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<pre>Internet Engineering Task Force (IETF) J. Jeong
Request for Comments: 8106 Sungkyunkwan University
Obsoletes: <a href="./rfc6106">6106</a> S. Park
Category: Standards Track Samsung Electronics
ISSN: 2070-1721 L. Beloeil
Orange
S. Madanapalli
NTT Data
March 2017
<span class="h1">IPv6 Router Advertisement Options for DNS Configuration</span>
Abstract
This document specifies IPv6 Router Advertisement (RA) options
(called "DNS RA options") to allow IPv6 routers to advertise a list
of DNS Recursive Server Addresses and a DNS Search List to IPv6
hosts.
This document, which obsoletes <a href="./rfc6106">RFC 6106</a>, defines a higher default
value of the lifetime of the DNS RA options to reduce the likelihood
of expiry of the options on links with a relatively high rate of
packet loss.
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 <a href="./rfc7841#section-2">Section 2 of RFC 7841</a>.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
<a href="http://www.rfc-editor.org/info/rfc8106">http://www.rfc-editor.org/info/rfc8106</a>.
<span class="grey">Jeong, et al. Standards Track [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc8106">RFC 8106</a> IPv6 DNS RA Options March 2017</span>
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
Provisions Relating to IETF Documents
(<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) 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.
Table of Contents
<a href="#section-1">1</a>. Introduction ....................................................<a href="#page-3">3</a>
<a href="#section-1.1">1.1</a>. Applicability Statements ...................................<a href="#page-3">3</a>
1.2. Coexistence of RA Options and DHCP Options for DNS
Configuration ..............................................<a href="#page-4">4</a>
<a href="#section-2">2</a>. Requirements Language ...........................................<a href="#page-4">4</a>
<a href="#section-3">3</a>. Terminology .....................................................<a href="#page-4">4</a>
<a href="#section-4">4</a>. Overview ........................................................<a href="#page-5">5</a>
<a href="#section-5">5</a>. Neighbor Discovery Extension ....................................<a href="#page-5">5</a>
<a href="#section-5.1">5.1</a>. Recursive DNS Server Option ................................<a href="#page-6">6</a>
<a href="#section-5.2">5.2</a>. DNS Search List Option .....................................<a href="#page-7">7</a>
<a href="#section-5.3">5.3</a>. DNS Configuration Procedure ................................<a href="#page-8">8</a>
<a href="#section-5.3.1">5.3.1</a>. Procedure in IPv6 Hosts .............................<a href="#page-9">9</a>
<a href="#section-5.3.2">5.3.2</a>. Warnings for DNS Options Configuration ..............<a href="#page-9">9</a>
<a href="#section-6">6</a>. Implementation Considerations ..................................<a href="#page-10">10</a>
<a href="#section-6.1">6.1</a>. DNS Repository Management .................................<a href="#page-10">10</a>
6.2. Synchronization between DNS Server List and
Resolver Repository .......................................<a href="#page-11">11</a>
6.3. Synchronization between DNS Search List and
Resolver Repository .......................................<a href="#page-12">12</a>
<a href="#section-7">7</a>. Security Considerations ........................................<a href="#page-12">12</a>
<a href="#section-7.1">7.1</a>. Security Threats ..........................................<a href="#page-12">12</a>
<a href="#section-7.2">7.2</a>. Recommendations ...........................................<a href="#page-13">13</a>
<a href="#section-8">8</a>. IANA Considerations ............................................<a href="#page-13">13</a>
<a href="#section-9">9</a>. References .....................................................<a href="#page-14">14</a>
<a href="#section-9.1">9.1</a>. Normative References ......................................<a href="#page-14">14</a>
<a href="#section-9.2">9.2</a>. Informative References ....................................<a href="#page-14">14</a>
<a href="#appendix-A">Appendix A</a>. Changes from <a href="./rfc6106">RFC 6106</a> .................................<a href="#page-17">17</a>
Acknowledgements ..................................................<a href="#page-18">18</a>
Authors' Addresses ................................................<a href="#page-19">19</a>
<span class="grey">Jeong, et al. Standards Track [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc8106">RFC 8106</a> IPv6 DNS RA Options March 2017</span>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
The purpose of this document is to standardize IPv6 Router
Advertisement (RA) options (DNS RA options) for DNS Recursive Server
Addresses used for DNS name resolution in IPv6 hosts, and also for a
DNS Search List (DNSSL) of domain suffixes.
