Internet Area Working Group D. Lamparter
Internet-Draft NetDEF, Inc.
Intended status: Experimental T. Fiebig
Expires: 25 October 2025 MPI-INF
23 April 2025
DHCPv4 Option for IPv4 routes with IPv6 nexthops
draft-equinox-intarea-dhcpv4-route4via6-01
Abstract
As a result of the shortage of IPv4 addresses, installations are
increasingly recovering IPv4 addresses from uses where they are not
strictly necessary. One such situation is in establishing next hops
for IPv4 routes, replacing this use with IPv6 addresses. This
document describes how to provision DHCP-configured hosts with their
routes in such a situation.
// This draft lives at https://github.com/eqvinox/dhc-route4via6
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 25 October 2025.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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and restrictions with respect to this document. Code Components
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Extended static route function . . . . . . . . . . . . . . . 3
3.1. Applicable next-hop behavior . . . . . . . . . . . . . . 4
3.2. Applicable destination prefix behavior . . . . . . . . . 5
4. Expected host behavior . . . . . . . . . . . . . . . . . . . 6
4.1. Singular address assignment . . . . . . . . . . . . . . . 6
4.2. Overlapping routes from other sources . . . . . . . . . . 6
4.3. Default route . . . . . . . . . . . . . . . . . . . . . . 7
4.4. Routes clashing with the connected subnet . . . . . . . . 7
5. DHCP Option encoding . . . . . . . . . . . . . . . . . . . . 7
5.1. Destination prefix . . . . . . . . . . . . . . . . . . . 8
5.2. Next-hop . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
Example encoded options . . . . . . . . . . . . . . . . . . . . . 11
Revision history (TO BE REMOVED) . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
IPv4 is currently (and will likely be for some time) in a situation
where IPv4 addresses are in short supply, but services still need to
be made available to users that do not yet have IPv6 connectivity.
In some cases, even the service side may not have IPv6 support yet.
In other cases some aspect of the service precludes using proxy-style
service delivery with translation technologies on either or both
sides. This leads to a need for fine-grained deployment of IPv4
connectivity with minimum wastage of addresses.
A particularly interesting improvement enabled by the extension
described here is the complete removal of IPv4 addresses from first-
hop routers acting as DHCPv4/v6 relays, while still providing IPv4
connectivity. In this scenario, the relay (assumed colocated with
the router) has no IPv4 address to use to communicate with the
client. An almost-working solution for this case is presented by
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[DHCPv6] with the [DHCP4o6] transport method. Since this mechanism
encapsulates IPv4 DHCP messages, all related IPv4 configuration can
be carried. However, DHCPv4 does not support a way to encode an IPv6
default gateway or other routes, which is necessary in this case.
If the router and relay are not co-located, the relay may have an
IPv4 address while the router does not. In this case, the option
described in this document could be carried in a plain IPv4 DHCP
message.
Note that the changes described in this document are to DHCPv4, not
DHCPv6.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Extended static route function
This document defines a DHCPv4 option container with 2 suboptions for
use within it. The container can occur multiple times, and the
suboptions can each occur zero, one, or multiple times in each
container. Each container represents one "fully-filled matrix" of
destination prefixes and next-hops, i.e. to be interpreted as a full
cartesian combination of the two sets. In detail, this means:
1. each option container is processed as one unit, building up a
list of destination prefixes and next-hops. It is expected that
the container will in most cases present one destination prefix
with one or two next-hops. However, DHCPv4 clients processing
the option MUST support processing multiple destination prefix
suboptions.
// *NB/TODO: this implies [DHCP-LONGOPT] is NOT applicable here.
Not
// sure if that's a good idea.*
2. when multiple destination prefixes occur in the same container,
all of the next-hops for that container MUST be processed for all
of the destination prefixes in that container.
3. if no destination prefix sub-option occurs in a container, the
IPv4 default route (0.0.0.0/0) is implicitly expressed and the
container MUST be processed as if the 0.0.0.0/0 destination
prefix was explicitly encoded.
