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IETF RFC 6731
Last modified on Wednesday, December 19th, 2012
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Internet Engineering Task Force (IETF) T. Savolainen
Request for Comments: 6731 Nokia
Category: Standards Track J. Kato
ISSN: 2070-1721 NTT
T. Lemon
Nominum, Inc.
December 2012
Improved Recursive DNS Server Selection for Multi-Interfaced Nodes
 Abstract
A multi-interfaced node is connected to multiple networks, some of
which might be utilizing private DNS namespaces. A node commonly
receives recursive DNS server configuration information from all
connected networks. Some of the recursive DNS servers might 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 recursive DNS servers to use. This document describes
DHCPv4 and DHCPv6 options that can be used to configure nodes with
information required to perform informed recursive DNS server
selection decisions.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6731.
Copyright Notice
Copyright (c) 2012 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
(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
Savolainen, et al. Standards Track PAGE 1 
RFC 6731 RDNSS Selection for MIF Nodes December 2012
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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Private Namespaces and Problems for Multi-Interfaced Nodes . . 4
2.1. Fully Qualified Domain Names with Limited Scopes . . . . . 4
2.2. Network-Interface-Specific IP Addresses . . . . . . . . . 5
2.3. A Problem Not Fully Solved by the Described Solution . . . 6
3. Deployment Scenarios . . . . . . . . . . . . . . . . . . . . . 7
3.1. CPE Deployment Scenario . . . . . . . . . . . . . . . . . 7
3.2. Cellular Network Scenario . . . . . . . . . . . . . . . . 7
3.3. VPN Scenario . . . . . . . . . . . . . . . . . . . . . . . 8
3.4. Dual-Stack Accesses . . . . . . . . . . . . . . . . . . . 8
4. Improved RDNSS Selection . . . . . . . . . . . . . . . . . . . 8
4.1. Procedure for Prioritizing RDNSSes and Handling
Responses . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2. RDNSS Selection DHCPv6 Option . . . . . . . . . . . . . . 11
4.3. RDNSS Selection DHCPv4 Option . . . . . . . . . . . . . . 13
4.4. Scalability Considerations . . . . . . . . . . . . . . . . 15
4.5. Limitations on Use . . . . . . . . . . . . . . . . . . . . 15
4.6. Coexistence of Various RDNSS Configuration Tools . . . . . 16
4.7. Considerations on Follow-Up Queries . . . . . . . . . . . 17
4.8. Closing Network Interfaces and Local Caches . . . . . . . 17
5. Example of a Node Behavior . . . . . . . . . . . . . . . . . . 17
6. Considerations for Network Administrators . . . . . . . . . . 19
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
8. Security Considerations . . . . . . . . . . . . . . . . . . . 20
8.1. Attack Vectors . . . . . . . . . . . . . . . . . . . . . . 20
8.2. Trust Levels of Network Interfaces . . . . . . . . . . . . 21
8.3. Importance of Following the Algorithm . . . . . . . . . . 21
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
9.1. Normative References . . . . . . . . . . . . . . . . . . . 21
9.2. Informative References . . . . . . . . . . . . . . . . . . 22
Appendix A. Possible Alternative Practices for RDNSS Selection . 23
A.1. Sending Queries Out on Multiple Interfaces in Parallel . . 23
A.2. Search List Option for DNS Forward Lookup Decisions . . . 23
A.3. More-Specific Routes for Reverse Lookup Decisions . . . . 24
A.4. Longest Matching Prefix for Reverse Lookup Decisions . . . 24
Appendix B. DNSSEC and Multiple Answers Validating with
Different Trust Anchors . . . . . . . . . . . . . . . 24
Appendix C. Pseudocode for RDNSS Selection . . . . . . . . . . . 24
Appendix D. Acknowledgements . . . . . . . . . . . . . . . . . . 29
Savolainen, et al. Standards Track PAGE 2
RFC 6731 RDNSS Selection for MIF Nodes December 2012
1. Introduction
A multi-interfaced node (MIF node) faces several problems a single-
homed node does not encounter, as is described in [RFC 6418]. This
document studies in detail the problems private namespaces might
cause for multi-interfaced nodes and provides a solution. The node
might be implemented as a host or as a router.
We start from the premise that network operators sometimes include
private, but still globally unique, namespaces in the answers they
provide from Recursive DNS Servers (RDNSSes) 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
RDNSSes have information other RDNSSes do not have. The node ought
to be able to query the RDNSS that can resolve the query regardless
of whether the answer comes from the public DNS or a private
namespace.
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 on only one network. The browser
therefore needs to be able to communicate over different network
interfaces, depending on the destination it is trying to reach.
Selection of the correct interface and source address is often
crucial in the networks using private namespaces. In such
deployments, the destination's IP addresses might only be reachable
on the network interface over which the destination's name was
resolved. Henceforth, the 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 (e.g., [RFC 4191] and [RFC 3442].
This document is organized in the following manner. Background
information about problem descriptions and example deployment
scenarios are included in Sections 2 and 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 Section 4. Section 6 contains normative
guidelines related to creation of private namespaces. The IANA
considerations are in Section 7. Informational Section 8 summarizes
identified security considerations.
Appendix A describes best current practices that are possible with
tools preceding this document and that are possibilities on networks
not supporting the solution described in this document. Appendix B
discusses a scenario where multiple answers are possible to validate,
Savolainen, et al. Standards Track PAGE 3
RFC 6731 RDNSS Selection for MIF Nodes December 2012
but with different trust anchors. Appendix C illustrates with
pseudocode the functionality described in Section 4.
