Network Working Group P. Saint-Andre
Internet-Draft &yet
Intended status: Standards Track M. Miller
Expires: August 8, 2014 Cisco Systems, Inc.
February 4, 2014
Domain Name Associations (DNA) in the Extensible Messaging and Presence
Protocol (XMPP)
draft-ietf-xmpp-dna-05
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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This Internet-Draft will expire on August 8, 2014.
Copyright Notice
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include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Flow Chart . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. A Simple Scenario . . . . . . . . . . . . . . . . . . . . . . 6
5. One-Way Authentication . . . . . . . . . . . . . . . . . . . 7
6. Piggybacking . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Assertion . . . . . . . . . . . . . . . . . . . . . . . . 8
6.2. Supposition . . . . . . . . . . . . . . . . . . . . . . . 9
7. Alternative Prooftypes . . . . . . . . . . . . . . . . . . . 10
7.1. DANE . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.2. POSH . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8. Secure Delegation and Multi-Tenancy . . . . . . . . . . . . . 12
9. Prooftype Model . . . . . . . . . . . . . . . . . . . . . . . 13
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
10.1. Well-Known URI for xmpp-client Service . . . . . . . . . 13
10.2. Well-Known URI for xmpp-server Service . . . . . . . . . 13
11. Security Considerations . . . . . . . . . . . . . . . . . . . 14
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
12.1. Normative References . . . . . . . . . . . . . . . . . . 14
12.2. Informative References . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
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) [RFC6120]. Because XMPP servers are typically identified by
DNS domain names, 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. In relation to such
associations, this document does the following:
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]
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3. Describes the flow of operations for establishing a domain name
association (DNA)
This document also provides guidelines for secure delegation. The
need for secure delegation arises because 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) using technologies such as DNS SRV records
[RFC2782]. If such delegation is not done in a secure manner, then
the domain name association cannot be authenticated.
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 1 and Server 2,
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 |
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| [B: Certificate Request] |
| [A: Client Certificate] |
| |
+-----------------------------------------------+
|
(A establishes DNA for b.example!)
|
+-------------AUTHENTICATION--------------------+
| | |
| {client certificate?} ----+ |
| | | |
| | yes no | |
| 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' |
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| to='a.example'/> |
| |
+-----------------------------------------------+
|
+-----------DNA DANCE AS ABOVE------------------+
| |
| DNS RESOLUTION, STREAM HEADERS, |
| TLS NEGOTIATION, AUTHENTICATION |
| |
+-----------------------------------------------+
|
+----------DIALBACK IDENTITY VERIFICATION-------+
| |
| A: <db:result from='a.example' |
| to='c.example' |
| type='valid'/> |
| |
+-----------------------------------------------+
|
|
(Section 6.2: Piggybacking Supposition)
|
|
+-----------SUBSEQUENT CONNECTION---------------+
| |
| B: <stream from='c.example' |
| to='rooms.a.example'> |
| |
| A: <stream from='rooms.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='rooms.a.example'/> |
| |
| B: <db:result from='rooms.a.example' |
| to='c.example' |
| type='valid'/> |
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| |
+-----------------------------------------------+
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 1 (a.example) and Server 2 (b.example):
1. Server 1 resolves the DNS domain name b.example.
2. Server 1 opens a TCP connection to the resolved IP address.
3. Server 1 sends an initial stream header to Server 2, asserting
that it is a.example:
<stream:stream from='a.example' to='b.example'>
4. Server 2 sends a response stream header to Server 1, asserting
that it is b.example:
<stream:stream from='b.example' to='a.example'>
5. The servers attempt TLS negotiation, during which Server 2
(acting as a TLS server) presents a PKIX certificate proving that
it is b.example and Server 1 (acting as a TLS client) presents a
PKIX certificate proving that it is a.example.
6. Server 1 checks the PKIX certificate that Server 2 provided and
Server 2 checks the PKIX certificate that Server 1 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 1 presents a PKIX certificate during TLS negotiation, which
enables the parties to complete mutual authentication.
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o There are no additional domains associated with Server 1 and
Server 2 (say, a subdomain rooms.a.example on Server 1 or a second
domain c.example on Server 2).
