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IETF RFC 6106
IPv6 Router Advertisement Options for DNS Configuration
Last modified on Tuesday, November 30th, 2010
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Internet Engineering Task Force (IETF) J. Jeong
Request for Comments: 6106 Brocade/ETRI
Obsoletes: 5006 S. Park
Category: Standards Track SAMSUNG Electronics
ISSN: 2070-1721 L. Beloeil
France Telecom R&D
S. Madanapalli
iRam Technologies
November 2010
IPv6 Router Advertisement Options for DNS Configuration
 Abstract
This document specifies IPv6 Router Advertisement options to allow
IPv6 routers to advertise a list of DNS recursive server addresses
and a DNS Search List to IPv6 hosts.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6106.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Jeong, et al. Standards Track PAGE 1 
RFC 6106 IPv6 RA DNS Options November 2010
Table of Contents
1. Introduction ....................................................3
1.1. Applicability Statements ...................................3
1.2. Coexistence of RA Options and DHCP Options for DNS
Configuration ..............................................4
2. Requirements Language ...........................................4
3. Terminology .....................................................4
4. Overview ........................................................5
5. Neighbor Discovery Extension ....................................5
5.1. Recursive DNS Server Option ................................6
5.2. DNS Search List Option .....................................7
5.3. Procedure of DNS Configuration .............................8
5.3.1. Procedure in IPv6 Host ..............................8
5.3.2. Warnings for DNS Options Configuration .............10
6. Implementation Considerations ..................................10
6.1. DNS Repository Management .................................10
6.2. Synchronization between DNS Server List and
Resolver Repository .......................................11
6.3. Synchronization between DNS Search List and
Resolver Repository .......................................12
7. Security Considerations ........................................13
7.1. Security Threats ..........................................13
7.2. Recommendations ...........................................14
8. IANA Considerations ............................................15
9. Acknowledgements ...............................................15
10. References ....................................................16
10.1. Normative References .....................................16
10.2. Informative References ...................................16
Appendix A. Changes from RFC 5006 ................................18
Jeong, et al. Standards Track PAGE 2
RFC 6106 IPv6 RA DNS Options November 2010
1. Introduction
The purpose of this document is to standardize an IPv6 Router
Advertisement (RA) option for DNS Recursive Server Addresses used for
the DNS name resolution in IPv6 hosts. This RA option was specified
in an earlier Experimental specification [RFC 5006]. This document is
also to define a new RA option for Domain Name Search Lists for an
enhanced DNS configuration. Thus, this document obsoletes [RFC 5006],
which only defines the RA option for DNS Recursive Server Addresses.
Neighbor Discovery (ND) for IP version 6 and IPv6 stateless address
autoconfiguration provide ways to configure either fixed or mobile
nodes with one or more IPv6 addresses, default routers, and some
other parameters [RFC 4861][RFC 4862]. Most Internet services are
identified by using a DNS name. The two RA options defined in this
document provide the DNS information needed for an IPv6 host to reach
Internet services.
It is infeasible to manually configure nomadic hosts each time they
connect to a different network. While a one-time static
configuration is possible, it is generally not desirable on general-
purpose hosts such as laptops. For instance, locally defined name
spaces would not be available to the host if it were to run its own
name server software directly connected to the global DNS.
The DNS information can also be provided through DHCP
[RFC 3315][RFC 3736][RFC 3646]. However, the access to DNS is a
fundamental requirement for almost all hosts, so IPv6 stateless
autoconfiguration cannot stand on its own as an alternative
deployment model in any practical network without any support for DNS
configuration.
These issues are not pressing in dual-stack networks as long as a DNS
server is available on the IPv4 side, but they become more critical
with the deployment of IPv6-only networks. As a result, this
document defines a mechanism based on IPv6 RA options to allow IPv6
hosts to perform the automatic DNS configuration.
1.1. Applicability Statements
RA-based DNS configuration is a useful alternative in networks where
an IPv6 host's address is autoconfigured through IPv6 stateless
address autoconfiguration and where there is either no DHCPv6
infrastructure at all or some hosts do not have a DHCPv6 client. The
intention is to enable the full configuration of basic networking
information for hosts without requiring DHCPv6. However, when in
Jeong, et al. Standards Track PAGE 3
RFC 6106 IPv6 RA DNS Options November 2010
many networks some additional information needs to be distributed,
those networks are likely to employ DHCPv6. In these networks, RA-
based DNS configuration may not be needed.