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 [RFC 2119].
2. Private Namespaces and Problems for Multi-Interfaced Nodes
This section describes two private namespace scenarios related to
node multi-interfacing for which the procedure described in Section 4
provides a solution. Additionally, Section 2.3 describes a problem
for which this document provides only a partial solution.
2.1. Fully Qualified Domain Names with Limited Scopes
A multi-interfaced node can be connected to one or more networks that
are using private namespaces. As an example, the node can
simultaneously open a Wireless LAN (WLAN) connection to the public
Internet, a cellular connection to an operator network, and a Virtual
Private Network (VPN) connection to an enterprise network. When an
application initiates a connection establishment to a Fully Qualified
Domain Name (FQDN), the node needs to be able to choose the right
RDNSS for making a successful DNS query. This is illustrated in
Figure 1. An FQDN for a public name can be resolved with any RDNSS,
but for an FQDN of the private name of an enterprise's or operator's
service, the node needs to be able to correctly select the right
RDNSS for the DNS resolution, i.e., do also network interface
selection already before destination's IP address is known.
Savolainen, et al. Standards Track PAGE 4
RFC 6731 RDNSS Selection for MIF Nodes December 2012
+---------------+
| RDNSS with | | Enterprise
+------+ | public + |----| Intranet
| | | enterprise's | |
| |===== VPN =======| private names | |
| | +---------------+ +----+
| MIF | | FW |
| node | +----+
| | +---------------+ |
| |----- WLAN ------| RDNSS with |----| Public
| | | public names | | Internet
| | +---------------+ +----+
| | | FW |
| | +---------------+ +----+
| |---- cellular ---| RDNSS with | |
+------+ | public + | | Operator
| operator's |----| Intranet
| private names | |
+---------------+
Figure 1: Private DNS Namespaces Illustrated
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 Figure 2. The node's
routing, source, and destination address selection mechanism has to
ensure the destination's IP address is only used in combination with
source IP addresses of the network interface on which the name was
resolved.
+--------------------| |
+------+ IPv6 | RDNSS A |------| IPv6
| |-- interface 1 --| saying Peer is | |
| | | at: 2001:0db8:0::1 | |
| MIF | +--------------------+ +------+
| node | | Peer |
| | +--------------------+ +------+
| | IPv6 | RDNSS B | |
| |-- interface 2 --| saying Peer is | |
+------+ | at: 2001:0db8:1::1 |------| IPv6
+--------------------+ |
Figure 2: Private DNS Namespaces and Different IP Addresses for an
FQDN on Interfaces 1 and 2
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RFC 6731 RDNSS Selection for MIF Nodes December 2012
A similar situation can happen with IPv6 protocol translation and
AAAA record synthesis [RFC 6147]. A synthetic AAAA record is
guaranteed to be valid only on the network on which it was
synthesized. Figure 3 illustrates a scenario where the peer's IPv4
address is synthesized into different IPv6 addresses by RDNSSes A and
B.
+-------------------| +-------+
+------+ IPv6 | RDNSS A |----| NAT64 |
| |-- interface 1 --| saying Peer is | +-------+
| | | at: A_Pref96:IPv4 | |
| MIF | +-------------------+ | +------+
| node | IPv4 +---| Peer |
| | +-------------------+ | +------+
| | IPv6 | RDNSS B | |
| |-- interface 2 --| saying Peer is | +-------+
+------+ | at: B_Pref96:IPv4 |----| NAT64 |
+-------------------+ +-------+
Figure 3: AAAA Synthesis Results in
Network-Interface-Specific IPv6 Addresses
It is worth noting that network-specific IP addresses can also cause
problems for a single-homed node, if the node retains DNS cache
during movement from one network to another. After the network
change, a node can have entries in its DNS cache that are no longer
correct or appropriate for its new network position.
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 sets of service offerings. In an even more
complex scenario, an FQDN identifies a 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 might be solvable only
by manual user intervention.
However, when DNS Security (DNSSEC) is used, the DNSSEC validation
procedure can provide assistance for selecting correct responses for
some, but not all, use cases. A node might prefer to use the DNS
answer that validates with the preferred trust anchor.
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RFC 6731 RDNSS Selection for MIF Nodes December 2012
3. Deployment Scenarios
This document has been written with three particular deployment
scenarios in mind. The first is a Customer Premises Equipment (CPE)
with two or more uplink Virtual Local Area Network (VLAN)
connections. The second scenario involves a cellular device with two
uplink Internet connections: WLAN and cellular. The third scenario
is for VPNs, where use of a local RDNSS might be preferred for
latency reasons, but the enterprise's RDNSS has to be used to resolve
private names used by the enterprise.
In this section, we are referring to the RDNSS preference values
defined in Section 4. The purpose of that is to illustrate when
administrators might choose to utilize the different preference
values.
3.1. CPE Deployment Scenario
A home gateway can have two uplink connections leading to different
networks, as described in [WITHOUT-IPV6NAT]. In the two-uplink
scenario, only one uplink connection leads to the Internet, while the
other 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 need only send default
queries to the RDNSS of the interface leading to the Internet and
queries related to the private namespace to the RDNSS of the private
network. This can be configured by setting the RDNSS of the private
network to know about listed domains and networks, but not to be a
default RDNSS.
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 RDNSS of each uplink ISP's RDNSS. It is a deployment
decision whether the updated hosts would be pointed to a DNS proxy or
to actual RDNSSes.
Depending on actual deployments, all VLAN connections might be
considered trusted.
