Network Working Group J. Jeong, Ed.
Internet-Draft ETRI/University of Minnesota
Expires: July 20, 2006 S. Park
SAMSUNG Electronics
L. Beloeil
France Telecom R&D
S. Madanapalli
SAMSUNG ISO
January 16, 2006
IPv6 Router Advertisement Option for DNS Configuration
draft-jeong-dnsop-ipv6-dns-discovery-07.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document specifies a new IPv6 Router Advertisement option to
allow IPv6 routers to advertise DNS recursive server addresses to
IPv6 hosts.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Applicability Statements . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Neighbor Discovery Extension . . . . . . . . . . . . . . . . . 4
5.1 Recursive DNS Server Option . . . . . . . . . . . . . . . 5
5.2 Procedure of DNS Configuration . . . . . . . . . . . . . . 7
5.2.1 Procedure in IPv6 Router . . . . . . . . . . . . . . . 7
5.2.2 Procedure in IPv6 Host . . . . . . . . . . . . . . . . 7
6. Implementation Considerations . . . . . . . . . . . . . . . . 7
6.1 DNS Server List Management . . . . . . . . . . . . . . . . 8
6.2 Synchronization between DNS Server List and Resolver
Repository . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
10.1 Normative References . . . . . . . . . . . . . . . . . . . 11
10.2 Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . 14
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1. Introduction
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 routes and some other
parameters [4][5]. To support the access to additional services in
the Internet that are identified by a DNS name, such as a web server,
the configuration of at least one recursive DNS server is also needed
for DNS name resolution.
It is infeasible for nomadic hosts, such as laptops, to have to enter
a DNS resolver each time they connect to a different wireless LAN
(WLAN) such as IEEE 802.11 a/b/g [10]-[13]. This document provides a
new way which uses a new IPv6 Router Advertisement option to allow
IPv6 routers to advertise DNS recursive server addresses to IPv6
hosts.
1.1 Applicability Statements
RA-based DNS configuration is useful in the networks where an IPv6
address is autoconfigured through IPv6 stateless address
autoconfiguration, such as SOHO, home networks, cellular networks
(e.g., 3GPP), MIPv6 (especially, HMIPv6), NEMO and MANET connected to
the Internet. Especially, the new RA option may be useful in some
mobile environments where the addresses of the RDNSSes are added or
deleted according to a mobile node's movement because the RA option
includes a lifetime field that allows the mobile node to delete the
expired entries for RDNSSes. This lifetime field can be configured
to a value that will require the mobile node to time out the RDNSS
address entry in the previous network and to switch over to another
RDNSS address in the same network. Therefore, the lifetime field can
allow the mobile node to use the RDNSSes announced in the network
where it is placed. As a result, the local RDNSS may provide the
mobile node with quicker recursive DNS resolution service than the
remote RDNSSes. Using the lifetime field differentiate RA approach
from DHCPv6 approach in that it allows mobile nodes to use local
RDNSSes rather than remote RDNSSes in order to being able to reduce
the DNS resolution delay [6]-[8].
2. Definitions
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 [3].
3. Terminology
This document uses the terminology described in [4][5]. In addition,
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three new terms are defined below:
o Recursive DNS Server (RDNSS): Server which provides a recursive
DNS resolution service.
o DNS Server List: Data structure for managing DNS Server
Information existing in IPv6 protocol stack in addition to
Neighbor Cache and Destination Cache for Neighbor Discovery [4].
o Resolver Repository: Configuration repository with RDNSS addresses
which a DNS resolver on the host uses for DNS name resolution,
such as Unix resolver file (i.e., /etc/resolv.conf) and Windows
registry.
4. Overview
This document defines a new ND option called RDNSS option that
contains the addresses of recursive DNS servers. Existing ND
transport mechanisms (i.e., advertisements and solicitations) are
used. This works in the same way that hosts learn about routers and
prefixes. An IPv6 host can configure the IPv6 addresses of one or
more RDNSSes via RA message periodically sent by router or solicited
by a Router Solicitation (RS).
Through the RDNSS option along with the prefix information option
based on the ND protocol [4][5], an IPv6 host can perform its network
configuration of its IPv6 address and RDNSS simultaneously without
needing a separate message exchange for the RDNSS information. The
RA option for RDNSS can be used on any network that supports the use
of ND.
This approach requires RDNSS information to be configured in the
routers sending the advertisements. The configuration of RDNSS
addresses in the routers can be done by manual configuration. The
automatic configuration or redistribution of RDNSS information is
possible by running a DHCPv6 client running on the router [6]-[8].