RA-based DNS configuration allows an IPv6 host to acquire the DNS
configuration (i.e., DNS recursive server addresses and DNS Search
List) for the link(s) to which the host is connected. Furthermore,
the host learns this DNS configuration from the same RA message that
provides configuration information for the link, thereby avoiding
also running DHCPv6.
The advantages and disadvantages of the RA-based approach are
discussed in [RFC 4339] along with other approaches, such as the DHCP
and well-known anycast address approaches.
1.2. Coexistence of RA Options and DHCP Options for DNS Configuration
Two protocols exist to configure the DNS information on a host, the
Router Advertisement options described in this document and the
DHCPv6 options described in [RFC 3646]. They can be used together.
The rules governing the decision to use stateful configuration
mechanisms are specified in [RFC 4861]. Hosts conforming to this
specification MUST extract DNS information from Router Advertisement
messages, unless static DNS configuration has been specified by the
user. If there is DNS information available from multiple Router
Advertisements and/or from DHCP, the host MUST maintain an ordered
list of this information as specified in Section 5.3.1.
2. Requirements Language
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 [RFC 2119].
3. Terminology
This document uses the terminology described in [RFC 4861] and
[RFC 4862]. In addition, four new terms are defined below:
o Recursive DNS Server (RDNSS): Server that provides a recursive DNS
resolution service for translating domain names into IP addresses
as defined in [RFC 1034] and [RFC 1035].
o RDNSS Option: IPv6 RA option to deliver the RDNSS information to
IPv6 hosts [RFC 4861].
Jeong, et al. Standards Track PAGE 4
RFC 6106 IPv6 RA DNS Options November 2010
o DNS Search List (DNSSL): The list of DNS suffix domain names used
by IPv6 hosts when they perform DNS query searches for short,
unqualified domain names.
o DNSSL Option: IPv6 RA option to deliver the DNSSL information to
IPv6 hosts.
o DNS Repository: Two data structures for managing DNS Configuration
Information in the IPv6 protocol stack in addition to Neighbor
Cache and Destination Cache for Neighbor Discovery [RFC 4861]. The
first data structure is the DNS Server List for RDNSS addresses
and the second is the DNS Search List for DNS search domain names.
o Resolver Repository: Configuration repository with RDNSS addresses
and a DNS Search List that a DNS resolver on the host uses for DNS
name resolution; for example, the Unix resolver file (i.e., /etc/
resolv.conf) and Windows registry.
4. Overview
This document standardizes the ND option called the RDNSS option
defined in [RFC 5006] that contains the addresses of recursive DNS
servers. This document also defines a new ND option called the DNSSL
option for the Domain Search List. This is to maintain parity with
the DHCPv6 options and to ensure that there is necessary
functionality to determine the search domains.
The existing ND message (i.e., Router Advertisement) is used to carry
this information. An IPv6 host can configure the IPv6 addresses of
one or more RDNSSes via RA messages. Through the RDNSS and DNSSL
options, along with the prefix information option based on the ND
protocol ([RFC 4861] and [RFC 4862]), an IPv6 host can perform the
network configuration of its IPv6 address and the DNS information
simultaneously without needing DHCPv6 for the DNS configuration. The
RA options for RDNSS and DNSSL can be used on any network that
supports the use of ND.
This approach requires the manual configuration or other automatic
mechanisms (e.g., DHCPv6 or vendor proprietary configuration
mechanisms) to configure the DNS information in routers sending the
advertisements. The automatic configuration of RDNSS addresses and a
DNS Search List in routers is out of scope for this document.
5. Neighbor Discovery Extension
The IPv6 DNS configuration mechanism in this document needs two new
ND options in Neighbor Discovery: (i) the Recursive DNS Server
(RDNSS) option and (ii) the DNS Search List (DNSSL) option.
