource record in question is expressed in its canonical form (see [RFC2535], section 8.1), which is uncompressed with all alphabetic characters in lower case. The first label of the FQDN in the query is then hashed using the MD5 algorithm, described in [RFC1321]. The first 32 bits of the resultant 128-bit hash is then appended to the prefix FF02:0:0:0:0:2::/96 to yield the 128-bit "solicited name multicast address". (Note: this procedure is intended to be the same as that specified in section 3 of "IPv6 Node Information Queries" [NodeInfo]). A responder that listens for queries for multiple names with different first labels will necessarily listen to multiple of these solicited name multicast addresses. For IPv4 LINKLOCAL addressing, section 2.4 of "Dynamic Configuration of IPv4 Link-Local Addresses" [IPV4Link] lays out the rules with respect to source address selection, TTL settings, and acceptable source/destination address combinations. IPv6 is described in [RFC2460]; IPv6 LINKLOCAL addressing is described in [RFC2373]. LLMNR queries and responses MUST obey the rules laid out in these documents. 2.5. TTL In composing an LLMNR response, the responder MUST set the Hop Limit field in the IPv6 header and the TTL field in IPv4 header of the LLMNR response to 255. The sender MUST verify that the Hop Limit field in IPv6 header and TTL field in IPv4 header of each response to the LLMNR query is set to 255. If it is not, then sender MUST ignore the response. Implementation note: In the sockets API for IPv4, the IP_TTL and IP_MULTICAST_TTL socket options are used to specifyset the TTL of outgoing unicast and multicast packets. The IP_RECVTTL socket option is available on some platforms to receiveretrieve the IPv4 TTL of received packets with recvmsg(). [RFC2292] specifies similar options for specifyingsetting and receivingretrieving the IPv6 Hop Limit. 2.4. TTL The responder should use2.6. Retransmissions In order to avoid synchronization, LLMNR queries and responses are delayed by a pre-configured TTL value in the records returned intime uniformly distributed between 0 and 200 ms. If the LLMNR query response. Due to the TTL minimalization necessary when caching an RRset, all TTLs in an RRset MUST be set tois not resolved within the same value. Intimeout interval (LLMNR_TIMEOUT), then a sender MAY repeat the additional and authority sectiontransmission of the response the responder includes the same records asa DNS server would insertquery in order to assure themselves that the responsequery has been received by a host capable of responding to the unicast DNSquery. 2.5. No/multiple responses TheSince a sender MUST anticipate receiving no replies to some LLMNR queries, in the event thatcannot know beforehand whether it will receive no responders are available within the linklocal multicast scope,response, one response, or more than one response to a query, it SHOULD wait for LLMNR_TIMEOUT in order to collect all possible responses, rather than considering the event that no positive non-null responses exist forquery answered after the transmitted query. If no positivefirst response is received,received. LLMNR implementations SHOULD dynamically estimate the timeout value (LLMNR_TIMEOUT) on a resolver treats it asper-interface basis, using the algorithms described in [RFC2988], with a response that no recordsminimum timeout value of the specified type300 ms. Repetition SHOULD NOT be attempted more than 3 times and class for the specified name exist (NXRRSET). While the responder MUSTSHOULD NOT respondbe repeated more often than once per second to queries for names it is not authoritative for, areduce unnecessary network traffic. 2.7. DNS TTL The responder MAY respond toshould use a query forpre-configured TTL value in the name it is authoritative for, even ifrecords returned in the type ofLLMNR query does not match a RR owned by the responder, with RCODE setresponse. Due to NXRRSET. For example, ifthe host has a AAAA RR, but no A RR, andTTL minimalization necessary when caching an A RR query is received, the host would respond withRRset, all TTLs in an RCODE set to NXRRSET. The senderRRset MUST anticipate receiving multiple repliesbe set to the same LLMNR query, in the event that several LLMNR enabled computers receivevalue. In the queryadditional and respond with valid answers. When this occurs,authority section of the responses MAY first be concatenated, and then treated inresponse the same manner that multiple RRs received fromresponder includes the same records as a DNS server would, ordinarily.would insert in the response to the unicast DNS query. 3. Usage model A host may be configured asLLMNR is a "sender", butpeer-to-peer name resolution protocol that is not a "responder" orintended as a "responder", butreplacement for DNS. By default, LLMNR requests SHOULD be sent only when no manual or automatic DNS configuration has been performed, when DNS servers do not respond, or when they respond to a "sender". However,query with RCODE=3 (Authoritative Name Error) or RCODE=0, and an empty answer section. As noted in [DNSPerf], even when DNS servers are configured, a host configured assignificant fraction of DNS queries do not receive a "responder" MUST at least useresponse, or result in a "sender's" capabilitynegative responses due to send LLMNR dynamic update requestsmissing inverse mappings or NS records that point to verifynonexistent or inappropriate hosts. Given this, support for LLMNR as a secondary name resolution mechanism has the uniquenesspotential to result in a large number of inappropriate queries without the names, as described in Section 5. An LLMNR "sender" MAY multicast requests for any name.following additional restrictions:  If that name is not qualified anda DNS query does not end inreceive a trailing dot, for the purposes ofresponse, prior to falling back to LLMNR, the implicit search order is as follows:  Request the name with the current domain appended.a DNS query SHOULD be retransmitted at least once.  