Internet DRAFT - draft-saintandre-xmpp-dna
draft-saintandre-xmpp-dna
Network Working Group P. Saint-Andre
Internet-Draft M. Miller
Intended status: Standards Track Cisco Systems, Inc.
Expires: October 17, 2013 April 15, 2013
Domain Name Associations (DNA) in the Extensible Messaging and Presence
Protocol (XMPP)
draft-saintandre-xmpp-dna-02
Abstract
This document improves the security of the Extensible Messaging and
Presence Protocol (XMPP) in two ways. First, it specifies how
"prooftypes" can establish a strong association between a domain name
and an XML stream. Second, it describes how to securely delegate a
source domain to a derived domain, which is especially important in
virtual hosting environments.
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 October 17, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
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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. Flow Chart . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. A Simple Scenario . . . . . . . . . . . . . . . . . . . . . . 5
5. One-Way Authentication . . . . . . . . . . . . . . . . . . . 6
6. Piggybacking . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Assertion . . . . . . . . . . . . . . . . . . . . . . . . 7
6.2. Supposition . . . . . . . . . . . . . . . . . . . . . . . 9
7. Alternative Prooftypes . . . . . . . . . . . . . . . . . . . 10
7.1. DANE . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.2. POSH . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8. Secure Delegation and Multi-Tenancy . . . . . . . . . . . . . 11
9. Prooftype Model . . . . . . . . . . . . . . . . . . . . . . . 12
10. Security Considerations . . . . . . . . . . . . . . . . . . . 12
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
12.1. Normative References . . . . . . . . . . . . . . . . . . 12
12.2. Informative References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
The need to establish a strong association between a domain name and
an XML stream arises in both client-to-server and server-to-server
communication using the Extensible Messaging and Presence Protocol
(XMPP), because XMPP servers are typically identified by DNS domain
names. However, a client or peer server needs to verify the identity
of a server to which it connects. To date, such verification has
been established based on information obtained from the Domain Name
System (DNS), the Public Key Infrastructure (PKI), or similar
sources. This document (1) generalizes the model currently in use so
that additional prooftypes can be defined, (2) provides a basis for
modernizing some prooftypes to reflect progress in underlying
technologies such as DNS Security [RFC4033], and (3) describes the
flow of operations for establishing a domain name association.
Furthermore, the process for resolving the domain name of an XMPP
service into the IP address at which an XML stream will be negotiated
(defined in [RFC6120]) can involve delegation of a source domain
(say, example.com) to a derived domain (say, hosting.example.net).
If such delegation is not done in a secure manner, then the domain
name association cannot be authenticated. Therefore, this document
provides guidelines for defining secure delegation methods.
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2. Terminology
This document inherits XMPP terminology from [RFC6120] and
[XEP-0220], DNS terminology from [RFC1034], [RFC1035], [RFC2782] and
[RFC4033], and security terminology from [RFC4949] and [RFC5280].
The terms "source domain", "derived domain", "reference identity",
and "presented identity" are used as defined in the "CertID"
specification [RFC6125]. The terms "permissive federation",
"verified federation", and "encrypted federation" are derived from
[XEP-0238], although we substitute the term "authenticated
federation" for the term "trusted federation" from that document.
3. Flow Chart
The following flow chart illustrates the protocol flow for
establishing domain name associations between Server A and Server B,
as described in the remaining sections of this document.
|
|
(Section 4: A Simple Scenario)
|
|
DNS RESOLUTION ETC.
|
+-------------STREAM HEADERS--------------------+
| |
| A: <stream from='a.example' to='b.example'> |
| |
| B: <stream from='b.example' to='a.example'> |
| |
+-----------------------------------------------+
|
+-------------TLS NEGOTIATION-------------------+
| |
| B: Server Certificate |
| [B: Certificate Request] |
| [A: Client Certificate] |
| |
+-----------------------------------------------+
|
(A establishes DNA for b.example!)
|
+-------------AUTHENTICATION--------------------+
| | |
| {client certificate?} ----+ |
| | | |
| | yes no | |
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| v | |
| SASL EXTERNAL | |
| (mutual auth!) | |
+------------------------------------|----------+
|
+----------------+
| B needs to auth A
|
(Section 5: One-Way Authentication)
|
|
DNS RESOLUTION ETC.
|
+-------------STREAM HEADERS--------------------+
| |
| B: <stream from='b.example' to='a.example'> |
| |
| A: <stream from='a.example' to='b.example'> |
| |
+-----------------------------------------------+
|
+-------------TLS NEGOTIATION-------------------+
| |
| A: Server Certificate |
| |
+-----------------------------------------------+
|
(B establishes DNA for a.example!)