3.2. Cellular Network Scenario
A cellular device can have both WLAN and cellular network interfaces
up. In such a case, it is often desirable to use WLAN by default,
except for the connections that the cellular network operator wants
to go over the cellular interface. The use of WLAN for DNS queries
Savolainen, et al. Standards Track PAGE 7
RFC 6731 RDNSS Selection for MIF Nodes December 2012
likely improves the power consumption of cellular devices and often
provides lower latency. The cellular network might utilize private
names; hence, the cellular device needs to ask for those through the
cellular interface. This can be configured by setting the RDNSS of
the cellular network to be of low preference and listing the domains
and networks related to the cellular network's private namespaces as
being available via the cellular network's RDNSS. This will cause a
node to send DNS queries by default to the RDNSS of the WLAN
interface (that is, by default, considered to be of medium
preference) and queries related to private namespaces to the RDNSS of
the cellular interface.
In this scenario, the cellular interface can be considered trusted
and WLAN oftentimes untrusted.
3.3. VPN Scenario
Depending on a deployment, there might be interest in using VPN only
for the traffic destined to a enterprise network. The enterprise
might be using private namespaces; hence, related DNS queries need to
be sent over VPN to the enterprise's RDNSS, while by default, the
RDNSS of a local access network might be used for all other traffic.
This can be configured by setting the RDNSS of the VPN interface to
be of low preference and listing the domains and networks related to
an enterprise network's private namespaces being available via the
RDNSS of the VPN interface. This will cause a node to send DNS
queries by default directly to the RDNSS of the WLAN interface (that
is, by default, considered to be of medium preference) and queries
related to private namespaces to the RDNSS of the VPN interface.
In this scenario, the VPN interface can be considered trusted and the
local access network untrusted.
3.4. Dual-Stack Accesses
In all three scenarios, one or more of the connected networks can
support both IPv4 and IPv6. In such a case, both or either of DHCPv4
and DHCPv6 can be used to learn RDNSS selection information.
4. Improved RDNSS Selection
This section describes DHCP options and a procedure that a (stub/
proxy) resolver can utilize for improved RDNSS selection in the face
of private namespaces and multiple simultaneously active network
interfaces. The procedure is subject to limitations of use as
described in Section 4.5. The pseudocode in Appendix C illustrates
how the improved RDNSS selection works.
Savolainen, et al. Standards Track PAGE 8
RFC 6731 RDNSS Selection for MIF Nodes December 2012
4.1. Procedure for Prioritizing RDNSSes and Handling Responses
A resolver SHALL build a preference list of RDNSSes it will contact
depending on the query. To build the list in an optimal way, a node
SHALL request for RDNSS selection information with the DHCP options
defined in Sections 4.2 and 4.3 before any DNS queries need to be
made. With help of the received RDNSS selection information, the
node can determine if any of the available RDNSSes have special
knowledge about specific domains needed for forward DNS lookups or
network addresses (later referred as "network") needed for reverse
DNS lookups.
A resolver lacking more specific information can assume that all
information is available from any RDNSS of any network interface.
The RDNSSes learned by other RDNSS address configuration methods can
be considered as default RDNSSes, but preference-wise, they MUST be
handled as medium preference RDNSSes (see also Section 4.6).
When a DNS query needs to be made, the resolver MUST give highest
preference to the RDNSSes explicitly known to serve a matching domain
or network. The resolver MUST take into account differences in trust
levels (see Section 8.2) of pieces of received RDNSS selection
information. The resolver MUST prefer RDNSSes of trusted interfaces.
The RDNSSes of untrusted interfaces can be of highest preference only
if the trusted interfaces specifically configures low preference
RDNSSes. The non-exhaustive list of cases in Figure 4 illustrates
how the different trust levels of received RDNSS selection
information influence the RDNSS selection logic. In Figure 4,
"Medium", "High", and "Low" indicate the explicitly configured
RDNSS's preference over other RDNSSes. The "Medium" preference is
also used with RDNSSes for which no explicit preference configuration
information is available. The "Specific domains" in Figure 4
indicate the explicitly configured "Domains and networks" private
namespace information that a particular RDNSS has.
A resolver MUST prioritize between equally trusted RDNSSes with the
help of the DHCP option preference field. The resolver MUST NOT
prioritize less trusted RDNSSes higher than trusted, even in the case
when a less trusted RDNSS would apparently have additional
information. In the case of all other things being equal, the
resolver can make the prioritization decision based on its internal
preferences.
Savolainen, et al. Standards Track PAGE 9
RFC 6731 RDNSS Selection for MIF Nodes December 2012
Information from | Information from | Resulting RDNSS
more trusted | less trusted | preference
interface A | interface B | selection
--------------------------+------------------------+-----------------
1. Medium preference | Medium preference | Default:
default | default | A, then B
--------------------------+------------------------+-----------------
2. Medium preference | High preference default| Default:
default | | A, then B
| Specific domains | Specific:
| | A, then B
--------------------------+------------------------+-----------------
3. Low preference default | Medium preference | Default:
| default | B, then A
--------------------------+------------------------+-----------------
4. Low preference default | Medium preference | Default:
| default | B, then A
Specific domains | | Specific:
| | A, then B
--------------------------+------------------------+-----------------
Figure 4: RDNSS Selection in the Case of Different Trust Levels
Because DNSSEC provides cryptographic assurance of the integrity of
DNS data, it is necessary to prefer data that can be validated under
DNSSEC over data that cannot. 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 RDNSS and to rely on that RDNSS to perform DNSSEC
validation (signaling that it has done so using the AD bit). DNSSEC
is necessary to detect forged responses, and without it any DNS
response could be forged or altered. Unless the DNS responses have
been validated with DNSSEC, a node cannot make a decision to prefer
data from any interface with any great assurance.
