Improved DNS Server Selection for Multi-Interfaced Nodes
draft-ietf-mif-dns-server-selection-07
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 6731.
|
|
|---|---|---|---|
| Authors | Jun-ya Kato , Ted Lemon , Teemu Savolainen | ||
| Last updated | 2011-10-25 (Latest revision 2011-10-19) | ||
| Replaces | draft-savolainen-mif-dns-server-selection | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 6731 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-mif-dns-server-selection-07
Internet Engineering Task Force T. Savolainen
Internet-Draft Nokia
Intended status: Standards Track J. Kato
Expires: April 27, 2012 NTT
T. Lemon
Nominum, Inc.
October 25, 2011
Improved DNS Server Selection for Multi-Interfaced Nodes
draft-ietf-mif-dns-server-selection-07
Abstract
A multi-interfaced node is connected to multiple networks, some of
which may be utilizing private DNS namespaces. A node commonly
receives DNS server configuration information from all connected
networks. Some of the DNS servers may have information about
namespaces other servers do not have. When a multi-interfaced node
needs to utilize DNS, the node has to choose which of the servers to
contact to. This document describes DHCPv4 and DHCPv6 options that
can be used to configure nodes with information required to perform
informed DNS server selection decisions.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 27, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
2. Private namespaces and problems for multi-interfaced nodes . . 5
2.1. Fully qualified domain names with limited scopes . . . . . 5
2.2. Network interface specific IP addresses . . . . . . . . . 6
2.3. A problem not fully solved by the described solution . . . 8
3. Deployment scenarios . . . . . . . . . . . . . . . . . . . . . 8
3.1. CPE deployment scenario . . . . . . . . . . . . . . . . . 8
3.2. Cellular network scenario . . . . . . . . . . . . . . . . 9
3.3. VPN scenario . . . . . . . . . . . . . . . . . . . . . . . 9
3.4. Dual-stack accesses . . . . . . . . . . . . . . . . . . . 9
4. Improved DNS server selection . . . . . . . . . . . . . . . . 9
4.1. Procedure for prioritizing DNS servers and handling
responses . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2. DNS server selection DHCPv6 option . . . . . . . . . . . . 12
4.3. DNS server selection DHCPv4 option . . . . . . . . . . . . 14
4.4. Limitations on use . . . . . . . . . . . . . . . . . . . . 16
4.5. Coexistence with RFC3646 and RFC1533 . . . . . . . . . . . 16
4.6. Considerations on follow-up queries . . . . . . . . . . . 17
5. Example of a node behavior . . . . . . . . . . . . . . . . . . 17
6. Scalability considerations . . . . . . . . . . . . . . . . . . 20
7. Considerations for network administrators . . . . . . . . . . 20
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
10. Security Considerations . . . . . . . . . . . . . . . . . . . 21
10.1. Attack vectors . . . . . . . . . . . . . . . . . . . . . . 21
10.2. Trust levels of network interfaces . . . . . . . . . . . . 21
10.3. Importance of following the algorithm . . . . . . . . . . 21
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
11.1. Normative References . . . . . . . . . . . . . . . . . . . 22
11.2. Informative References . . . . . . . . . . . . . . . . . . 22
Appendix A. Possible alternative practices for DNS server
selection . . . . . . . . . . . . . . . . . . . . . . 23
A.1. Sending queries out on multiple interfaces in parallel . . 23
A.2. Search list option for DNS forward lookup decisions . . . 24
A.3. More specific routes for reverse lookup decision . . . . . 24
A.4. Longest matching prefix for reverse lookup decision . . . 24
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Appendix B. DNSSEC and multiple answers validating with
different trust anchors . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
A multi-interfaced node faces several problems a single-homed node
does not encounter, as is described in
[I-D.ietf-mif-problem-statement]. This document studies in detail
the problems private namespaces may cause for multi-interfaced nodes
and provides a solution. The node may be implemented as a host or as
a router.
We start from the premise that network operators sometimes include
private namespaces in the answers they provide from DNS servers, and
that those private namespaces are at least as useful to nodes as the
answers from the public DNS. When private namespaces are visible for
a node, some DNS servers have information other servers do not have.
The node ought to be able to ask right server for the information it
needs.
An example of an application that benefits from multi-interfacing is
a web browser that commonly accesses many different destinations,
each of which is available only on one network. The browser
therefore needs to be able to communicate over different network
interfaces, depending on the destination it is trying to reach.
