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DNSEXT Working Group Levon Esibov
INTERNET-DRAFT Bernard Aboba
Category: Standards Track Dave Thaler
<draft-ietf-dnsext-mdns-19.txt> Microsoft
12 May 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.
Esibov, Aboba & Thaler Standards Track [Page 1]
INTERNET-DRAFT LLMNR 12 May 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 .............................. 5
2.3 Unicast queries ................................. 6
2.4 Addressing ...................................... 7
2.5 Off-link detection .............................. 8
2.6 Retransmissions ................................. 8
2.7 DNS TTL ......................................... 9
3. Usage model ........................................... 9
3.1 Unqualified names ............................... 10
3.2 LLMNR configuration ............................. 10
4. Conflict resolution ................................... 12
4.1 Considerations for multiple interfaces .......... 13
4.2 API issues ...................................... 14
5. Security considerations ............................... 15
5.1 Scope restriction ............................... 15
5.2 Usage restriction ............................... 16
5.3 Cache and port separation ....................... 17
5.4 Authentication .................................. 17
6. IANA considerations ................................... 17
7. Normative References .................................. 17
8. Informative References ................................ 18
Acknowledgments .............................................. 19
Authors' Addresses ........................................... 19
Intellectual Property Statement .............................. 20
Full Copyright Statement ..................................... 20
Esibov, Aboba & Thaler Standards Track [Page 2]
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1. Introduction
This document discusses Link Local Multicast Name Resolution (LLMNR),
which operates on a separate port from the Domain Name System (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 errors, as described in Section 3.
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 home gateway, then the network either has DNS and DHCPv4
servers or the home gateway provides DHCPv4 and DNS server
functionality. By providing Dynamic Host Configuration Service for
IPv4 (DHCPv4), as well as a DNS server with support for dynamic DNS,
which is also authoritative for the A RRs of local hosts, it is possible
to support resolution of local host names over IPv4.
For small IPv6 networks, equivalent functionality can be provided by
implementing Dynamic Host Configuration Service for IPv6 (DHCPv6) for
DNS configuration [DHCPv6DNS], as well providing a DNS server with
support for dynamic DNS, which is also authoritative for the AAAA RRs of
local hosts, it is possible to support the resolution of local host
names over IPv6 as well as IPv4.
In the future, LLMNR may be defined to support greater than link-scope
multicast. This would occur if LLMNR deployment is successful, the
assumption that LLMNR is not needed on multiple links proves incorrect,
and multicast routing becomes ubiquitous. For example, it is not clear
that this assumption will be valid in large ad hoc networking scenarios.
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 mechanisms.
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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
"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
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. Typically a host is
configured as both a sender and a responder. However, a
host may be configured as a sender, but not a responder
or as a responder, but not a 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
A typical sequence of events for LLMNR usage is as follows:
[1] A sender needs to resolve a query for a name "host.example.com",
so it sends an LLMNR query to the link-scope 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 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.
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2.1. Sender behavior
A sender sends an LLMNR query for any legal resource record type (e.g.
A/AAAA, SRV, PTR, etc.) to the link-scope multicast address. As
described in Section 2.3, a sender may also send a unicast query. An
LLMNR sender MAY send a request for any name.
The RD (Recursion Desired) bit MUST NOT be set in a query. If a
responder receives a query with the header containing RD set bit, the
responder MUST ignore the RD bit.
The sender MUST anticipate receiving no replies to some LLMNR queries,
in the event that no responders are available within the link-scope or
in the event no positive non-null responses exist for the transmitted
query. If no positive response is received, a resolver treats it as a
response that no records of the specified type and class exist for the
specified name (it is treated the same as a response with RCODE=0 and an
empty answer section).
2.2. Responder behavior
A responder MUST listen on UDP port TBD on the link-scope multicast
address(es) 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. A host configured as a responder MUST act as a sender
to verify the uniqueness of names as described in Section 4.
Responders MUST NOT respond to LLMNR queries for names they are not
authoritative for, except in the event of a discovered conflict, as
described in Section 4. 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, a computer "host.example.com." configured to respond to
LLMNR queries is authoritative for the name "host.example.com.". On
receiving an LLMNR A/AAAA resource record query for the name
"host.example.com." the host authoritatively responds with A/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
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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) an LLMNR query
for an A resource 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 NOT 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 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's LLMNR cache contains an NS resource record that
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enables the sender to send a query directly to the hosts
authoritative for the name in the query, or
c. The sender queries for a PTR RR.
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
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 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 224.0.0.251. The IPv6 link-scope multicast address a
given responder listens to, and to which a sender sends all queries, is
TBD.
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2.5. Off-link detection
The source address of LLMNR queries and responses MUST be "on link".
The destination address of an LLMNR query MUST be a link-scope multicast
address or an "on link" unicast address; 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 the LLMNR
multicast address; if it was sent to another multicast address, then the
query MUST be silently discarded.