Jeong, et al. Standards Track PAGE 5
RFC 6106 IPv6 RA DNS Options November 2010
5.1. Recursive DNS Server Option
The RDNSS option contains one or more IPv6 addresses of recursive DNS
servers. All of the addresses share the same Lifetime value. If it
is desirable to have different Lifetime values, multiple RDNSS
options can be used. Figure 1 shows the format of the RDNSS option.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: Addresses of IPv6 Recursive DNS Servers :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Recursive DNS Server (RDNSS) Option Format
Fields:
Type 8-bit identifier of the RDNSS option type as assigned
by the IANA: 25
Length 8-bit unsigned integer. The length of the option
(including the Type and Length fields) is in units of
8 octets. The minimum value is 3 if one IPv6 address
is contained in the option. Every additional RDNSS
address increases the length by 2. The Length field
is used by the receiver to determine the number of
IPv6 addresses in the option.
Lifetime 32-bit unsigned integer. The maximum time, in
seconds (relative to the time the packet is sent),
over which this RDNSS address MAY be used for name
resolution. Hosts MAY send a Router Solicitation to
ensure the RDNSS information is fresh before the
interval expires. In order to provide fixed hosts
with stable DNS service and allow mobile hosts to
prefer local RDNSSes to remote RDNSSes, the value of
Lifetime SHOULD be bounded as
MaxRtrAdvInterval <= Lifetime <= 2*MaxRtrAdvInterval
where MaxRtrAdvInterval is the Maximum RA Interval
defined in [RFC 4861]. A value of all one bits
(0xffffffff) represents infinity. A value of zero
means that the RDNSS address MUST no longer be used.
Jeong, et al. Standards Track PAGE 6
RFC 6106 IPv6 RA DNS Options November 2010
Addresses of IPv6 Recursive DNS Servers
One or more 128-bit IPv6 addresses of the recursive
DNS servers. The number of addresses is determined
by the Length field. That is, the number of
addresses is equal to (Length - 1) / 2.
5.2. DNS Search List Option
The DNSSL option contains one or more domain names of DNS suffixes.
All of the domain names share the same Lifetime value. If it is
desirable to have different Lifetime values, multiple DNSSL options
can be used. Figure 2 shows the format of the DNSSL option.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: Domain Names of DNS Search List :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: DNS Search List (DNSSL) Option Format
Fields:
Type 8-bit identifier of the DNSSL option type as assigned
by the IANA: 31
Length 8-bit unsigned integer. The length of the option
(including the Type and Length fields) is in units of
8 octets. The minimum value is 2 if at least one
domain name is contained in the option. The Length
field is set to a multiple of 8 octets to accommodate
all the domain names in the field of Domain Names of
DNS Search List.
Lifetime 32-bit unsigned integer. The maximum time, in
seconds (relative to the time the packet is sent),
over which this DNSSL domain name MAY be used for
name resolution. The Lifetime value has the same
semantics as with the RDNSS option. That is, Lifetime
SHOULD be bounded as follows:
MaxRtrAdvInterval <= Lifetime <= 2*MaxRtrAdvInterval.
Jeong, et al. Standards Track PAGE 7
RFC 6106 IPv6 RA DNS Options November 2010
A value of all one bits (0xffffffff) represents
infinity. A value of zero means that the DNSSL
domain name MUST no longer be used.
Domain Names of DNS Search List
One or more domain names of DNS Search List that MUST
be encoded using the technique described in Section
3.1 of [RFC 1035]. By this technique, each domain
name is represented as a sequence of labels ending in
a zero octet, defined as domain name representation.
For more than one domain name, the corresponding
domain name representations are concatenated as they
are. Note that for the simple decoding, the domain
names MUST NOT be encoded in a compressed form, as
described in Section 4.1.4 of [RFC 1035]. Because the
size of this field MUST be a multiple of 8 octets,
for the minimum multiple including the domain name
representations, the remaining octets other than the
encoding parts of the domain name representations
MUST be padded with zeros.
Note: An RDNSS address or a DNSSL domain name MUST be used only as
long as both the RA router Lifetime (advertised by a Router
Advertisement message [RFC 4861]) and the corresponding option
Lifetime have not expired. The reason is that in the current
network to which an IPv6 host is connected, the RDNSS may not be
currently reachable, that the DNSSL domain name is not valid any
more, or that these options do not provide service to the host's
current address (e.g., due to network ingress filtering
[RFC 2827][RFC 5358]).
