| Rfc | 5505 |
|---|
| Title | Principles of Internet Host Configuration |
|---|
| Author | B. Aboba, D. Thaler, L.
Andersson, S. Cheshire |
|---|
| Date | May 2009 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | INFORMATIONAL |
|---|
|
Network Working Group B. Aboba
Request for Comments: 5505 D. Thaler
Category: Informational L. Andersson
S. Cheshire
Internet Architecture Board
May 2009
Principles of Internet Host Configuration
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (c) 2009 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 in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Abstract
This document describes principles of Internet host configuration.
It covers issues relating to configuration of Internet-layer
parameters, as well as parameters affecting higher-layer protocols.
Table of Contents
1. Introduction ....................................................3
1.1. Terminology ................................................3
1.2. Internet Host Configuration ................................4
1.2.1. Internet-Layer Configuration ........................4
1.2.2. Higher-Layer Configuration ..........................6
2. Principles ......................................................7
2.1. Minimize Configuration .....................................7
2.2. Less Is More ...............................................7
2.3. Minimize Diversity .........................................8
2.4. Lower-Layer Independence ...................................9
2.5. Configuration Is Not Access Control .......................11
3. Additional Discussion ..........................................12
3.1. Reliance on General-Purpose Mechanisms ....................12
3.2. Relationship between IP Configuration and Service
Discovery .................................................13
3.2.1. Fate Sharing .......................................14
3.3. Discovering Names versus Addresses ........................15
3.4. Dual-Stack Issues .........................................15
3.5. Relationship between Per-Interface and Per-Host
Configuration .............................................16
4. Security Considerations ........................................17
4.1. Configuration Authentication ..............................18
5. Informative References .........................................19
Appendix A. Acknowledgments .......................................24
Appendix B. IAB Members at the Time of This Writing ...............24
1. Introduction
This document describes principles of Internet host [STD3]
configuration. It covers issues relating to configuration of
Internet-layer parameters, as well as parameters affecting higher-
layer protocols.
In recent years, a number of architectural questions have arisen, for
which we provide guidance to protocol developers:
o The protocol layers and general approaches that are most
appropriate for configuration of various parameters.
o The relationship between parameter configuration and service
discovery.
o The relationship between per-interface and per-host configuration.
o The relationship between network access authentication and host
configuration.
o The desirability of supporting self-configuration of parameters or
avoiding parameter configuration altogether.
o The role of link-layer protocols and tunneling protocols in
Internet host configuration.
The role of the link-layer and tunneling protocols is particularly
important, since it can affect the properties of a link as seen by
higher layers (for example, whether privacy extensions [RFC4941] are
available to applications).
1.1. Terminology
link
A communication facility or medium over which nodes can
communicate at the link layer, i.e., the layer immediately below
IP. Examples are Ethernets (simple or bridged), Point-to-Point
Protocol (PPP) links, X.25, Frame Relay, or ATM networks as well
as Internet- or higher-layer "tunnels", such as tunnels over IPv4
or IPv6 itself.
on link
An address that is assigned to an interface on a specified link.
off link
The opposite of "on link"; an address that is not assigned to any
interfaces on the specified link.
mobility agent
Either a home agent or a foreign agent [RFC3344] [RFC3775].
1.2. Internet Host Configuration
1.2.1. Internet-Layer Configuration
Internet-layer configuration is defined as the configuration required
to support the operation of the Internet layer. This includes
configuration of per-interface and per-host parameters, including IP
address(es), subnet prefix(es), default gateway(s), mobility
agent(s), boot service configuration and other parameters:
IP address(es)
Internet Protocol (IP) address configuration includes both
configuration of link-scope addresses as well as global addresses.
Configuration of IP addresses is a vital step, since practically
all of IP networking relies on the assumption that hosts have IP
address(es) associated with (each of) their active network
interface(s). Used as the source address of an IP packet, these
IP addresses indicate the sender of the packet; used as the
destination address of a unicast IP packet, these IP addresses
indicate the intended receiver.
The only common example of IP-based protocols operating without an
IP address involves address configuration, such as the use of
DHCPv4 [RFC2131] to obtain an address. In this case, by
definition, DHCPv4 is operating before the host has an IPv4
address, so the DHCP protocol designers had the choice of either
using IP without an IP address, or not using IP at all. The
benefits of making IPv4 self-reliant, configuring itself using its
own IPv4 packets, instead of depending on some other protocol,
outweighed the drawbacks of having to use IP in this constrained
mode. Use of IP for purposes other than address configuration can
safely assume that the host will have one or more IP addresses,
which may be self-configured link-local addresses [RFC3927]
[RFC4862], or other addresses configured via DHCP or other means.
Subnet prefix(es)
Once a subnet prefix is configured on an interface, hosts with an
IP address can exchange unicast IP packets directly with on-link
hosts within the same subnet prefix.
Default gateway(s)
Once a default gateway is configured on an interface, hosts with
an IP address can send unicast IP packets to that gateway for
forwarding to off-link hosts.
Mobility agent(s)
While Mobile IPv4 [RFC3344] and Mobile IPv6 [RFC3775] include
their own mechanisms for locating home agents, it is also possible
for mobile nodes to utilize dynamic home agent configuration.