Request just the name. This is the behavior suggested by [RFC1536].A sender SHOULD send LLMNR uses this technique to resolvequeries only for names that are either unqualified host names.or exist within the default domain.  A responder with both linklocal and routable addresses MUST respond to LLMNR queries for A/AAAA RRs only with routable address(es). This encourages use of routable address(es) for establishment of new connections. 3.1. Unqualified names The same host MAY use LLMNR queries for the resolution of unqualified host names, and conventional DNS queries for resolution of other DNS names. If a DNS server is running on a host that supports LLMNR, the DNS server MUST respond to LLMNR queries only for the RRSets owned by the host on which the server is running, but MUST NOT respond for other records for which the server is authoritative. A sender MUST NOT send a unicast LLMNR query except when: a. A sender repeats a query after it received a response to the previous LLMNR query with the TC bit set, or b. The sender's LLMNR cache contains an NS resource record that enables the sender to send a query directly to the hosts authoritative for thename is not qualified and does not end in the query. A responder witha name "host.example.com." configured to respond to the LLMNR queries is authoritativetrailing dot, for the name "host.example.com.". For example, when a responder withpurposes of LLMNR, the name "host.example.com." receives an A type LLMNR query forimplicit search order is as follows:  Request the name "host.example.com." it authoritatively responds towith the query. 3.1.current domain appended.  Request just the name. This is the behavior suggested by [RFC1536]. LLMNR uses this technique to resolve unqualified host names. 3.2. LLMNR configuration 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. By default, LLMNR requests SHOULD be sent only when no manual or automatic DNSSince IPv4 and IPv6 utilize distinct configuration has been performed, when DNS servers do not respond, or when they respond tomechanisms, it is possible for a query with an RCODE setdual stack host to NXRRSET. Where DHCPv4 or DHCPv6 is implemented, DHCP options canbe usedconfigured 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 configure LLMNR onthe configured DNS server over IPv4. However, an interface. The LLMNR Enable Option, described in [LLMNREnable], canIPv6-only host unconfigured with a DNS server suitable for use over IPv6 will be usedunable to explicitly enable or disable use of LLMNR on an interface. The LLMNR Enable Option doesresolve names using DNS. Since automatic IPv6 DNS configuration mechanisms such as [DHCPv6DNS] and [DNSDisc] are not determine whether or in which orderyet widely deployed, and not all DNS itself is used for name resolution. The orderservers support IPv6, lack of IPv6 DNS configuration may be a common problem in which variousthe short term, and LLMNR may prove useful in enabling linklocal name resolution mechanisms should be used can be specified using the Name Service Search Option for DHCP, [RFC2937]. Aover IPv6. For example, a home gateway may implement a DNS proxy and DHCPv4, but not DHCPv6 for DNS configuration [DHCPv6DNS].[DHCPv6DNS] or [DNSDisc]. In such a circumstance, IPv6-only hosts will not be configured with a DNS server. Where the DNS proxy does not support dynamic client update over IPv6 or DHCPv6-based dynamic update of the DNS proxy, the home gateway will not be able to dynamically register the names of IPv6 hosts. As a result, the DNS proxy will respond to AAAA RR queries sent over IPv4 or IPv6 with an RCODE of NXRRSET.authoritative name error (RCODE=3). This prevents hosts from resolving the names of IPv6-only hosts on the local link. In this situation, LLMNR over IPv6 can be used for resolution of dynamic names. 3.1.1. ConsistencyWhere 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 configurationLLMNR 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]. 3.2.1. Configuration consistency It is possible that DNS servers and/or DNSconfiguration 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 go down. 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. In order to minimize inconsistencies, the following practices are recommended: Periodic retryUnless unconfigured hosts periodically retry configuration, an outage in the DNS configuration mechanism will result in hosts continuing to prefer 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 two (2) minutes is recommended. DNS failover By default, LLMNR queries are not sent unless DNS is not configured, configured DNS servers have not responded, or respond with an RCODE of NXRRSET. However, where all configured DNS servers fail, LLMNR queries will be sent.4. Sequence of events The sequence of events for LLMNR usage is as follows: 1. If a sender needs to resolve a query for a name "host.example.com", then it sends a LLMNR query to the LINKLOCAL multicast address. 