In deployments where private namespaces are present, selection of
correct route and destination and source addresses for the actual IP
connection is crucial as well, as the resolved destination's IP
addresses may be only usable on the network interface over which the
name was resolved on. Hence solution described in this document is
assumed to be commonly used in combination with tools for delivering
additional routing and source and destination address selection
policies.
This document is organized in the following manner. Background
information about problem descriptions and example deployment
scenarios are included in Section 2 and Section 3. Section 4
contains all normative descriptions for DHCP options and node
behavior. Informative Section 5 illustrates behavior of a node
implementing functionality described in the Section 4. Section 6
contains informational considerations about scalability. Section 7
contains normative guidelines related to creation of private
namespaces. Informational Section 10 summarizes identified security
considerations.
The Appendix A describes best current practices possible with tools
preceding this document and that may be possibilities on networks not
supporting the solution described in this document.
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1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Private namespaces and problems for multi-interfaced nodes
This section describes two node multi-interfacing related private
namespace scenarios for which the procedure described in Section 4
provides a solution for. Additionally, Section 2.3 describes a
problem for which this document provides only partial solution.
2.1. Fully qualified domain names with limited scopes
A multi-interfaced node may be connected to one or more networks that
are using private namespaces. As an example, the node may have
simultaneously open a wireless LAN (WLAN) connection to the public
Internet, cellular connection to an operator network, and a virtual
private network (VPN) connection to a corporate network. When an
application initiates a connection establishment to an FQDN, the node
needs to be able to choose the right DNS server for making a
successful DNS query. This is illustrated in the figure 1. An FQDN
for a public name can be resolved with any DNS server, but for an
FQDN of corporation's or operator's service's private name the node
needs to be able to correctly select the right DNS server for the DNS
resolution, i.e. do also network interface selection already before
destination's IP address is known.
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+---------------+
| DNS server w/ | | Corporate
+------+ | public + |----| Intranet
| | | corporation's | |
| |===== VPN =======| private names | |
| | +---------------+ +----+
| MIF | | FW |
| node | +----+
| | +---------------+ |
| |----- WLAN ------| DNS server w/ |----| Public
| | | public names | | Internet
| | +---------------+ +----+
| | | FW |
| | +---------------+ +----+
| |---- cellular ---| DNS server w/ | |
+------+ | public + | | Operator
| operator's |----| Intranet
| private names | |
+---------------+
Private DNS namespaces illustrated
Figure 1
2.2. Network interface specific IP addresses
In the second problem an FQDN is valid and resolvable via different
network interfaces, but to different and not necessarily globally
reachable IP addresses, as is illustrated in the figure 2. Node's
routing and source and destination address selection mechanism must
ensure the destination's IP address is only used in combination with
source IP addresses of the network interface the name was resolved
on.
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+--------------------| |
+------+ IPv6 | DNS server A |------| IPv6
| |-- interface 1 --| saying Peer is | |
| | | at: 2001:0db8:0::1 | |
| MIF | +--------------------+ +------+
| node | | Peer |
| | +--------------------+ +------+
| | IPv6 | DNS server B | |
| |-- interface 2 --| saying Peer is | |
+------+ | at: 2001:0db8:1::1 |------| IPv6
+--------------------+ |
Private DNS namespaces and different IP addresses for an FQDN on
interfaces 1 and 2.
Figure 2
Similar situation can happen with IPv6 protocol translation and AAAA
record synthesis [RFC6147]. A synthetic AAAA record is guaranteed to
be valid only on a network it was synthesized on. Figure 3
illustrates a scenario where the peer's IPv4 address is synthesized
into different IPv6 addresses by DNS servers A and B.
+-------------------| +-------+
+------+ IPv6 | DNS server A |----| NAT64 |
| |-- interface 1 --| saying Peer is | +-------+
| | | at: A_Pref96:IPv4 | |
| MIF | +-------------------+ | +------+
| node | IPv4 +---| Peer |
| | +-------------------+ | +------+
| | IPv6 | DNS server B | |
| |-- interface 2 --| saying Peer is | +-------+
+------+ | at: B_Pref96:IPv4 |----| NAT64 |
+-------------------+ +-------+
AAAA synthesis results in network interface specific IPv6 addresses.
Figure 3
It is worth noting is that network specific IP addresses can cause
problems also for a single-homed node, if the node retains DNS cache
during movement from one network to another. After the network
change, a node may have entries in its DNS cache that are no longer
correct or appropriate for its new network position.
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2.3. A problem not fully solved by the described solution
A more complex scenario is an FQDN, which in addition to possibly
resolving into network interface specific IP addresses, identifies on
different network interfaces completely different peer entities with
potentially different set of service offerings. In even more complex
scenario, an FQDN identifies unique peer entity, but one that
provides different services on its different network interfaces. The
solution described in this document is not able to tackle these
higher layer issues. In fact, these problems may be solvable only by
manual user intervention.
