Network Working Group T. Lemon
Internet-Draft Nominum, Inc.
Intended status: Best Current Practice T. Mrugalski
Expires: August 18, 2014 Internet Systems Consortium, Inc.
February 14, 2014
Customizing DHCP Configuration on the Basis of Network Topology
draft-ietf-dhc-topo-conf-01
Abstract
DHCP servers have evolved over the years to provide significant
functionality beyond that which is described in the DHCP base
specifications. One aspect of this functionality is support for
context-specific configuration information. This memo describes some
such features and makes recommendations as to how they can be used.
Status of This Memo
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This Internet-Draft will expire on August 18, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Locality . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Simple Subnetted Network . . . . . . . . . . . . . . . . . . 7
5. Relay agent running on a host . . . . . . . . . . . . . . . . 9
6. Cascade relays . . . . . . . . . . . . . . . . . . . . . . . 9
7. Regional Configuration Example . . . . . . . . . . . . . . . 9
8. Dynamic Lookup . . . . . . . . . . . . . . . . . . . . . . . 11
9. Relay Agent Configurations . . . . . . . . . . . . . . . . . 12
10. Mutliple subnets on the same link . . . . . . . . . . . . . . 12
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
12. Security Considerations . . . . . . . . . . . . . . . . . . . 13
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
14.1. Normative References . . . . . . . . . . . . . . . . . . 13
14.2. Informative References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
The DHCPv4 [RFC2131] and DHCPv6 [RFC3315] protocol specifications
describe how addresses can be allocated to clients based on network
topology information provided by the DHCP relay infrastructure.
Address allocation decisions are integral to the allocation of
addresses and prefixes in DHCP.
The DHCP protocol also describes mechanisms for provisioning devices
with additional configuration information; for example, DNS [RFC1034]
server addresses, default DNS search domains, and similar
information.
Although it was the intent of the authors of these specifications
that DHCP servers would provision devices with configuration
information appropriate to each device's location on the network,
this practice was never documented, much less described in detail.
Existing DHCP server implementations do in fact provide such
capabilities; the goal of this document is to describe those
capabilities for the benefit both of operators and of protocol
designers who may wish to use DHCP as a means for configuring their
own services, but may not be aware of the capabilities provided by
modern DHCP servers.
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2. Terminology
an IP address with a scope of use wider than the local link.
provider edge router. The provider router closest to the customer.
customer premise equipment device. Typically a router belonging to
the customer that connects directly to the provider link.
3. Locality
Figure 1 illustrates a simple hierarchy of network links with Link D
serving as a backbone to which the DHCP server is attached.
Figure 2 illustrates a more complex case. Although some of its
aspects are unlikely to be seen in an actual production networks,
they are beneficial for explaining finer aspects of the DHCP
protocols.
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Link A Link B
|===+===========| |===========+======|
| |
| |
+---+---+ +---+---+
| relay | | relay |
| A | | B |
+---+---+ +---+---+
| |
| Link C |
|===+==========+=================+======|
|
|
+----+---+ +--------+
| router | | DHCP |
| A | | Server |
+----+---+ +----+---+
| |
| |
| Link D |
|==============+=================+======|
|
|
+----+---+
| router |
| B |
+----+---+
|
|
|===+==========+=================+======|
| Link E |
| |
+---+---+ +---+---+
| relay | | relay |
| C | | D |
+---+---+ +---+---+
| |
| |
|===+===========| |===========+======|
Link F Link G
Figure 1: A simple network
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Link A Link B Link H
|===+==========| |=========+======| |======+======|
| | |
| | |
+---+---+ +---+---+ +---+---+
| relay | | relay | | relay |
| A | | B | | G |
+---+---+ +---+---+ +---+---+
| | |
| Link C | | Link J
|===+==========+==============+======| |======+======|
| |
| |
+----+---+ +--------+ +---+---+
| router | | DHCP | | relay |
| A | | Server | | F |
+----+---+ +----+---+ +---+---+
| | |
| | |
| Link D | |
|==============+=========+=======+=============+======|
| |
| |
+----+---+ +---+---+
| router | | relay |
| B | | E |
+----+---+ +---+---+
| |
| |
|===+==========+=========+=======+======|
| Link E |
| |
+---+---+ +---+---+
| relay | | relay |
| C | | D |
+---+---+ +---+---+
| |
| |
|===+===========| |===========+======|
Link F Link G
Figure 2: Complex network
This diagram allows us to represent a variety of different network
configurations and illustrate how existing DHCP servers can provide
configuration information customized to the particular location from
which a client is making its request.
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It's important to understand the background of how DHCP works when
considering this diagram. DHCP clients are assumed not to have
routable IP addresses when they are attempting to obtain
configuration information.
