Softwire Provisioning
using DHCPv4 Over DHCPv6Deutsche Telekom AGCTO-ATI, Landgrabenweg 151BonnNRW53227Germanyian.farrer@telekom.deTsinghua UniversityBeijing100084P.R. China+86-10-6278-5822sunqi.ietf@gmail.comTsinghua UniversityBeijing100084P.R. China+86-10-6260-3059yong@csnet1.cs.tsinghua.edu.cnTsinghua UniversityBeijing100084P.R. China+86-10-6278-5822lh.sunlinh@gmail.com
Internet
DHCWGDHCPv4 over DHCPv6 (RFC7341) is a mechanism for dynamically
configuring IPv4 for use as an over-the-top service in a IPv6-only
network. Softwires are an example of such a service. For DHCPv4 over DHCPv6
(DHCP 4o6) to function with some IPv4-over-IPv6 softwire mechanisms
and deployment scenarios (e.g., RFC7596 or RFC7597), the operator
needs to know the IPv6 address that the client will use as the
source of IPv4-in-IPv6 softwire tunnel. This address, in
conjunction with the client's IPv4 address, and (in some deployments)
the Port Set ID are used to create a binding table entry in the
operator's softwire tunnel concentrator. This memo defines a
DHCPv6 option to convey IPv6 parameters for establishing the softwire
tunnel and a DHCPv4 option (to be used only with DHCP 4o6) to
communicate the source tunnel IPv6 address between the DHCP 4o6
client and server. It is designed to work in conjunction with
the IPv4 address allocation process.
DHCPv6 Options for Configuration of Softwire Address and
Port-Mapped Clients (RFC7598) describes a deterministic
DHCPv6 based mechanism for provisioning softwires. This document
updates "DHCPv6 Options for Configuration of Softwire Address
and Port-Mapped Clients" (RFC7598), allowing OPTION_S46_BR (90)
to be enumerated in the DHCPv6 client's Option Request Option (ORO)
request and appear directly within subsequent messages sent by
the DHCPv6 server.
Deterministic IPv4-over-IPv6 transition technologies require
that elements are pre-configured with binding rules for routing
traffic to clients. This places a constraint on the choice of
address used as the client's softwire source address: it must
use a pre-determined prefix which is usually
configured on the home gateway device.
describes a DHCPv6 based mechanism for provisioning such
deterministic softwires.
A dynamic provisioning model, such as using DHCPv4
over DHCPv6 (DHCP 4o6) allows much more
flexibility in the location of the IPv4-over-IPv6 softwire source
address. In this model, the IPv6 address is dynamically
communicated back to the service provider allowing the
corresponding softwire configuration to be created in the border
router (BR).
The DHCP 4o6 client and softwire client could be run on end
devices attached to a network segment using any routable IPv6
prefix allocated to an end-user, located anywhere within an arbitrary
home network topology. Dynamic allocation also helps to optimize
IPv4 resource usage as only clients which are actively renewing
their IPv4 lease hold on to the address.
This document describes a mechanism for dynamically provisioning
softwires created using DHCP 4o6, including provisioning the
client with the address of the softwire border router (BR) and
informing the service provider of client's binding between
the dynamically allocated IPv4 address and Port Set ID
and the IPv6 address that the softwire Initiator will use for
accessing IPv4-over-IPv6 services.
The mechanism operates alongside the DHCP 4o6 message flows to
communicate the binding information over the IPv6-only network.
The DHCP 4o6 server provides a single point in the network
which holds the current client binding information. The service
provider can then use this binding information to provision
other functional elements, such as the BR(s).
The mechanism described in this document is only suitable for use for
provisioning softwire clients via DHCP 4o6. The options described here
are only applicable within the DHCP 4o6 message exchange process.
Current softwire technologies suitable for extending to incorporate
DHCP 4o6 with dynamic IPv4 address leasing include
and .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
when, and only when, they appear in all capitals, as shown here.In order to provision a softwire, both IPv6 and IPv4 configuration
needs to be passed to the client. To map this to the DHCP 4o6
configuration process, the IPv6 configuration is carried in
DHCPv6 options , carried
inside the DHCPv6 message DHCPV4-RESPONSE (21) sent by the server.