A node SHALL send requests to RDNSSes in the order defined by the
preference list until an acceptable reply is received, all replies
are received, or a timeout 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 RDNSSes on the preference
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, the first received
response from any RDNSS can be considered acceptable. A DNSSEC-aware
node MAY always contact all RDNSSes in an attempt to receive a
response that can be validated, but contacting all RDNSSes is not
Savolainen, et al. Standards Track PAGE 10
RFC 6731 RDNSS Selection for MIF Nodes December 2012
mandated for the default case as that would consume excess resources
in some deployments.
In the case of a validated NXDOMAIN response being received from an
RDNSS that can provide answers for the queried name, a node MUST NOT
accept non-validated replies from other RDNSSes (see Appendix B for
considerations related to multiple trust anchors).
4.2. RDNSS Selection DHCPv6 Option
DHCPv6 option described below can be used to inform resolvers what
RDNSS can be contacted when initiating forward or reverse DNS lookup
procedures. This option is DNS record type agnostic and applies, for
example, equally to both A and AAAA queries.
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_RDNSS_SELECTION | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| DNS-recursive-name-server (IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |prf| |
+-+-+-+-+-+-+-+-+ Domains and networks |
| (variable length) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: DHCPv6 Option for Explicit Domain Configuration
option-code: OPTION_RDNSS_SELECTION (74)
option-len: Length of the option in octets
DNS-recursive-name-server: An IPv6 address of RDNSS
Reserved: Field reserved for the future. MUST be set to zero and
MUST be ignored on receipt.
Savolainen, et al. Standards Track PAGE 11
RFC 6731 RDNSS Selection for MIF Nodes December 2012
prf: RDNSS preference:
01 High
00 Medium
11 Low
10 Reserved
Reserved preference value (10) MUST NOT be sent. On receipt,
the Reserved value MUST be treated as Medium preference (00).
Domains and networks: The list of domains for forward DNS lookup and
networks for reverse DNS lookup about which
the RDNSS has special knowledge. Field MUST
be encoded as specified in Section 8 of
[RFC 3315]. A special domain of "." is used to
indicate capability to resolve global names
and act as a default RDNSS. Lack of a "."
domain on the list indicates that the RDNSS
only has information related to listed domains
and networks. Networks for reverse mapping
are encoded as defined for IP6.ARPA [RFC 3596]
or IN-ADDR.ARPA [RFC 2317].
A node SHOULD include the Option Request Option (OPTION_ORO
[RFC 3315]) in a DHCPv6 request with the OPTION_RDNSS_SELECTION option
code to inform the DHCPv6 server about the support for the improved
RDNSS selection logic. The DHCPv6 server receiving this information
can then choose to provision RDNSS addresses only with
OPTION_RDNSS_SELECTION.
OPTION_RDNSS_SELECTION contains one or more domains of which the
related RDNSS has particular knowledge. The option can occur
multiple times in a single DHCPv6 message, if multiple RDNSSes are to
be configured. This can be the case, for example, if a network link
has multiple RDNSSes for reliability purposes.
The list of networks MUST cover all the domains configured in this
option. The length of the included networks SHOULD be as long as
possible to avoid potential collision with information received on
other option instances or with options received from DHCP servers of
other network interfaces. Overlapping networks are interpreted so
that the resolver can use any of the RDNSSes for queries matching the
networks.
If OPTION_RDNSS_SELECTION contains an RDNSS 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
Savolainen, et al. Standards Track PAGE 12
RFC 6731 RDNSS Selection for MIF Nodes December 2012
obtained information. However, the node SHOULD NOT extend the
lifetime of earlier information either. When a conflicting RDNSS
address is learned from a less trusted interface, the node MUST
ignore the option.
Like the RDNSS options of [RFC 3646], OPTION_RDNSS_SELECTION MUST NOT
appear in any other than the following DHCPv6 messages: Solicit,
Advertise, Request, Renew, Rebind, Information-Request, and Reply.
The client SHALL periodically refresh information learned with
OPTION_RDNSS_SELECTION. The information SHALL be refreshed on link-
state changes, such as those caused by node mobility, and when
renewing lifetimes of IPv6 addresses configured with DHCPv6.
Additionally, the DHCPv6 Information Refresh Time Option, as
specified in [RFC 4242], can be used to control the update frequency.
4.3. RDNSS Selection DHCPv4 Option
The DHCPv4 option described below can be used to inform resolvers
which RDNSS can be contacted when initiating forward or reverse DNS
lookup procedures. This option is DNS record type agnostic and
applies, for example, equally to both A and AAAA queries.
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) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: DHCPv4 Option for Explicit Domain Configuration
option-code: RDNSS Selection (146)
option-len: Length of the option in octets
Reserved: Field reserved for the future. MUST be set to zero and
MUST be ignored on receipt.
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RFC 6731 RDNSS Selection for MIF Nodes December 2012
prf: RDNSS preference:
01 High
00 Medium
11 Low
10 Reserved
Reserved preference value (10) MUST NOT be sent. On receipt,
the Reserved value MUST be treated as Medium preference (00).
Primary DNS-recursive-name-server's IPv4 address: Address of a
primary RDNSS
Secondary DNS-recursive-name-server's IPv4 address: Address of a
secondary RDNSS
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 about which
the RDNSSes have special knowledge. Field
MUST be encoded as specified in Section 8 of
[RFC 3315]. A special domain of "." is used to
indicate capability to resolve global names
and act as the default RDNSS. Lack of a "."
domain on the list indicates that RDNSSes only
have information related to listed domains and
networks. Networks for reverse mapping are
encoded as defined for IP6.ARPA [RFC 3596] or
IN-ADDR.ARPA [RFC 2317].