5.3. Procedure of DNS Configuration
The procedure of DNS configuration through the RDNSS and DNSSL
options is the same as with any other ND option [RFC 4861].
5.3.1. Procedure in IPv6 Host
When an IPv6 host receives DNS options (i.e., RDNSS option and DNSSL
option) through RA messages, it processes the options as follows:
o The validity of DNS options is checked with the Length field; that
is, the value of the Length field in the RDNSS option is greater
than or equal to the minimum value (3), and the value of the
Length field in the DNSSL option is greater than or equal to the
minimum value (2).
Jeong, et al. Standards Track PAGE 8
RFC 6106 IPv6 RA DNS Options November 2010
o If the DNS options are valid, the host SHOULD copy the values of
the options into the DNS Repository and the Resolver Repository in
order. Otherwise, the host MUST discard the options. Refer to
Section 6 for the detailed procedure.
When the IPv6 host has gathered a sufficient number (e.g., three) of
RDNSS addresses (or DNS search domain names), it SHOULD maintain
RDNSS addresses (or DNS search domain names) by the sufficient number
such that the latest received RDNSS or DNSSL is more preferred to the
old ones; that is, when the number of RDNSS addresses (or DNS search
domain names) is already the sufficient number, the new one replaces
the old one that will expire first in terms of Lifetime. As an
exceptional case, if the received RDNSS addresses (or DNS search
domain names) already exist in the IPv6 host, their Lifetime fields
update their Expiration-time, that is, when the corresponding DNS
information expires in the IPv6 host; note that when the Lifetime
field has zero, the corresponding RDNSS (or DNS search domain name)
is deleted from the IPv6 host. Except for this update, the IPv6 host
SHOULD ignore other RDNSS addresses (or DNS search domain names)
within an RDNSS (or a DNSSL) option and/or additional RDNSS (or
DNSSL) options within an RA. Refer to Section 6 for the detailed
procedure. Note that the sufficient number is a system parameter, so
it can be determined by a local policy. Also, separate parameters
can be specified for the sufficient number of RDNSS addresses and
that of DNS search domain names, respectively. In this document,
three is RECOMMENDED as a sufficient number considering both the
robust DNS query and the reasonably time-bounded recognition of the
unreachability of DNS servers.
In the case where the DNS options of RDNSS and DNSSL can be obtained
from multiple sources, such as RA and DHCP, the IPv6 host SHOULD keep
some DNS options from all sources. Unless explicitly specified for
the discovery mechanism, the exact number of addresses and domain
names to keep is a matter of local policy and implementation choice.
However, the ability to store at least three RDNSS addresses (or
DNSSL domain names) from at least two different sources is
RECOMMENDED. The DNS options from Router Advertisements and DHCP
SHOULD be stored into the DNS Repository and Resolver Repository so
that information from DHCP appears there first and therefore takes
precedence. Thus, the DNS information from DHCP takes precedence
over that from RA for DNS queries. On the other hand, for DNS
options announced by RA, if some RAs use the Secure Neighbor
Discovery (SEND) protocol [RFC 3971] for RA security, they MUST be
preferred over those that do not use SEND. Refer to Section 7 for
the detailed discussion on SEND for RA DNS options.
Jeong, et al. Standards Track PAGE 9
RFC 6106 IPv6 RA DNS Options November 2010
5.3.2. Warnings for DNS Options Configuration
There are two warnings for DNS options configuration: (i) warning for
multiple sources of DNS options and (ii) warning for multiple network
interfaces. First, in the case of multiple sources for DNS options
(e.g., RA and DHCP), an IPv6 host can configure its IP addresses from
these sources. In this case, it is not possible to control how the
host uses DNS information and what source addresses it uses to send
DNS queries. As a result, configurations where different information
is provided by different sources may lead to problems. Therefore,
the network administrator needs to configure DNS options in multiple
sources in order to prevent such problems from happening.
Second, if different DNS information is provided on different network
interfaces, this can lead to inconsistent behavior. The IETF is
working on solving this problem for both DNS and other information
obtained by multiple interfaces [MIF-PROBLEM][MIF-PRACTICE].
6. Implementation Considerations
Note: This non-normative section gives some hints for implementing
the processing of the RDNSS and DNSSL options in an IPv6 host.
For the configuration and management of DNS information, the
advertised DNS configuration information can be stored and managed in
both the DNS Repository and the Resolver Repository.