OPTION_S46_BR (90) is used to provision the remote IPv6 address for
the softwire border router (see below).
OPTION_S46_BIND_IPV6_PREFIX (TBD1), is optionally sent by the
DHCP 4o6 server to indicate to the client a preferred IPv6
prefix for binding the received IPv4 configuration and sourcing
tunnel traffic. This may be necessary if there are multiple IPv6
prefixes in use in the customer network (e.g., Unique Local Addresses
(ULAs)), or if the specific IPv4-over-IPv6 transition mechanism
requires the use of a particular prefix for any reason.
IPv4 configuration is carried in DHCPv4 messages ,
(inside the DHCP 4o6 option OPTION_DHCPV4_MSG (87)) using the mechanism
described in .
In order for the client to communicate the softwire source
address, a new DHCPv4 option OPTION_DHCP4O6_S46_SADDR (TBD2) is
defined in this document. This is included in DHCPREQUEST
messages sent by the client and is stored by the server for the
lifetime of the IPv4 address lease.
Section 4.2 of defines option
OPTION_S46_BR(90) for communicating remote softwire border
relay (BR) IPv6 address(es) to a client, but mandates that
the option can only be used when encapsulated within one of
the softwire container options: OPTION_S46_CONT_MAPE (94) or
OPTION_S46_CONT_LW(96). From Section 3 of
:
"Softwire46 DHCPv6 clients that receive provisioning options that
are not encapsulated in container options MUST silently ignore these
options."This document updates , removing
this restriction for OPTION_S46_BR (90), allowing it to be
enumerated in the client's ORO request and appear directly
within subsequent messages sent by the DHCPv6 server.
The following diagram shows the relevant extensions to the
successful DHCP 4o6 IPv4 allocation client/server message flow for
the softwire source address function. The full process, including
error handling is described in .
In each step, the DHCPv6 portion of the message and any relevant
option is shown above the arrow. The DHCP 4o6 content of the
message and its relevant options are below the arrow. All the
DHCPv4 messages are encapsulated in DHCPV4-QUERY (20) or
DHCPV4-RESPONSE (21) messages. Where relevant, the necessary
options and their contents are shown.
The client constructs a DHCPv6
'DHCPV4-QUERY(20)' message. This message contains two
options: DHCPv6 OPTION_ORO (6) and OPTION_DHCPV4_MSG (87).
OPTION_ORO lists '90' (OPTION_S46_BR) and 'TBD1'
(OPTION_S46_BIND_IPV6_PREFIX). OPTION_DHCPV4_MSG contains a
DHCPv4 DHCPDISCOVER message.
The server responds with a DHCPv6
'DHCPV4-RESPONSE (21)' message. This message contains an
OPTION_S46_BR (90) containing the IPv6 address of the BR for
the client's softwire configuration. The message may also
optionally contain OPTION_S46_BIND_IPV6_PREFIX (TBD1).
OPTION_DHCPV4_MSG contains a DHCPv4 DHCPOFFER message. The
DHCPv4 message contains an available IPv4 address.
The client sends with a DHCPv6
'DHCPV4-QUERY(20)' message containing a DHCPv4 DHCPREQUEST
message with the requested IPv4 address and
OPTION_DHCP4O6_S46_SADDR (TBD2) with the IPv6
address which the client will use as its softwire source
address.
The server sends a DHCPv6
'DHCPV4-RESPONSE (21)' message. OPTION_DHCPV4_MSG contains a
DHCPv4 DHCPACK message with the allocated IPv4 address.
OPTION_DHCP4O6_S46_SADDR with the client's bound softwire
source address is included.
The format of DHCPv6 Source Binding Prefix hint option is as
follows:option-code: OPTION_S46_BIND_IPV6_PREFIX (TBD1)option-length: 1 + length of bind-ipv6-prefix, specified
in bytes.
bindprefix6-len: 8-bit field expressing the bit mask length
of the IPv6 prefix specified in bind-ipv6-prefix. Valid
values are 0 to 128.
bind-ipv6-prefix: The IPv6 prefix indicating the preferred
prefix for the client to bind the received IPv4 configuration
to. The length is (bindprefix6-len + 7) / 8. The field is
padded on the right with zero bits up to the next octet
boundary when bind-ipv6-prefix is not evenly divisible by
8. These padding bits are ignored by the receiver (see
).