The RDNSS Selection option contains one or more domains of which the
primary and secondary RDNSSes have particular knowledge. 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)" [RFC 3396]. When
multiple options are present, the data portions of all option
instances are concatenated together.
The list of networks MUST cover all the domains configured in this
option. The length of the included networks SHOULD be as long as
possible to avoid potential collision with information received on
other option instances or with options received from DHCP servers of
other network interfaces. Overlapping networks are interpreted so
that the resolver can use any of the RDNSSes for queries matching the
networks.
Savolainen, et al. Standards Track PAGE 14
RFC 6731 RDNSS Selection for MIF Nodes December 2012
If the RDNSS Selection option contains an RDNSS 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 extend
the lifetime of earlier information either. When a conflicting RDNSS
address is learned from a less trusted interface, the node MUST
ignore the option.
The client SHALL periodically refresh information learned with the
RDNSS Selection option. The information SHALL be refreshed on link-
state changes, such as those caused by node mobility, and when
extending the lease of IPv4 addresses configured with DHCPv4.
4.4. Scalability Considerations
The general size limitations of the DHCP messages limit the number of
domains and networks that can be carried inside of these RDNSS
selection options. The DHCP options for RDNSS selection are best
suited for those deployments where relatively few and carefully
selected domains and networks are enough.
4.5. Limitations on Use
The RDNSS selection option SHOULD NOT be enabled by default. (In
this section, "RDNSS selection option" refers to the DHCPv4 RDNSS
Selection option and the DHCPv6 OPTION_RDNSS_SELECTION.) The option
can be used in the following environments:
1. The RDNSS selection option is delivered across a secure, trusted
channel.
2. The RDNSS selection option is not secured, but the client on a
node does DNSSEC validation.
3. The RDNSS selection option is not secured, the resolver does
DNSSEC validation, and the client communicates with the resolver
configured with the RDNSS selection option over a secure, trusted
channel.
4. The IP address of the RDNSS that is being recommended in the
RDNSS selection option is known and trusted by the client; that
is, the RDNSS selection option serves not to introduce the client
to a new RDNSS, but rather to inform it that the RDNSS it has
already been configured to trust is available to it for resolving
certain domains.
Savolainen, et al. Standards Track PAGE 15
RFC 6731 RDNSS Selection for MIF Nodes December 2012
As the DHCP by itself cannot tell whether it is using a secure,
trusted channel, or whether the client on a node is performing DNSSEC
validation, this option cannot be used without being explicitly
enabled. The functionality can be enabled for an interface via
administrative means, such as by provisioning tools or manual
configuration. Furthermore, the functionality can be automatically
enabled by a client on a node that knows it is performing DNSSEC
validation or by a node that is configured or hard-coded to trust
certain interfaces (see Section 8.2).
4.6. Coexistence of Various RDNSS Configuration Tools
The DHCPv4 RDNSS Selection option and the DHCPv6
OPTION_RDNSS_SELECTION are designed to coexist with each other and
with other tools used for RDNSS address configuration.
For RDNSS selection purposes, information received from all tools
MUST be combined together into a single list, as discussed in
Section 4.1.
It can happen that DHCPv4 and DHCPv6 are providing conflicting RDNSS
selection information on the same or on equally trusted interfaces.
In such a case, DHCPv6 MUST be preferred unless DHCPv4 is utilizing
additional security frameworks for protecting the messages.
The RDNSSes learned via tools other than the DHCPv4 RDNSS Selection
option and the DHCPv6 OPTION_RDNSS_SELECTION MUST be handled as
default RDNSSes, with medium preference, when building a list of
RDNSSes to talk to (see Section 4.1).
The non-exhaustive list of possible other sources for RDNSS address
configuration are:
(1) DHCPv6 OPTION_DNS_SERVERS defined in [RFC 3646].
(2) DHCPv4 Domain Server option defined in [RFC 2132].
(3) IPv6 Router Advertisement RDNSS Option defined in [RFC 6106].
When the RDNSS selection option contains a default RDNSS address and
other sources are providing RNDSS addresses, the resolver MUST make
the decision about which one to prefer based on the RDNSS preference
field value. If the RDNSS selection option defines medium
preference, then the RDNSS from the RDNSS selection option SHALL be
selected.
If multiple sources are providing same RDNSS(es) IP address(es), each
address MUST be added to the RDNSS list only once.
Savolainen, et al. Standards Track PAGE 16
RFC 6731 RDNSS Selection for MIF Nodes December 2012
If a node had indicated support for OPTION_RDNSS_SELECTION in a
DHCPv6 request, the DHCPv6 server MAY omit sending of
OPTION_DNS_SERVERS. This enables offloading use case where the
network administrator wishes to only advertise low preference default
RDNSSes.
4.7. 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 RDNSS 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
RDNSS(es) of the same interface, or to the same RDNSS, irrespectively
of the FQDN received. Otherwise, referrals can fail.
4.8. Closing Network Interfaces and Local Caches
Cached information related to private namespaces can become obsolete
after the network interface over which the information was learned is
closed (Section 2.2) or a new parallel network interface is opened
that alters RDNSS selection preferences. An implementation SHOULD
ensure obsolete information is not retained in these events. One
implementation approach to avoid unwanted/obsolete responses from the
local cache is to manage per-interface DNS caches or have interface
information stored in the DNS cache. An alternative approach is to
perform, possibly selective, DNS cache flushing on interface change
events.