For IPv4, an "on link" address is defined as a link-local address 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 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, 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 uniformly distributed between 0 and 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
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the query in order to assure that it was received by a host capable of
responding to it. 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 estimate the timeout value
(LLMNR_TIMEOUT) based on the last response received, on a per-interface
basis, using the algorithms described in [RFC2988], with a minimum
timeout value of 300 ms. 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.
2.7. DNS TTL
The responder should use a pre-configured TTL value in the records
returned in the LLMNR query response. Due to the TTL minimalization
necessary when caching an RRset, all TTLs in an RRset MUST be set to the
same value. In the additional and authority section of the response the
responder includes the same records as a DNS server would insert in the
response to the unicast DNS query.
3. Usage model
LLMNR is a peer-to-peer name resolution protocol that is not intended as
a replacement 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 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
significant fraction of DNS queries do not receive a response, or result
in a negative responses due to missing inverse mappings or NS records
that point to nonexistent or inappropriate hosts. Given this, support
for LLMNR as a secondary name resolution mechanism has the potential to
result in a large number of inappropriate queries without the following
additional restrictions:
[1] If a DNS query does not receive a response, prior to falling
back to LLMNR, the query SHOULD be retransmitted at least
once.
[2] Where a DNS server is configured, by default a sender
SHOULD send LLMNR queries only for names that are either
unqualified or exist within the default domain. Where no
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DNS server is configured, an LLMNR query MAY be sent for
any name.
[3] A responder with both link-local 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.
[4] A sender SHOULD send LLMNR queries for PTR RRs
via unicast, as specified in Section 2.3.
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
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.
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 name is not qualified and does not end in a trailing dot, for the
purposes of LLMNR, the implicit search order is as follows:
[1] Request the name with the current domain appended.
[2] 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.
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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. Since
automatic IPv6 DNS configuration mechanisms (such as [DHCPv6DNS] and
[DNSDisc]) are not yet widely deployed, and not all DNS servers support
IPv6, 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.
For example, a home network may provide a DHCPv4 server but may not
support DHCPv6 for DNS configuration [DHCPv6DNS]. In such a
circumstance, IPv6-only hosts will not be configured with a DNS server.
Where a DNS server is configured but does not support dynamic client
update over IPv6 or DHCPv6-based dynamic update, hosts on the home
network will not be able to dynamically register or resolve the names of
local IPv6 hosts. 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.
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].
3.2.1. Configuration consistency
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 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.
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Unless 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.
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.
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 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 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 configuration,
the host MUST verify resource record uniqueness on that interface. To
accomplish this, the host MUST send an LLMNR query for each UNIQUE
resource record.
By default, a host SHOULD be configured to behave as though all RRs are
UNIQUE. Uniqueness verification is carried out when the host:
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- starts up or
- is configured to respond to the LLMNR queries on an interface or
- 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 another interface. If this
is the case, then 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 such
a situation, name conflicts are detected when a sender receives more
than one response to its LLMNR multicast 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 UDP 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 mechanism described above. Based on the result, the host detects
whether there is a name conflict and acts accordingly.
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.
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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.
Host myhost will then send the first responder's response 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.
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.
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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 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:
[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.
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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/AAAA query for a popular Internet host), and 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 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 Section 3, LLMNR is intended for usage in a limited set of
scenarios.
If an interface has been configured via any automatic configuration
mechanism which is able to supply DNS configuration information, then
LLMNR SHOULD NOT be used as the primary name resolution mechanism on
that interface, although it MAY be used as a name resolution mechanism
of last resort.
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 it send queries to that address.
Use of LLMNR as a 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 name resolution mechanism of last resort.
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5.3. Cache and port separation
In order to prevent responses to LLMNR queries from polluting the DNS
cache, LLMNR implementations MUST use a distinct, isolated cache for
LLMNR on each interface. The use of separate caches is most effective
when LLMNR is used as a name resolution mechanism of last resort, since
this minimizes the opportunities for poisoning the LLMNR cache, and
decreases reliance on it.
LLMNR operates on a separate port from DNS, reducing the likelihood that
a DNS server will unintentionally respond to an LLMNR query.
5.4. Authentication
LLMNR 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 utilizes a link-scope multicast IPv4 address (224.0.0.251) that
has been previously allocated to LLMNR by IANA. It also requires
allocation of a link-scope multicast IPv6 address.
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., et al., "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.
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[RFC2373] Hinden, R. and S. Deering, "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. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000.
8. Informative References
[RFC1536] Kumar, A., et. al., "DNS Implementation Errors and
Suggested Fixes", RFC 1536, October 1993.
[RFC2292] Stevens, W. and M. Thomas, "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. 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.
[DHCPv6DNS] Droms, R., "A Guide to Implementing Stateless DHCPv6
Service", Internet draft (work in progress), draft-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] 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-07.txt, August 2002.
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[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
Phone: +1 425 703 8835
EMail: dthaler@microsoft.com
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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-19.txt>, and expires
November 22, 2003.
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