OPTION_S46_BIND_IPV6_PREFIX is a singleton. Servers MUST NOT
send more than one instance of the OPTION_S46_BIND_IPV6_PREFIX
option.
The format of DHCPv4 over DHCPv6 softwire source address
option is as follows:option-code: OPTION_DHCP4O6_S46_SADDR (TBD2)option-length: 16.softwire-ipv6-src-address: 16 bytes long; The IPv6
address that is associated (either being requested
for binding or currently bound) with the client's IPv4
configuration.
NB - The function of OPTION_DHCP4O6_S46_SADDR may
seem similar to the DHCPv4 message's 'chaddr' field, or the
Client Identifier (61) option in that it provides a lower-layer
address which is unique that the server can use for identifying
the client. However, as both of these are required to remain
constant throughout the address lease lifetime, they cannot be
used with the mechanism described in this document. This is
because the client may only be able to construct the IPv6
address to use as the source address after it has received the
first DHCPV4-RESPONSE message from the server containing
OPTION_S46_BIND_IPV6_PREFIX.
A client requiring dynamic softwire configuration first enables
DHCP 4o6 configuration using the method described in Section 5
of . If OPTION_DHCP4_O_DHCP6_SERVER is
received in the corresponding REPLY message, the client MAY
continue with the configuration process described below.
Before the dynamic softwire configuration process can commence,
the client MUST be configured with a suitable IPv6 prefix to be used
as the local softwire endpoint. This could be obtained using
DHCPv6, RA/PIO or another mechanism.
When constructing the initial DHCP 4o6 DHCPDISCOVER message,
the client includes a DHCPv6 OPTION_ORO (6) within the options
field of the DHCP-QUERY message. OPTION_ORO contains the option
codes for OPTION_S46_BR (90) and
OPTION_S46_BIND_IPV6_PREFIX (TBD1).
On receipt of the DHCP 4o6 server's reply (a DHCPV4-RESPONSE
containing a DHCPOFFER message), the client checks
the contents of the DHCPv4-RESPONSE for the presence of a valid
OPTION_S46_BR option. If this option is not present, or does not
contain at least one valid IPv6 address for a BR, then the client
MUST discard the message, as without the address of the BR the
client cannot configure the softwire and so has no interface to
request IPv4 configuration for.
The DHCPV4-RESPONSE message may also include
OPTION_S46_BIND_IPV6_PREFIX, which is used by the operator
to indicate a preferred prefix that the client should use to
bind IPv4 configuration to. If received, the client first
checks the option according to . If
valid, the client uses this prefix as the 'IPv6 binding prefix'
and follows to the process described in Section 5.1
of in order to select an active IPv6
prefix to construct the softwire. If no match is found,
or the client doesn't receive OPTION_S46_BIND_IPV6_PREFIX the
client MAY select any valid IPv6 prefix (of a suitable scope)
to use as the tunnel source.
Once the client has selected a suitable prefix, it MAY
use either an existing IPv6 address that is already configured
on an interface, or create a new address specifically for use
as the softwire source address (e.g., using an Interface
Identifier constructed as per Section 6 of
). If a new address is being created,
the client MUST complete configuration of the new address,
performing duplicate address detection (if required) before
proceeding.
The client then constructs a DHCPV4-QUERY message containing
a DHCPv4 DHCPREQUEST message. OPTION_DHCP4O6_S46_SADDR is
included in the options field of the DHCPREQUEST message
with the IPv6 address of its softwire source address in the
softwire-ipv6-src-address field.
When the client receives a DHCPv4 DHCPACK message from the
server, it checks the IPv6 address in OPTION_DHCP4O6_S46_SADDR
against its active softwire source address. If they match,
the allocation process has concluded. If there is a discrepancy
then the process described in
is followed.
If the client receives a DHCPv4 DHCPNAK message from
the server, then the configuration process has been unsuccessful.
The client then restarts the process from Step 1 of
.
Whenever the client attempts to extend the lease time
of the IPv4 address, OPTION_DHCP4O6_S46_SADDR with the IPv6
address of its softwire source address in the
softwire-ipv6-src-address field MUST be included in the DHCPREQUEST
message.