|
|
(Section 6.1: Piggybacking Assertion)
|
|
+----------DIALBACK IDENTITY ASSERTION----------+
| |
| B: <db:result from='c.example' |
| to='a.example'/> |
| |
+-----------------------------------------------+
|
+-----------DNA DANCE AS ABOVE------------------+
| |
| DNS RESOLUTION, STREAM HEADERS, |
| TLS NEGOTIATION, AUTHENTICATION |
| |
+-----------------------------------------------+
|
+----------DIALBACK IDENTITY VERIFICATION-------+
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| |
| A: <db:result from='a.example' |
| to='c.example' |
| type='valid'/> |
| |
+-----------------------------------------------+
|
|
(Section 6.2: Piggybacking Supposition)
|
|
+-----------SUBSEQUENT CONNECTION---------------+
| |
| B: <stream from='c.example' |
| to='chatrooms.a.example'> |
| |
| A: <stream from='chatrooms.a.example' |
| to='c.example'> |
| |
+-----------------------------------------------+
|
+-----------DNA DANCE AS ABOVE------------------+
| |
| DNS RESOLUTION, STREAM HEADERS, |
| TLS NEGOTIATION, AUTHENTICATION |
| |
+-----------------------------------------------+
|
+-----------DIALBACK OPTIMIZATION---------------+
| |
| B: <db:result from='c.example' |
| to='chatrooms.a.example'/> |
| |
| B: <db:result from='chatrooms.a.example' |
| to='c.example' |
| type='valid'/> |
| |
+-----------------------------------------------+
4. A Simple Scenario
To illustrate the problem, consider the simplified order of events
(see [RFC6120] for details) in establishing an XML stream between
Server A (a.example) and Server B (b.example):
1. Server A resolves the DNS domain name b.example.
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2. Server A opens a TCP connection to the resolved IP address.
3. Server A sends an initial stream header to Server B, asserting
that it is a.example:
<stream:stream from='a.example' to='b.example'>
4. Server B sends a response stream header to Server A, asserting
that it is b.example:
<stream:stream from='b.example' to='a.example'>
5. The servers attempt TLS negotiation, during which Server B
(acting as a TLS server) presents a PKIX certificate proving that
it is b.example and Server A (acting as a TLS client) presents a
PKIX certificate proving that it is a.example.
6. Server A checks the PKIX certificate that Server B provided and
Server B checks the PKIX certificate that Server A provided; if
these proofs are consistent with the XMPP profile of the matching
rules from [RFC6125], each server accepts that there is a strong
domain name association between its stream to the other party and
the DNS domain name of the other party.
Several simplifying assumptions underlie the happy scenario just
outlined:
o Server A presents a PKIX certificate during TLS negotiation, which
enables the parties to complete mutual authentication.
o There are no additional domains associated with Server A and
Server B (say, a subdomain chatrooms.a.example on Server A or a
second domain c.example on Server B).
o The server administrators are able to obtain PKIX certificates in
the first place.
o The server administrators are running their own XMPP servers,
rather than using hosting services.
Let's consider each of these "wrinkles" in turn.
5. One-Way Authentication
If Server A does not present its PKIX certificate during TLS
negotiation (perhaps because it wishes to verify the identity of
Server B before presenting its own credentials), Server B is unable
to mutually authenticate Server A. Therefore, Server B needs to
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negotiate and authenticate a stream to Server A, just as Server A has
done:
1. Server B resolves the DNS domain name a.example.
2. Server B opens a TCP connection to the resolved IP address.
3. Server B sends an initial stream header to Server A, asserting
that it is b.example:
<stream:stream from='b.example' to='a.example'>
4. Server A sends a response stream header to Server B, asserting
that it is a.example:
<stream:stream from='a.example' to='b.example'>
5. The servers attempt TLS negotiation, during which Server A
(acting as a TLS server) presents a PKIX certificate proving that
it is a.example.
6. Server B checks the PKIX certificate that Server A provided; if
it is consistent with the XMPP profile of the matching rules from
[RFC6125], Server B accepts that there is a strong domain name
association between its stream to Server A and the DNS domain
name a.example.
Unfortunately, now the servers are using two TCP connections instead
of one, which is somewhat wasteful. However, there are ways to tie
the authentication achieved on the second TCP connection to the first
TCP connection; see [XEP-0288] for further discussion.
6. Piggybacking
6.1. Assertion
Consider the common scenario in which Server B hosts not only
b.example but also a second domain c.example. If a user of Server B
associated with c.example wishes to communicate with a friend at
a.example, Server B needs to send XMPP stanzas from the domain
c.example rather than b.example. Although Server B could open an new
TCP connection and negotiate new XML streams for the domain pair of
c.example and a.example, that too is wasteful. Server B already has
a connection to a.example, so how can it assert that it would like to
add a new domain pair to the existing connection?
The traditional method for doing so is the Server Dialback protocol,
first specified in [RFC3920] and since moved to [XEP-0220]. Here,
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Server B can send a <db:result/> element for the new domain pair over
the existing stream.