IPv6 Neighbor Discovery (ND) and IPv6 Stateless Address
Autoconfiguration (SLAAC) provide ways to configure either fixed or
mobile nodes with one or more IPv6 addresses, default routers, and
some other parameters [<a href="./rfc4861" title=""Neighbor Discovery for IP version 6 (IPv6)"">RFC4861</a>] [<a href="./rfc4862" title=""IPv6 Stateless Address Autoconfiguration"">RFC4862</a>].
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
namespaces would not be available to the host if it were to run its
own recursive name server directly connected to the global DNS.
The DNS information can also be provided through DHCPv6 [<a href="./rfc3315" title=""Dynamic Host Configuration Protocol for IPv6 (DHCPv6)"">RFC3315</a>]
[<a href="./rfc3736" title=""Stateless Dynamic Host Configuration Protocol (DHCP) Service for IPv6"">RFC3736</a>] [<a href="./rfc3646" title=""DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)"">RFC3646</a>]. However, access to DNS is a fundamental
requirement for almost all hosts, so IPv6 SLAAC 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 DNS RA options to allow IPv6
hosts to perform automatic DNS configuration.
<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>. Applicability Statements</span>
RA-based DNS configuration is a useful alternative in networks where
an IPv6 host's address is autoconfigured through IPv6 SLAAC and where
either (i) there is no DHCPv6 infrastructure at all or (ii) 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, for networks that need to distribute
additional information, DHCPv6 is likely to be employed. 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 the DNSSL)
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.
<span class="grey">Jeong, et al. Standards Track [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc8106">RFC 8106</a> IPv6 DNS RA Options March 2017</span>
The advantages and disadvantages of the RA-based approach are
discussed in [<a href="./rfc4339" title=""IPv6 Host Configuration of DNS Server Information Approaches"">RFC4339</a>] along with other approaches, such as the DHCP
and well-known anycast address approaches.
<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>. Coexistence of RA Options and DHCP Options for DNS Configuration</span>
Two protocols exist to configure the DNS information on a host: the
RA options specified in this document and the DHCPv6 options
specified in [<a href="./rfc3646" title=""DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)"">RFC3646</a>]. They can be used together. The rules
governing the decision to use stateful configuration mechanisms are
specified in [<a href="./rfc4861" title=""Neighbor Discovery for IP version 6 (IPv6)"">RFC4861</a>]. Hosts conforming to this specification MUST
extract DNS information from RA messages, unless static DNS
configuration has been specified by the user. If there is DNS
information available from multiple RAs and/or from DHCP, the host
MUST maintain an ordered list of this information as specified in
<a href="#section-5.3.1">Section 5.3.1</a>.
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Requirements Language</span>
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 [<a href="./rfc2119" title=""Key words for use in RFCs to Indicate Requirement Levels"">RFC2119</a>].
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Terminology</span>
This document uses the terminology defined in [<a href="./rfc4861" title=""Neighbor Discovery for IP version 6 (IPv6)"">RFC4861</a>] and
[<a href="./rfc4862" title=""IPv6 Stateless Address Autoconfiguration"">RFC4862</a>]. In addition, six new terms are defined below:
o Recursive DNS Server (RDNSS): A server that provides a recursive
DNS resolution service for translating domain names into IP
addresses or resolving PTR records as defined in [<a href="./rfc1034" title=""Domain names - concepts and facilities"">RFC1034</a>] and
[<a href="./rfc1035" title=""Domain names - implementation and specification"">RFC1035</a>].
o RDNSS Option: An IPv6 RA option to deliver the RDNSS information
to IPv6 hosts [<a href="./rfc4861" title=""Neighbor Discovery for IP version 6 (IPv6)"">RFC4861</a>].
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: An 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 the
Neighbor Cache and Destination Cache for Neighbor Discovery
<span class="grey">Jeong, et al. Standards Track [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc8106">RFC 8106</a> IPv6 DNS RA Options March 2017</span>
[<a href="./rfc4861" title=""Neighbor Discovery for IP version 6 (IPv6)"">RFC4861</a>]. The first data structure is the DNS Server List for
RDNSS addresses, and the second is the DNSSL for DNS search domain
names.
o Resolver Repository: Configuration repository with RDNSS addresses
and a DNSSL that a DNS resolver on the host uses for DNS name
resolution -- for example, the UNIX resolver file (i.e.,
/etc/resolv.conf) and the Windows registry.