Boot service configuration
Boot service configuration is defined as the configuration
necessary for a host to obtain and perhaps also to verify an
appropriate boot image. This is appropriate for disk-less hosts
looking to obtain a boot image via mechanisms such as the Trivial
File Transfer Protocol (TFTP) [RFC1350], Network File System (NFS)
[RFC3530], and Internet Small Computer Systems Interface (iSCSI)
[RFC3720] [RFC4173]. It also may be useful in situations where it
is necessary to update the boot image of a host that supports a
disk, such as in the Preboot Execution Environment [PXE]
[RFC4578]. While strictly speaking, boot services operate above
the Internet layer, where boot service is used to obtain the
Internet-layer code, it may be considered part of Internet-layer
configuration. While boot service parameters may be provided on a
per-interface basis, loading and verification of a boot image
affects behavior of the host as a whole.
Other IP parameters
Internet-layer parameter configuration also includes configuration
of per-host parameters (e.g., hostname) and per-interface
parameters (e.g., IP Time-To-Live (TTL) to use in outgoing
packets, enabling/disabling of IP forwarding and source routing,
and Maximum Transmission Unit (MTU)).
1.2.2. Higher-Layer Configuration
Higher-layer configuration is defined as the configuration required
to support the operation of other components above the Internet-
layer. This includes, for example:
Name Service Configuration
The configuration required for the host to resolve names. This
includes configuration of the addresses of name resolution
servers, including IEN 116 [IEN116], Domain Name System (DNS),
Windows Internet Name Service (WINS), Internet Storage Name
Service (iSNS) [RFC4171] [RFC4174], and Network Information
Service (NIS) servers [RFC3898], and the setting of name
resolution parameters such as the DNS domain and search list
[RFC3397], the NetBIOS node type, etc. It may also include the
transmission or setting of the host's own name. Note that link-
local name resolution services (such as NetBIOS [RFC1001], Link-
Local Multicast Name Resolution (LLMNR) [RFC4795], and multicast
DNS (mDNS) [mDNS]) typically do not require configuration.
Once the host has completed name service configuration, it is
capable of resolving names using name resolution protocols that
require configuration. This not only allows the host to
communicate with off-link hosts whose IP addresses are not known,
but, to the extent that name services requiring configuration are
utilized for service discovery, also enables the host to discover
services available on the network or elsewhere. While name
service parameters can be provided on a per-interface basis, their
configuration will typically affect behavior of the host as a
whole.
Time Service Configuration
Time service configuration includes configuration of servers for
protocols such as the Simple Network Time Protocol (SNTP) and the
Network Time Protocol (NTP). Since accurate determination of the
time may be important to operation of the applications running on
the host (including security services), configuration of time
servers may be a prerequisite for higher-layer operation.
However, it is typically not a requirement for Internet-layer
configuration. While time service parameters can be provided on a
per-interface basis, their configuration will typically affect
behavior of the host as a whole.
Other service configuration
This can include discovery of additional servers and devices, such
as printers, Session Initiation Protocol (SIP) proxies, etc. This
configuration will typically apply to the entire host.
2. Principles
This section describes basic principles of Internet host
configuration.
2.1. Minimize Configuration
Anything that can be configured can be misconfigured. Section 3.8 of
"Architectural Principles of the Internet" [RFC1958] states: "Avoid
options and parameters whenever possible. Any options and parameters
should be configured or negotiated dynamically rather than manually."
That is, to minimize the possibility of configuration errors,
parameters should be automatically computed (or at least have
reasonable defaults) whenever possible. For example, the Path
Maximum Transmission Unit (PMTU) can be discovered, as described in
"Packetization Layer Path MTU Discovery" [RFC4821], "TCP Problems
with Path MTU Discovery" [RFC2923], "Path MTU discovery" [RFC1191],
and "Path MTU Discovery for IP version 6" [RFC1981].
Having a protocol design with many configurable parameters increases
the possibilities for misconfiguration of those parameters, resulting
in failures or other sub-optimal operation. Eliminating or reducing
configurable parameters helps lessen this risk. Where configurable
parameters are necessary or desirable, protocols can reduce the risk
of human error by making these parameters self-configuring, such as
by using capability negotiation within the protocol, or by automated
discovery of other hosts that implement the same protocol.
2.2. Less Is More
The availability of standardized, simple mechanisms for general-
purpose Internet host configuration is highly desirable.
"Architectural Principles of the Internet" [RFC1958] states,
"Performance and cost must be considered as well as functionality"
and "Keep it simple. When in doubt during design, choose the
simplest solution."
To allow protocol support in many types of devices, it is important
to minimize the footprint requirement. For example, IP-based
protocols are used on a wide range of devices, from supercomputers to
small low-cost devices running "embedded" operating systems. Since
the resources (e.g., memory and code size) available for host
configuration may be very small, it is desirable for a host to be
able to configure itself in as simple a manner as possible.