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4. if no next-hop sub-option occurs in a container, the "source of
DHCP packet" function is encoded as with the unspecified-address
(::) next-hop described below. The container MUST be processed
as if a :: next-hop were explicitly encoded.
5. both sub-options may be absent, the rules in the previous two
bullet points MUST be applied in combination. This is in fact
expected to be a relatively common setup and is intentionally
made available with a space-preserving encoding.
6. the container can be repeated an arbitrary number of times with
different suboptions. Any single destination prefix MUST NOT be
expressed in more than one container. If encountering duplicate
destination prefixes, clients MAY pick one at random, or MAY
merge nexthops; but this is considered a network side
configuration error. However, note that this refers only to
repeating the exact same destination prefix and prefix length. A
differing prefix length presents a distinct destination prefix
and MUST be processed independently.
7. next-hops MUST NOT repeat within the same container, but the same
next-hop MAY occur in any number of containers and MUST be
processed normally in that case.
8. if multiple next-hops are expressed in the same container, this
represents equal-cost multipath routes across the set of next-
hops. Clients processing the option MUST install at least one of
the listed nexthops but MAY choose any subset at their
discretion, e.g. if their capability of installing ECMP is
limited to some number of next-hops (or is 1, i.e. no ECMP
support.)
3.1. Applicable next-hop behavior
Outlined in [IANA-IPv6], not all IPv6 addresses are valid for use
when encoded as next-hop and some have specific functionality
[IANA-IPv6-SPECIAL] attached to them as follows:
1. the unspecified-address nexthop (which is also implied by the
absence of any nexthop suboption) indicates that the routes in
the container should use the DHCP packet's source address as
nexthop. When [DHCP4o6] is in use, hosts MUST retrieve the IPv6
source address of the DHCPv6 packet carrying the DHCPV4-RESPONSE
message.
// TODO: does it really make sense to support IPv4 here? Maybe
only
// allow this with DHCP4o6?
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2. the Discard-Only Address block (0100::/64) [DISCARD] MAY be used
to express unreachable destinations, in particular if only
limited but not global IPv4 connectivity is available. If this
is used, it MUST be the only next-hop suboption in one container,
clients SHOULD ignore the entire container if this is not
respected. If a client is unable to mark destinations as
unreachable in its routing table, it MAY ignore the container and
SHOULD indicate a client configuration issue in its
administrative interfaces.
3. any unicast IPv6 address MAY be used as next-hop. This
specifically also covers link-local addresses, which the client
MUST support and MUST associate with the link that it has
received the DHCP packets on.
4. // TODO: is ::ffff:192.0.2.123 an IPv4 nexthop? Is this worth
// supporting explicitly, and then saying that the other static
route
// / default gateway options should be ignored?
5. the following types/ranges of addresses are invalid and MUST NOT
be used; no client behavior is specified if any are present in a
container:
* the loopback address (::1)
// TODO: express other directly-connected IPv4 hosts with
this?
* any multicast address (ff00::/8)
* any address with a reserved allocation
3.2. Applicable destination prefix behavior
Some IPv4 prefixes, due to their function given in [IANA-IPv4], do
not make sense to use with this option. DHCPv4 servers MUST NOT
encode and DHCPv4 clients MUST ignore the following prefixes as well
as any more-specific prefixes within them:
* 0.0.0.0/8 (note that 0.0.0.0/0 is less specific than this, and
thus valid)
* 127.0.0.0/8
* 224.0.0.0/4
* 255.255.255.255/32
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Behavior for 240.0.0.0/4 is outside the scope of this document.
4. Expected host behavior
The option described in this document is intended to be implemented
on hosts supporting IPv4 routes with IPv6 nexthops as described in
[v4overv6]. Hosts that do not support the behavior described there
MUST NOT request and MUST ignore the option described in this
document.
Hosts that support [v4overv6] behavior and acquire their
configuration from [DHCP] SHOULD implement the option described here.
4.1. Singular address assignment
While not limited to this case, this option is expected and intended
to be used with assigning a singular IPv4 address to a DHCPv4 client.