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. Overview</span>
This document standardizes an ND option called the "RDNSS option",
which contains the addresses of RDNSSes. This document also
standardizes an ND option called the "DNSSL option", which contains
the DNSSL. 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., RA) 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 [<a href="./rfc4861" title=""Neighbor Discovery for IP version 6 (IPv6)"">RFC4861</a>]
[<a href="./rfc4862" title=""IPv6 Stateless Address Autoconfiguration"">RFC4862</a>], 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 networks that support the use of ND.
This approach requires manual configuration or 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 DNSSL in routers
is out of scope for this document.
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. Neighbor Discovery Extension</span>
The IPv6 DNS configuration mechanism described in this document needs
two ND options in Neighbor Discovery: (i) the RDNSS option and
(ii) the DNSSL option.
<span class="grey">Jeong, et al. Standards Track [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc8106">RFC 8106</a> IPv6 DNS RA Options March 2017</span>
<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>. Recursive DNS Server Option</span>
The RDNSS option contains one or more IPv6 addresses of RDNSSes. 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: RDNSS Option Format
Fields:
Type 8-bit identifier of the RDNSS option type as assigned by
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 received) over which
these RDNSS addresses MAY be used for name resolution.
The value of Lifetime SHOULD by default be at least
3 * MaxRtrAdvInterval, where MaxRtrAdvInterval is the
maximum RA interval as defined in [<a href="./rfc4861" title=""Neighbor Discovery for IP version 6 (IPv6)"">RFC4861</a>]. A value of
all one bits (0xffffffff) represents infinity. A value
of zero means that the RDNSS addresses MUST no longer
be used.
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Addresses of IPv6 Recursive DNS Servers
One or more 128-bit IPv6 addresses of the RDNSSes. The
number of addresses is determined by the Length field.
That is, the number of addresses is equal to
(Length - 1) / 2.
Note: The addresses for RDNSSes in the RDNSS option MAY be link-local
addresses. Such link-local addresses SHOULD be registered in
the Resolver Repository along with the corresponding link zone
indices of the links that receive the RDNSS option(s) for them.
The link-local addresses MAY be represented in the Resolver
Repository with their link zone indices in the textual format
for scoped addresses as described in [<a href="./rfc4007" title=""IPv6 Scoped Address Architecture"">RFC4007</a>]. When a
resolver sends a DNS query message to an RDNSS identified by a
link-local address, it MUST use the corresponding link.
The rationale of the default value of the Lifetime field is as
follows. The Router Lifetime field, set by AdvDefaultLifetime,
has the default of 3 * MaxRtrAdvInterval as specified in
[<a href="./rfc4861" title=""Neighbor Discovery for IP version 6 (IPv6)"">RFC4861</a>], so such a default or a larger default can allow for
the reliability of DNS options even under the loss of RAs on
links with a relatively high rate of packet loss. Note that
the ratio of AdvDefaultLifetime to MaxRtrAdvInterval is the
number of unsolicited multicast RAs sent by the router. Since
the DNS option entries can survive for at most three
consecutive losses of RAs containing DNS options, the default
value of the Lifetime lets the DNS option entries be resilient
to packet-loss environments.
<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>. DNS Search List Option</span>
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 :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Figure 2: DNSSL Option Format
Fields:
Type 8-bit identifier of the DNSSL option type as assigned by
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 "Domain Names of DNS Search List" field.
Lifetime 32-bit unsigned integer. The maximum time in seconds
(relative to the time the packet is received) over which
these DNSSL domain names MAY be used for name resolution.
The Lifetime value has the same semantics as the
semantics for the RDNSS option. That is, Lifetime SHOULD
by default be at least 3 * MaxRtrAdvInterval. A value of
all one bits (0xffffffff) represents infinity. A value
of zero means that the DNSSL domain names MUST no longer
be used.
Domain Names of DNS Search List
One or more domain names of the DNSSL that MUST be
encoded as described in <a href="./rfc1035#section-3.1">Section 3.1 of [RFC1035]</a>. With
this technique, each domain name is represented as a
sequence of labels ending in a zero octet, defined as a
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 the
compressed form described in <a href="./rfc1035#section-4.1.4">Section 4.1.4 of [RFC1035]</a>.