One interesting example is IP support in preboot execution
environments. Since by definition boot configuration is required in
hosts that have not yet fully booted, it is often necessary for pre-
boot code to be executed from Read Only Memory (ROM), with minimal
available memory. Many hosts do not have enough space in this ROM
for even a simple implementation of TCP, so in the Preboot Execution
Environment (PXE) the task of obtaining a boot image is performed
using the User Datagram Protocol over IP (UDP/IP) [RFC768] instead.
This is one reason why Internet-layer configuration mechanisms
typically depend only on IP and UDP. After obtaining the boot image,
the host will have the full facilities of TCP/IP available to it,
including support for reliable transport protocols, IPsec, etc.
In order to reduce complexity, it is desirable for Internet-layer
configuration mechanisms to avoid dependencies on higher layers.
Since embedded devices may be severely constrained on how much code
they can fit within their ROM, designing a configuration mechanism in
such a way that it requires the availability of higher-layer
facilities may make that configuration mechanism unusable in such
devices. In fact, it cannot even be guaranteed that all Internet-
layer facilities will be available. For example, the minimal version
of IP in a host's boot ROM may not implement IP fragmentation and
reassembly.
2.3. Minimize Diversity
The number of host configuration mechanisms should be minimized.
Diversity in Internet host configuration mechanisms presents several
problems:
Interoperability
As configuration diversity increases, it becomes likely that a
host will not support the configuration mechanism(s) available on
the network to which it has attached, creating interoperability
problems.
Footprint
For maximum interoperability, a host would need to implement all
configuration mechanisms used on all the link layers it supports.
This increases the required footprint, a burden for embedded
devices. It also leads to lower quality, since testing resources
(both formal testing, and real-world operational use) are spread
more thinly -- the more different configuration mechanisms a
device supports, the less testing each one is likely to undergo.
Redundancy
To support diversity in host configuration mechanisms, operators
would need to support multiple configuration services to ensure
that hosts connecting to their networks could configure
themselves. This represents an additional expense for little
benefit.
Latency
As configuration diversity increases, hosts supporting multiple
configuration mechanisms may spend increasing effort to determine
which mechanism(s) are supported. This adds to configuration
latency.
Conflicts
Whenever multiple mechanisms are available, it is possible that
multiple configurations will be returned. To handle this, hosts
would need to merge potentially conflicting configurations. This
would require conflict-resolution logic, such as ranking of
potential configuration sources, increasing implementation
complexity.
Additional traffic
To limit configuration latency, hosts may simultaneously attempt
to obtain configuration by multiple mechanisms. This can result
in increasing on-the-wire traffic, both from use of multiple
mechanisms as well as from retransmissions within configuration
mechanisms not implemented on the network.
Security
Support for multiple configuration mechanisms increases the attack
surface without any benefit.
2.4. Lower-Layer Independence
"Architectural Principles of the Internet" [RFC1958] states,
"Modularity is good. If you can keep things separate, do so."
It is becoming increasingly common for hosts to support multiple
network access mechanisms, including dialup, wireless, and wired
local area networks; wireless metropolitan and wide area networks;
etc. The proliferation of network access mechanisms makes it
desirable for hosts to be able to configure themselves on multiple
networks without adding configuration code specific to each new link
layer.
As a result, it is highly desirable for Internet host configuration
mechanisms to be independent of the underlying lower layer. That is,
only the link-layer protocol (whether it be a physical link or a
virtual tunnel link) should be explicitly aware of link-layer
parameters (although those link-layer parameters may be configured by
general Internet-layer mechanisms). Introduction of lower-layer
dependencies increases the likelihood of interoperability problems
and adds Internet-layer configuration mechanisms that hosts need to
implement.
Lower-layer dependencies can be best avoided by keeping Internet host
configuration above the link layer, thereby enabling configuration to
be handled for any link layer that supports IP. In order to provide
media independence, Internet host configuration mechanisms should be
link-layer protocol independent.
While there are examples of Internet-layer configuration within the
link layer (such as in PPP IPv4CP [RFC1332] and "Mobile radio
interface Layer 3 specification; Core network protocols; Stage 3
(Release 5)" [3GPP-24.008]), this approach has disadvantages. These
include the extra complexity of implementing different mechanisms on
different link layers and the difficulty in adding new higher-layer
parameters that would require defining a mechanism in each link-layer
protocol.
For example, "PPP Internet Protocol Control Protocol Extensions for
Name Server Addresses" [RFC1877] was developed prior to the
definition of the DHCPINFORM message in "Dynamic Host Configuration
Protocol" [RFC2131]; at that time, Dynamic Host Configuration
Protocol (DHCP) servers had not been widely implemented on access
devices or deployed in service provider networks. While the design
of IPv4CP was appropriate in 1992, it should not be taken as an
example that new link-layer technologies should emulate. Indeed, in
order to "actively advance PPP's most useful extensions to full
standard, while defending against further enhancements of
questionable value", "IANA Considerations for the Point-to-Point
Protocol (PPP)" [RFC3818] changed the allocation of PPP numbers
(including IPv4CP extensions) so as to no longer be "first come first
served".