2. A responder responds to this query only if it is authoritative for the domain name "host.example.com". The responder sends a response to the sender via unicast over UDP. 3. Upon the reception of the response, the sender verifies that the Hop Limit field in IPv6 header or TTL field in IPv4 header (depending on the protocol used) of the response is set to 255.Conflict resolution The sender then verifies compliance with the addressing requirements for IPv4, described in [IPV4Link], and IPv6, describedMUST anticipate receiving multiple replies to the same LLMNR query, in [RFC2373]. If these conditions are met, thenthe sender usesevent that several LLMNR enabled computers receive the query and cachesrespond with valid answers. When this occurs, the returned response. If not,responses MAY first be concatenated, and then treated in the sender ignores the response and continues waiting forsame manner that multiple RRs received from the response. 5. Conflict resolutionsame DNS server would, ordinarily. 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 hostname. Every responder that responds to a LLMNR query and/or dynamic update request 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 respond to 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 cache, the host MUST verify resource record uniqueness on that interface. To accomplish this, the host MUST send a dynamic LLMNR update request for each new UNIQUE resource record. FormatThe format of the dynamic LLMNR update request is identical to the format of the dynamic DNS update requestthat specified in [RFC2136]. 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 configured to respond to the LLMNR queries on somean interface or - is configured to respond to the LLMNR queries using additional UNIQUE resource records. Below we describe theThe data to be specified in the dynamic update request:request is as follows: Header section contains values according to [RFC2136]. Zone section The zone name in the zone section MUST be set to the name of the UNIQUE record. The zone type in the zone section MUST be set to SOA. The zone class in the zone section MUST be set to the class of the UNIQUE record. Prerequisite section This section MUST contain a record set whose semantics are described in [RFC2136], Section 2.4.3 "RRset Does Not Exist",Exist" (NXRRSET), requesting that RRs with the NAME and TYPE of the UNIQUE record do not exist. Update section This section MUST be left empty. Additional section This section is set according to [RFC2136]. When a host that owns a UNIQUE record receives a dynamic update request that requests that the UNIQUE resource record set does not exist, the host MUST respond via unicast with the YXRRSET error, according to the rules described in Section 3 of [RFC2136]. After the client receives an YXRRSET response to its dynamic update request stating that a UNIQUE resource record does not exist, the host MUST check whether the response arrived on another interface. If this is the case, then the client can use the UNIQUE resource record in response to LLMNR queries and dynamic update requests. If not, then it MUST NOT use the UNIQUE resource record in response to LLMNR queries and dynamic update requests. Note that this name conflict detection mechanism doesn't prevent name conflicts when previously partitioned segments are connected by a bridge. In such a situation, name conflicts are detected when a sender receives more than one response to its LLMNR query. In this case, the sender sends the first response that it received to all responders that responded to this query except the first one, using unicast. A host that receives a query response containing a UNIQUE resource record that it owns, even if it didn't send such a query, MUST verify that no other host within the LLMNR scope is authoritative for the same name, using the dynamic LLMNR update request mechanism described above. Based on the result, the host detects whether there is a name conflict and acts as described above. 184.108.40.206. 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 wish 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 5.4. The situation is illustrated in figure 1 below:1. ---------- ---------- | | | | [A] [myhost] [myhost] Figure 1. LINKLOCAL 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 below.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 will then forward a response from the first responder to the second responder, who will attempt to verify the uniqueness of host RR for its name, but will not discover a conflict, since the conflicting host resides on a different link. Therefore it will continue using its name. Indeed, host myhost cannot distinguish between the situation shown in Figure 2, and that shown in Figure 3 where no conflict exists: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. 220.127.116.11. 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. 6.5. 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 and an attacker only needs to be misconfigured to answer an LLMNR query with incorrect information. In order to address the security vulnerabilities, the following mechanisms are contemplated:  Scope restrictions.  Usage restrictions.  Cache and port separation.  Authentication. These techniques are described in the following sections. 18.104.22.168. 