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.
<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>. DNS Configuration Procedure</span>
The procedure for DNS configuration through the RDNSS and DNSSL
options is the same as it is with any other ND option [<a href="./rfc4861" title=""Neighbor Discovery for IP version 6 (IPv6)"">RFC4861</a>].
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<span class="h4"><a class="selflink" id="section-5.3.1" href="#section-5.3.1">5.3.1</a>. Procedure in IPv6 Hosts</span>
When an IPv6 host receives DNS options (i.e., RDNSS and DNSSL
options) 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 satisfies the
requirement that (Length - 1) % 2 == 0. The value of the Length
field in the DNSSL option is greater than or equal to the minimum
value (2). Also, the validity of the RDNSS option is checked with
the "Addresses of IPv6 Recursive DNS Servers" field; that is, the
addresses should be unicast addresses.
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
<a href="#section-6">Section 6</a> for the detailed procedure.
In the case where the DNS information of RDNSS and DNSSL can be
obtained from multiple sources, such as RAs 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 as a local configuration option. However, in
the case of multiple sources, the ability to store a total of at
least three RDNSS addresses (or DNSSL domain names) from the multiple
sources is RECOMMENDED. The DNS options from RAs and DHCP SHOULD be
stored in 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 RAs for DNS queries. On the other hand, for DNS
options announced by RAs, if some RAs use the Secure Neighbor
Discovery (SEND) protocol [<a href="./rfc3971" title=""SEcure Neighbor Discovery (SEND)"">RFC3971</a>] for RA security, they MUST be
preferred over those that do not use SEND. Also, DNS options
announced by RAs via SEND MUST be preferred over those announced by
unauthenticated DHCP [<a href="./rfc3118" title=""Authentication for DHCP Messages"">RFC3118</a>]. Refer to <a href="#section-7">Section 7</a> for a detailed
discussion of SEND for DNS RA options.
<span class="h4"><a class="selflink" id="section-5.3.2" href="#section-5.3.2">5.3.2</a>. Warnings for DNS Options Configuration</span>
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., RAs 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
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information is provided by different mechanisms for autoconfiguration
may lead to problems. Therefore, the network administrator needs to
carefully configure different DNS options in the multiple mechanisms
for autoconfiguration in order to minimize the impact of such
problems [<a href="#ref-DHCPv6-SLAAC">DHCPv6-SLAAC</a>].
Second, if different DNS information is provided on different network
interfaces, this can lead to inconsistent behavior. The IETF worked
on solving this problem for both DNS and other information obtained
from multiple interfaces [<a href="./rfc6418" title=""Multiple Interfaces and Provisioning Domains Problem Statement"">RFC6418</a>] [<a href="./rfc6419" title=""Current Practices for Multiple-Interface Hosts"">RFC6419</a>] and standardized a
DHCP-based solution for RDNSS selection for multi-interfaced nodes as
described in [<a href="./rfc6731" title=""Improved Recursive DNS Server Selection for Multi-Interfaced Nodes"">RFC6731</a>].
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Implementation Considerations</span>
The implementation considerations in this document include the
following three: (i) DNS repository management, (ii) synchronization
between the DNS Server List and the Resolver Repository, and
(iii) synchronization between the DNSSL and the Resolver Repository.
Note: The implementations that are updated according to this document
will still interoperate with the existing implementations
according to [<a href="./rfc6106" title=""IPv6 Router Advertisement Options for DNS Configuration"">RFC6106</a>]. This is because the main change in
this document is the increase of the default Lifetime of DNS
options, considering lossy links.
<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>. DNS Repository Management</span>
For DNS repository management, the following two data structures
SHOULD be synchronized with the Resolver Repository: (i) the DNS
Server List, which keeps the list of RDNSS addresses and (ii) the
DNSSL, which 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:
o RDNSS address for DNS Server List: IPv6 address of the RDNSS that
is available for recursive DNS resolution service in the network
advertising the RDNSS option.
o DNSSL domain name for DNSSL: DNS suffix domain name that is used
to perform DNS query searches for short, unqualified domain names.
o Expiration-time for DNS Server List or DNSSL: 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 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
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field is updated to have a new Expiration-time. When the current
time becomes larger than Expiration-time, this entry is regarded
as expired, so it should not be used any more. Note that the DNS
information for the RDNSS and DNSSL options need not be dropped if
the expiry of the RA router lifetime happens. This is because
these options have their own lifetime values.