However, when DNSSEC is used, the DNSSEC validation procedure may
provide assistance for selecting correct responses for some, but not
all, use cases. A node may prefer to use the DNS answer that
validates with the preferred trust anchor.
3. Deployment scenarios
This document has been written with three particular deployment
scenarios in mind. First being a Consumer Premises Equipment (CPE)
with two or more uplink VLAN connections. Second scenario involves a
cellular device with two uplink Internet connections: WLAN and
cellular. Third scenario is for VPNs, where use of local DNS server
may be preferred for latency reasons, but corporate DNS server must
be used to resolve private names used by the corporation.
3.1. CPE deployment scenario
A home gateway may have two uplink connections leading to different
networks, as is described in
[I-D.ietf-v6ops-ipv6-multihoming-without-ipv6nat]. In the two
uplinks scenario only one uplink connection leads to the Internet,
while another uplink connection leads to a private network utilizing
private namespaces.
It is desirable that the CPE does not have to send DNS queries over
both uplink connections, but instead CPE should send default queries
to the DNS server of the interface leading to the Internet, and
queries related to private namespace to the DNS server of the private
network.
In this scenario the legacy hosts can be supported by deploying DNS
proxy on the CPE and configuring hosts in the LAN to talk to the DNS
proxy. However, updated hosts would be able to talk directly to the
correct DNS servers of each uplink ISP's DNS server. It is
deployment decision whether the updated hosts would be pointed to DNS
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proxy or to actual DNS servers.
Depending on actual deployments, all VLAN connections might be
considered as trusted.
3.2. Cellular network scenario
A cellular device may have both WLAN and cellular network interfaces
up. In such a case it is often desirable to use WLAN by default,
except for those connections cellular network operator wants to go
over cellular interface. The cellular network may utilize private
names and hence the cellular device needs to ask for those through
the cellular interface.
In this scenario cellular interface can be considered trusted and
WLAN oftentimes untrusted.
3.3. VPN scenario
Depending on a deployment, there may be interest to use VPN only for
the traffic destined to a corporate network. The corporation may be
using private namespace, and hence related DNS queries should be send
over VPN to the corporate DNS server, while by default a DNS server
of a local access network may be used.
In this scenario VPN interface can be considered trusted and local
access network untrusted.
3.4. Dual-stack accesses
In all three scenarios one or more of the connected networks may
support both IPv4 and IPv6. In such a case both or either of DHCPv4
and DHCPv6 can be used to learn DNS server selection information.
4. Improved DNS server selection
This section describes DHCP options and a procedure that a (stub /
proxy) resolver may utilize for improved DNS server selection in the
face of private namespaces and multiple simultaneously active network
interfaces.
4.1. Procedure for prioritizing DNS servers and handling responses
A resolver SHALL build a priority list of DNS servers it will contact
to depending on the query. To build the list in an optimal way, a
node SHOULD ask with DHCP which DNS servers of each network interface
are most likely to be able to successfully serve forward lookup
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requests matching to specific domain or reverse (PTR record) lookup
requests matching to specific network addresses (later referred as
"network"). For security reasons the DNS server selection
information MUST be used only when it is safe to do so, see
Section 4.4 for details.
The node SHOULD create a node specific route for the DNS server
addresses learned via DHCP. The route must point to the interface
DNS server address was learned on. This is required to ensure DNS
queries are sent out via the right network interface.
A resolver lacking more specific information shall assume that all
information is available from any DNS server of any network
interface. The DNS servers learnt by other DNS server address
configuration methods MUST be handled as medium priority default
servers.
When a DNS query needs to be made, the resolver SHOULD give highest
precedence to the DNS servers explicitly known to serve matching
domain or network. The resolver MUST take into account differences
in trust levels of pieces of received DNS server selection
information. The resolver MUST prefer DNS servers of trusted
interfaces. The DNS servers of untrusted interfaces may be of
highest priority only if trusted interfaces specifically configure
low priority DNS servers. The non-exhaustive list on figure 4
illustrates how the different trust levels of received DNS server
selection information SHOULD influence the DNS server selection
logic.
Trustworthiness of an interface and configuration information
received over the interface is implementation and/or node deployment
dependent. Trust may be based on, for example, on the nature of an
interface. For example, an authenticated and encrypted VPN or layer
2 connections to a trusted home network may be considered as trusted,
and an unauthenticated and unencrypted connection to an unknown
visited network may be considered as untrusted. In some occasions an
interface may be considered trusted only if explicitly configured to
be trusted.
