Network Working Group A.K. Vijayabhaskar
Internet-Draft Hewlett-Packard
Expires: April 17, 2004 16 Oct 2003
The Name Service Search Option for DHCPv6
draft-ietf-dhc-dhcpv6-opt-nss-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document describes a new option called Name Service Search Option
to specify the order in which name services should be consulted by the
client when resolving host names and other information.
1. Introduction
DHCPv6 specification [1] allows DHCPv6 server to convey various kinds
of name services to the DHCPv6 clients through the options like DNS
Servers Option [2], NIS Servers Option [3] and NIS+ Servers Option [3].
The clients can use multiple name services. Currently, there is no
way by which the client can be notified with the search order for the
name services. This document defines a new option by which the server
can pass the search order for the name services to the clients.
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2. Requirements
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in RFC 2119 [5]
3. Terminology
This document uses terminology specific to IPv6 and DHCPv6 as defined
in "Terminology" section of the DHCPv6 specification [1].
4. Name Service Search Option
The Name Service Search Option lists the one or more option codes of
the name services in the order of preference. The first listed name
service will be first used for resolution.
The format of the Name Service Search Option is as shown below:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_NSS | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NS Option code | NS Option code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code: OPTION_NSS (tbd)
option-len: Length of the 'NS Option code' fields in octets; It must
be a multiple of 2.
NS Option code: 2 bytes option code of Name service represented
in network byte order.
The following list gives the various name services and their
corresponding option codes.
Name Service Option code
Domain Name Servers Option OPT_DNS_SERVERS
Network Information Servers Option OPT_NIS_SERVERS
NetBIOS over TCP/IP Name Server Option OPT_NBNS_SERVERS
Network Information Service+ Servers Option OPT_NISP_SERVERS
The value of OPT_DNS_SERVERS is defined in [2] and the values of
OPT_NIS_SERVERS and OPT_NISP_SERVERS are defined in [3]. Currently
the values are `tbd`. IANA has been requested to assign a value for
these options. Currently, there is no DHCPv6 option to deliver NetBIOS
over TCP/IP Name Server (NBNS) [4] addresses to the clients. So, the
value of OPT_NBNS_SERVERS is `tbd`.
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The NS Option code of 0 is to indicate that the client should refer
to local naming information (e.g., an /etc/hosts file on a UNIX
machine).
Example:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_NSS | 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPT_DNS_SERVERS | OPT_NIS_SERVERS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This means, the client has to first use DNS, then NIS and then the
local naming information like /etc/hosts for name service.
5. Client Behaviour
The client should use this option to create search list for name
service.
i) If the DHCPv6 server doesn't provide the information about one or
more name services listed in this option and these service are
supported in the client and the client is configured to access these
services, it can either use the information about these name services
configured by some other mechanism external to DHCPv6 (or) it can try
to obtain the information about these name services from the
alternative DHCPv6 servers (or) it can ignore these name services
and keep the remaining name services in the search list in the same
relative order as before. For example, assume that the server provides
the search list as (1) DNS (2) NIS+ and (3) NetBIOS and the server
has not provided NIS+ server addresses to the client. The client
supports NIS+ and it is configured to use NIS+. The client can
either get the information about NIS+ servers through some other
mechanisms like manual configuration database (or) it can contact
alternative DHCPv6 servers to obtain NIS+ server addresses. If both
mechanisms doesn't succeed, it can ignore NIS+ and keep (1) DNS
and (2) NetBIOS in its search list with DNS and NetBIOS in same
relative order as before.
ii) The client may receive this option with some name services, which
are not supported by client (or) it has not been configured to access.
In this case, it can ignore these name services and create the search
list removing these name services with relative order of the remaining
name services not changed.
This specification does mandate any strict rules to be followed
for the cases i) and ii). However, it provides the guidelines
which will be helpful for the implementors. For all these cases
the client can interpret this option in a system-specific manner
whose specification is outside the scope of this document.
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6. Appearance of this option
The Name Service Search option MUST NOT appear in other than the
following messages: Solicit, Advertise, Request, Renew, Rebind,
Information-Request and Reply.
The option number for this option MAY appear in the Option Request
Option [1] in the following messages: Solicit, Request, Renew,
Rebind, Information-Request and Reconfigure.
7. Security Considerations
The Name Service Search Option may be used by an intruder DHCPv6
server to provide a search list which contain only the name services
which are not supported in the client. This will make the clients
unable to resolve the names and other information.
The Name Service Search Option may be used by an intruder DHCPv6
server to provide a search list with lower precedence given to the
service which provide better results. For example, local naming
information like /etc/hosts, which doesn't have adequate information
for name and other resolutions, will be given higher precedence in the
search list. This will degrade the performance of the applications
running on the client as they have to perform lookup multiple times
with different name services to resolve the names and other
information. If these applications are very critical like banking
services, which require higher throughput, this will result in Denial
Of Service.
To avoid attacks through this option, the DHCP client SHOULD use
authenticated DHCP (see section "Authentication of DHCP messages"
in the DHCPv6 specification [1]).
8. IANA Considerations
IANA is requested to assign an option code to the following options
from the option-code space defined in "DHCPv6 Options" section of the
DHCPv6 specification [1].
Option Name Value Described in
OPTION_NSS tbd Section 4
OPTION_NBNS_SERVERS tbd Section 4
9. Normative References
[1] Bound, J., Carney, M., Perkins, C., Lemon, T., Volz, B. and R.
Droms (ed.), "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
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10. Informative References
[2] R. (ed.) Droms. DNS Configuration options for DHCPv6. Internet
Draft, Internet Engineering Task Force, August 2003. Work in
progress.
[3] A. K. Vijayabhaskar. NIS Configuration Options for DHCPv6.
Internet Draft, Internet Engineering Task Force, Oct 2003. Work
in progress.
[4] NetBIOS Working Group, "Protocol Standard for a NetBIOS Service
on a TCP/UDP transport: Detailed Specifications", STD 19, RFC
1002, March 1987.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
Author's Address
Vijayabhaskar A K
Hewlett-Packard STSD-I
29, Cunningham Road
Bangalore - 560052
India
Phone: +91-80-2053085
E-Mail: vijayak@india.hp.com
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society. The idea of this specification is based on
RFC 2937, September 2000.
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