<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>. Synchronization between DNS Server List and Resolver Repository</span>
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) in 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:
o Step (a): Receive and parse the RDNSS option(s). For the RDNSS
addresses in each RDNSS option, perform Steps (b) through (d).
o 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 scenario. That is, the
RDNSS can resign from its DNS service because the machine running
the RDNSS is out of service intentionally or unintentionally.
Also, in the renumbering scenario, 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).
o Step (c): For each RDNSS address, if it already exists in the DNS
Server List and the RDNSS option's Lifetime field is not set to
zero, then just update the value of the Expiration-time field
according to the procedure specified in the third bullet of
<a href="#section-6.1">Section 6.1</a>. Otherwise, go to Step (d).
o 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 as the first one in
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 <a href="#section-5.3.1">Section 5.3.1</a>),
delete from the DNS Server List the entry with the shortest
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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 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.
<span class="h3"><a class="selflink" id="section-6.3" href="#section-6.3">6.3</a>. Synchronization between DNS Search List and Resolver Repository</span>
When an IPv6 host receives the information of multiple DNSSL domain
names within a network through an RA message with DNSSL option(s), it
stores the DNSSL domain names (in order) in both the DNSSL 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 the
same as the processing of RDNSS option(s) as described in
<a href="#section-6.2">Section 6.2</a>.
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. Security Considerations</span>
In this section, we analyze security threats related to DNS options
and then make recommendations to cope with such security threats.
<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>. Security Threats</span>
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
attacker can let IPv6 hosts resolve a hostname without a DNS suffix
into an unintended host's IP address with a fraudulent DNSSL. These
attacks are similar to ND attacks specified in [<a href="./rfc4861" title=""Neighbor Discovery for IP version 6 (IPv6)"">RFC4861</a>] that use
Redirect or Neighbor Advertisement messages to redirect traffic to
individual addresses of malicious parties.
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However, the security of these RA options for DNS configuration does
not affect ND protocol security [<a href="./rfc4861" title=""Neighbor Discovery for IP version 6 (IPv6)"">RFC4861</a>]. 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.
<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>. Recommendations</span>
The Secure Neighbor Discovery (SEND) protocol [<a href="./rfc3971" title=""SEcure Neighbor Discovery (SEND)"">RFC3971</a>] is designed
as a security mechanism for ND. In this case, ND can use SEND to
allow all the ND options, including the RDNSS and DNSSL options, to
be automatically signed with digital signatures.
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 DHCPv6 servers [<a href="./rfc7610" title=""DHCPv6-Shield: Protecting against Rogue DHCPv6 Servers"">RFC7610</a>]. 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 RA messages that include the RDNSS and
DNSSL options from unauthorized nodes [<a href="./rfc6104" title=""Rogue IPv6 Router Advertisement Problem Statement"">RFC6104</a>] [<a href="./rfc6105" title=""IPv6 Router Advertisement Guard"">RFC6105</a>].
An attacker may provide a bogus DNSSL 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 [<a href="./rfc1535" title=""A Security Problem and Proposed Correction With Widely Deployed DNS Software"">RFC1535</a>], [<a href="./rfc1536" title=""Common DNS Implementation Errors and Suggested Fixes"">RFC1536</a>], and [<a href="./rfc3646" title=""DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)"">RFC3646</a>].
<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>. IANA Considerations</span>
The RDNSS option defined in this document uses the IPv6 Neighbor
Discovery Option type assigned by IANA as follows:
Option Name Type
-----------------------------------
Recursive DNS Server Option 25
The DNSSL option defined in this document uses the IPv6 Neighbor
Discovery Option type assigned by IANA as follows:
Option Name Type
-----------------------------------
DNS Search List Option 31
These options are registered in the "IPv6 Neighbor Discovery Option
Formats" registry [<a href="#ref-ICMPv6" title=""Internet Control Message Protocol version 6 (ICMPv6) Parameters"">ICMPv6</a>].
<span class="grey">Jeong, et al. Standards Track [Page 13]</span></pre>
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<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>. References</span>
<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>. Normative References</span>
[<a id="ref-RFC2119">RFC2119</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>,
DOI 10.17487/RFC2119, March 1997,
<<a href="http://www.rfc-editor.org/info/rfc2119">http://www.rfc-editor.org/info/rfc2119</a>>.