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.
Savolainen, et al. Standards Track PAGE 17
RFC 6731 RDNSS Selection for MIF Nodes December 2012
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 | | |
| +----------+ | |
| | | |
Figure 7: Illustration of Learning Domains
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 the
DHCPv6 server.
3. The node stores the learned domains and IPv6 networks for later
use.
4. The node opens its second network interface 2.
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 the DHCPv6 server.
Savolainen, et al. Standards Track PAGE 18
RFC 6731 RDNSS Selection for MIF Nodes December 2012
6. The node stores the learned domains and networks for later use.
Figure 8 illustrates how a resolver uses the learned domain
information. Network information use for reverse lookups is not
illustrated, but that would be similar to the example in Figure 8.
Application Node RDNSS RDNSS
on interface 1 on interface 2
| | | |
(1) |--Name REQ-->| | |
| | | |
| +----------------+ | |
(2) | | RDNSS | | |
| | prioritization | | |
| +----------------+ | |
| | | |
(3) | |------------DNS resolution------>|
| |<--------------------------------|
| | | |
(4) |<--Name resp-| | |
| | | |
Figure 8: Example on Choosing Interface Based on Domain
Flow explanations:
1. An application makes a request for resolving an FQDN, e.g.,
'private.domain2.example.com'.
2. A node creates list of RDNSSes to contact and uses configured
RDNSS selection information and stored domain information on
prioritization decisions.
3. The node has chosen interface 2, as it was learned earlier from
DHCPv6 that the interface 2 has domain 'domain2.example.com'.
The node then resolves the requested name using interface 2's
RDNSS to an IPv6 address.
4. The node replies to the application with the resolved IPv6
address.
6. Considerations for Network Administrators
Network administrators deploying private namespaces can assist
advanced nodes in their RDNSS selection process by providing the
information described within this document.
Savolainen, et al. Standards Track PAGE 19
RFC 6731 RDNSS Selection for MIF Nodes December 2012
Private namespaces MUST be globally unique in order to keep DNS
unambiguous and henceforth avoid 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 RDNSSes only if the same operator
provides a SOA record for example.com.
It is RECOMMENDED for administrators utilizing this tool to deploy
DNSSEC for their zone in order to counter attacks against private
namespaces.
7. IANA Considerations
Per this memo, IANA has assigned two new option codes.
The first option code has been assigned for the DHCPv4 RDNSS
Selection option (146) from the "BOOTP Vendor Extensions and DHCP
Options" registry in the group "Dynamic Host Configuration Protocol
(DHCP) and Bootstrap Protocol (BOOTP) Parameters".
The second option code is requested to be assigned for the DHCPv6
OPTION_RDNSS_SELECTION (74) from the "DHCP Option Codes" registry in
the group "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)".
8. Security Considerations
8.1. Attack Vectors
It is possible that attackers might try to utilize the DHCPv4 RDNSS
Selection option or the DHCPv6 OPTION_RDNSS_SELECTION 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 the attacker's RDNSS as the highest preference default
RDNSS.
The best countermeasure for nodes is to implement validating DNSSEC-
aware resolvers. Trusting validation done by an RDNSS is a
possibility only if a node trusts the RDNSS and can use a secure
channel for DNS messages.
Savolainen, et al. Standards Track PAGE 20
RFC 6731 RDNSS Selection for MIF Nodes December 2012
8.2. Trust Levels of Network Interfaces
Trustworthiness of an interface and configuration information
received over the interface is implementation and/or node deployment
dependent, and the details of determining that trust are beyond the
scope of this specification. Trust might, for example, be based on
the nature of the interface: an authenticated and encrypted VPN, or a
layer 2 connection to a trusted home network or to a trusted cellular
network, might be considered trusted, while an unauthenticated and
unencrypted connection to an unknown visited network would likely be
considered untrusted.
In many cases, an implementation might not be able to determine trust
levels without explicit configuration provided by the user or the
node's administrator. Therefore, for example, an implementation
might not by default trust configuration received even over VPN
interfaces. In some occasions, standards defining organizations that
are specific to access network technology might be able to define
trust levels as part of the system design work.
8.3. Importance of Following the Algorithm
Section 4 uses normative language for describing a node's internal
behavior in order to ensure that nodes will not open up new attack
vectors by accidental use of RDNSS selection options. During the
standards work, consensus was that it is safer to not always enable
this option by default, but only when deemed useful and safe.
9. References
9.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[RFC 2317] Eidnes, H., de Groot, G., and P. Vixie, "Classless IN-
ADDR.ARPA delegation", BCP 20, RFC 2317, March 1998.
[RFC 3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC 3396] Lemon, T. and S. Cheshire, "Encoding Long Options in the
Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396,
November 2002.
Savolainen, et al. Standards Track PAGE 21
RFC 6731 RDNSS Selection for MIF Nodes December 2012
[RFC 3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", RFC 3596,
October 2003.
[RFC 4242] Venaas, S., Chown, T., and B. Volz, "Information Refresh
Time Option for Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 4242, November 2005.
9.2. Informative References
[RFC 3397] Aboba, B. and S. Cheshire, "Dynamic Host Configuration
Protocol (DHCP) Domain Search Option", RFC 3397,
November 2002.
[RFC 3442] Lemon, T., Cheshire, S., and B. Volz, "The Classless
Static Route Option for Dynamic Host Configuration
Protocol (DHCP) version 4", RFC 3442, December 2002.
[RFC 3646] Droms, R., "DNS Configuration options for Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
December 2003.
[RFC 4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, November 2005.
