One document matched: draft-cheshire-pcp-unsupp-family-03.txt
Differences from draft-cheshire-pcp-unsupp-family-02.txt
PCP working group S. Cheshire
Internet-Draft Apple
Updates: 6887 (if approved) June 12, 2013
Intended status: Standards Track
Expires: December 14, 2013
Update to the PCP specification
draft-cheshire-pcp-unsupp-family-03
Abstract
The Port Control Protocol allows clients to request mappings in NAT
gateways and firewalls. This document specifies the PCP
UNSUPP_FAMILY error code, which enables PCP servers to inform clients
when the requested external address family is not supported. This
document also removes the requirement for the PCP server to validate
the mapping nonce, which proved to be unhelpful in practice.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 14, 2013.
Copyright Notice
Copyright (c) 2013 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
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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.
1. Terminology
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
"Key words for use in RFCs to Indicate Requirement Levels" [RFC2119].
2. PCP Unsupported Family Error
The Port Control Protocol [RFC6887] allows clients to request
mappings in NAT gateways and firewalls. A client can request a
mapping to an external IPv6 address or to an external IPv4 address.
The client signifies which family of external address it desires by
the type of address it puts into the Suggested External Address
field.
If the client wants an external IPv6 address, then it populates the
Suggested External Address field with a native IPv6 address. In the
overwhelmingly common case where the client doesn't know the external
address when it makes its initial request, this will be the all-zeros
IPv6 address (::).
If the client wants an external IPv4 address, then it populates the
Suggested External Address field with an IPv4-mapped IPv6 address
(the first 80 bits set to zero, the next 16 set to one). In the
overwhelmingly common case where the client doesn't know the NAT's
external address when it makes its initial request, this will be the
all-zeros IPv4 address (::ffff:0:0).
Note that while the specific address placed in the Suggested External
Address field is merely a suggestion that the PCP server is free to
ignore, the address family is not. If the suggested address cannot
be provided, another address of the same family SHOULD be provided if
possible, but if the suggested address *family* cannot be provided by
this PCP server, it MUST return a PCP error reply containing the
UNSUPP_FAMILY error code.
Many gateway devices, particularly early ones, may not be able to
provide both external address families. For example, an IPv4-only
NAT cannot provide an external IPv6 address.
Even with gateway devices that can support both external address
families, the ability to provide the an external address of the
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requested family may depend on the family of the client's internal
address. For example, a gateway that supports native IPv6, and
traditional NAT44, but not NAT64, can provide mappings from an
internal IPv6 address to an external IPv6 address (typically the same
address when no address translation is being performed), and can
provide mappings from an internal IPv4 address to an external IPv4
address, but not mappings from an internal IPv6 address to an
external IPv4 address. When such a gateway receives a request to map
an internal IPv6 address to an external IPv4 address it MUST return
the UNSUPP_FAMILY error code.
Note that it is possible and valid for a given internal address and
port to have two mappings simultaneously, one to an external IPv4
address and one to an external IPv6 address. The handling of
outbound packets is determined by the outbound destination address;
for example, an outbound IPv6 packet addressed to an IPv6 address in
the NAT64 gateway's IPv6 address pool is translated to the
corresponding IPv4 packet before forwarding; an outbound IPv6 packet
addressed to some other routable IPv6 address is forwarded
unmodified.
A client that can handle both IPv6 and IPv4 external addresses MAY
send two requests, and then determine its behaviour based on the
responses it receives. For example, if the client requests and
receives an IPv6 external address, it may create a DNS AAAA record
giving that IPv6 address. If the client requests and receives an
IPv4 external address, it may create a DNS address record giving that
IPv4 address. If the client requests and receives both families of
external address, it may create both DNS records. Or, if one
external address is sufficient for the client, then it MAY first
request its preferred address family, and only if that fails with an
UNSUPP_FAMILY error, request the other family.
2.1. Implications for RFC 6887
Various sections of the PCP specification [RFC6887] describe clients
and servers identifying a mapping by examining the three-tuple of
{ internal port, protocol, internal address } in a request or reply.
For example:
If the internal port, protocol, and internal address match an
existing static mapping (which will have no nonce), then a PCP
reply is sent giving the external address and port of that static
mapping, using the nonce from the PCP request. The server does
not record the nonce.
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It is possible that a mapping might already exist for a requested
internal address, protocol, and port. If so, the PCP server takes
the following actions...
If no mapping exists for the internal address, protocol, and port,
and the PCP server is able to create a mapping using the suggested
external address and port, it SHOULD do so.
After performing common PCP response processing, the response is
further matched with a previously sent MAP request by comparing
the internal IP address (the destination IP address of the PCP
response, or other IP address specified via the THIRD_PARTY
option), the protocol, the internal port, and the mapping nonce.
Other fields are not compared, because the PCP server sets those
fields.
Everywhere that RFC 6887 refers to using the "internal port,
protocol, and internal address" to identify a particular mapping, it
should be read to mean the four-tuple of
{ int port, protocol, internal address, external address family }.