In environments where the DNS information is stored in user space and
ND runs in the kernel, it is necessary to synchronize the DNS
information (i.e., RDNSS addresses and DNS search domain names) in
kernel space and the Resolver Repository in user space. For the
synchronization, an implementation where ND works in the kernel
should provide a write operation for updating DNS information from
the kernel to the Resolver Repository. One simple approach is to
have a daemon (or a program that is called at defined intervals) that
keeps monitoring the Lifetimes of RDNSS addresses and DNS search
domain names all the time. Whenever there is an expired entry in the
DNS Repository, the daemon can delete the corresponding entry from
the Resolver Repository.
6.1. DNS Repository Management
For DNS repository management, the kernel or user-space process
(depending on where RAs are processed) should maintain two data
structures: (i) DNS Server List that keeps the list of RDNSS
addresses and (ii) DNS Search List that keeps the list of DNS search
domain names. Each entry in these two lists consists of a pair of an
RDNSS address (or DNSSL domain name) and Expiration-time as follows:
Jeong, et al. Standards Track PAGE 10
RFC 6106 IPv6 RA DNS Options November 2010
o RDNSS address for DNS Server List: IPv6 address of the Recursive
DNS Server, which is available for recursive DNS resolution
service in the network advertising the RDNSS option.
o DNSSL domain name for DNS Search List: DNS suffix domain names,
which are used to perform DNS query searches for short,
unqualified domain names in the network advertising the DNSSL
option.
o Expiration-time for DNS Server List or DNS Search List: The time
when this entry becomes invalid. Expiration-time is set to the
value of the Lifetime field of the RDNSS option or DNSSL option
plus the current system time. Whenever a new RDNSS option with
the same address (or DNSSL option with the same domain name) is
received on the same interface as a previous RDNSS option (or
DNSSL option), this field is updated to have a new Expiration-
time. When Expiration-time becomes less than the current system
time, this entry is regarded as expired.
6.2. Synchronization between DNS Server List and Resolver Repository
When an IPv6 host receives the information of multiple RDNSS
addresses within a network (e.g., campus network and company network)
through an RA message with RDNSS option(s), it stores the RDNSS
addresses (in order) into both the DNS Server List and the Resolver
Repository. The processing of the RDNSS consists of (i) the
processing of RDNSS option(s) included in an RA message and (ii) the
handling of expired RDNSSes. The processing of RDNSS option(s) is as
follows:
Step (a): Receive and parse the RDNSS option(s). For the RDNSS
addresses in each RDNSS option, perform Steps (b) through (d).
Step (b): For each RDNSS address, check the following: If the
RDNSS address already exists in the DNS Server List and the RDNSS
option's Lifetime field is set to zero, delete the corresponding
RDNSS entry from both the DNS Server List and the Resolver
Repository in order to prevent the RDNSS address from being used
any more for certain reasons in network management, e.g., the
termination of the RDNSS or a renumbering situation. That is, the
RDNSS can resign from its DNS service because the machine running
the RDNSS is out of service intentionally or unintentionally.
Also, under the renumbering situation, the RDNSS's IPv6 address
will be changed, so the previous RDNSS address should not be used
any more. The processing of this RDNSS address is finished here.
Otherwise, go to Step (c).
Jeong, et al. Standards Track PAGE 11
RFC 6106 IPv6 RA DNS Options November 2010
Step (c): For each RDNSS address, if it already exists in the DNS
Server List, then just update the value of the Expiration-time
field according to the procedure specified in the third bullet of
Section 6.1. Otherwise, go to Step (d).
Step (d): For each RDNSS address, if it does not exist in the DNS
Server List, register the RDNSS address and Lifetime with the DNS
Server List and then insert the RDNSS address in front of the
Resolver Repository. In the case where the data structure for the
DNS Server List is full of RDNSS entries (that is, has more
RDNSSes than the sufficient number discussed in Section 5.3.1),
delete from the DNS Server List the entry with the shortest
Expiration-time (i.e., the entry that will expire first). The
corresponding RDNSS address is also deleted from the Resolver
Repository. For the ordering of RDNSS addresses in an RDNSS
option, position the first RDNSS address in the RDNSS option as
the first one in the Resolver Repository, the second RDNSS address
in the option as the second one in the repository, and so on.