A resolver SHOULD prioritize between equally trusted DNS servers with
help of the DHCP option preference field. The resolver MUST NOT
prioritize less trusted DNS servers higher than trusted, even in the
case of less trusted server would apparently have additional
information. In the case of all other things being equal the
resolver shall make the prioritization decision based on its internal
preferences.
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Information from | Information from | Resulting DNS
more trusted | less trusted | server priority
interface A | interface B | selection
--------------------------+------------------------+--------------------
1. Medium priority | Medium priority | Default: A, then B
default | default |
--------------------------+------------------------+--------------------
2. Medium priority | High priority default | Default: A, then B
default | High priority specific | Specific: A, then B
--------------------------+------------------------+--------------------
3. Low priority default | Medium priority | Default: B, then A
| default |
--------------------------+------------------------+--------------------
4. Low priority default | Medium priority | Default: B, then A
High priority specific | default | Specific: A, then B
--------------------------+------------------------+--------------------
Figure 4: DNS server selection in the case of different trust levels
Because DNSSEC provides cryptographic assurance of the integrity of
DNS data, data that can be validated under DNSSEC is necessarily to
be preferred over data that cannot be. There are two ways that a
node can determine that data is valid under DNSSEC. The first is to
perform DNSSEC validation itself. The second is to have a secure
connection to an authenticated DNS server, and to rely on that DNS
server to perform DNSSEC validation (signalling that it has done so
using the AD bit). If a DNS response is not proven to be unmolested
using DNSSEC, then a node cannot make a decision to prefer data from
any interface with any great assurance: any response could be forged,
and there is no way to detect the forgery without DNSSEC.
A node SHALL send requests to DNS servers in the order defined by the
priority list until an acceptable reply is received, all replies are
received, or a time out occurs. In the case of a requested name
matching to a specific domain or network rule accepted from any
interface, a DNSSEC-aware resolver MUST NOT proceed with a reply that
cannot be validated using DNSSEC until all DNS servers on the
priority list have been contacted or timed out. This protects
against possible redirection attacks. In the case of the requested
name not matching to any specific domain or network, first received
response from any DNS server MAY be considered acceptable. A DNSSEC-
aware node MAY always contact all DNS server in an attempt to receive
a response that can be validated, but contacting all DNS servers is
not mandated for the default case as in some deployments that would
consume excess resources.
The resolver SHOULD avoid sending queries over different network
interfaces in parallel as that wastes resources such as energy. The
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amount of wasted energy can be significant, for example when radio
interfaces has to be started just for the queries.
In the case of validated NXDOMAIN response being received from a DNS
server that can provide answers for the queried name a node MUST NOT
accept non-validated replies from other DNS servers (see Appendix B
for considerations related to multiple trust anchors.
4.2. DNS server selection DHCPv6 option
DHCPv6 option described below can be used to inform resolvers which
DNS server should be contacted when initiating forward or reverse DNS
lookup procedures.
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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_DNS_SERVER_SELECT | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| DNS-recursive-name-server (IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |prf| |
+-+-+-+-+-+-+-+-+ Domains and networks |
| (variable length) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code: OPTION_DNS_SERVER_SELECT (TBD)
option-len: Length of the option in octets
DNS-recursive-name-server: An IPv6 address of a recursive DNS server
Reserved: Field reserved for the future. MUST be set to zero.
prf: DNS server preference, for selecting between
equally trusted DNS servers:
01 High
00 Medium
11 Low
10 Reserved
Domains and networks: The list of domains for forward DNS
lookup and networks for reverse DNS lookup the DNS server
has special knowledge about. Field MUST be encoded as
specified in Section "Representation and use of
domain names" of [RFC3315].
Special domain of "." is used to indicate
capability to resolve global names and act as a
default name server. Lack of "."
domain on the list indicates DNS server only has
information related to listed domains and networks.
Networks for reverse mapping are encoded as
defined for ip6.arpa [RFC3152] or in-addr.arpa [RFC2317].
DHCPv6 option for explicit domain configuration
Figure 5
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A node SHOULD include an OPTION_ORO option in a DHCPv6 request with
the OPTION_DNS_SERVER_SELECT option code to inform the DHCPv6 server
about the support for the improved DNS server selection logic.
DHCPv6 server receiving this information MAY then choose to provision
DNS server addresses only with the OPTION_DNS_SERVER_SELECT.