The reason for making this assumption is that one of the functions of
DHCP is to bootstrap the DHCP client's IP address configuration; if
the client does not yet have an IP address configuration, it cannot
route packets to an off-link DHCP server, and so some kind of relay
mechanism is required.
The details of how this works are different between DHCPv4 and
DHCPv6, but the essence is the same: whether or not the client
actually has an IP configuration, it generally communicates with the
DHCP server by sending its requests to a DHCP relay agent on the
local link; this relay agent, which has a routable IP address, then
forwards the DHCP requests to the DHCP server. In some cases in
DHCPv4, when a DHCP client has a routable IPv4 address, the message
is unicast to the DHCP server rather than going through a relay
agent. In DHCPv6 that is also possible in case where server is
configured with server unicast option and clients are able to take
advantage of it. In such case once the clients get their (presumably
global) addresses, they are able to contact server directly,
bypassing relays. It should be noted that such a mode is completely
controllable by administrators in DHCPv6. (They may simply choose to
not configure server unicast option, thus forcing clients to send
their messages always via relay agents).
In either case, the DHCP server is able to obtain an IP address that
it knows is on-link for the link to which the DHCP client is
connected: either the DHCPv4 client's routable IPv4 address, or the
relay agent's IP address on the link to which the client is
connected.
In DHCPv6 protocol, there are two mechanisms defined in [RFC3315]
that allow server to distinguish which link the relay agent is
connected to. The first mechanism is a link-address field in the
RELAY-FORW and RELAY-REPL messages. Somewhat contrary to its name,
relay agents insert an address that is typically global and can be
used to uniquely identify the link on which the client is located.
In normal circumstances this is the solution that is easiest to
maintain. It requires, however, for the relay agent to have an
address configured on its client-facing interface. If for whatever
reason that is not feasible (e.g. because the relay agent does not
have a global address), the relay agent includes an interface-id
option that identifies the link clients are connected to. It is up
to administrator to make sure that the interface-id is unique within
his administrative domain.
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DHCPv6 also has support for more finely grained link identification,
using Lightweight DHCPv6 Relay Agents [RFC6221] (LDRA). In this
case, in addition to receiving an IPv6 address that is on-link for
the link to which the client is connected, the DHCPv6 server also
receives an Interface Identifier option from the relay agent that can
be used to more precisely identify the client's location on the
network.
What this means in practice is that the DHCP server in all cases has
sufficient information to pinpoint, at the very least, the layer 3
link to which the client is connected, and in some cases which layer
2 link the client is connected to, when the layer 3 link is
aggregated out of multiple layer 2 links.
In all cases, then, the DHCP server will have a link-identifying IP
address, and in some cases it may also have a link-specific
identifier. It should be noted that there is no guarantee that the
link-specific identifier will be unique outside the scope of the
link-identifying IP address.
It is also possible for link-specific identifiers to be nested, so
that the actual identifier that identifies the link is an aggregate
of two or more link-specific identifiers sent by a set of LDRAs in a
chain; in general this functions exactly as if a single identifier
were received from a single LDRA, so we do not treat it specially in
the discussion below, but sites that use chained LDRA configurations
will need to be aware of this when configuring their DHCP servers.
So let's examine the implications of this in terms of how a DHCP
server can deliver targeted supplemental configuration information to
DHCP clients.
4. Simple Subnetted Network
Consider Figure 1 in the context of a simple subnetted network. In
this network, there are four leaf subnets: links A, B, F and G, on
which DHCP clients will be configured. Relays A, B, C and D in this
example are represented in the diagram as IP routers with an embedded
relay function, because this is a very typical configuration, but the
relay function can also be provided in a separate server on each
link.
In a simple network like this, there may be no need for link-specific
configuration in DHCPv6, since local routing information is delivered
through router advertisements. However, in IPv4, it is very typical
to configure the default route using DHCP; in this case, the default
route will be different on each link. In order to accomplish this,
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the DHCP server will need link-specific configuration for the default
route.
To illustrate, we will use an example from a hypothetical DHCP server
that uses a simple JSON notation for configuration. Although we know
of no DHCP server that uses this specific syntax, every commercial
DHCP server provides similar functionality.
{"prefixes":
{"10.0.0.0/24": {"options": {"routers": ["10.0.0.1"]}
"on-link": ["a"]}}
"10.0.1.0/24": {"options": {"routers": ["10.0.1.1"}}
"on-link": ["b"]}
"10.0.2.0/24": {"options": {"routers": ["10.0.2.1"}}
"on-link": ["f"]}
"10.0.3.0/24": {"options": {"routers": ["10.0.3.1"}}
"on-link": ["g"]}}
Figure 2
Figure 3
In Figure 3, we see a configuration example for this scenario: a set
of prefixes, each of which has a set of options and a list of links
for which it is on-link. We have defined one option for each prefix:
a routers option. This option contains a list of values; each list
only has one value, and that value is the IP address of the router
specific to the prefix.