The automatic configuration of RDNSS addresses in the routers is out
of scope in this document.
The preference field of RDNSS option allows IPv6 hosts to select a
primary RDNSS among several RDNSSes; this can be used for load
balancing of RDNSSes.
5. Neighbor Discovery Extension
The IPv6 DNS configuration mechanism in this document needs a new ND
option in Neighbor Discovery, Recursive DNS Server (RDNSS) option.
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5.1 Recursive DNS Server Option
RDNSS option contains one or more IPv6 addresses of recursive DNS
servers. All of the addresses share the same preference and lifetime
values. If it is desirable to have different preference and lifetime
values, multiple RDNSS options can be used.
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 | Pref |S| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: Addresses of IPv6 Recursive DNS Servers :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Recursive DNS Server (RDNSS) Option Format
Figure 1 shows the format of RDNSS option.
Fields:
Type 8-bit identifier of the option type (TBD: IANA)
Option Name Type
RDNSS option (TBD)
Length 8-bit unsigned integer. The length of the
option (including the type and length fields)
in units of 8 octets. The minimum value is 0x03
if one IPv6 address is contained in the option.
Every additional RDNSS address increases the
length by 0x02. The length field is used by
the receiver to determine the number of IPv6
addresses in the option.
Pref The preference of an RDNSS. A 4-bit unsigned
integer. A decimal value of 15 indicates the
highest preference. A value of zero means
unspecified. The default value of preference
may be from 8 to 11 for manually configured or
RA-derived RDNSSes. A preference less than 8
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means less preferred than manual configured
RDNSS and a preference greater than 11 means
more preferred.
S 1-bit "Service open" flag. When set, it indicates
that RDNSS(es) in the option can be available for
IPv6 hosts when they are detached from the origin
subnet where they obtained the RDNSSes. This flag
SHOULD be set only if the routers or firewalls in
the network allow DNS Query messages to be routed
to the destination RDNSS without being filtered
out, and the RDNSS is configured to perform
recursive queries for all hosts regardless of
their addresses.
Lifetime 32-bit unsigned integer. The maximum time, in
seconds (relative to the time the packet is sent),
over which this RDNSS is 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 the
stable DNS service and allow mobile hosts to
prefer local RDNSSes to remote RDNSSes, the value
of lifetime should be at least as long as the
Maximum RA Interval (MaxRtrAdvInterval) in
RFC 2461, and be at most as long as two times
MaxRtrAdvInterval; Lifetime SHOULD be bounded as
follows:
MaxRtrAdvInterval<=Lifetime<=2*MaxRtrAdvInterval.
A value of all one bits (0xffffffff) represents
infinity. A value of zero means that the RDNSS
MUST no longer be used.
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. For the maximum number of
RDNSS addresses in one RDNSS option, at most
three is recommended since three RDNSSes are
enough for DNS resolution service. See Section
5.2 for how parts of RDNSSes in one RDNSS option
can be selected by a host.
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5.2 Procedure of DNS Configuration
The procedure of DNS configuration through RDNSS option is the same
as any other ND option [4].
5.2.1 Procedure in IPv6 Router
An IPv6 router SHOULD include RDNSS option(s) in every solicited RA.
When the router participates in routing and forwarding data packets,
RDNSS option SHOULD be sent along with Prefix Information option
included in unsolicited RAs. Otherwise, the RDNSS option MUST NOT be
advertised.
Since one RDNSS option is recommended to have at most three RDNSSes,
the router MAY send more than one RDNSS option if it would like to
advertise more than three RDNSSes.
5.2.2 Procedure in IPv6 Host
When an IPv6 host receives RDNSS option through RA, it checks whether
the option is valid;
o If the RDNSS option is present, the host SHOULD copy the option's
value into the DNS Server List and the Resolver Repository as long
as the value of Length field is greater than or equal to the
minimum value (0x03). The host MAY ignore additional RDNSS
addresses within an RDNSS option and/or additional RDNSS options
within an RA when it has gathered a sufficient number of RDNSSes.
It is expected that most implementations will use at least two or
three RDNSSes, but few will use a fourth or subsequent RDNSS.
o If the RDNSS option is present but invalid (e.g., it has the
length less than 0x03), the host SHOULD discard the option.
6. Implementation Considerations
Note
This non-normative section gives some hints for implementing the
processing of RDNSS option in IPv6 host.
For the configuration and management of RDNSS information, the
advertised RDNSS addresses can be stored and managed in both the DNS
Server List and the Resolver Repository.