This ordering allows the RDNSS addresses in the RDNSS option to be
preferred according to their order in the RDNSS option for the DNS
name resolution. The processing of these RDNSS addresses is
finished here.
The handling of expired RDNSSes is as follows: Whenever an entry
expires in the DNS Server List, the expired entry is deleted from the
DNS Server List, and also the RDNSS address corresponding to the
entry is deleted from the Resolver Repository.
6.3. Synchronization between DNS Search List and Resolver Repository
When an IPv6 host receives the information of multiple DNSSL domain
names within a network (e.g., campus network and company network)
through an RA message with DNSSL option(s), it stores the DNSSL
domain names (in order) into both the DNS Search List and the
Resolver Repository. The processing of the DNSSL consists of (i) the
processing of DNSSL option(s) included in an RA message and (ii) the
handling of expired DNSSLs. The processing of DNSSL option(s) is as
follows:
Step (a): Receive and parse the DNSSL option(s). For the DNSSL
domain names in each DNSSL option, perform Steps (b) through (d).
Step (b): For each DNSSL domain name, check the following: If the
DNSSL domain name already exists in the DNS Search List and the
DNSSL option's Lifetime field is set to zero, delete the
corresponding DNSSL entry from both the DNS Search List and the
Resolver Repository in order to prevent the DNSSL domain name from
being used any more for certain reasons in network management,
Jeong, et al. Standards Track PAGE 12
RFC 6106 IPv6 RA DNS Options November 2010
e.g., the termination of the RDNSS or a renaming situation. That
is, the RDNSS can resign from its DNS service because the machine
running the RDNSS is out of service intentionally or
unintentionally. Also, under the renaming situation, the DNSSL
domain names will be changed, so the previous domain names should
not be used any more. The processing of this DNSSL domain name is
finished here. Otherwise, go to Step (c).
Step (c): For each DNSSL domain name, if it already exists in the
DNS Server List, then just update the value of the Expiration-time
field according to the procedure specified in the third bullet of
Section 6.1. Otherwise, go to Step (d).
Step (d): For each DNSSL domain name, if it does not exist in the
DNS Search List, register the DNSSL domain name and Lifetime with
the DNS Search List and then insert the DNSSL domain name in front
of the Resolver Repository. In the case where the data structure
for the DNS Search List is full of DNSSL domain name entries (that
is, has more DNSSL domain names than the sufficient number
discussed in Section 5.3.1), delete from the DNS Server List the
entry with the shortest Expiration-time (i.e., the entry that will
expire first). The corresponding DNSSL domain name is also
deleted from the Resolver Repository. For the ordering of DNSSL
domain names in a DNSSL option, position the first DNSSL domain
name in the DNSSL option as the first one in the Resolver
Repository, the second DNSSL domain name in the option as the
second one in the repository, and so on. This ordering allows the
DNSSL domain names in the DNSSL option to be preferred according
to their order in the DNSSL option for the DNS domain name used by
the DNS query. The processing of these DNSSL domain name is
finished here.
The handling of expired DNSSLs is as follows: Whenever an entry
expires in the DNS Search List, the expired entry is deleted from
the DNS Search List, and also the DNSSL domain name corresponding
to the entry is deleted from the Resolver Repository.
7. Security Considerations
In this section, we analyze security threats related to DNS options
and then suggest recommendations to cope with such security threats.
7.1. Security Threats
For the RDNSS option, an attacker could send an RA with a fraudulent
RDNSS address, misleading IPv6 hosts into contacting an unintended
DNS server for DNS name resolution. Also, for the DNSSL option, an
Jeong, et al. Standards Track PAGE 13
RFC 6106 IPv6 RA DNS Options November 2010
attacker can let IPv6 hosts resolve a host name without a DNS suffix
into an unintended host's IP address with a fraudulent DNS Search
List.
These attacks are similar to Neighbor Discovery attacks that use
Redirect or Neighbor Advertisement messages to redirect traffic to
individual addresses of malicious parties. That is, as a rogue
router, a malicious node on a LAN can promiscuously receive packets
for any router's Media Access Control (MAC) address and send packets
with the router's MAC address as the source MAC address in the Layer
2 (L2) header. As a result, L2 switches send packets addressed to
the router to the malicious node. Also, this attack can send
redirects that tell the hosts to send their traffic somewhere else.