This implies that the Subnet Mask option defined in [DHCP-OPT] will
have the value 255.255.255.255.
DHCPv4 clients implementing the option described in this document
MUST process such a Subnet Mask option value as assigning a single
address. There is no network or broadcast address for this "single-
sized" pseudo-subnet. No IPv4 addresses are expressed to be on-link
for the purposes of [ARP] (though they MAY become so due to
additional, e.g. local configuration assigning additional addresses
to the interface.)
Whether the address is bound to the interface or host (strong vs.
weak host model), and whether to perform or skip [DADv4] for the
address is beyond the scope of this document.
4.2. Overlapping routes from other sources
[RFC3442] documents a mechanism to communicate a set of routes and
their nexthops over DHCP. The original DHCP "router" option (code 3)
may communicate a default router. If either of these options is
used, the routes communicated may overlap.
To get consistent and unsurprising behavior, this document places the
follwing expectations on the host:
// TODO: redundant paragraph/merge with text above, needs some
// merging/editing.
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* Routes that describe distinct destination prefixes MUST be handled
independently. This includes routes that differ only in prefix
length. As a result, the routing table MAY contain a mix of IPv4
routes with IPv4 nexthops as well as IPv6 nexthops. Standard
longest prefix match behavior MUST be observed.
* If routes with the same destination prefix are described both with
previously existing methods as well as the options documented
here, the route described by the latter MUST be used and the
routes with IPv4 nexthops MUST be discarded. This notably
includes "unreachable" routes described here; a route with an IPv4
nexthop for such a destination MUST still be discarded.
* Multiple routes for the same destination prefix with different
nexthops of the same address family SHOULD be combined into a
single route for equal-cost multipath behavior, if the host
supports this. If ECMP routes are not supported, the host MUST
deterministically choose one of the routes. This MAY be done by
using the first or last option as seen in DHCP packet order, or by
choosing the numerically lowest or highest nexthop.
4.3. Default route
The default route is expressed here as a route for 0.0.0.0/0, which
is also implied by the absence of any destination prefix suboption.
There is no distinct special encoding for a default gateway, any
nexthop for 0.0.0.0/0 MUST be treated as if it were a default
gateway.
4.4. Routes clashing with the connected subnet
// (only applicable if NOT assigning a single IPv4 address as /32)
// TODO: determine what behavior is reasonable here. (The client is
// likely to be given a /32 subnet mask anyway.)
5. DHCP Option encoding
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (TBA1) | Length | Suboptions |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
: ... :
Type TBA1 (field defined in [DHCP-OPT])
Length as defined in [DHCP-OPT]
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Suboptions zero or more suboptions as defined below
5.1. Destination prefix
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 1 | Length | R | prefixlen |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 prefix (0 - 4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 1
Length as defined in [DHCP-OPT]
R Reserved bits. MUST be sent as zero, MUST be ignored on receipt.
prefixlen IPv4 prefix length, integer value from 0 to 32 (inclusive)
IPv4 prefix The route's destination prefix, encoded in as few bytes
necessary for the given prefixlen value, i.e. calculate length as
ceil(prefixlen / 8).
// TODO: maybe just ditch minimum length encoding, make it match
// other DHCPv4 bits?
Valid values are described in Section 3.2.
If the Length field indicates additional data past the IPv4 prefix
value, clients MUST ignore it. Future documents MAY introduce other
behavior here and servers MUST NOT send any such data until such a
point.
5.2. Next-hop
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 2 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 addresses (n*16 octets) |
: ... :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 2
Length as defined in [DHCP-OPT], a multiple of 16.
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IPv6 addresses One or more IPv6 addresses specifying the nexthop for
this route. Refer to Section 3.1 for valid values and associated
behavior.
// TODO: is this a list, or do we just repeat the suboption? In
// theory, this could have sub-suboptions, but unlikely to need?
6. Security Considerations
_TBD_
7. Privacy Considerations
_TBD_
8. IANA Considerations
A codepoint from the "BOOTP Vendor Extensions and DHCP Options"
registry is requested for use with the container option described in
Section 5.