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 1 does not present its PKIX certificate during TLS
negotiation (perhaps because it wishes to verify the identity of
Server 2 before presenting its own credentials), Server 2 is unable
to mutually authenticate Server 1. Therefore, Server 2 needs to
negotiate and authenticate a stream to Server 1, just as Server 1 has
done:
1. Server 2 resolves the DNS domain name a.example.
2. Server 2 opens a TCP connection to the resolved IP address.
3. Server 2 sends an initial stream header to Server 1, asserting
that it is b.example:
<stream:stream from='b.example' to='a.example'>
4. Server 1 sends a response stream header to Server 2, asserting
that it is a.example:
<stream:stream from='a.example' to='b.example'>
5. The servers attempt TLS negotiation, during which Server 1
(acting as a TLS server) presents a PKIX certificate proving that
it is a.example.
6. Server 2 checks the PKIX certificate that Server 1 provided; if
it is consistent with the XMPP profile [RFC6120] of the matching
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rules from [RFC6125], Server 2 accepts that there is a strong
domain name association between its stream to Server 1 and the
DNS domain name a.example.
At this point 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 2 hosts not only
b.example but also a second domain c.example (a "multi-tenanted"
environment). If a user of Server 2 associated with c.example wishes
to communicate with a friend at a.example, Server 2 needs to send
XMPP stanzas from the domain c.example rather than b.example.
Although Server 2 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 2 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,
Server 2 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 2'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 1 needs to obtain some kind of
proof that Server 2 really is also c.example. It can do the same
thing that it did before:
1. Server 1 resolves the DNS domain name c.example.
2. Server 1 opens a TCP connection to the resolved IP address (which
might be the same IP address as for b.example).
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3. Server 1 sends an initial stream header to Server 2, asserting
that it is a.example:
<stream:stream from='a.example' to='c.example'>
4. Server 2 sends a response stream header to Server 1, asserting
that it is c.example:
<stream:stream from='c.example' to='a.example'>
5. The servers attempt TLS negotiation, during which Server 2
(acting as a TLS server) presents a PKIX certificate proving that
it is c.example.
6. Server 1 checks the PKIX certificate that Server 2 provided; if
it is consistent with the XMPP profile [RFC6120] of the matching
rules from [RFC6125], Server 1 accepts that there is a strong
domain name association between its stream to Server 2 and the
DNS domain name c.example.
Now that Server 1 accepts the domain name association, it informs
Server 2 of that fact:
<db:result from='a.example' to='c.example' type='valid'/>
The parties can then terminate the second connection, since it was
used only for Server 1 to associate a stream over the same IP:port
combination with the domain name c.example (the 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 1 provides XMPP services not only for
a.example but also for a subdomain such as a groupchat service at
rooms.a.example (see [XEP-0045]). If a user from c.example at Server
2 wishes to join a room on the groupchat sevice, Server 2 needs to
send XMPP stanzas from the domain c.example to the domain
rooms.a.example rather than a.example. Therefore, Server 2 needs to
negotiate and authenticate a stream to rooms.a.example:
1. Server 2 resolves the DNS domain name rooms.a.example.
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2. Server 2 opens a TCP connection to the resolved IP address.
3. Server 2 sends an initial stream header to Server 1 acting as
rooms.a.example, asserting that it is b.example:
<stream:stream from='b.example' to='rooms.a.example'>
4. Server 1 sends a response stream header to Server 2, asserting
that it is rooms.a.example:
<stream:stream from='rooms.a.example' to='b.example'>
5. The servers attempt TLS negotiation, during which Server 1
(acting as a TLS server) presents a PKIX certificate proving that
it is rooms.a.example.
6. Server 2 checks the PKIX certificate that Server 1 provided; if
it is consistent with the XMPP profile [RFC6120] of the matching
rules from [RFC6125], Server 2 accepts that there is a strong
domain name association between its stream to Server 1 and the
DNS domain name rooms.a.example.
As before, the parties now have two TCP connections open. So that
they can close the now-redundant connection, Server 2 sends a
dialback key to Server 1 over the new connection.
<db:result from='c.example' to='rooms.a.example'>
some-dialback-key
</db:result>
Server 1 then informs Server 2 that it accepts the domain name
association:
<db:result from='rooms.a.example' to='c.example' type='valid'/>
Server 2 can now close the connection over which it tested the domain
name association for rooms.a.example.