[<a id="ref-RFC4861">RFC4861</a>] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", <a href="./rfc4861">RFC 4861</a>,
DOI 10.17487/RFC4861, September 2007,
<<a href="http://www.rfc-editor.org/info/rfc4861">http://www.rfc-editor.org/info/rfc4861</a>>.
[<a id="ref-RFC4862">RFC4862</a>] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", <a href="./rfc4862">RFC 4862</a>,
DOI 10.17487/RFC4862, September 2007,
<<a href="http://www.rfc-editor.org/info/rfc4862">http://www.rfc-editor.org/info/rfc4862</a>>.
[<a id="ref-RFC1035">RFC1035</a>] Mockapetris, P., "Domain names - implementation and
specification", STD 13, <a href="./rfc1035">RFC 1035</a>, DOI 10.17487/RFC1035,
November 1987, <<a href="http://www.rfc-editor.org/info/rfc1035">http://www.rfc-editor.org/info/rfc1035</a>>.
[<a id="ref-RFC4007">RFC4007</a>] Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and
B. Zill, "IPv6 Scoped Address Architecture", <a href="./rfc4007">RFC 4007</a>,
DOI 10.17487/RFC4007, March 2005,
<<a href="http://www.rfc-editor.org/info/rfc4007">http://www.rfc-editor.org/info/rfc4007</a>>.
<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>. Informative References</span>
[<a id="ref-RFC1034">RFC1034</a>] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, <a href="./rfc1034">RFC 1034</a>, DOI 10.17487/RFC1034, November 1987,
<<a href="http://www.rfc-editor.org/info/rfc1034">http://www.rfc-editor.org/info/rfc1034</a>>.
[<a id="ref-RFC3315">RFC3315</a>] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", <a href="./rfc3315">RFC 3315</a>, DOI 10.17487/RFC3315,
July 2003, <<a href="http://www.rfc-editor.org/info/rfc3315">http://www.rfc-editor.org/info/rfc3315</a>>.
[<a id="ref-RFC3736">RFC3736</a>] Droms, R., "Stateless Dynamic Host Configuration Protocol
(DHCP) Service for IPv6", <a href="./rfc3736">RFC 3736</a>, DOI 10.17487/RFC3736,
April 2004, <<a href="http://www.rfc-editor.org/info/rfc3736">http://www.rfc-editor.org/info/rfc3736</a>>.
[<a id="ref-RFC3646">RFC3646</a>] Droms, R., Ed., "DNS Configuration options for Dynamic
Host Configuration Protocol for IPv6 (DHCPv6)", <a href="./rfc3646">RFC 3646</a>,
DOI 10.17487/RFC3646, December 2003,
<<a href="http://www.rfc-editor.org/info/rfc3646">http://www.rfc-editor.org/info/rfc3646</a>>.
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[<a id="ref-RFC6106">RFC6106</a>] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
"IPv6 Router Advertisement Options for DNS Configuration",
<a href="./rfc6106">RFC 6106</a>, DOI 10.17487/RFC6106, November 2010,
<<a href="http://www.rfc-editor.org/info/rfc6106">http://www.rfc-editor.org/info/rfc6106</a>>.
[<a id="ref-RFC4339">RFC4339</a>] Jeong, J., Ed., "IPv6 Host Configuration of DNS Server
Information Approaches", <a href="./rfc4339">RFC 4339</a>, DOI 10.17487/RFC4339,
February 2006, <<a href="http://www.rfc-editor.org/info/rfc4339">http://www.rfc-editor.org/info/rfc4339</a>>.
[<a id="ref-RFC3971">RFC3971</a>] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
"SEcure Neighbor Discovery (SEND)", <a href="./rfc3971">RFC 3971</a>,
DOI 10.17487/RFC3971, March 2005,
<<a href="http://www.rfc-editor.org/info/rfc3971">http://www.rfc-editor.org/info/rfc3971</a>>.
[<a id="ref-RFC3118">RFC3118</a>] Droms, R., Ed., and W. Arbaugh, Ed., "Authentication for
DHCP Messages", <a href="./rfc3118">RFC 3118</a>, DOI 10.17487/RFC3118, June 2001,
<<a href="http://www.rfc-editor.org/info/rfc3118">http://www.rfc-editor.org/info/rfc3118</a>>.