// Editor note: 2 places of TBA1
A registry is requested to be created for the sub-options in the
option above.
// TBD: proper wording for this, and fill in values 1 & 2
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[DHCP] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://www.rfc-editor.org/rfc/rfc2131>.
[DHCP-OPT] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
<https://www.rfc-editor.org/rfc/rfc2132>.
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[DHCP-LONGOPT]
Lemon, T. and S. Cheshire, "Encoding Long Options in the
Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396,
DOI 10.17487/RFC3396, November 2002,
<https://www.rfc-editor.org/rfc/rfc3396>.
[v4overv6] Chroboczek, J., Kumari, W. A., and T. Høiland-Jørgensen,
"IPv4 routes with an IPv6 next hop", Work in Progress,
Internet-Draft, draft-chroboczek-intarea-v4-via-v6-03, 20
January 2025, <https://datatracker.ietf.org/doc/html/
draft-chroboczek-intarea-v4-via-v6-03>.
[DISCARD] Hilliard, N. and D. Freedman, "A Discard Prefix for IPv6",
RFC 6666, DOI 10.17487/RFC6666, August 2012,
<https://www.rfc-editor.org/rfc/rfc6666>.
[IANA-IPv4]
IANA, "IPv4 Address Space Registry",
<https://www.iana.org/assignments/ipv4-address-space/>.
[IANA-IPv6]
IANA, "Internet Protocol Version 6 Address Space",
<https://www.iana.org/assignments/ipv6-address-space/>.
[IANA-IPv6-SPECIAL]
IANA, "IPv6 Special-Purpose Address Registry",
<https://www.iana.org/assignments/iana-ipv6-special-
registry/>.
9.2. Informative References
[RFC3442] Lemon, T., Cheshire, S., and B. Volz, "The Classless
Static Route Option for Dynamic Host Configuration
Protocol (DHCP) version 4", RFC 3442,
DOI 10.17487/RFC3442, December 2002,
<https://www.rfc-editor.org/info/rfc3442>.
[DHCPv6] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
Richardson, M., Jiang, S., Lemon, T., and T. Winters,
"Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 8415, DOI 10.17487/RFC8415, November 2018,
<https://www.rfc-editor.org/info/rfc8415>.
[DHCP4o6] Sun, Q., Cui, Y., Siodelski, M., Krishnan, S., and I.
Farrer, "DHCPv4-over-DHCPv6 (DHCP 4o6) Transport",
RFC 7341, DOI 10.17487/RFC7341, August 2014,
<https://www.rfc-editor.org/info/rfc7341>.
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[ARP] Plummer, D., "An Ethernet Address Resolution Protocol: Or
Converting Network Protocol Addresses to 48.bit Ethernet
Address for Transmission on Ethernet Hardware", STD 37,
RFC 826, DOI 10.17487/RFC0826, November 1982,
<https://www.rfc-editor.org/rfc/rfc826>.
[DADv4] Cheshire, S., "IPv4 Address Conflict Detection", RFC 5227,
DOI 10.17487/RFC5227, July 2008,
<https://www.rfc-editor.org/rfc/rfc5227>.
Acknowledgements
The authors would like to acknowledge and thank Tomek Mrugalski for
very extensive comments, and in particular pointing out the proper
way to use DHCP options.
Comments and feedback has been received and appreciated from Ole
Troan.
Example encoded options
_TBD: outdated examples removed, will be re-added_
Revision history (TO BE REMOVED)
* -01: scrap single-option encoding, use container instead, and
reference special-purpose IPv6 addresses (e.g. for discard)
* -00:
Authors' Addresses
David 'equinox' Lamparter
NetDEF, Inc.
San Jose,
United States of America
Email: equinox@diac24.net, equinox@opensourcerouting.org
Tobias Fiebig
Max-Planck-Institut fuer Informatik
Campus E14
66123 Saarbruecken
Germany
Phone: +49 681 9325 3527
Email: tfiebig@mpi-inf.mpg.de
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