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. In the absence of authentication, LLMNR reduces the exposure to such threats by ignoring LLMNR query response packets received from off-link senders. In all received responses, the Hop Limit field in IPv6 and the TTL field in IPv4 are verified to contain 255, the maximum legal value. Since routers decrement the Hop Limit on all packets they forward, received packets containing a Hop Limit of 255 must have originated from a neighbor. While restricting ignoring packets received from off-link senders reduces the level of vulnerability, it does not eliminate it. There 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. 22.214.171.124. Usage restriction As noted in Section 3.1,3, LLMNR is intended for usage in scenarios wherea DNS server is not configured, DNS servers do not respond to queries or respond with RCODElimited set to NXRRSET.of scenarios. If an interface has been configured via any automatic configuration mechanism which is able to supply DNS configuration information, then LLMNR MUSTSHOULD NOT be used as the primary name resolution mechanism on that interface, although it MAY be used as a secondary mechanism when DNS servers do not respond to queries, or respond with RCODE set to NXRRSET.mechanism. 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 LINKLOCAL multicast address, nor will it send queries to that address. Use of LLMNR as a secondary name resolution mechanism increases security vulnerabilities. For example, if an LLMNR query is sent whenever a DNS server does not respond in a timely way, then an attacker can execute a denial of service attack on the DNS server(s) and then poison the LLMNR cache by responding to the resulting LLMNR queries with incorrect information. 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 best thought of as a secondary name resolution mechanism. 126.96.36.199. 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 the 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. 188.8.131.52. Authentication LLMNR does not require use of DNSSEC, and as a result, responses to LLMNR queries MAY NOT be authenticated. 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. 7.6. IANA Considerations This specification does not create any new name spaces for IANA administration. LLMNR requires allocation of a port.port for both TCP and UDP. LLMNR utilizes a link scope multicast IPv4 address (184.108.40.206) that has been previously allocated to LLMNR by IANA. 8.It also requires allocation of a link scope multicast IPv6 address, for use with queries of types other than A/AAAA. 7. 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. [RFC2136] Vixie, P., Thomson, S., Rekhter, Y., Bound, J., "Dynamic Updates in the Domain Name System (DNS UPDATE)", RFC 2136, April 1997. [RFC2365] Meyer, D., "Administratively Scoped IP Multicast", BCP 23, RFC 2365, July 1998. [RFC2373] Hinden, R., Deering, S., "IP Version 6 Addressing Architecture", RFC 2373, July 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. [RFC2988] Paxson, V., Allman, M., "Computing TCP's Retransmission Timer", RFC 2988, November 2000. [IPV4Link] Cheshire, S., Aboba, B.,B.,Guttman, E., "Dynamic Configuration of IPv4 Link-Local Addresses", Internet draft (work in progress), draft-ietf-zeroconf-ipv4-linklocal-05.txt, November 2001. [LLMNREnable] Guttman, E., "DHCP LLMNR Enable Option", Internet draft (work in progress), draft-guttman-mdns-enable-02.txt, Aprildraft-ietf-zeroconf- ipv4-linklocal-07.txt, August 2002. 9.8. Informative References [RFC1536] Kumar, A., et. al. "DNS Implementation Errors and Suggested Fixes", RFC 1536, October 1993. [RFC2292] Stevens, W., Thomas, M., "Advanced Sockets API for IPv6", RFC 2292, February 1998. [RFC2434] Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. [RFC2553] Gilligan, R., Thomson, S., Bound, J., Stevens, W., "Basic Socket Interface Extensions for IPv6", RFC 2553, March 1999. [RFC2937] Smith, C., "The Name Service Search Option for DHCP", RFC 2937, September 2000. [DHCPv6DNS] Droms, R., Narten, T., and Aboba, B. "Using"A Guide to Implementing Stateless DHCPv6 for DNS Configuration in Hosts", draft-droms-dnsconfig- dhcpv6-01.txt,Service", Internet draft (work in progress), Marchdraft-droms- dhcpv6-stateless-guide-01.txt, October 2002. [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] Thaler, D.,Durand, A., Hagino, I., "IPv6 StatelessThaler, D., "Well known site local unicast addresses to communicate with recursive DNS Discovery",servers", Internet draft (work in progress), draft-ietf-ipngwg-dns- discovery-03.txt, November 2001.draft-ietf- ipv6-dns-discovery-07.txt, October 2002. [LLMNREnable] Guttman, E., "DHCP LLMNR Enable Option", Internet draft (work in progress), draft-guttman-mdns-enable-02.txt, April 2002. [NodeInfo] Crawford, Matt,M., "IPv6 Node Information Queries", Internet draft (work in progress), draft-ietf-ipn-gwg-icmp-name- lookups-08.txt, July 2001.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, Robert Elz, Rob Austein, James Gilroy, Olafur Gudmundsson, Erik Guttman, Myron Hattig, Thomas Narten, 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: firstname.lastname@example.org Bernard Aboba Microsoft Corporation One Microsoft Way Redmond, WA 98052 Phone: +1 425 706 6605 EMail: email@example.com Dave Thaler Microsoft Corporation One Microsoft Way Redmond, WA 98052 Phone: +1 425 703 8835 EMail: firstname.lastname@example.org 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. 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