In IPv6, where link-layer-independent mechanisms such as stateless
autoconfiguration [RFC4862] and stateless DHCPv6 [RFC3736] are
available, PPP IPv6CP [RFC5072] configures an Interface-Identifier
that is similar to a Media Access Control (MAC) address. This
enables PPP IPv6CP to avoid duplicating DHCPv6 functionality.
However, Internet Key Exchange Version 2 (IKEv2) [RFC4306] utilizes
the same approach as PPP IPv4CP by defining a Configuration Payload
for Internet host configuration for both IPv4 and IPv6. While the
IKEv2 approach reduces the number of packet exchanges, "Dynamic Host
Configuration Protocol (DHCPv4) Configuration of IPsec Tunnel Mode"
[RFC3456] points out that leveraging DHCP has advantages in terms of
address management integration, address pool management,
reconfiguration, and fail-over.
Extensions to link-layer protocols for the purpose of Internet-,
transport-, or application-layer configuration (including server
configuration) should be avoided. Such extensions can negatively
affect the properties of a link as seen by higher layers. For
example, if a link-layer protocol (or tunneling protocol) configures
individual IPv6 addresses and precludes using any other addresses,
then applications that want to use privacy extensions [RFC4941] may
not function well. Similar issues may arise for other types of
addresses, such as Cryptographically Generated Addresses [RFC3972].
Avoiding lower-layer dependencies is desirable even where the lower
layer is link independent. For example, while the Extensible
Authentication Protocol (EAP) may be run over any link satisfying its
requirements (see Section 3.1 of [RFC3748]), many link layers do not
support EAP and therefore Internet-layer configuration mechanisms
that depend on EAP would not be usable on links that support IP but
not EAP.
2.5. Configuration Is Not Access Control
Network access authentication and authorization is a distinct problem
from Internet host configuration. Therefore, network access
authentication and authorization is best handled independently of the
Internet and higher-layer configuration mechanisms.
Having an Internet- or higher-layer protocol authenticate clients is
appropriate to prevent resource exhaustion of a scarce resource on
the server (such as IP addresses or prefixes), but not for preventing
hosts from obtaining access to a link. If the user can manually
configure the host, requiring authentication in order to obtain
configuration parameters (such as an IP address) has little value.
Network administrators who wish to control access to a link can
better achieve this using technologies like Port-Based Network Access
Control [IEEE-802.1X]. Note that client authentication is not
required for Stateless DHCPv6 [RFC3736] since it does not result in
allocation of any limited resources on the server.
3. Additional Discussion
3.1. Reliance on General-Purpose Mechanisms
Protocols should either be self-configuring (especially where fate
sharing is important), or use general-purpose configuration
mechanisms (such as DHCP or a service discovery protocol, as noted in
Section 3.2). The choice should be made taking into account the
architectural principles discussed in Section 2.
Taking into account the general-purpose configuration mechanisms
currently available, we see little need for development of additional
general-purpose configuration mechanisms.
When defining a new host parameter, protocol designers should first
consider whether configuration is indeed necessary (see Section 2.1).
If configuration is necessary, in addition to considering fate
sharing (see Section 3.2.1), protocol designers should consider:
1. The organizational implications for administrators. For example,
routers and servers are often administered by different sets of
individuals, so that configuring a router with server parameters
may require cross-group collaboration.
2. Whether the need is to configure a set of interchangeable servers
or to select a particular server satisfying a set of criteria.
See Section 3.2.
3. Whether IP address(es) should be configured, or name(s). See
Section 3.3.
4. If IP address(es) are configured, whether IPv4 and IPv6 addresses
should be configured simultaneously or separately. See Section
3.4.
5. Whether the parameter is a per-interface or a per-host parameter.
For example, configuration protocols such as DHCP run on a per-
interface basis and hence are more appropriate for per-interface
parameters.
6. How per-interface configuration affects host-wide behavior. For
example, whether the host should select a subset of the per-
interface configurations, or whether the configurations are to
merged, and if so, how this is done. See Section 3.5.
3.2. Relationship between IP Configuration and Service Discovery
Higher-layer configuration often includes configuring server
addresses. The question arises as to how this differs from "service
discovery" as provided by Service Discovery protocols such as
"Service Location Protocol, Version 2" (SLPv2) [RFC2608] or "DNS-
Based Service Discovery" (DNS-SD) [DNS-SD].
In Internet host configuration mechanisms such as DHCP, if multiple
server instances are provided, they are considered interchangeable.
For example, in a list of time servers, the servers are considered
interchangeable because they all provide the exact same service --
telling you the current time. In a list of local caching DNS
servers, the servers are considered interchangeable because they all
should give you the same answer to any DNS query. In service
discovery protocols, on the other hand, a host desires to find a
server satisfying a particular set of criteria, which may vary by
request. When printing a document, it is not the case that any
printer will do. The speed, capabilities, and physical location of
the printer matter to the user.
Information learned via DHCP is typically learned once, at boot time,
and after that may be updated only infrequently (e.g., on DHCP lease
renewal), if at all. This makes DHCP appropriate for information
that is relatively static and unchanging over these time intervals.