[RFC 4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
[RFC 6106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
"IPv6 Router Advertisement Options for DNS Configuration",
RFC 6106, November 2010.
[RFC 6147] 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.
[RFC 6418] Blanchet, M. and P. Seite, "Multiple Interfaces and
Provisioning Domains Problem Statement", RFC 6418,
November 2011.
[WITHOUT-IPV6NAT]
Troan, O., Miles, D., Matsushima, S., Okimoto, T., and D.
Wing, "IPv6 Multihoming without Network Address
Translation", Work in Progress, February 2012.
Savolainen, et al. Standards Track PAGE 22
RFC 6731 RDNSS Selection for MIF Nodes December 2012
Appendix A. Possible Alternative Practices for RDNSS Selection
On some private namespace deployments, explicit policies for RDNSS
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 can 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 RDNSSes are
preferred.
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 via a
cheaper interface. Also, load on RDNSSes 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 others and prefer those, without forgetting that
fallback is required for cases when the 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 [RFC 6106] or
DHCP search list options -- DHCPv4 Domain Search Option number 119
[RFC 3397] and DHCPv6 Domain Search List Option number 24 [RFC 3646].
The node behavior is very similar to that illustrated in the example
in Section 5. While these options are not intended to be used in
RDNSS selection, they can be used by the nodes as hints for smarter
RDNSS prioritization purposes in order to increase likelihood of fast
and successful DNS queries.
Overloading of existing DNS search list options is not without
problems: resolvers would obviously use the domains learned from
search lists for name resolution purposes. This might 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.
Savolainen, et al. Standards Track PAGE 23
RFC 6731 RDNSS Selection for MIF Nodes December 2012
A.3. More-Specific Routes for Reverse Lookup Decisions
[RFC 4191] defines how more-specific routes can be provisioned for
nodes. This information is not intended to be used in RDNSS
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. An RDNSS configured via the same interface as
more-specific routes is more likely capable to answer reverse lookup
questions correctly than an RDNSS of another interface. The
likelihood of success is possibly higher if an RDNSS address is
received in the same RA [RFC 6106] as the more-specific route
information.
A.4. Longest Matching Prefix for Reverse Lookup Decisions
A node can utilize the longest matching prefix approach when deciding
which RDNSS to contact for reverse lookup purposes. Namely, the node
can send a DNS query to an RDNSS learned over an interface having a
longest matching prefix to the address being queried. This approach
can help in cases where Unique Local Addressing (ULA) [RFC 4193]
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 the
order 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.
Appendix C. Pseudocode for RDNSS Selection
This section illustrates the RDNSS selection logic in C-style
pseudocode. The code is not intended to be usable as such; it is
only here for illustration purposes.
The beginning of the whole procedure is a call to "dns_query"
function with a query and list of RDNSSes given as parameters.
Savolainen, et al. Standards Track PAGE 24
RFC 6731 RDNSS Selection for MIF Nodes December 2012
/* This is a structure that holds all information related to an RDNSS.*/
/* Here we include only the information related for this illustration.*/
struct rdnss
{
int prf; /* Preference of an RDNSS. */
int interface; /* Type of an interface RDNSS was learned over. */
struct d_and_n; /* Domains and networks information for this RDNSS. */
};
int has_special_knowledge( const struct rdnss *rdnss,
const char *query)
{
/* This function matches the query to the domains and networks
information of the given RDNSS. The function returns TRUE
if the query matches the domains and networks; otherwise, FALSE. */
/* The implementation of this matching function
is left for reader, or rather writer. */
/* return TRUE if query matches rdnss->d_and_n, otherwise FALSE. */
}
const struct rdnss* compare_rdnss_prf( const struct rdnss *rdnss_1,
const struct rdnss *rdnss_2 )
{
/* This function compares preference values of two RDNSSes and
returns the more preferred RDNSS. The function prefers rdnss_1
in the case of equal preference values. */
if (rdnss_1->prf == HIGH_PRF) return rdnss_1;
if (rdnss_2->prf == HIGH_PRF) return rdnss_2;
if (rdnss_1->prf == MED_PRF) return rdnss_1;
if (rdnss_2->prf == MED_PRF) return rdnss_2;
return rdnss_1;
}
const struct rdnss* compare_rdnss_trust( const struct rdnss *rdnss_1,
const struct rdnss *rdnss_2 )
{
/* This function compares trust of the two given RDNSSes. The trust
is based on the trust on the interface RDNSS was learned on. */
/* If the interface is the same, the trust is also the same,
and hence, function will return NULL to indicate lack of
difference in trust. */
if (rdnss_1->interface == rdnss_2->interface) return NULL;
Savolainen, et al. Standards Track PAGE 25
RFC 6731 RDNSS Selection for MIF Nodes December 2012
/* Otherwise, implementation-specific rules define which interface
is considered more secure than the other. The rules shown here