<db:result from='c.example' to='a.example'>
some-dialback-key
</db:result>
This element functions as Server B's assertion that it is (also)
c.example, and thus is functionally equivalent to the 'from' address
of an initial stream header as previously described.
In response to this assertion, Server A needs to obtain some kind of
proof that Server B really is also c.example. It can do the same
thing that it did before:
1. Server A resolves the DNS domain name c.example.
2. Server A opens a TCP connection to the resolved IP address (which
might be the same IP address as for b.example).
3. Server A sends an initial stream header to Server B, asserting
that it is a.example:
<stream:stream from='a.example' to='c.example'>
4. Server B sends a response stream header to Server A, asserting
that it is c.example:
<stream:stream from='c.example' to='a.example'>
5. The servers attempt TLS negotiation, during which Server B
(acting as a TLS server) presents a PKIX certificate proving that
it is c.example.
6. Server A checks the PKIX certificate that Server B provided; if
it is consistent with the XMPP profile of the matching rules from
[RFC6125], Server A accepts that there is a strong domain name
association between its stream to Server B and the DNS domain
name c.example.
Now that Server A accepts the domain name association, it informs
Server B of that fact:
<db:result from='a.example' to='c.example' type='valid'/>
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The parties can then terminate the second connection, since it was
used only for Server A to associate a stream over the same IP:port
combination with the domain name c.example (dialback key links the
original stream to the new association).
6.2. Supposition
Piggybacking can also occur in the other direction. Consider the
common scenario in which Server A provides XMPP services not only for
a.example but also for a subdomain such as a groupchat service at
chatrooms.a.example (see [XEP-0045]). If a user from c.example at
Server B wishes to join a room on the groupchat sevice, Server B
needs to send XMPP stanzas from the domain c.example to the domain
chatrooms.a.example rather than a.example. Therefore, Server B needs
to negotiate and authenticate a stream to chatrooms.a.example:
1. Server B resolves the DNS domain name chatrooms.a.example.
2. Server B opens a TCP connection to the resolved IP address.
3. Server B sends an initial stream header to Server A acting as
chatrooms.a.example, asserting that it is b.example:
<stream:stream from='b.example' to='chatrooms.a.example'>
4. Server A sends a response stream header to Server B, asserting
that it is chatrooms.a.example:
<stream:stream from='chatrooms.a.example' to='b.example'>
5. The servers attempt TLS negotiation, during which Server A
(acting as a TLS server) presents a PKIX certificate proving that
it is chatrooms.a.example.
6. Server B checks the PKIX certificate that Server A provided; if
it is consistent with the XMPP profile of the matching rules from
[RFC6125], Server B accepts that there is a strong domain name
association between its stream to Server A and the DNS domain
name chatrooms.a.example.
As before, the parties now have two TCP connections open. So that
they can close the now-redundant connection, Server B sends a
dialback key to Server A over the new connection.
<db:result from='c.example' to='chatrooms.a.example'>
some-dialback-key
</db:result>
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Server A then informs Server B that it accepts the domain name
association:
<db:result from='chatrooms.a.example' to='c.example' type='valid'/>
Server B can now close the connection over which it tested the domain
name association for chatrooms.a.example.
7. Alternative Prooftypes
The foregoing protocol flows assumed that domain name associations
were proved using the standard PKI prooftype specified in [RFC6120]:
that is, the server's proof consists of a PKIX certificate that is
checked according to a profile of the matching rules from [RFC6125],
the client's verification material is obtained out of band in the
form of a trusted root, and secure DNS is not necessary.
However, sometimes XMPP server administrators are unable or unwilling
to obtain valid PKIX certificates for their servers (e.g., the
administrator of im.cs.podunk.example can't receive certification
authority verification messages sent to
mailto:hostmaster@podunk.example, or hosting.example.net does not
want to take on the liability of holding the certificate and private
key for example.com). In these circumstances, prooftypes other than
PKIX are desirable. Two alternatives have been defined so far: DANE
and POSH.
7.1. DANE
In the DANE prooftype, the server's proof consists of a PKIX
certificate that is compared as an exact match or a hash of either
the SubjectPublicKeyInfo or the full certificate, and the client's
verification material is obtained via secure DNS.
The DANE prooftype is based on [I-D.ietf-dane-srv]. For XMPP
purposes, the following rules apply:
o If there is no SRV resource record, pursue the fallback methods
described in [RFC6120].
o The 'to' address of the initial stream header SHOULD be used to
determine the domain name of the TLS client's reference
identifier, whereas use of the TLS Server Name Indication is
OPTIONAL (as previously discussed in [RFC6120]).