PCP clients and servers that only support one external address family
can continue to use the previous three-tuple
{ internal port, protocol, internal address } to identify a mapping,
since they only support one external address family, and unilaterally
reject requests and responses containing the unsupported family. For
PCP servers this means rejecting requests containing the unsupported
address family via the UNSUPP_FAMILY error code. For PCP clients
this should be a non-issue because a PCP client should never receive
a reply containing an external address family it didn't request, but
should a client receive such a reply from a misbehaving PCP server
offering an external address family the client did not request, the
client MUST silently ignore the erroneous reply.
An implication of this update to the PCP specification is that when
renewing a mapping, a PCP client MUST include a suggested external
address of the correct family, so that the gateway device can
identify which mapping is being renewed. Ideally a PCP client SHOULD
record the previously-granted external address and use that as the
suggested external address in its renewal request, to facilitate
recovery in the event of gateway state loss, but at the very least a
PCP client MUST provide an all-zeroes suggested external address of
the correct family (just as it must have indicated the desired
address family in its initial request that created the mapping).
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3. New Nonce Check Behavior
The PCP specification [RFC6887] states that if a client requests a
mapping (or renews a mapping, which is the same thing, from the
server's point of view) and the requested mapping already exists, but
with a different nonce, then the server returns a NOT_AUTHORIZED
error.
This has proved to be problematic. The nonce exists primarily to
allow the client to validate that the responses it receives from the
PCP server are legitimate, and have not been fraudulently generated
by an off-path attacker. Requiring the PCP server to also verify
that the nonce matches causes unnecessary failures.
For example, if a client reboots or otherwise suffers a loss of
state, it may not have a record of nonces it previously used.
Suppose this client then requests a mapping from an external IPv4
address to its internal IP address at TCP port 22, so that it can
receive ssh logins. If the same client machine had previously
requested such a mapping with a different nonce, then the new request
will fail with a NOT_AUTHORIZED error. This is unhelpful and
misleading. The client does in fact have a mapping. Incoming
connection requests to its external address and port will in fact be
forwarded to it at port 22. The PCP server is simply refusing to
tell the client what the external address and port are, hindering the
client's ability to use the mapping.
The same scenario also exists in the case where (i) a different
internal host had previously requested a mapping to its internal port
22, (ii) that host then left the network, and (iii) the newly vacated
internal IP address is then assigned to new host. When this happens,
the new host will be unable to usefully request a mapping to its
internal port 22 until the old mapping expires. The new host will
actually have a mapping to its internal port 22, and will actually
receive incoming connection requests arriving at the external address
and port, but the PCP server will refuse to tell the client what the
external address and port are, thereby hindering the new host from
communicating that external address and port to the peer it wishes to
receive connections from. This is not helpful.
This PCP security check does not prevent the new host from learning
the external address and port by other circuitous means. For
example, the new host could discover the external address and port by
sending outbound traffic a destination it controls, and having that
destination report back the source address and port.
Furthermore, this PCP security check is inconsistent with other PCP
behaviour. It makes PCP behave differently for explicit dynamic
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versus other kinds of mappings. Indeed, requests matching static
mappings are not subjected to the nonce check and will result in a
response containing the static mapping's current state. There is no
reason that MAP requests matching a dynamic mapping should return
less information.
Therefore, the nonce check behaviour decribed below MUST be
implemented instead.
3.1. Nonce Check for MAP Requests
If operating in the Simple Threat Model (Section 18.1 of PCP
specification [RFC6887]), and the internal port, protocol, internal
address, and external address family match an existing explicit
dynamic mapping, but the mapping nonce does not match, then the
existing mapping nonce is updated to the new nonce, the lifetime is
updated (subject to the PCP server policy for maximum and minimum
lifetimes) and an appropriate PCP reply is sent, using the new nonce
and assigned lifetime.
This specification makes no statement about mapping nonce with the
Advanced Threat Model.
3.2. Nonce Check for PEER Requests
If operating in the Simple Threat Model (Section 18.1 of PCP
specification [RFC6887]), and the internal port, protocol, internal
address, and external address family match a mapping that already
exists, but the mapping nonce does not match (that is, a previous
PEER request was processed), then the existing mapping nonce is
updated to the new nonce, the lifetime is updated (subject to the PCP
server policy for maximum and minimum lifetimes) and an appropriate
PCP reply is sent, using the new nonce and assigned lifetime.
This specification makes no statement about mapping nonce with the
Advanced Threat Model.
3.3. Returning NOT_AUTHORIZED error
A NOT_AUTHORIZED error should still be returned, as described in
Section 15.1 of PCP specification [RFC6887], when a PCP client
attemps to delete a static mapping (i.e., a mapping created outside
of PCP itself) or an outbound (implicit or PEER-created) mapping.
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4. IANA Considerations
IANA should allocate the following PCP Result Code:
14 UNSUPP_FAMILY: Unsupported external address family, e.g., IPv6 in
a NAT that handles only IPv4. This is a long lifetime error.
5. Security Considerations
This new error code leaks no sensitive information and creates no new
security vulnerabilities.
6. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6887] Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
Selkirk, "Port Control Protocol (PCP)", RFC 6887,
April 2013.
Author's Address
Stuart Cheshire
Apple Inc.
1 Infinite Loop
Cupertino, California 95014
USA
Phone: +1 408 974 3207
Email: cheshire@apple.com
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