[<a id="ref-RFC6104">RFC6104</a>] Chown, T. and S. Venaas, "Rogue IPv6 Router Advertisement
Problem Statement", <a href="./rfc6104">RFC 6104</a>, DOI 10.17487/RFC6104,
February 2011, <<a href="http://www.rfc-editor.org/info/rfc6104">http://www.rfc-editor.org/info/rfc6104</a>>.
[<a id="ref-RFC6105">RFC6105</a>] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
Mohacsi, "IPv6 Router Advertisement Guard", <a href="./rfc6105">RFC 6105</a>,
DOI 10.17487/RFC6105, February 2011,
<<a href="http://www.rfc-editor.org/info/rfc6105">http://www.rfc-editor.org/info/rfc6105</a>>.
[<a id="ref-RFC7610">RFC7610</a>] Gont, F., Liu, W., and G. Van de Velde, "DHCPv6-Shield:
Protecting against Rogue DHCPv6 Servers", <a href="https://www.rfc-editor.org/bcp/bcp199">BCP 199</a>,
<a href="./rfc7610">RFC 7610</a>, DOI 10.17487/RFC7610, August 2015,
<<a href="http://www.rfc-editor.org/info/rfc7610">http://www.rfc-editor.org/info/rfc7610</a>>.
[<a id="ref-RFC1535">RFC1535</a>] Gavron, E., "A Security Problem and Proposed Correction
With Widely Deployed DNS Software", <a href="./rfc1535">RFC 1535</a>,
DOI 10.17487/RFC1535, October 1993,
<<a href="http://www.rfc-editor.org/info/rfc1535">http://www.rfc-editor.org/info/rfc1535</a>>.
[<a id="ref-RFC1536">RFC1536</a>] Kumar, A., Postel, J., Neuman, C., Danzig, P., and S.
Miller, "Common DNS Implementation Errors and Suggested
Fixes", <a href="./rfc1536">RFC 1536</a>, DOI 10.17487/RFC1536, October 1993,
<<a href="http://www.rfc-editor.org/info/rfc1536">http://www.rfc-editor.org/info/rfc1536</a>>.
[<a id="ref-DHCPv6-SLAAC">DHCPv6-SLAAC</a>]
Liu, B., Jiang, S., Gong, X., Wang, W., and E. Rey,
"DHCPv6/SLAAC Interaction Problems on Address and
DNS Configuration", Work in Progress,
<a href="./draft-ietf-v6ops-dhcpv6-slaac-problem-07">draft-ietf-v6ops-dhcpv6-slaac-problem-07</a>, August 2016.
<span class="grey">Jeong, et al. Standards Track [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc8106">RFC 8106</a> IPv6 DNS RA Options March 2017</span>
[<a id="ref-RFC6418">RFC6418</a>] Blanchet, M. and P. Seite, "Multiple Interfaces and
Provisioning Domains Problem Statement", <a href="./rfc6418">RFC 6418</a>,
DOI 10.17487/RFC6418, November 2011,
<<a href="http://www.rfc-editor.org/info/rfc6418">http://www.rfc-editor.org/info/rfc6418</a>>.
[<a id="ref-RFC6419">RFC6419</a>] Wasserman, M. and P. Seite, "Current Practices for
Multiple-Interface Hosts", <a href="./rfc6419">RFC 6419</a>, DOI 10.17487/RFC6419,
November 2011, <<a href="http://www.rfc-editor.org/info/rfc6419">http://www.rfc-editor.org/info/rfc6419</a>>.
[<a id="ref-RFC6731">RFC6731</a>] Savolainen, T., Kato, J., and T. Lemon, "Improved
Recursive DNS Server Selection for Multi-Interfaced
Nodes", <a href="./rfc6731">RFC 6731</a>, DOI 10.17487/RFC6731, December 2012,
<<a href="http://www.rfc-editor.org/info/rfc6731">http://www.rfc-editor.org/info/rfc6731</a>>.
[<a id="ref-ICMPv6">ICMPv6</a>] IANA, "Internet Control Message Protocol version 6
(ICMPv6) Parameters",
<<a href="http://www.iana.org/assignments/icmpv6-parameters/">http://www.iana.org/assignments/icmpv6-parameters/</a>>.