Boot-time discovery of server addresses is appropriate for service
types where there are a small number of interchangeable servers that
are of interest to a large number of clients. For example, listing
time servers in a DHCP packet is appropriate because an organization
may typically have only two or three time servers, and most hosts
will be able to make use of that service. Listing all the printers
or file servers at an organization is a lot less useful, because the
list may contain hundreds or thousands of entries, and on a given day
a given user may not use any of the printers in that list.
Service discovery protocols can support discovery of servers on the
Internet, not just those within the local administrative domain. For
example, see "Remote Service Discovery in the Service Location
Protocol (SLP) via DNS SRV" [RFC3832] and DNS-Based Service Discovery
[DNS-SD]. Internet host configuration mechanisms such as DHCP, on
the other hand, typically assume the server or servers in the local
administrative domain contain the authoritative set of information.
For the service discovery problem (i.e., where the criteria varies on
a per-request basis, even from the same host), protocols should
either be self-discovering (if fate sharing is critical), or use a
general-purpose service discovery mechanism.
In order to avoid a dependency on multicast routing, it is necessary
for a host to either restrict discovery to services on the local link
or to discover the location of a Directory Agent (DA). Since the DA
may not be available on the local link, service discovery beyond the
local link is typically dependent on a mechanism for configuring the
DA address or name. As a result, service discovery protocols can
typically not be relied upon for obtaining basic Internet-layer
configuration, although they can be used to obtain higher-layer
configuration parameters.
3.2.1. Fate Sharing
If a server (or set of servers) is needed to get a set of
configuration parameters, "fate sharing" (Section 2.3 of [RFC1958])
is preserved if those parameters are ones that cannot be usefully
used without those servers being available. In this case,
successfully obtaining those parameters via other means has little
benefit if they cannot be used because the required servers are not
available. The possibility of incorrect information being configured
is minimized if there is only one machine that is authoritative for
the information (i.e., there is no need to keep multiple
authoritative servers in sync). For example, learning default
gateways via Router Advertisements provides perfect fate sharing.
That is, gateway addresses can be obtained if and only if they can
actually be used. Similarly, obtaining DNS server configuration from
a DNS server would provide fate sharing since the configuration would
only be obtainable if the DNS server were available.
While fate sharing is a desirable property of a configuration
mechanism, in some situations fate sharing may not be possible. When
utilized to discover services on the local link, service discovery
protocols typically provide for fate sharing, since hosts providing
service information typically also provide the services. However,
this is no longer the case when service discovery is assisted by a
Directory Agent (DA). First of all, the DA's list of operational
servers may not be current, so it is possible that the DA may provide
clients with service information that is out of date. For example, a
DA's response to a client's service discovery query may contain stale
information about servers that are no longer operational. Similarly,
recently introduced servers might not yet have registered themselves
with the DA. Furthermore, the use of a DA for service discovery also
introduces a dependency on whether the DA is operational, even though
the DA is typically not involved in the delivery of the service.
Similar limitations exist for other server-based configuration
mechanisms such as DHCP. Typically DHCP servers do not check for the
liveness of the configuration information they provide, and do not
discover new configuration information automatically. As a result,
there is no guarantee that configuration information will be current.
Section 3.3 of "IPv6 Host Configuration of DNS Server Information
Approaches" [RFC4339] discusses the use of well-known anycast
addresses for discovery of DNS servers. The use of anycast addresses
enables fate sharing, even where the anycast address is provided by
an unrelated server. However, in order to be universally useful,
this approach would require allocation of one or more well-known
anycast addresses for each service. Configuration of more than one
anycast address is desirable to allow the client to fail over faster
than would be possible from routing protocol convergence.
3.3. Discovering Names vs. Addresses
In discovering servers other than name resolution servers, it is
possible to either discover the IP addresses of the server(s), or to
discover names, each of which may resolve to a list of addresses.
It is typically more efficient to obtain the list of addresses
directly, since this avoids the extra name resolution steps and
accompanying latency. On the other hand, where servers are mobile,
the name-to-address binding may change, requiring a fresh set of
addresses to be obtained. Where the configuration mechanism does not
support fate sharing (e.g., DHCP), providing a name rather than an
address can simplify operations, assuming that the server's new
address is manually or automatically updated in the DNS; in this
case, there is no need to re-do parameter configuration, since the
name is still valid. Where fate sharing is supported (e.g., service
discovery protocols), a fresh address can be obtained by re-
initiating parameter configuration.
In providing the IP addresses for a set of servers, it is desirable
to distinguish which IP addresses belong to which servers. If a
server IP address is unreachable, this enables the host to try the IP
address of another server, rather than another IP address of the same
server, in case the server is down. This can be enabled by
distinguishing which addresses belong to the same server.
3.4. Dual-Stack Issues
One use for learning a list of interchangeable server addresses is
for fault tolerance, in case one or more of the servers are
unresponsive. Hosts will typically try the addresses in turn, only
attempting to use the second and subsequent addresses in the list if
the first one fails to respond quickly enough. In such cases, having
the list sorted in order of expected likelihood of success will help
clients get results faster. For hosts that support both IPv4 and
IPv6, it is desirable to obtain both IPv4 and IPv6 server addresses
within a single list. Obtaining IPv4 and IPv6 addresses in separate
lists, without indicating which server(s) they correspond to,
requires the host to use a heuristic to merge the lists.