The malicious node can send unsolicited RA or Neighbor Advertisement
(NA) replies, answer RS or Neighbor Solicitation (NS) requests, etc.
Thus, the attacks related to RDNSS and DNSSL are similar to both
Neighbor Discovery attacks and attacks against unauthenticated DHCP,
as both can be used for both "wholesale" traffic redirection and more
specific attacks.
However, the security of these RA options for DNS configuration does
not affect ND protocol security [RFC 4861]. This is because learning
DNS information via the RA options cannot be worse than learning bad
router information via the RA options. Therefore, the vulnerability
of ND is not worse and is a subset of the attacks that any node
attached to a LAN can do independently of ND.
7.2. Recommendations
The Secure Neighbor Discovery (SEND) protocol [RFC 3971] is used as a
security mechanism for ND. It is RECOMMENDED that ND use SEND to
allow all the ND options including the RDNSS and DNSSL options to be
automatically included in the signatures. Through SEND, the
transport for the RA options is integrity protected; that is, SEND
can prevent the spoofing of these DNS options with signatures. Also,
SEND enables an IPv6 host to verify that the sender of an RA is
actually a router authorized to act as a router. However, since any
valid SEND router can still insert RDNSS and DNSSL options, the
current SEND cannot verify which one is or is not authorized to send
the options. Thus, this verification of the authorized routers for
ND options will be required. [CSI-SEND-CERT] specifies the usage of
extended key for the certificate deployed in SEND. This document
defines the roles of routers (i.e., routers acting as proxy and
address owner) and explains the authorization of the roles. The
mechanism in this document can be extended to verify which routers
are authorized to insert RDNSS and DNSSL options.
Jeong, et al. Standards Track PAGE 14
RFC 6106 IPv6 RA DNS Options November 2010
It is common for network devices such as switches to include
mechanisms to block unauthorized ports from running a DHCPv6 server
to provide protection from rogue DHCP servers. That means that an
attacker on other ports cannot insert bogus DNS servers using DHCPv6.
The corresponding technique for network devices is RECOMMENDED to
block rogue Router Advertisement messages including the RDNSS and
DNSSL options from unauthorized nodes.
An attacker may provide a bogus DNS Search List option in order to
cause the victim to send DNS queries to a specific DNS server when
the victim queries non-FQDNs (fully qualified domain names). For
this attack, the DNS resolver in IPv6 hosts can mitigate the
vulnerability with the recommendations mentioned in [RFC 1535],
[RFC 1536], and [RFC 3646].
8. IANA Considerations
The RDNSS option defined in this document uses the IPv6 Neighbor
Discovery Option type defined in RFC 5006 [RFC 5006], which was
assigned by the IANA as follows:
Option Name Type
Recursive DNS Server Option 25
The IANA has assigned a new IPv6 Neighbor Discovery Option type for
the DNSSL option defined in this document:
Option Name Type
DNS Search List Option 31
These options have been registered in the "Internet Control Message
Protocol version 6 (ICMPv6) Parameters" registry
(http://www.iana.org).
9. Acknowledgements
This document has greatly benefited from inputs by Robert Hinden,
Pekka Savola, Iljitsch van Beijnum, Brian Haberman, Tim Chown, Erik
Nordmark, Dan Wing, Jari Arkko, Ben Campbell, Vincent Roca, and Tony
Cheneau. The authors sincerely appreciate their contributions.
Jeong, et al. Standards Track PAGE 15
RFC 6106 IPv6 RA DNS Options November 2010
10. References
10.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 4861] Narten, T., Nordmark, E., Simpson, W., and H.
Soliman, "Neighbor Discovery for IP version 6
(IPv6)", RFC 4861, September 2007.
[RFC 4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6
Stateless Address Autoconfiguration", RFC 4862,
September 2007.
[RFC 1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
10.2. Informative References
[RFC 1034] Mockapetris, P., "Domain names - concepts and
facilities", STD 13, RFC 1034, November 1987.
[RFC 3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration
Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC 3736] Droms, R., "Stateless Dynamic Host Configuration
Protocol (DHCP) Service for IPv6", RFC 3736,
April 2004.