The OPTION_DNS_SERVER_SELECT contains one or more domains the related
DNS server has particular knowledge of. The option can occur
multiple times in a single DHCPv6 message, if multiple DNS servers
are to be configured.
IPv6 networks should cover all the domains configured in this option.
Networks should be as long as possible to avoid potential collision
with information received on other option instances or with options
received from DHCPv6 servers of other network interfaces.
Overlapping IPv6 networks are interpreted so that the resolver can
use any of the DNS servers for queries matching the networks.
If the OPTION_DNS_SERVER_SELECT contains a DNS server address already
learned from other DHCPv6 servers of the same network, and contains
new domains or networks, the node SHOULD append the information to
the information received earlier. The node MUST NOT remove
previously obtained information. However, the node SHOULD NOT extent
lifetime of earlier information either. In the case of conflicting
DNS server address is learned from less trusted interface, the node
MUST ignore the option.
As the DNS options of [RFC3646], the OPTION_DNS_SERVER_SELECT option
MUST NOT appear in any other than the following DHCPv6 messages:
Solicit, Advertise, Request, Renew, Rebind, Information-Request, and
Reply.
The information conveyed in OPTION_DNS_SERVER_SELECT is considered
valid until changed or refreshed by general events that trigger
DHCPv6 action. In the event that it is desired for the client to
request a refresh of the information, use of generic DHCPv6
Information Refresh Time Option, as specified in [RFC4242] is
RECOMMENDED.
4.3. DNS server selection DHCPv4 option
DHCPv4 option described below can be used to inform resolvers which
DNS server should be contacted when initiating forward or reverse DNS
lookup procedures.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| CODE | Len | Reserved |prf| Primary .. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .. DNS-recursive-name-server's IPv4 address | Secondary .. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .. DNS-recursive-name-server's IPv4 address | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
+ Domains and networks |
| (variable length) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code: OPTION_DNS_SERVER_SELECT (TBD)
option-len: Length of the option in octets
Reserved: Field reserved for the future. MUST be set to zero.
prf: DNS servers preference, for selecting between
equally trusted DNS servers:
01 High
00 Medium
11 Low
10 Reserved
Primary DNS-recursive-name-server's IPv4 address: Address of
a primary recursive DNS server
Secondary DNS-recursive-name-server's IPv4 address: Address of
a secondary recursive DNS server or 0.0.0.0 if
not configured.
Domains and networks: The list of domains for forward DNS lookup
and networks for reverse DNS lookup the DNS servers
have special knowledge about. Field MUST be encoded as
specified in Section "Representation and use of
domain names" of [RFC3315].
Special domain of "." is used to indicate
capability to resolve global names and act as
default name servers. Lack of "."
domain on the list indicates DNS servers only have
information related to listed domains and networks.
Networks for reverse mapping are encoded as
defined for ip6.arpa [RFC3152] or in-addr.arpa [RFC2317].
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DHCPv4 option for explicit domain configuration
Figure 6
The OPTION_DNS_SERVER_SELECT contains one or more domains the primary
and secondary DNS servers have particular knowledge of. If the
length of the domains and networks field causes option length to
exceed the maximum permissible for a single option (255 octets), then
multiple options MAY be used, as described in "Encoding Long Options
in the Dynamic Host Configuration Protocol (DHCPv4)" [RFC3396]. When
multiple options are present, the data portions of all option
instances are concatenated together.
If the OPTION_DNS_SERVER_SELECT contains a DNS server address already
learned from other DHCPv4 servers of the same network, and contains
new domains or networks, the node SHOULD append the information to
the information received earlier. The node MUST NOT remove
previously obtained information. However, the node SHOULD NOT extent
lifetime of earlier information either. In the case of conflicting
DNS server address is learned from less trusted interface, the node
MUST ignore the option.
4.4. Limitations on use
Use of OPTION_DNS_SERVER_SELECT is ideal in the following
environments, but SHOULD NOT be enabled by default otherwise:
1. The server selection option is delivered across a secure, trusted
channel.
2. The server selection option is not secured, but the client on a
node does DNSSEC validation.
3. The server selection option is not secured, the resolver does
DNSSEC validation, and the client communicates with the resolver
configured with server selection option over a secure, trusted
channel.
4. The DNS server IP address that is being recommended in the server
selection option is known and trusted by the client; that is, the
server selection option serves not to introduce the client to a new
server, but rather to inform it that a server it has already been
configured to trust is available to it for resolving certain domains.
