Internet Draft J. Jeong
ETRI/University of Minnesota
S. Park
SAMSUNG Electronics
L. Beloeil
France Telecom R&D
S. Madanapalli
SAMSUNG ISO
Expires: August 2005 15 February 2005
IPv6 DNS Configuration based on Router Advertisement
draft-jeong-dnsop-ipv6-dns-discovery-04.txt
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Abstract
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This document specifies the steps which an IPv6 host takes in
deciding how to configure the addresses of recursive DNS servers for
DNS name resolution. The way of discovering recursive DNS servers is
based on Router Advertisement message.
Conventions used in this document
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].
Table of Contents
1. Introduction....................................................2
2. Terminology.....................................................3
3. Overview........................................................3
4. Neighbor Discovery Extension....................................4
4.1. Recursive DNS Server Option................................4
5. Procedure of DNS Configuration..................................5
6. Configuration of DNS Information................................6
6.1. DNS Server Cache Management................................6
6.2. Synchronization between DNS Server Cache and Resolver File.7
6.3. DNS Resolution.............................................8
6.4. Implementation Considerations..............................8
7. Applicability Statements........................................8
8. Open Issues.....................................................9
9. Security Considerations.........................................9
10. Acknowledgements...............................................9
11. Normative References..........................................10
12. Informative References........................................10
13. Authors' Addresses............................................10
14. Intellectual Property Statement...............................11
Full Copyright Statement..........................................12
Acknowledgement...................................................12
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.
Because IPv6 address is too long to remember, some ways to
configure IPv6 recursive DNS servers are required for the sake of
user's convenience [9].
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This document defines the process of DNS configuration based on IPv6
Router Advertisement (RA) to find out the addresses of recursive DNS
servers within the local network.
2. Terminology
This document uses the terminology described in [4][5]. In addition,
three new terms are defined below:
Recursive DNS Server (RDNSS)
A Recursive DNS Server is a name server that offers the recursive
service of DNS name resolution.
DNS Server Cache
DNS Server Cache is a data structure for managing DNS Server
Information existing in IPv6 protocol stack in addition to
Neighbor Cache and Destination Cache for Neighbor Discovery [4].
Resolver File
Resolver File is a configuration file which DNS resolver on the
host uses for DNS name resolution, e.g., /etc/resolv.conf in UNIX.
3. Overview
RA approach for RDNSS is to define a new ND option called RDNSS
option that contains a recursive DNS server address. Existing ND
transport mechanisms (i.e., advertisements and solicitations) are
used. This works in the same way that nodes 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).
This approach needs RDNSS information to be configured in the
routers doing the advertisements. The configuration of RDNSS
address can be performed manually by operator or other ways, such
as automatic configuration through DHCPv6 client running on the
router [6]-[8]. When advertising more than one RDNSS option, an
RA message includes as many RDNSS options as RDNSSes.
Through ND protocol and RDNSS option along with prefix information
option, an IPv6 host can perform its network configuration of its
IPv6 address and RDNSS simultaneously [4][5]. The RA option for
RDNSS can be used on any network that supports the use of ND.
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However, it is worth noting that some link layers (e.g., WLAN)
need to acknowledge multicast packets, which may increase the
amount of link-layer traffic [9].
The RA approach is useful in some mobile environments where the
addresses of the RDNSSes are changing because the RA option
includes a lifetime field that allows client to use RDNSSes nearer
to the client. This can be configured to a value that will
require the client to time out the entry and switch over to
another RDNSS address (see Section 6.1 and 6.2).
The preference value of RDNSS, included in RDNSS option, allows
IPv6 hosts to select a primary RDNSS among several RDNSSes; this
can be used for load balancing of RDNSSes (see Section 6.1 and
6.2).
4. Neighbor Discovery Extension
The IPv6 DNS configuration mechanism in this document needs a new ND
option in Neighbor Discovery, Recursive DNS Server (RDNSS) option.
4.1. Recursive DNS Server Option
RDNSS option contains the IPv6 address of the recursive DNS server.
When advertising more than one RDNSS option, an RA message includes
as many RDNSS options as DNS servers. Figure 1 shows the format of
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 | Pref | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: IPv6 Address of RDNSS :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1. Recursive DNS Server (RDNSS) Option Format
Fields:
Type 8-bit identifier of the option type (TBD: IANA)
Option Name Type
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RDNSS option (TBD)
Length 8-bit unsigned integer. The length of the
option (including the type and length fields)
in units of 8 octets SHOULD be 0x03 (3 x 8 = 24
octets).
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 field can be used for load
balancing of DNS queries with multiple RDNSSes
according to local policy.
Lifetime 32-bit unsigned integer. The maximum time, in
seconds, over which this RDNSS is used for name
resolution. Hosts should contact the source
of this information, router, before this time
interval expires. A value of all one bits
(0xffffffff) represents infinity. A value of
zero means that the RDNSS must no longer be used.
IPv6 Address of RDNSS
128-bit IPv6 address of the recursive DNS server.