[RFC 3646] Droms, R., "DNS Configuration options for Dynamic
Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 3646, December 2003.
[RFC 5006] Jeong, J., Park, S., Beloeil, L., and S.
Madanapalli, "IPv6 Router Advertisement Option for
DNS Configuration", RFC 5006, September 2007.
[RFC 4339] Jeong, J., "IPv6 Host Configuration of DNS Server
Information Approaches", RFC 4339, February 2006.
[RFC 3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander,
"SEcure Neighbor Discovery (SEND)", RFC 3971,
March 2005.
Jeong, et al. Standards Track PAGE 16
RFC 6106 IPv6 RA DNS Options November 2010
[RFC 5358] Damas, J. and F. Neves, "Preventing Use of Recursive
Nameservers in Reflector Attacks", BCP 140,
RFC 5358, October 2008.
[RFC 2827] Ferguson, P. and D. Senie, "Network Ingress
Filtering: Defeating Denial of Service Attacks which
employ IP Source Address Spoofing", BCP 38,
RFC 2827, May 2000.
[RFC 1535] Gavron, E., "A Security Problem and Proposed
Correction With Widely Deployed DNS Software",
RFC 1535, October 1993.
[RFC 1536] Kumar, A., Postel, J., Neuman, C., Danzig, P., and
S. Miller, "Common DNS Implementation Errors and
Suggested Fixes", RFC 1536, October 1993.
[MIF-PROBLEM] Blanchet, M. and P. Seite, "Multiple Interfaces
Problem Statement", Work in Progress, August 2010.
[MIF-PRACTICE] Wasserman, M. and P. Seite, "Current Practices for
Multiple Interface Hosts", Work in Progress,
August 2010.
[CSI-SEND-CERT] Gagliano, R., Krishnan, S., and A. Kukec,
"Certificate profile and certificate management for
SEND", Work in Progress, October 2010.
Jeong, et al. Standards Track PAGE 17
RFC 6106 IPv6 RA DNS Options November 2010
Appendix A. Changes from RFC 5006
The following changes were made from RFC 5006 "IPv6 Router
Advertisement Option for DNS Configuration":
o Added the DNS Search List (DNSSL) option to support the
advertisement of DNS suffixes used in the DNS search along with
RDNSS option in RFC 5006.
o Clarified the coexistence of RA options and DHCP options for DNS
configuration.
o Modified the procedure in IPv6 host:
* Clarified the procedure for DNS options in an IPv6 host.
* Specified a sufficient number of RDNSS addresses or DNS search
domain names as three.
* Specified a way to deal with DNS options from multiple sources,
such as RA and DHCP.
o Modified the implementation considerations for DNSSL option
handling.
o Modified the security considerations to consider more attack
scenarios and the corresponding possible solutions.
o Modified the IANA considerations to require another IPv6 Neighbor
Discovery Option type for the DNSSL option.
Jeong, et al. Standards Track PAGE 18
RFC 6106 IPv6 RA DNS Options November 2010
Authors' Addresses
Jaehoon Paul Jeong
Brocade Communications Systems/ETRI
6000 Nathan Ln N
Plymouth, MN 55442
USA
Phone: +1 763 268 7173
Fax: +1 763 268 6800
EMail: pjeong@brocade.com
URI: http://www.cs.umn.edu/~jjeong/
Soohong Daniel Park
Digital Media & Communications R&D Center
SAMSUNG Electronics
416 Maetan-3dong, Yeongtong-Gu
Suwon, Gyeonggi-Do 443-742
Korea
Phone: +82 31 279 8876
EMail: soohong.park@samsung.com
Luc Beloeil
France Telecom R&D
42, rue des coutures
BP 6243
14066 CAEN Cedex 4
France
Phone: +33 2 40 44 97 40
EMail: luc.beloeil@orange-ftgroup.com
Syam Madanapalli
iRam Technologies
#H304, Shriram Samruddhi, Thubarahalli
Bangalore - 560066
India
EMail: smadanapalli@gmail.com
Jeong, et al. Standards Track PAGE 19
IPv6 Router Advertisement Options for DNS Configuration
RFC TOTAL SIZE: 45181 bytes
PUBLICATION DATE: Tuesday, November 30th, 2010
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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