For example, assume there are two servers, A and B, each with one
IPv4 address and one IPv6 address. If the first address the host
should try is (say) the IPv6 address of server A, then the second
address the host should try, if the first one fails, would generally
be the IPv4 address of server B. This is because the failure of the
first address could be due to either server A being down, or some
problem with the host's IPv6 address, or a problem with connectivity
to server A. Trying the IPv4 address next is preferred since the
reachability of the IPv4 address is independent of all potential
failure causes.
If the list of IPv4 server addresses were obtained separately from
the list of IPv6 server addresses, a host trying to merge the lists
would not know which IPv4 addresses belonged to the same server as
the IPv6 address it just tried. This can be solved either by
explicitly distinguishing which addresses belong to which server or,
more simply, by configuring the host with a combined list of both
IPv4 and IPv6 addresses. Note that the same issue can arise with any
mechanism (e.g., DHCP, DNS, etc.) for obtaining server IP addresses.
Configuring a combined list of both IPv4 and IPv6 addresses gives the
configuration mechanism control over the ordering of addresses, as
compared with configuring a name and allowing the host resolver to
determine the address list ordering. See "Dynamic Host Configuration
Protocol (DHCP): IPv4 and IPv6 Dual-Stack Issues" [RFC4477] for more
discussion of dual-stack issues in the context of DHCP.
3.5. Relationship between Per-Interface and Per-Host Configuration
Parameters that are configured or acquired on a per-interface basis
can affect behavior of the host as a whole. Where only a single
configuration can be applied to a host, the host may need to
prioritize the per-interface configuration information in some way
(e.g., most trusted to least trusted). If the host needs to merge
per-interface configuration to produce a host-wide configuration, it
may need to take the union of the per-host configuration parameters
and order them in some way (e.g., highest speed interface to lowest
speed interface). Which procedure is to be applied and how this is
accomplished may vary depending on the parameter being configured.
Examples include:
Boot service configuration
While boot service configuration can be provided on multiple
interfaces, a given host may be limited in the number of boot
loads that it can handle simultaneously. For example, a host not
supporting virtualization may only be capable of handling a single
boot load at a time, or a host capable of supporting N virtual
machines may only be capable of handling up to N simultaneous boot
loads. As a result, a host may need to select which boot load(s)
it will act on, out of those configured on a per-interface basis.
This requires that the host prioritize them (e.g., most to least
trusted).
Name service configuration
While name service configuration is provided on a per-interface
basis, name resolution configuration typically will affect
behavior of the host as a whole. For example, given the
configuration of DNS server addresses and searchlist parameters on
each interface, the host determines what sequence of name service
queries is to be sent on which interfaces.
Since the algorithms used to determine per-host behavior based on
per-interface configuration can affect interoperability, it is
important for these algorithms to be understood by implementers. We
therefore recommend that documents defining per-interface mechanisms
for acquiring per-host configuration (e.g., DHCP or IPv6 Router
Advertisement options) include guidance on how to deal with multiple
interfaces. This may include discussions of the following items:
1. Merging. How are per-interface configurations combined to produce
a per-host configuration? Is a single configuration selected, or
is the union of the configurations taken?
2. Prioritization. Are the per-interface configurations prioritized
as part of the merge process? If so, what are some of the
considerations to be taken into account in prioritization?
4. Security Considerations
Secure IP configuration presents a number of challenges. In addition
to denial-of-service and man-in-the-middle attacks, attacks on
configuration mechanisms may target particular parameters. For
example, attackers may target DNS server configuration in order to
support subsequent phishing or pharming attacks such as those
described in "New trojan in mass DNS hijack" [DNSTrojan]. A number
of issues exist with various classes of parameters, as discussed in
Section 2.6, Section 4.2.7 of "IPv6 Neighbor Discovery (ND) Trust
Models and Threats" [RFC3756], Section 1.1 of "Authentication for
DHCP Messages" [RFC3118], and Section 23 of "Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)" [RFC3315]. Given the
potential vulnerabilities, hosts often restrict support for DHCP
options to the minimum set required to provide basic TCP/IP
configuration.
Since boot configuration determines the boot image to be run by the
host, a successful attack on boot configuration could result in an
attacker gaining complete control over a host. As a result, it is
particularly important that boot configuration be secured.
Approaches to boot configuration security are described in
"Bootstrapping Clients using the Internet Small Computer System
Interface (iSCSI) Protocol" [RFC4173] and "Preboot Execution
Environment (PXE) Specification" [PXE].
4.1. Configuration Authentication
The techniques available for securing Internet-layer configuration
are limited. While it is technically possible to perform a very
limited subset of IP networking operations without an IP address, the
capabilities are severely restricted. A host without an IP address
cannot receive conventional unicast IP packets, only IP packets sent
to the broadcast or a multicast address. Configuration of an IP
address enables the use of IP fragmentation; packets sent from the
unknown address cannot be reliably reassembled, since fragments from
multiple hosts using the unknown address might be reassembled into a
single IP packet. Without an IP address, it is not possible to take
advantage of security facilities such as IPsec, specified in
"Security Architecture for the Internet Protocol" [RFC4301] or
Transport Layer Security (TLS) [RFC5246]. As a result, configuration
security is typically implemented within the configuration protocols
themselves.