<span class="grey">Jeong, et al. Standards Track [Page 16]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc8106">RFC 8106</a> IPv6 DNS RA Options March 2017</span>
<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>. Changes from <a href="./rfc6106">RFC 6106</a></span>
The following changes were made from <a href="./rfc6106">RFC 6106</a> ("IPv6 Router
Advertisement Options for DNS Configuration"):
o This document allows a higher default value of the lifetime of the
DNS RA options than <a href="./rfc6106">RFC 6106</a> in order to avoid the frequent expiry
of the options on links with a relatively high rate of packet
loss; at the same time, this document also makes additional
clarifications. The lifetime's lower bound of
2 * MaxRtrAdvInterval was shown to lead to the expiry of these
options on links with a relatively high rate of packet loss. To
avoid this problem, this revision relaxes the lower bound and sets
a higher default value of 3 * MaxRtrAdvInterval.
o The text regarding the generation of a Router Solicitation message
to ensure that the RDNSS information is fresh before the expiry of
the RDNSS option is removed in order to prevent multicast traffic
on the link from increasing.
o The addresses for RDNSSes in the RDNSS option can be not only
global addresses but also link-local addresses. The link-local
addresses for RDNSSes should be registered in the Resolver
Repository along with the corresponding link zone indices.
o <a href="./rfc6106">RFC 6106</a> recommended that the number of RDNSS addresses that
should be learned and maintained through the RDNSS RA option
should be limited to three. This document removes that
recommendation; thus, the number of RDNSS addresses to maintain is
determined by an implementer's local policy.
o <a href="./rfc6106">RFC 6106</a> recommended that the number of DNS search domains that
should be learned and maintained through the DNSSL RA option
should be limited to three. This document removes that
recommendation; thus, when the set of unique DNSSL values are not
equivalent, none of them may be ignored for hostname lookups
according to an implementer's local policy.
o The guidance of the specific implementation for the
synchronization of the DNS Repository and Resolver Repository in
the kernel space and user space is removed.
o The key words "SHOULD" and "RECOMMENDED" (<a href="./rfc2119">RFC 2119</a>) are removed in
the recommendation of using SEND as a security mechanism for ND.
Instead of using these key words, SEND is specified as only a
possible security mechanism for ND.
<span class="grey">Jeong, et al. Standards Track [Page 17]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-18" ></span>
<span class="grey"><a href="./rfc8106">RFC 8106</a> IPv6 DNS RA Options March 2017</span>
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, Tony
Cheneau, Fernando Gont, Jen Linkova, Ole Troan, Mark Smith, Tatuya
Jinmei, Lorenzo Colitti, Tore Anderson, David Farmer, Bing Liu, and
Tassos Chatzithomaoglou. The authors sincerely appreciate their
contributions.
This document was supported by an Institute for Information &
communications Technology Promotion (IITP) grant funded by the Korean
government (MSIP) [10041244, Smart TV 2.0 Software Platform].
<span class="grey">Jeong, et al. Standards Track [Page 18]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc8106">RFC 8106</a> IPv6 DNS RA Options March 2017</span>
Authors' Addresses
Jaehoon Paul Jeong
Department of Software
Sungkyunkwan University
2066 Seobu-Ro, Jangan-Gu
Suwon, Gyeonggi-Do 16419
Republic of Korea
Phone: +82 31 299 4957
Fax: +82 31 290 7996
Email: pauljeong@skku.edu
URI: <a href="http://iotlab.skku.edu/people-jaehoon-jeong.php">http://iotlab.skku.edu/people-jaehoon-jeong.php</a>
Soohong Daniel Park
Software R&D Center
Samsung Electronics
Seoul R&D Campus D-Tower, 56, Seongchon-Gil, Seocho-Gu
Seoul 06765
Republic of Korea
Email: soohong.park@samsung.com
Luc Beloeil
Orange
5 rue Maurice Sibille
BP 44211
44042 Nantes Cedex 1
France
Phone: +33 2 28 56 11 84
Email: luc.beloeil@orange.com
Syam Madanapalli
NTT Data
#H304, Shriram Samruddhi, Thubarahalli
Bangalore 560066
India
Phone: +91 959 175 7926
Email: smadanapalli@gmail.com
Jeong, et al. Standards Track [Page 19]
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