When the DHCP server receives a request, it searches the list of
prefixes for one that encloses the link-identifying IP address
provided by the client or relay agent. The DHCP server then examines
the options list associated with that prefix and returns those
options to the client.
So for example a client connected to link A in the example would have
a link-identifying IP address within the 10.0.0.0/24 prefix, so the
DHCP server would match it to that prefix. Based on the
configuration, the DHCP server would then return a routers option
containing a single IP address: 10.0.0.1. A client on link F would
have a link-identifying address in the 10.0.2.0/24 prefix, and would
receive a routers option containing the IP address 10.0.2.1.
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5. Relay agent running on a host
Relay agent is a DHCP software that may be run on any IP node.
Although it is typically run on a a router, it doesn't have to be
one. Relay agent can be run on a host connected to two links. That
case is presented in Figure 2. There is router B that is connected
to links D and E. At the same time there is also a host that is
connected to the same links. The relay agent software is running on
that host. That is uncommon, but legal configuration.
6. Cascade relays
Let's observe another case shown in Figure 2. Note that in typical
configuration, the clients connected to link G will send their
requests to relay D which will forward its packets directly to the
DHCP server. That is typical, but not the only possible
configuration. It is possible to configure relay agent D to forward
client messages to relay E which in turn will send it to the DHCP
server. This configuration is sometimes referred to as cascade relay
agents.
Note that the relaying mechanism works differently in DHCPv4 and in
DHCPv6. In DHCPv4 only the first relay is able to set the GIADDR
field in the DHCPv4 packet. Any following relays that receive that
packet will not change it as the server needs GIADDR information from
the first relay (i.e. the closest to the client). Server will send
the response back to the GIADDR address, which is the address of the
first relay agent that seen the client's message. That means that
the client messages travel on a different path than the server's
responses. A message from client connected to link G will travel via
relay D, relay E and to the server. A response message will be sent
from the server to relay D which will send it to the client on link
G.
Relaying in DHCPv6 is more structured. Each relay agent encapsulates
a packet that is destined to the server and sends it towards the
server. Depending on the configuration that can be server's unicast
address, a multicast address or next relay agent address. The next
relay repeats the encapsulation process. Although the resulting
packet is more complex (may have up to 32 levels of encapsulation if
traveled through 32 relays), every relay may insert its own options
and it is clear which relay agent inserted which option.
7. Regional Configuration Example
In this example, link C is a regional backbone for an ISP. Link E is
also a regional backbone for that ISP. Relays A, B, C and D are PE
routers, and Links A, B, F and G are actually link aggregators with
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individual layer 2 circuits to each customer--for example, the relays
might be DSLAMs or cable head-end systems. At each customer site we
assume there is a single CPE device attached to the link.
We further assume that links A, B, F and G are each addressed by a
single prefix, although it would be equally valid for each CPE device
to be numbered on a separate prefix.
In a real-world deployment, there would likely be many more than two
PE routers connected to each regional backbone; we have kept the
number small for simplicity.
In this example, the goal is to configure all the devices within a
region with server addresses local to that region, so that service
traffic does not have to be routed between regions unnecessarily.
{"prefixes":
{"2001:DB8:0:0::/40": {"on-link": ["A"]}}
"2001:DB8:100:0::/40": {"on-link": ["B"]}
"2001:DB8:200:0::/40": {"on-link": ["F"]}
"2001:DB8:300:0::/40": {"on-link": ["G"]}}
{"links":
{"A": {"region": "omashu"},
"B": {"region": "omashu"},
"F": {"region": "gaoling"},
"G": {"region": "gaoling"}}}
{"regions":
{"omashu": {"options":
{"sip-servers": ["sip.omashu.example.org"],
"dns-servers": ["dns1.omashu.example.org",
"dns2.omashu.example.org"]}},
"gaoling": {"options":
{"sip-servers": ["sip.gaoling.example.org"],
"dns-servers": ["dns1.gaoling.example.org",
"dns2.gaoling.example.org"]}}}}
Figure 3
In this example, when a request comes in to the DHCP server with a
link-identifying IP address in the 2001:DB8:0:0::/40 prefix, it is
identified as being on link A. The DHCP server then looks on the
list of links to see what region the client is in. Link A is
identified as being in omashu. The DHCP server then looks up omashu
in the set of regions, and discovers a list of region-specific
options.