4.5. Coexistence with RFC3646 and RFC1533
The OPTION_DNS_SERVER_SELECT is designed to coexist with DHCPv6
OPTION_DNS_SERVERS defined in [RFC3646] and DHCPv4 Domain Name Server
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Option defined in [RFC1533]. The DNS servers configured via
OPTION_DNS_SERVERS or Domain Name Server Option MUST be considered as
default name servers with medium preference. When both options are
received from the same network interface and the
OPTION_DNS_SERVER_SELECT contains default DNS server address, the
resolver MUST make the decision which one to prefer based on DNS
preference field value. If OPTION_DNS_SERVER_SELECT defines medium
preference then DNS server from OPTION_DNS_SERVER_SELECT SHALL be
selected.
If OPTION_DNS_SERVERS or Domain Name Server Option and
OPTION_DNS_SERVER_SELECT contain the same DNS server(s) IP
address(es), a node SHALL add the same address of a DNS server to the
server list only once.
If a node had indicated support for OPTION_DNS_SERVER_SELECT in
DHCPv6 request, the DHCPv6 server may choose to omit sending of
OPTION_DNS_SERVERS. This enables offloading use case where network
administrator wishes to only advertise low priority default DNS
servers.
4.6. Considerations on follow-up queries
Any follow-up queries that are performed on the basis of an answer
received on an interface MUST continue to use the same interface,
irrespective of the DNS server selection settings on any other
interface. For example, if a node receives a reply with a canonical
name (CNAME) or delegation name (DNAME) the follow-up queries MUST be
sent to DNS server(s) of the same interface, or to same DNS server,
irrespectively of the FQDN received. Otherwise referrals may fail.
5. Example of a node behavior
Figure 7 illustrates node behavior when it initializes two network
interfaces for parallel usage and learns domain and network
information from DHCPv6 servers.
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Application Node DHCPv6 server DHCPv6 server
on interface 1 on interface 2
| | |
| +-----------+ |
(1) | | open | |
| | interface | |
| +-----------+ |
| | |
(2) | |---option REQ-->|
| |<--option RESP--|
| | |
| +-----------+ |
(3) | | store | |
| | domains | |
| +-----------+ |
| | |
| +-----------+ |
(4) | | open | |
| | interface | |
| +-----------+ |
| | | |
(5) | |---option REQ------------------->|
| |<--option RESP-------------------|
| | | |
| +----------+ | |
(6) | | store | | |
| | domains | | |
| +----------+ | |
| | | |
Illustration of learning domains
Figure 7
Flow explanations:
1. A node opens its first network interface
2. The node obtains domain 'domain1.example.com' and IPv6 network
'0.8.b.d.0.1.0.0.2.ip6.arpa' for the new interface 1 from DHCPv6
server
3. The node stores the learned domains and IPv6 networks for later
use
4. The node opens its second network interface 2
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5. The node obtains domain 'domain2.example.com' and IPv6 network
information, say '1.8.b.d.0.1.0.0.2.ip6.arpa' for the new
interface 2 from DHCPv6 server
6. The node stores the learned domains and networks for later use
Figure 8 below illustrates how a resolver uses the learned domain
information. Network information use for reverse lookups is not
illustrated, but that would go as the figure 7 example.
Application Node DHCPv6 server DHCPv6 server
on interface 1 on interface 2
| | | |
(1) |--Name REQ-->| | |
| | | |
| +----------------+ | |
(2) | | DNS server | | |
| | prioritization | | |
| +----------------+ | |
| | | |
(3) | |------------DNS resolution------>|
| |<--------------------------------|
| | | |
(4) |<--Name resp-| | |
| | | |
Example on choosing interface based on domain
Figure 8
Flow explanations:
1. An application makes a request for resolving an FQDN, e.g.
'private.domain2.example.com'
2. A node creates list of DNS servers to contact to and uses
configured DNS server information and stored domain information
on prioritization decisions.
3. The node has chosen interface 2, as from DHCPv6 it was learned
earlier that the interface 2 has domain 'domain2.example.com'.
The node then resolves the requested name using interface 2's DNS
server to an IPv6 address
4. The node replies to application with the resolved IPv6 address
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6. Scalability considerations
The size limitations of DHCP messages limit the number of domains and
networks that can be carried in configuration options. Including the
domains and networks in a DHCP option is best suited for deployments
where relatively few carefully selected domains and networks are
adequate.
7. Considerations for network administrators
Network administrators deploying private namespaces should assist
advanced nodes in their DNS server selection process by providing
information described within this document.
Private namespaces MUST be globally unique in order to keep DNS
unambiguous and henceforth avoiding caching related issues and
destination selection problems (see Section 2.3). Exceptions to this
rule are domains utilized for local name resolution (such as .local).