PPP [RFC1661] does not support secure negotiation within IPv4CP
[RFC1332] or IPv6CP [RFC5072], enabling an attacker with access to
the link to subvert the negotiation. In contrast, IKEv2 [RFC4306]
provides encryption, integrity, and replay protection for
configuration exchanges.
Where configuration packets are only expected to originate on
particular links or from particular hosts, filtering can help control
configuration spoofing. For example, a wireless access point usually
has no reason to forward broadcast DHCP DISCOVER packets to its
wireless clients, and usually should drop any DHCP OFFER packets
received from those wireless clients, since, generally speaking,
wireless clients should be requesting addresses from the network, not
offering them. To prevent spoofing, communication between the DHCP
relay and servers can be authenticated and integrity protected using
a mechanism such as IPsec.
Internet-layer secure configuration mechanisms include SEcure
Neighbor Discovery (SEND) [RFC3971] for IPv6 stateless address
autoconfiguration [RFC4862], or DHCP authentication for stateful
address configuration. DHCPv4 [RFC2131] initially did not include
support for security; this was added in "Authentication for DHCP
Messages" [RFC3118]. DHCPv6 [RFC3315] included security support.
However, DHCP authentication is not widely implemented for either
DHCPv4 or DHCPv6.
Higher-layer configuration can make use of a wider range of security
techniques. When DHCP authentication is supported, higher-layer
configuration parameters provided by DHCP can be secured. However,
even if a host does not support DHCPv6 authentication, higher-layer
configuration via Stateless DHCPv6 [RFC3736] can still be secured
with IPsec.
Possible exceptions can exist where security facilities are not
available until later in the boot process. It may be difficult to
secure boot configuration even once the Internet layer has been
configured, if security functionality is not available until after
boot configuration has been completed. For example, it is possible
that Kerberos, IPsec, or TLS will not be available until later in the
boot process; see "Bootstrapping Clients using the Internet Small
Computer System Interface (iSCSI) Protocol" [RFC4173] for discussion.
Where public key cryptography is used to authenticate and integrity-
protect configuration, hosts need to be configured with trust anchors
in order to validate received configuration messages. For a node
that visits multiple administrative domains, acquiring the required
trust anchors may be difficult.
5. Informative References
[3GPP-24.008] 3GPP TS 24.008 V5.8.0, "Mobile radio interface Layer 3
specification; Core network protocols; Stage 3 (Release
5)", June 2003.
[DNSTrojan] Goodin, D., "New trojan in mass DNS hijack", The
Register, December 5, 2008,
http://www.theregister.co.uk/2008/12/05/
new_dnschanger_hijacks/
[IEN116] J. Postel, "Internet Name Server", IEN 116, August
1979, http://www.ietf.org/rfc/ien/ien116.txt
[IEEE-802.1X] Institute of Electrical and Electronics Engineers,
"Local and Metropolitan Area Networks: Port-Based
Network Access Control", IEEE Standard 802.1X-2004,
December 2004.
[DNS-SD] Cheshire, S., and M. Krochmal, "DNS-Based Service
Discovery", Work in Progress, September 2008.
[mDNS] Cheshire, S. and M. Krochmal, "Multicast DNS", Work in
Progress, September 2008.
[PXE] Henry, M. and M. Johnston, "Preboot Execution
Environment (PXE) Specification", September 1999,
http://www.pix.net/software/pxeboot/archive/pxespec.pdf
[RFC768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC1001] NetBIOS Working Group in the Defense Advanced Research
Projects Agency, Internet Activities Board, and End-
to-End Services Task Force, "Protocol standard for a
NetBIOS service on a TCP/UDP transport: Concepts and
methods", STD 19, RFC 1001, March 1987.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC
1191, November 1990.
[RFC1332] McGregor, G., "The PPP Internet Protocol Control
Protocol (IPCP)", RFC 1332, May 1992.
[RFC1350] Sollins, K., "The TFTP Protocol (Revision 2)", STD 33,
RFC 1350, July 1992.
[RFC1661] Simpson, W., Ed., "The Point-to-Point Protocol (PPP)",
STD 51, RFC 1661, July 1994.
[RFC1877] Cobb, S., "PPP Internet Protocol Control Protocol
Extensions for Name Server Addresses", RFC 1877,
December 1995.
[RFC1958] Carpenter, B., Ed., "Architectural Principles of the
Internet", RFC 1958, June 1996.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU
Discovery for IP version 6", RFC 1981, August 1996.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC2608] Guttman, E., Perkins, C., Veizades, J., and M. Day,
"Service Location Protocol, Version 2", RFC 2608, June
1999.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC
2923, September 2000.
[RFC3118] Droms, R., Ed., and W. Arbaugh, Ed., "Authentication
for DHCP Messages", RFC 3118, June 2001.