5. Procedure of DNS Configuration
The processing of RDNSS option is handled like any other ND option
and would happen when an RA is received. No new procedure is needed.
Figure 2 shows the procedure of DNS configuration for IPv6 host on
the basis of IPv6 RA message.
IPv6 Host Router
| |
(a)|(-------------------------RS------------------------->)|
(b)|<-----------------RA w/ RDNSS option(s)----------------|
(c)| Processing of RA |
(d)| Stateless Address Autoconfiguration |
(e)| DNS Configuration based on RA option |
(f)| (DNS Configuration based on DHCP option) |
Figure 2. Procedure of IPv6 Host DNS Configuration
The procedure consists of the following steps:
Step (a) : IPv6 Host sends RS (Router Solicitation) message to get an
RA message. It is optional.
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Step (b) : For the RS message sent by IPv6 Host, Router sends an RA
message, which contains Prefix Information option for
stateless address autoconfiguration and RDNSS options
for DNS servers.
Step (c) : If there are not any Prefix Information option and RDNSS
in RA message, IPv6 Host goes to Step (f).
Step (d) : If there is Prefix Information option in RA message, IPv6
Host performs stateless address autoconfiguration on the
basis of the prefix included in the option. If the
stateless autoconfiguration fails, IPv6 Host goes to Step
(f).
Step (e) : If there is an RDNSS option in RA message, IPv6 Host
stores the RDNSS address in both its DNS Server Cache and
resolver configuration file.
Step (f) : If M (Managed address configuration) flag is set on, IPv6
Host MUST perform stateful address autoconfiguration
through DHCPv6 [4]-[6]. If no RDNSS option is included in
the RA message, IPv6 Host MAY perform DNS configuration
through DHCPv6 [6]-[8] regardless of whether the O flag is
set or not.
6. Configuration of DNS Information
The RDNSS addresses are announced by RDNSS options in RA message.
The newly discovered RDNSS addresses are stored and managed in both
DNS Server Cache and Resolver File.
6.1. DNS Server Cache Management
The DNS configuration in this document needs a new DNS Server Cache
in IPv6 protocol stack in addition to Neighbor Cache and Destination
Cache for Neighbor Discovery [4]. Each entry of DNS Server Cache
consists of RDNSS address, Preference, Expire-time, and Onsite-flag
as follows:
- RDNSS address:
RDNSS address indicates the recursive DNS server in the site.
- Preference:
Preference, delivered in RDNSS option, is used for an IPv6 host to
decide which RDNSS it will use as a primary RDNSS among the
announced RDNSSes. This field can be used for load balancing of
DNS queries with multiple RDNSSes within an autonomous site.
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- Expire-time:
Lifetime, delivered in RDNSS option, is used to give the time when
this entry becomes invalid. Expire-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.
- Onsite-flag:
Onsite-flag is set on while Expire-time is less than the current
system time; in other words, while this entry is valid. When
Expire-time becomes greater than the current system time, this
flag is set off. When Expire-time becomes less than the current
system time again through the receipt of another RDNSS option, the
flag is set on. The entry of which Onsite-flag is off is not
deleted immediately, but used for DNS resolution in the site where
the IPv6 host is a mobile node and no RDNSS is provided.
Therefore, the IPv6 host MAY use the RDNSS of the previous site
even though the RDNSS is in the remote network.
To limit the storage needed for the DNS Server Cache, a node MAY
need to garbage-collect old entries. However, care must be taken
to insure that sufficient space is always present to hold the
working set of active entries. Any LRU-based policy that only
reclaims entries that have expired should be adequate for garbage-
collecting unused entries [4]. For example, when the replacement
is necessary, the IPv6 host can choose one of which Onsite-flag is
off and of which Expire-time is the least.
6.2. Synchronization between DNS Server Cache and Resolver File
When an IPv6 host receives the information of multiple RDNSSes within
a site through an RA message with RDNSS options, it stores the RDNSS
addresses in order into both DNS Server Cache and Resolver File. The
processing of the RDNSS option included in RA message is as follows:
Step (a) : Receive and parse RDNSS options.
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 DNS Server Cache
and Resolver File. In the case where there are several
routers advertising RDNSS option(s) in a subnet, "Pref"
field is used to arrange the information.
Step (c) : For each RDNSS option, check the following: If each value
of "Pref" and "Lifetime" fields is set to zero, delete the
corresponding RDNSS entry from both DNS Server Cache and
Resolver File in order to let the RDNSS be not used any
more for certain reason in network management, e.g., the
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breakdown of the RDNSS and a renumbering situation.
Step (d) : Delete each entry of which Onsite-flag is set off from DNS
Server Cache and the RDNSS address corresponding to the
entry from Resolver File. However, in mobile environment,
in order that a mobile node can still use the RDNSS of the
previous site when the node moves into another site and no
RDNSS is available there, it MAY be allowed to maintain
the entry of which Onsite-flag is off in both DNS Server
Cache and Resolver File. In this case, invalid entries
can be deleted according to LRU-based policy.