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The DHCP server then resolves the domain names listed in the options
and sends a sip-server option containing the IP addresses that the
resolver returned for sip.omashu.example.org, and a dns-server option
containing the IP addresses returned by the resolver for
dns1.omashu.example.org and dns2.omashu.example.org.
Similarly, if the DHCP server receives a request from a DHCP client
where the link-identifying IP address is contained by the prefix
2001:DB8:300:0::/40, then the DHCP server identifies the client as
being connected to link G. The DHCP server then identifies link G as
being in the gaoling region, and returns the sip-servers and dns-
servers options specific to that region.
As with the previous example, the exact configuration syntax and
structure shown above does not precisely match what existing DHCP
servers do, but the behavior illustrated in this example can be
accomplished with all existing commercial DHCP servers.
8. Dynamic Lookup
In the Regional example, the configuration listed several domain
names as values for the sip-servers and dns-servers options. The
wire format of both of these options contains one or more IPv6
addresses--there is no way to return a domain name to the client.
This was understood to be an issue when the original DHCP protocol
was defined, and historical implementations even from the very early
days would accept domain names and resolve them. Some early DHCP
implementations, particularly those based on earlier BOOTP
implementations, had very limited capacity for reconfiguration.
However, all modern commercial DHCP servers handle name resolution by
querying the resolver each time a DHCP packet comes in. This means
that if DHCP servers and DNS servers are managed by different
administrative entities, there is no need for the administrators of
the DHCP servers and DNS servers to communicate when changes are
made. When changes are made to the DNS server, these changes are
immediately and automatically adopted by the DHCP server. Similarly,
when DHCP server configurations change, DNS server administrators
need not be aware of this.
It's worth noting that DNS is not the only way to resolve names, and
not all DHCP servers support other techniques (e.g., NIS+ or WINS).
However, since these protocols have all but vanished from common use,
this won't be an issue in new deployments.
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9. Relay Agent Configurations
It's worth mentioning that although we talk about relay agents and
routers in this document mostly as if they are the same device, this
is by no means required by the DHCP protocol. The relay agent is
simply a service that operates on a link, receiving link-local
multicasts or broadcasts and relaying them, using IP routing, to a
DHCP server. As long as the relay has an IP address on the link, and
a default route or more specific route through which it can reach a
DHCP server, it need not be a router, or even have multiple
interfaces.
10. Mutliple subnets on the same link
There are scenarios where there is more than one subnet from the same
protocol family (i.e. two or more IPv4 subnets or two or more IPv6
subnets) configured on the same layer 3 link. One example is a slow
network renumbering where some services are migrated to the new
addressing scheme, but some aren't yet. Second example is a cable
network, where cable modems and the devices connected behind them are
connected to the same layer 2 link. However, operators want the
cable modems and user devices to get addresses from distinct address
spaces, so users couldn't easily access their modems management
interfaces. Such a configuration is often referred to as shared
subnets.
To support such an configuration, additional differentiating
information is required. Many DHCP server implementations offer a
feature that is typically called client classification. The server
segregates incoming packets into one or more classes based on certain
packet characteristics, e.g. presence or value of certains options or
even a match between existing options. Servers require additional
information to handle such configuration, as it can't use the
topographical property of the relay addresses alone to properly
choose a subnet. Such information is always implementation specific.
11. Acknowledgments
Thanks to Dave Thaler for suggesting that even though "everybody
knows" how DHCP servers are deployed in the real world, it might be
worthwhile to have an IETF document that explains what everybody
knows, because in reality not everybody is an expert in how DHCP
servers are administered.
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12. Security Considerations
This document explains existing practice with respect to the use of
Dynamic Host Configuration Protocol [RFC2131] and Dynamic Host
Configuration Protocol Version 6 [RFC3315]. The security
considerations for these protocols are described in their
specifications and in related documents that extend these protocols.
This document introduces no new functionality, and hence no new
security considerations.
13. IANA Considerations
The IANA is hereby absolved of any requirement to take any action in
relation to this document.
14. References
14.1. Normative References
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
14.2. Informative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC6221] Miles, D., Ooghe, S., Dec, W., Krishnan, S., and A.
Kavanagh, "Lightweight DHCPv6 Relay Agent", RFC 6221, May
2011.
Authors' Addresses
Ted Lemon
Nominum, Inc.
2000 Seaport Blvd
Redwood City, CA 94063
USA
Phone: +1-650-381-6000
Email: Ted.Lemon@nominum.com
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Tomek Mrugalski
Internet Systems Consortium, Inc.
950 Charter Street
Redwood City, CA 94063
USA
Phone: +1 650 423 1345
Email: tomasz.mrugalski@gmail.com
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