[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T.,
Perkins, C., and M. Carney, "Dynamic Host Configuration
Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3344] Perkins, C., Ed., "IP Mobility Support for IPv4", RFC
3344, August 2002.
[RFC3397] Aboba, B. and S. Cheshire, "Dynamic Host Configuration
Protocol (DHCP) Domain Search Option", RFC 3397,
November 2002.
[RFC3456] Patel, B., Aboba, B., Kelly, S., and V. Gupta, "Dynamic
Host Configuration Protocol (DHCPv4) Configuration of
IPsec Tunnel Mode", RFC 3456, January 2003.
[RFC3530] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
Beame, C., Eisler, M., and D. Noveck, "Network File
System (NFS) version 4 Protocol", RFC 3530, April 2003.
[RFC3720] Satran, J., Meth, K., Sapuntzakis, C., Chadalapaka, M.,
and E. Zeidner, "Internet Small Computer Systems
Interface (iSCSI)", RFC 3720, April 2004.
[RFC3736] Droms, R., "Stateless Dynamic Host Configuration
Protocol (DHCP) Service for IPv6", RFC 3736, April
2004.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and
H. Levkowetz, Ed., "Extensible Authentication Protocol
(EAP)", RFC 3748, June 2004.
[RFC3756] Nikander, P., Ed., Kempf, J., and E. Nordmark, "IPv6
Neighbor Discovery (ND) Trust Models and Threats", RFC
3756, May 2004.
[RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility
Support in IPv6", RFC 3775, June 2004.
[RFC3818] Schryver, V., "IANA Considerations for the Point-to-
Point Protocol (PPP)", BCP 88, RFC 3818, June 2004.
[RFC3832] Zhao, W., Schulzrinne, H., Guttman, E., Bisdikian, C.,
and W. Jerome, "Remote Service Discovery in the Service
Location Protocol (SLP) via DNS SRV", RFC 3832, July
2004.
[RFC3898] Kalusivalingam, V., "Network Information Service (NIS)
Configuration Options for Dynamic Host Configuration
Protocol for IPv6 (DHCPv6)", RFC 3898, October 2004.
[RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
Configuration of IPv4 Link-Local Addresses", RFC 3927,
May 2005.
[RFC3971] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
"SEcure Neighbor Discovery (SEND)", RFC 3971, March
2005.
[RFC3972] Aura, T., "Cryptographically Generated Addresses
(CGA)", RFC 3972, March 2005.
[RFC4171] Tseng, J., Gibbons, K., Travostino, F., Du Laney, C.,
and J. Souza, "Internet Storage Name Service (iSNS)",
RFC 4171, September 2005.
[RFC4173] Sarkar, P., Missimer, D., and C. Sapuntzakis,
"Bootstrapping Clients using the Internet Small
Computer System Interface (iSCSI) Protocol", RFC 4173,
September 2005.
[RFC4174] Monia, C., Tseng, J., and K. Gibbons, "The IPv4 Dynamic
Host Configuration Protocol (DHCP) Option for the
Internet Storage Name Service", RFC 4174, September
2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4306] Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
Protocol", RFC 4306, December 2005.
[RFC4339] Jeong, J., Ed., "IPv6 Host Configuration of DNS Server
Information Approaches", RFC 4339, February 2006.
[RFC4477] Chown, T., Venaas, S., and C. Strauf, "Dynamic Host
Configuration Protocol (DHCP): IPv4 and IPv6 Dual-Stack
Issues", RFC 4477, May 2006.
[RFC4578] Johnston, M. and S. Venaas, Ed., "Dynamic Host
Configuration Protocol (DHCP) Options for the Intel
Preboot eXecution Environment (PXE)", RFC 4578,
November 2006.
[RFC4795] Aboba, B., Thaler, D., and L. Esibov, "Link-local
Multicast Name Resolution (LLMNR)", RFC 4795, January
2007.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path
MTU Discovery", RFC 4821, March 2007.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
[RFC5072] Varada, S., Ed., Haskins, D., and E. Allen, "IP Version
6 over PPP", RFC 5072, September 2007.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.2", RFC 5246, August
2008.
[STD3] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123, October
1989.
Appendix A. Acknowledgments
Elwyn Davies, Bob Hinden, Pasi Eronen, Jari Arkko, Pekka Savola,
James Kempf, Ted Hardie, and Alfred Hoenes provided valuable input on
this document.
Appendix B. IAB Members at the Time of This Writing
Loa Andersson
Gonzalo Camarillo
Stuart Cheshire
Russ Housley
Olaf Kolkman
Gregory Lebovitz
Barry Leiba
Kurtis Lindqvist
Andrew Malis
Danny McPherson
David Oran
Dave Thaler
Lixia Zhang
Authors' Addresses
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
EMail: bernarda@microsoft.com
Dave Thaler
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
EMail: dthaler@microsoft.com
Loa Andersson
Ericsson AB
EMail: loa.andersson@ericsson.com
Stuart Cheshire
Apple Computer, Inc.
1 Infinite Loop
Cupertino, CA 95014
EMail: cheshire@apple.com