Across the lifetime of the leased IPv4 address, it is possible that
the client's IPv6 address will change, e.g., if there is an IPv6 re-
numbering event.
In this situation, the client MUST inform the server of the
new address. This is done by sending a DHCPREQUEST message
containing OPTION_DHCP4O6_S46_SADDR with the new IPv6
source address.
When the client receives a DHCPv4 DHCPACK message from the
server, it checks the IPv6 address in OPTION_DHCP4O6_S46_SADDR
against its active softwire source address. If they match,
the allocation process has concluded. If there is a discrepancy
then the process described in
is followed.
If the client receives a DHCPv4 DHCPNAK message in response
from the server, then the change of the bound IPv6 Softwire
source address has been unsuccessful. In this case, the client
MUST stop using the new IPv6 source address. The client then
restarts the process from Step 1 of .
When the client no longer requires the IPv4 resource, it
sends a DHCPv4 DHCPRELEASE message to the server. As the
options field is unused in this message type, OPTION_DHCP4O6_S46_SADDR
is not included.
On receipt of the OPTION_S46_BIND_IPV6_PREFIX option, the
client makes the following validation checks:
The received bindprefix6-len value is not larger than 128.
The number of bytes received in the bind-ipv6-prefix field is consistent
with the received bindprefix6-len value (calculated as described in
).
If either check fails, the receiver discards the invalid
option and proceeds to attempt configuration as if the option had
not been received.
The receiver MUST only use bits from the bind-ipv6-prefix field up
to the value specified in the bindprefix6-len when performing the
longest prefix match. bind-ipv6-prefix bits beyond this value
MUST be ignored.
If the client receives a DHCPACK message with an
OPTION_DHCP4O6_S46_SADDR containing an IPv6 address which
differs from its active softwire source address, the client
SHOULD wait for a randomized time interval and then resend the
DHCPREQUEST message with the correct softwire source
address. Section 4.1 descibes
the retransmission backoff interval process.
The default minimum time for the client to attempt retransmission
is 60 seconds. If, after this time has expired, the client has not
received a DHCPACK message with the correct bound IPv6 address,
client MAY send a DHCPRELEASE message and re-start the
process described in .
The re-try interval should be configurable and aligned with any
server policy defining the minimum time interval for client
address updates as described in
.
describes a mechanism for using DHCPv4
to distribute dynamic, shared IPv4 addresses to clients. The
mechanism described in this document is compatible with IPv4
address sharing, and can be enabled by following the
process described in Section 6 of .
Beyond the normal DHCP 4o6 functionality defined in
, the server MUST also store the IPv6
softwire source address of the client in the leasing address
database, alongside the IPv4 address and client identifier.
An OPTION_DHCP4O6_S46_SADDR containing the bound softwire source
address MUST be sent in every DHCPACK message sent by the
server.
The binding entry between the client's IPv6 softwire source
address and the leased IPv4 address is valid as long as the IPv4
lease remains valid.
In the event that the server receives a DHCPREQUEST message
for an active IPv4 lease containing a OPTION_DHCP4O6_S46_SADDR
with an IPv6 address which differs from the address which
is currently stored, the server updates the stored softwire
source address with the new address supplied by the client,
and sends a DHCPACK message containing the updated softwire
source address in OPTION_DHCP4O6_S46_SADDR.
The server MAY implement a policy enforcing a minimum time
interval between a client updating its softwire source
IPv6 address. If a client attempts to update the softwire
source IPv6 address before the minimum time has expired,
the server can either silently drop the client's
message or send back a DHCPACK message containing the
existing IPv6 address binding in OPTION_DHCP4O6_S46_SADDR.
If implemented, the default minimum client source address
update interval is 60 seconds.
In order for traffic to be forwarded correctly,
each CE's softwire IPv6 source addresses must be unique.
To ensure this, on receipt of every client DHCPREQUEST
message containing OPTION_DHCP4O6_S46_SADDR, the
DHCP 4o6 server MUST check the received IPv6 address
against all existing CE source addresses stored for
active client IPv4 leases. If there is a match,
then the client's source address MUST NOT be stored
or updated.Depending on where the client and server
are in the address leasing lifecycle, the DHCP 4o6
server then takes the following action:
If the DHCP 4o6 does not have a current, active
IPv4 address lease for the client, then the DHCP
address allocation process has not been succesful.