In environments where the RDNSS information is stored in user space
and ND runs in the kernel, it is necessary to synchronize the DNS
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Server List for RDNSSes in kernel space and the Resolver Repository
in user space. For the synchronization, the implementation where ND
works in the kernel should provide the write operation for updating
RDNSS information from the kernel to the Resolver Repository. One
simple approach to perform this is to have a daemon around (or a
program that is called at the defined intervals) that keeps
monitoring the lifetime of RDNSSes all the time. Whenever there is
an expired entry in the DNS Server List, the daemon can delete the
corresponding entry from the Resolver Repository.
6.1 DNS Server List Management
The kernel or user-space process (depending on where RAs are
processed) should maintain a data structure called DNS Server List
which keeps the list of RDNSSes. Each entry of DNS Server List
consists of RDNSS address, Preference, Expiration-time, and Service-
open-flag as follows:
o RDNSS address: IPv6 address of Recursive DNS Server which is
available for recursive DNS resolution service in the network
advertising the RDNSS option; such a network is called site in
this document.
o Preference: Preference field in RDNSS option allowing IPv6 hosts
to select a primary RDNSS among several RDNSSes; the other RDNSSes
except for the primary one can be used as backup.
o Expiration-time: Expiration time field giving the time when this
entry becomes invalid. Expiration-time is set to the value of
Lifetime field of RDNSS option plus the current system time.
Whenever a new RDNSS option with the same address is received,
this field is updated to have a new expiration time. When
Expiration-time becomes less than the current system time, this
entry is regared as expired. The decision about whether to delete
the expired entry depends on its Service-open-flag (See the
explanation for Service-open-flag).
o Service-open-flag (S flag): Flag for deciding whether to delete
the expired entry. It is set to the value of "Service open" flag
of RDNSS option. When the entry has expired and Service-open-flag
is 0, the expired entry is deleted from the DNS Server List.
Otherwise, the entry is maintained. That is, Service-open-flag
set to 1 allows expired entry to be maintained. It may be useful
when an IPv6 host is nomadic or mobile node. Service-open-flag
allows an IPv6 host to continue to use expired RDNSSes located in
other networks which it moved from. When there is no available
RDNSS in the new network (or subnet), the IPv6 host can still use
the remote RDNSSes which it used for DNS name resolution before.
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A host MAY delete expired entries in order to limit the storage
needed for the DNS Server List. Any least recently used (LRU)
policy that reclaims entries that have expired with Service-open-
flag set to 0 can be adopted for replacing the expired entries
with the entries for newly announced RDNSSes [4]. For example,
when the replacement is necessary, the IPv6 host can choose one
whose Service-open-flag is turned off and whose Expiration-time is
the least.
Note
When the S flag is not set, an RDNSS may only be used as long as both
the RA router lifetime and the RDNSS option lifetime have not
expired. When the S flag is set, the RDNSS may continue to be used
after either or both of these lifetimes have expired and there are
not any more eligible RDNSSes (with still valid lifetimes or learned
through DHCP or manual configuration) are available [6]-[8]. RDNSSes
with expired lifetimes should be used as a last resort only, as they
may not be currently reachable or the RDNSS may not provide service
to the host's current address.
6.2 Synchronization between DNS Server List and Resolver Repository
When an IPv6 host receives the information of multiple RDNSSes within
a site 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 option included in
RA message is as follows:
Step (a): Receive and parse RDNSS option(s). Process only the
first three RDNSS addresses in each RDNSS option if one RDNSS
option has more than three RDNSS addresses.
Step (b): Arrange the addresses of RDNSSes in a descending order,
starting with the biggest value of "Pref" field of the RDNSS
option and store them in both the DNS Server List and the Resolver
Repository by sorting the entries, including newly added entries,
in the descending order of preference value. In the case where
there are several routers advertising RDNSS option(s) in a subnet,
"Pref" field is used to arrange the information. When the
preference is the same, the RDNSS announced earlier is more
preferred. Also, when one RDNSS option has multiple RDNSSes with
the same preference, keep the order or the addresses in the
option, so the first is preferred.
Step (c): For each RDNSS option, check the following: If each
value of "Lifetime" field is set to zero, regardless of the value
of 'S' flag, delete the corresponding RDNSS entries from both the
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DNS Server List and the Resolver Repository in order to let the
RDNSSes be not used any more for certain reasons in network
management, e.g., the breakdown of the RDNSS and a renumbering
situation.