Private namespaces MUST only consist of subdomains of domains for
which the relevant operator provides authoritative name service.
Thus, subdomains of example.com are permitted in the private
namespace served by an operator's DNS servers only if the same
operator provides an SOA record for example.com.
To counter against attacks against private namespaces, administrators
utilizing this tool SHOULD deploy DNSSEC for their zone.
8. Acknowledgements
The author would like to thank following people for their valuable
feedback and improvement ideas: Mark Andrews, Jari Arkko, Marcelo
Bagnulo, Brian Carpenter, Stuart Cheshire, Lars Eggert, Tomohiro
Fujisaki, Peter Koch, Suresh Krishnan, Murray Kucherawy, Edward
Lewis, Kurtis Lindqvist, Arifumi Matsumoto, Erik Nordmark, Steve
Padgett, Fabien Rapin, Matthew Ryan, Dave Thaler, Margaret Wasserman,
Dan Wing, and Dec Wojciech. Ted Lemon and Julien Laganier receive
special thanks for their contributions to security considerations.
This document was prepared using xml2rfc template and the related
web-tool.
9. IANA Considerations
This memo requests IANA to assign two new option codes. First option
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code is requested to be assigned for DHCPv4 DNS Server Selection
option (TBD) from the DHCP option code space defined in section "New
DHCP option codes" of RFC 2939. Second option code is requested to
be assigned to the DHCPv6 DNS Server Selection option (TBD) from the
DHCPv6 option code space defined in section "IANA Considerations" of
RFC 3315.
10. Security Considerations
10.1. Attack vectors
It is possible that attackers might try to utilize
OPTION_DNS_SERVER_SELECT option to redirect some or all DNS queries
sent by a resolver to undesired destinations. The purpose of an
attack might be denial-of-service, preparation for man-in-the-middle
attack, or something akin.
Attackers might try to lure specific traffic by advertising domains
and networks from very small to very large scope or simply by trying
to place attacker's DNS server as the highest priority default
server.
The best countermeasure for nodes is to implement validating DNSSEC
aware resolvers. Trusting on validation done by a DNS server is a
possibility only if a node trusts the DNS server and can use a secure
channel for DNS messages.
10.2. Trust levels of network interfaces
Decision on trust levels of network interfaces depends very much on
deployment scenario and types of network interfaces. For example,
unmanaged WLAN may be considered less trustworthy than managed
cellular or VPN connections. An implementation may not be able to
determine trust levels without explicit configuration provided by
user or administrator. Therefore, for example, an implementation may
not by default trust configuration received even over VPN interfaces.
The decision on levels of trust may be made by implementation, by
node administrators, or for example by other standards defining
organizations as part of system design work.
10.3. Importance of following the algorithm
The Section 4 uses normative language for describing node internal
behavior in order to ensure nodes would not open up new attack
vectors by accidental use of DNS server selection options. During
the standards work consensus was that it is safer to not to enable
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this option always by default, but only when deemed useful and safe.
11. References
11.1. Normative References
[RFC1533] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 1533, October 1993.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2317] Eidnes, H., de Groot, G., and P. Vixie, "Classless IN-
ADDR.ARPA delegation", BCP 20, RFC 2317, March 1998.
[RFC3152] Bush, R., "Delegation of IP6.ARPA", BCP 49, RFC 3152,
August 2001.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3396] Lemon, T. and S. Cheshire, "Encoding Long Options in the
Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396,
November 2002.
[RFC3736] Droms, R., "Stateless Dynamic Host Configuration Protocol
(DHCP) Service for IPv6", RFC 3736, April 2004.
[RFC4242] Venaas, S., Chown, T., and B. Volz, "Information Refresh
Time Option for Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 4242, November 2005.
11.2. Informative References
[I-D.ietf-mif-problem-statement]
Blanchet, M. and P. Seite, "Multiple Interfaces and
Provisioning Domains Problem Statement",
draft-ietf-mif-problem-statement-15 (work in progress),
May 2011.
[I-D.ietf-v6ops-ipv6-multihoming-without-ipv6nat]
Troan, O., Miles, D., Matsushima, S., Okimoto, T., and D.
Wing, "IPv6 Multihoming without Network Address
Translation",
draft-ietf-v6ops-ipv6-multihoming-without-ipv6nat-01 (work
in progress), August 2011.