The server returns a DHCPNAK message to the client.
If the DHCP 4o6 does have a current, active
IPv4 address lease, then the source address update
process (see )
has not been successful. The DHCP 4o6 server
can either silently drop the client's message or
return a DHCPACK message containing the existing IPv6
address binding in OPTION_DHCP4O6_S46_SADDR.
Security considerations which are applicable to
are also applicable here.A rogue client could attempt to use the mechanism described
in to redirect IPv4 traffic
intended for another client to itself. This would be performed by
sending a DHCPREQUEST message for another client's active IPv4
lease containing the attacker's softwire IPv6 address in
OPTION_DHCP4O6_S46_SADDR.For such an attack to be effective, the attacker would
need to know both the client identifier and active IPv4
address lease currently in use by another client. This could
be attempted in three ways:One customer learning the active IPv4 address lease
and client identifier of another customer via snooping
the DHCP4o6 message flow between the client
and server. The mechanism described in this document is
intended for use in a typical ISP network topology with
a dedicated layer-2 access network per-client,
meaning that snooping of another client's traffic is
not possible. If the access network is a shared
medium then it provisioning softwire clients using
dynamic DHCP4o6 as described here is
NOT RECOMMENDED.Learning the active IPv4 address lease and client identifier
via snooping the DHCP4o6 message flow between the client
and server in the aggregation or core ISP network.
In this case, the attacker requires a level
of access to the ISP's infrastructure that means they can already
intercept or interfere with traffic flows to the client.An attacker could attempt to brute-force guessing the IPv4
lease address and client identifier tuple. The risk of
this can be reduced by using a client identifier format
which is not easily guessable, e.g., by using a random
based client identifier (see
Section 3.5).
An attacker could attempt to redirect existing flows to a
client unable to process the traffic. This type of attack can
be prevented by implementing network ingress
filtering in conjunction with the BR source address validation processes
described in Section 5.2 and
Section 8.1.
A client may attempt to overload the server by sending
multiple source address update messages (see )
in a short time frame. This risk can be reduced by implementing
a server policy enforcing a minimum time interval between client
address changes as described in .
describes anonymity profiles for
DHCP clients. These considerations and recommendations are
also applicable to clients implementing the mechanism described
in this document. As DHCP4o6 only uses DHCPv6 as a stateless
transport for DHCPv4 messages, the "Anonymity Profile for DHCPv4"
described in Section 3 is most relevant here.
In addition to the considerations given in
, the mechanism that the
client uses for constructing the interface identifier
for its IPv6 softwire source address
(see ), could result in the
device being trackable across different networks
and sessions, e.g., if the client's softwire
IID is immutable.This can be mitigated by constructing the softwire source
IPv6 address as per Section 6 of .
Here, the address' IID contains only the allocated IPv4 address
(and port set identifier if is
being used). This means no additional client information is
exposed to the DHCP4o6 server, and will also mean
that the IID will change as the leased
IPv4 address changes (e.g., between sessions when Section 3.5
of is implemented).IANA is requested to assign the OPTION_S46_BIND_IPV6_PREFIX (TBD1)
option code from the DHCPv6 "Option Codes" registry maintained at
http://www.iana.org/assignments/dhcpv6-parameters.IANA is requested to assign the OPTION_DHCP4O6_S46_SADDR (TBD2) option
code from the "BOOTP Vendor Extensions and DHCP Options" registry maintained
at http://www.iana.org/assignments/bootp-dhcp-parameters.IANA is requested to update the entry for DHCPv6 Option S46_BR (90) in the
Option Codes table at
https://www.iana.org/assignments/dhcpv6-parameters as follows:Old Entry:New Entry:IANA is also requested to make a new entry for OPTION_S46_BIND_IPV6_PREFIX
(TBD1) in the Option Codes table at
https://www.iana.org/assignments/dhcpv6-parameters:The authors would like to thank Ted Lemon, Lishan Li, Tatuya
Jinmei, Jonas Gorski and Razvan Becheriu for their contributions
and comments.
Network Ingress Filtering:
Defeating Denial of Service Attacks which employ
IP Source Address Spoofing
https://tools.ietf.org/html/bcp38
IETF