Step (d): Delete each expired entry whose S flag is set off from
DNS Server List and the RDNSS address corresponding to the entry
from the Resolver Repository. However, in mobile environment, in
order that a mobile node can still use the RDNSS of the previous
site when the host moves into another site and no RDNSS is
available there, it MAY be allowed to maintain the entry whose S
flag is on in both the DNS Server List and the Resolver
Repository. The expired RDNSS entry is regarded as having lower
preference than the valid entries regardless of preference values.
The sorting for expired RDNSS entries with the S flag set on is
performed in both the DNS Server List and the Resolver Repository
based on preference values. In the case where the data structure
for the DNS Server List is full of RDNSS entries, the expired
entries MAY be deleted according to the LRU-based policy specified
in Section 6.1.
7. Security Considerations
The security of RA option for RDNSS is the same as ND protocol
security [4]. The RA option does not add any new vulnerability.
It should be noted that 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. A malicious node on a LAN can promiscuously
receive packets for any router's MAC address and send packets with
the router's MAC address as the source MAC address in the L2 header.
As a result, the 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 NA replies, answer RS or NS requests, etc.
Also, an attacker could configure a host to send out RA with a
fraudulent RDNSS address, which is presumably and easier avenue of
attack than becoming a rogue router and having to process all traffic
for the subnet. It is necessary to disable the RA RDNSS option
administatively to avoid this problem. All of this can be done
independently of implementing ND. Therefore, the RA option for RDNSS
does not add to the vulnerability.
If Secure Neighbor Discovery (SEND) protocol is used as the security
mechanism for ND, all the ND options including RDNSS option are also
automatically included in the signatures [9], so the RDNSS transport
is integrity-protected. However, since any valid SEND node can still
insert RDNSS options, SEND cannot verify who is or is not authorized
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to send the options.
8. IANA Considerations
Note
This section will be removed after the assignment of RDNSS option
type.
The IANA should assign a new IPv6 Neighbor Discovery Option type for
the RDNSS option defined in this document. The IANA registry for
these options is:
http://www.iana.org/assignments/icmpv6-parameters
9. Acknowledgements
This draft has greatly benefited from inputs by Robert Hinden, Pekka
Savola, Iljitsch van Beijnum, Brian Haberman and Tim Chown. The
authors appreciate their contribution.
10. References
10.1 Normative References
[1] Bradner, S., "IETF Rights in Contributions", RFC 3978,
March 2005.
[2] Bradner, S., "Intellectual Property Rights in IETF Technology",
RFC 3668, February 2004.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
[5] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
10.2 Informative References
[6] Droms, R., Ed., "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[7] Droms, R., "Stateless Dynamic Host Configuration Protocol
(DHCP) Service for IPv6", RFC 3736, April 2004.
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[8] Droms, R., Ed., "DNS Configuration options for Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
December 2003.
[9] Arkko, J., Ed., "SEcure Neighbor Discovery (SEND)", RFC 3971,
March 2005.
[10] ANSI/IEEE Std 802.11, "Part 11: Wireless LAN Medium Access
Control (MAC) and Physical Layer (PHY) Specifications",
March 1999.
[11] IEEE Std 802.11a, "Part 11: Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) specifications: High-speed
Physical Layer in the 5 GHZ Band", September 1999.
[12] IEEE Std 802.11b, "Part 11: Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) specifications: Higher-Speed
Physical Layer Extension in the 2.4 GHz Band", September 1999.
[13] IEEE P802.11g/D8.2, "Part 11: Wireless LAN Medium Access
Control (MAC) and Physical Layer (PHY) specifications: Further
Higher Data Rate Extension in the 2.4 GHz Band", April 2003.
Authors' Addresses
Jaehoon Paul Jeong (editor)
ETRI/Department of Computer Science and Engineering
University of Minnesota
117 Pleasant Street SE
Minneapolis, MN 55455
US
Phone: +1 651 587 7774
Fax: +1 612 625 2002
Email: jjeong@cs.umn.edu
URI: http://www.cs.umn.edu/~jjeong/
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Soohong Daniel Park
Mobile Platform Laboratory
SAMSUNG Electronics
416 Maetan-3dong, Yeongtong-Gu
Suwon, Gyeonggi-Do 443-742
Korea
Phone: +82 31 200 4508
Email: soohong.park@samsung.com
Luc Beloeil
France Telecom R&D
42, rue des coutures
BP 6243
14066 CAEN Cedex 4
France
Phone: +33 02 3175 9391
Email: luc.beloeil@francetelecom.com
Syam Madanapalli
AMSUNG India Software Operations
J. P. Techno Park, 3/1
Millers Road
Bangalore 560052
India
Phone: +91 80 51197777
Email: syam@samsung.com
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Acknowledgment
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