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[I-D.wing-behave-dns64-config]
Wing, D., "IPv6-only and Dual Stack Hosts on the Same
Network with DNS64", draft-wing-behave-dns64-config-03
(work in progress), February 2011.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC3397] Aboba, B. and S. Cheshire, "Dynamic Host Configuration
Protocol (DHCP) Domain Search Option", RFC 3397,
November 2002.
[RFC3646] Droms, R., "DNS Configuration options for Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
December 2003.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, November 2005.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
[RFC6106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
"IPv6 Router Advertisement Options for DNS Configuration",
RFC 6106, November 2010.
[RFC6147] Bagnulo, M., Sullivan, A., Matthews, P., and I. van
Beijnum, "DNS64: DNS Extensions for Network Address
Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
April 2011.
Appendix A. Possible alternative practices for DNS server selection
On some private namespace deployments explicit policies for DNS
server selection are not available. This section describes ways for
nodes to mitigate the problem by sending wide-spread queries and by
utilizing possibly existing indirect information elements as hints.
A.1. Sending queries out on multiple interfaces in parallel
A possible current practice is to send DNS queries out of multiple
interfaces and pick up the best out of the received responses. A
node SHOULD implement DNSSEC in order to be able to reject responses
that cannot be validated. Selection between legitimate answers is
implementation specific, but replies from trusted servers should be
preferred.
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A downside of this approach is increased consumption of resources.
Namely power consumption if an interface, e.g. wireless, has to be
brought up just for the DNS query that could have been resolved also
via cheaper interface. Also load on DNS servers is increased.
However, local caching of results mitigates these problems, and a
node might also learn interfaces that seem to be able to provide
'better' responses than other and prefer those - without forgetting
fallback required for cases when node is connected to more than one
network using private namespaces.
A.2. Search list option for DNS forward lookup decisions
A node can learn the special domains of attached network interfaces
from IPv6 Router Advertisement DNS Search List Option [RFC6106] or
DHCP search list options; DHCPv4 Domain Search Option number 119
[RFC3397] and DHCPv6 Domain Search List Option number 24 [RFC3646].
The node behavior is very similar as is illustrated in the example at
Section 5. While these options are not intended to be used in DNS
server selection, they may be used by the nodes as hints for smarter
DNS server prioritization purposes in order to increase likelihood of
fast and successful DNS query.
Overloading of existing DNS search list options is not without
problems: resolvers would obviously use the domains learned from
search lists also for name resolution purposes. This may not be a
problem in deployments where DNS search list options contain few
domains like 'example.com, private.example.com', but can become a
problem if many domains are configured.
A.3. More specific routes for reverse lookup decision
[RFC4191] defines how more specific routes can be provisioned for
nodes. This information is not intended to be used in DNS server
selection, but nevertheless a node can use this information as a hint
about which interface would be best to try first for reverse lookup
procedures. A DNS server configured via the same interface as more
specific routes is more likely capable to answer reverse lookup
questions correctly than DNS server of an another interface. The
likelihood of success is possibly higher if DNS server address is
received in the same RA [RFC6106] as the more specific route
information.
A.4. Longest matching prefix for reverse lookup decision
A node may utilize the longest matching prefix approach when deciding
which DNS server to contact for reverse lookup purposes. Namely, the
node may send a DNS query to a DNS server learned over an interface
having longest matching prefix to the address being queried. This
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approach can help in cases where ULA [RFC4193] addresses are used and
when the queried address belongs to a node or server within the same
network (for example intranet).
Appendix B. DNSSEC and multiple answers validating with different trust
anchors
When validating DNS answers with DNSSEC, a validator might order the
list of trust anchors it uses to start validation chains, in terms of
the node's preferences for those trust anchors. A node could use
this ability in order to select among alternative DNS results from
different interfaces. Suppose that a node has a trust anchor for the
public DNS root, and also has a special-purpose trust anchor for
example.com. An answer is received on interface i1 for
www.example.com, and the validation for that succeeds by using the
public trust anchor. Also, an answer is received on interface i2 for
www.example.com, and the validation for that succeeds by using the
trust anchor for example.com. In this case, the node has evidence
for relying on i2 for answers in the example.com zone.
Authors' Addresses
Teemu Savolainen
Nokia
Hermiankatu 12 D
TAMPERE, FI-33720
FINLAND
Email: teemu.savolainen@nokia.com
Jun-ya Kato
NTT
9-11, Midori-Cho 3-Chome Musashino-Shi
TOKYO, 180-8585
JAPAN
Email: kato@syce.net
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Ted Lemon
Nominum, Inc.
2000 Seaport Boulevard
Redwood City, CA 94063
USA
Phone: +1 650 381 6000
Email: Ted.Lemon@nominum.com
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