7.2. POSH
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In the POSH (PKIX Over Secure HTTP) prooftype, the server's proof
consists of a PKIX certificate that is checked according to the rules
from [RFC6120] and [RFC6125], the client's verification material is
obtained by retrieving the PKIK certificate over HTTPS at a well-
known URI [RFC5785], and secure DNS is not necessary since the HTTPS
retrieval mechanism relies on the chain of trust from the public key
infrastructure.
POSH is fully defined in [I-D.miller-xmpp-posh-prooftype].
8. Secure Delegation and Multi-Tenancy
One common method for deploying XMPP services is multi-tenancy or
virtual hosting: e.g., the XMPP service for example.com is actually
hosted at hosting.example.net. Such an arrangement is relatively
convenient in XMPP given the use of DNS SRV records [RFC2782], such
as the following pointer from example.com to hosting.example.net:
_xmpp-server._tcp.example.com. 0 IN SRV 0 0 5269 hosting.example.net
Secure connections with multi-tenancy can work using the PKIX
prooftype on a small scale if the provider itself wishes to host
several domains (e.g., several related domains such as jabber-
de.example and jabber-ch.example). However, in practice the security
of multi-tenancy has been found to be unwieldy when the provider
hosts large numbers of XMPP services on behalf of multiple customers.
Typically there are two main reasons for this state of affairs: the
service provider (say, hosting.example.net) wishes to limit its
liability and therefore does not wish to hold the certificate and
private key for the customer (say, example.com) and the customer
wishes to improve the security of the service and therefore does not
wish to share its certificate and private key with service provider.
As a result, server-to-server communications to example.com go
unencrypted or the communications are TLS-encrypted but the
certificates are not checked (which is functionally equivalent to a
connection using an anonymous key exchange). This is also true of
client-to-server communications, forcing end users to override
certificate warnings or configure their clients to accept
certificates for hosting.example.net instead of example.com. The
fundamental problem here is that if DNSSEC is not used then the act
of delegation via DNS SRV records is inherently insecure.
[I-D.ietf-dane-srv] explains how to use DNSSEC for secure delegation
with the DANE prooftype and [I-D.miller-xmpp-posh-prooftype] explains
how to use HTTPS redirects for secure delegation with the POSH
prooftype.
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9. Prooftype Model
In general, a DNA prooftype conforms to the following definition:
prooftype: A mechanism for proving an association between a domain
name and an XML stream, where the mechanism defines (1) the nature
of the server's proof, (2) the matching rules for comparing the
client's verification material against the server's proof, (3) how
the client obtains its verification material, and (4) whether the
mechanism depends on secure DNS.
The PKI, DANE, and POSH prooftypes adhere to this model. In
addition, other prooftypes are possible (examples might include PGP
keys rather than PKIX certificates, or a token mechanism such as
Kerberos or OAuth).
Some prooftypes depend on (or are enhanced by) secure DNS and
therefore also need to describe how secure delegation occurs for that
prooftype.
10. Security Considerations
This document supplements but does not supersede the security
considerations of [RFC6120] and [RFC6125]. Relevant security
considerations can also be found in [I-D.ietf-dane-srv] and
[I-D.miller-xmpp-posh-prooftype].
11. IANA Considerations
This document has no actions for the IANA.
12. References
12.1. Normative References
[I-D.ietf-dane-srv]
Finch, T., "Using DNS-Based Authentication of Named
Entities (DANE) TLSA records with SRV and MX records.",
draft-ietf-dane-srv-02 (work in progress), February 2013.
[I-D.miller-xmpp-posh-prooftype]
Saint-Andre, P., "Using PKIX over Secure HTTP (POSH) as a
Prooftype for XMPP Domain Name Associations", draft-
miller-xmpp-posh-prooftype-03 (work in progress), February
2013.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
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[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, May 2005.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC
4949, August 2007.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785, April
2010.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, March 2011.
[XEP-0220]
Miller, J., Saint-Andre, P., and P. Hancke, "Server
Dialback", XSF XEP 0220, August 2012.
12.2. Informative References
[RFC3920] Saint-Andre, P., Ed., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 3920, October 2004.
[XEP-0045]
Saint-Andre, P., "Multi-User Chat", XSF XEP 0045, February
2012.
[XEP-0238]
Saint-Andre, P., "XMPP Protocol Flows for Inter-Domain
Federation", XSF XEP 0238, March 2008.
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[XEP-0288]
Hancke, P. and D. Cridland, "Bidirectional Server-to-
Server Connections", XSF XEP 0288, August 2012.
Authors' Addresses
Peter Saint-Andre
Cisco Systems, Inc.
1899 Wynkoop Street, Suite 600
Denver, CO 80202
USA
Email: psaintan@cisco.com
Matthew Miller
Cisco Systems, Inc.
1899 Wynkoop Street, Suite 600
Denver, CO 80202
USA
Email: mamille2@cisco.com
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