As a last resort of DNS name resolution, an IPv6 host can use the
RDNSSes manually configured by its user in its Resolver File either
when it cannot get the information of RDNSSes from local network or
when there is no valid RDNSS address in DNS Server Cache.
6.3. DNS Resolution
Whenever DNS resolver has to resolve a DNS name which is not cached
in its local DNS cache, it can use one of the RDNSSes listed in
Resolver File, which is synchronized with DNS Server Cache. It
refers to RDNSS in order from the first RDNSS among the RDNSSes in
Resolver File.
6.4. Implementation Considerations
The current ND framework should be modified due to the
synchronization between DNS Server Cache for RDNSSes in kernel
space and Resolver File in user space. Because it is unacceptable
to write and rewrite the Resolver file (e.g., resolv.conf) from
the kernel, another approach is needed. One simple approach to
solve 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. However, such a daemon is not necessary
with the current ND framework.
7. Applicability Statements
RA-based DNS configuration is useful in the networks where IPv6
address is autoconfigured through IPv6 stateless address
autoconfiguration, such as SOHO, home networks, celluar networks
(e.g., 3GPP), MIPv6 (especially, HMIPv6), NEMO and MANET connected to
the Internet. Especially, the RA approach is useful in some mobile
environments where the addresses of the RDNSSes are changing because
the RA option includes a lifetime field that allows client to use
RDNSSes nearer to the client. This can be configured to a value that
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will require the client to time out the entry and switch over to
another RDNSS in current local network.
8. Open Issues
There might be some issues regarding RA-based DNS configuration as
follows:
o Can the RA approach be used safely in WLAN networks? Because
multicast is unreliable in WLAN networks, some RA messages advertised
by a router can be lost [9].
o How can we optimize bandwidth on the link, especially the link of
cellular networks, such as 3GPP network?
o Is the use of "Pref" or "Lifetime" fields useful?
o Is it useful to advertise the well-known anycast addresses for
RDNSSes through RA message? The well-known anycast addresses can be
set in RA configuration and be announced to IPv6 hosts through RDNSS
options in order to reduce the configuration effort of users [10].
o How to implement RA-based DNS configuration?
9. 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.
In the 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. All of this can be done independently of implementing ND.
Therefore, the RA option for RDNSS does not add to the vulnerability.
Security issues regarding the ND protocol have been discussed at IETF
SEND (Securing Neighbor Discovery) Working Group [11].
10. Acknowledgements
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This draft has greatly benefited from inputs by Robert Hinden,
Pekka Savola, and Tim Chown. The authors appreciate their
contribution.
11. Normative References
[1] S. Bradner, "Intellectual Property Rights in IETF Technology",
RFC 3668, February 2004.
[2] S. Bradner, "IETF Rights in Contributions", RFC 3667,
February 2004.
[3] S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] T. Narten, E. Nordmark and W. Simpson, "Neighbor Discovery for
IP version 6 (IPv6)", RFC 2461, December 1998.
[5] S. Thomson and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
12. Informative References
[6] R. Droms et al., "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[7] R. Droms, "Stateless Dynamic Host Configuration Protocol (DHCP)
Service for IPv6", RFC 3736, April 2004.
[8] R. Droms et al., "DNS Configuration options for Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)", RFC 3646, December
2003.
[9] J. Jeong et al., "IPv6 Host Configuration of DNS Server
Information Approaches", draft-ietf-dnsop-ipv6-dns-configuration
-04.txt, September 2004, Work in Progress.
[10] M. Ohta, "Preconfigured DNS Server Addresses", draft-ohta-
preconfigured-dns-01.txt, February 2004, Work in Progress.
[11] J. Arkko et al., "SEcure Neighbor Discovery (SEND)", draft-
ietf-send-ndopt-06.txt, July 2004, Work in Progress.
13. Authors' Addresses
Jaehoon Paul Jeong
ETRI/University of Minnesota at Twin Cities
117 Pleasant Street SE
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Minneapolis, MN 55455
USA
Phone: +1 651 587 7774
EMail: jjeong@cs.umn.edu
Soohong Daniel Park
Mobile Platform Laboratory,
SAMSUNG Electronics
Korea
Phone: +82 31 200 3728
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
Network Systems Division,
SAMSUNG India Software Operations
India
Phone: +91 80 555 0555
EMail: syam@samsung.com
14. Intellectual Property Statement
The following intellectual property notice is copied from RFC3668,
Section 5.
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Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology
described in this document or the extent to which any license
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Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
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attempt made to obtain a general license or permission for the use
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The IETF invites any interested party to bring to its attention
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Full Copyright Statement
The following copyright notice is copied from RFC3667, Section 5.4.
It describes the applicable copyright for this document.
Copyright (C) The Internet Society (2005). This document is
subject to the rights, licenses and restrictions contained in BCP
78, and except as set forth therein, the authors retain all their
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This document and the information contained herein are provided on
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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