7. Alternative Prooftypes
The foregoing protocol flows assumed that domain name associations
were proved using the standard PKI prooftype specified in [RFC6120]:
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that is, the server's proof consists of a PKIX certificate that is
checked according to the XMPP profile [RFC6120] 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. As one example,
a certificate authority (CA) might try to send email messages to
authoritative mailbox names [RFC2142], but the administrator of a
subsidiary service such as im.cs.podunk.example can't receive email
sent to mailto:hostmaster@podunk.example. As another example, a
hosting provider such as hosting.example.net might not want to take
on the liability of holding the certificate and private key for a
tenant such as example.com (or the tenant might not want the hosting
provider to hold its certificate and private key). In these
circumstances, prooftypes other than PKIX are desirable. As
described below, two alternatives have been defined so far: DNS-Based
Authentication of Named Entities (DANE) and and PKIX Over Secure HTTP
(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 makes use of the DNS-Based Authentication of Named
Entities [RFC6698], specifically the use of DANE with DNS SRV records
[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 Use the 'to' address of the initial stream header to determine the
domain name of the TLS client's reference identifier (since use of
the TLS Server Name Indication is purely discretionary in XMPP, as
mentioned in [RFC6120]).
7.2. POSH
In the POSH 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.
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POSH is defined in [I-D.ietf-xmpp-posh]. For XMPP purposes, the
well-known URIs [RFC5785] to be used are:
o "/.well-known/posh._xmpp-client._tcp.json" for client-to-server
connections
o "/.well-known/posh._xmpp-server._tcp.json" for server-to-server
connections
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 tenants.
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 tenant (say, example.com) and the tenant 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.
The specification for use of SRV and MX records with DANE
[I-D.ietf-dane-srv] explains how to use DNSSEC for secure delegation
with the DANE prooftype, and the POSH specification
[I-D.ietf-xmpp-posh] explains how to use HTTPS redirects for secure
delegation with the POSH prooftype.
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9. Prooftype Model
In general, a domain name association (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 thus
also need to describe how they ensure secure delegation.
10. IANA Considerations
The POSH specification [I-D.ietf-xmpp-posh] provides guidelines for
registering the well-known URIs [RFC5785] of protocols that make use
of POSH. This specification registers two such URIs, for which the
completed registration templates follow.
10.1. Well-Known URI for xmpp-client Service
This specification registers the well-known URI "posh._xmpp-
client._tcp.json" in the Well-Known URI Registry as defined by
[RFC5785].
URI suffix: posh._xmpp-client._tcp.json
Change controller: IETF
Specification document(s): [[ this document ]]
10.2. Well-Known URI for xmpp-server Service
This specification registers the well-known URI "posh._xmpp-
server._tcp.json" in the Well-Known URI Registry as defined by
[RFC5785].
URI suffix: posh._xmpp-server._tcp.json
Change controller: IETF
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Specification document(s): [[ this document ]]
11. 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.ietf-xmpp-posh].
12. References
12.1. Normative References
[I-D.ietf-dane-srv]
Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
Based Authentication of Named Entities (DANE) TLSA records
with SRV and MX records.", draft-ietf-dane-srv-03 (work in
progress), December 2013.
[I-D.ietf-xmpp-posh]
Miller, M. and P. Saint-Andre, "PKIX over Secure HTTP
(POSH)", draft-ietf-xmpp-posh-00 (work in progress),
February 2014.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[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.
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[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.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012.
[XEP-0220]
Miller, J., Saint-Andre, P., and P. Hancke, "Server
Dialback", XSF XEP 0220, September 2013.
12.2. Informative References
[RFC2142] Crocker, D., "MAILBOX NAMES FOR COMMON SERVICES, ROLES AND
FUNCTIONS", RFC 2142, May 1997.
[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.
[XEP-0288]
Hancke, P. and D. Cridland, "Bidirectional Server-to-
Server Connections", XSF XEP 0288, September 2013.
Authors' Addresses
Peter Saint-Andre
&yet
Email: ietf@stpeter.im
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Matthew Miller
Cisco Systems, Inc.
1899 Wynkoop Street, Suite 600
Denver, CO 80202
USA
Email: mamille2@cisco.com
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