are only for illustrative purposes and must be overwritten by
real implementations. */
if (rdnss_1->interface == IF_VPN) return rdnss_1;
if (rdnss_2->interface == IF_VPN) return rdnss_2;
if (rdnss_1->interface == IF_CELLULAR) return rdnss_1;
if (rdnss_2->interface == IF_CELLULAR) return rdnss_2;
if (rdnss_1->interface == IF_WLAN) return rdnss_1;
if (rdnss_2->interface == IF_WLAN) return rdnss_2;
/* Both RDNSSes are from unknown interfaces, so return NULL as
trust-based comparison is impossible. */
return NULL;
}
int compare_rdnsses ( const struct rdnss *rdnss_1,
const struct rdnss *rdnss_2,
const char *query)
{
/* This function compares two RDNSSes and decides which one is more
preferred for resolving the query. If the rdnss_1 is more
preferred, the function returns TRUE; otherwise, FALSE. */
const struct rdnss *more_trusted_rdnss = NULL;
const struct rdnss *less_trusted_rdnss = NULL;
/* Find out if either RDNSS is more trusted. */
more_trusted_rdnss = compare_rdnss_trust( rdnss_1, rdnss_2 );
/* Check if either was more trusted. */
if (more_trusted_rdnss)
{
/* Check which RDNSS was less trusted. */
less_trusted_rdnss =
more_trusted_rdnss == rdnss_1 ? rdnss_2 : rdnss_1;
/* If the more trusted interface is not of low preference
or has special knowledge about the query, or the more
trusted is more preferred and the less trusted has no special
information, prefer more trusted. Otherwise, prefer less trusted. */
if (more_trusted_rdnss->prf != LOW_PRF ||
has_special_knowledge( more_trusted_rdnss, query ) ||
(compare_rdnss_prf( more_trusted_rdnss, less_trusted_rdnss)
== more_trusted_rdnss &&
!has_special_knowledge( less_trusted_rdnss, query)))
Savolainen, et al. Standards Track PAGE 26
RFC 6731 RDNSS Selection for MIF Nodes December 2012
{
/* If the more_trusted_rdnss was rdnss_1, return TRUE. */
return more_trusted_rdnss == rdnss_1 ? TRUE : FALSE;
}
else
{
/* If the more_trusted_rdnss was rdnss_1, return TRUE. */
return less_trusted_rdnss == rdnss_1 ? TRUE : FALSE;
}
}
else
{
/* There is no trust difference between RDNSSes; therefore, prefer the
RDNSS that has special knowledge. If both have specific knowledge,
then prefer the rdnss_1. */
if (has_special_knowledge( rdnss_1, query )) return TRUE;
if (has_special_knowledge( rdnss_2, query )) return FALSE;
/* Neither had special knowledge. Therefore, return TRUE if
rdnss_1 is more preferred; otherwise, return FALSE */
return compare_rdnss_prf( rdnss_1 , rdnss_2 )
== rdnss_1 ? TRUE : FALSE;
}
}
void bubble_sort_rdnsses( struct rdnss rdnss_list[],
const int rdnsses,
const char* query)
{
/* This function implements a bubble sort to arrange
RDNSSes in rdnss_list into preference order. */
int i;
int swapped = 0;
struct rdnss rdnss_swap;
do
{
/* Clear swapped-indicator. */
swapped = FALSE;
/* Go through the RDNSS list. */
for (i = 0; i < rdnsses-1; i++)
{
/* Check if the next two items are in the right order, i.e.,
more preferred before less preferred. */
if (compare_rdnsses( &rdnss_list[i],
&rdnss_list[i+1], query) == FALSE)
Savolainen, et al. Standards Track PAGE 27
RFC 6731 RDNSS Selection for MIF Nodes December 2012
{
/* The order between two was not right, so swap these two RDNSSes. */
rdnss_swap = rdnss_list[i];
rdnss_list[i] = rdnss_list[i+1];
rdnss_list[i+1] = rdnss_swap;
swapped = TRUE;
}
}
} while (swapped);
/* No more swaps, which means the rdnss_list is now sorted
into preference order. */
}
struct hostent *dns_query( struct rdnss rdnss_list[],
const int rdnsses,
const char* query )
{
/* Perform address resolution for the query. */
int i;
struct hostent response;
/* Sort the RDNSSes into preference order. */
/* This is the function with which this pseudocode starts. */
bubble_sort_rdnsses( &rdnss_list[0], rdnsses, query );
/* Go thourgh all RDNSSes or until valid response is found. */
for (i = 0; i < rdnsses; i++)
{
/* Use the highest preference RDNSS first. */
response = send_and_validate_dns_query( rndss_list[i], query);
/* Check if DNSSEC validation is in use, and if so, validate the
received response. */
if (dnssec_in_use)
{
response = dnssec_validate(response);
/* If response is validated, use that. Otherwise, proceed to next
RDNSS. */
if (response) return response;
else continue;
}
/* If acceptable response has been found, return it. */
if (response) return response;
}
Savolainen, et al. Standards Track PAGE 28
RFC 6731 RDNSS Selection for MIF Nodes December 2012
return NULL;
}
Appendix D. Acknowledgements
The authors would like to thank the following people for their
valuable feedback and improvement ideas: Mark Andrews, Jari Arkko,
Marcelo Bagnulo, Brian Carpenter, Stuart Cheshire, Lars Eggert,
Stephan Farrell, Tomohiro Fujisaki, Brian Haberman, Peter Koch,
Suresh Krishnan, Murray Kucherawy, Barry Leiba, Edward Lewis, Kurtis
Lindqvist, Arifumi Matsumoto, Erik Nordmark, Steve Padgett, Fabien
Rapin, Matthew Ryan, Robert Sparks, Dave Thaler, Sean Turner,
Margaret Wasserman, Dan Wing, and Dec Wojciech. Ted Lemon and Julien
Laganier receive special thanks for their contributions to security
considerations.
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
Ted Lemon
Nominum, Inc.
2000 Seaport Boulevard
Redwood City, CA 94063
USA
Phone: +1 650 381 6000
EMail: Ted.Lemon@nominum.com
Savolainen, et al. Standards Track PAGE 29
RFC TOTAL SIZE: 68882 bytes
PUBLICATION DATE: Wednesday, December 19th, 2012
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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