Network Working Group D. New
Request for Comments: 3620 October 2003
Category: Standards Track
The TUNNEL Profile
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This memo describes a Blocks Extensible Exchange Protocol (BEEP)
profile that allows a BEEP peer to serve as an application-layer
proxy. It allows authorized users to access services through a
firewall.
Table of Contents
1. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 One-Hop Example. . . . . . . . . . . . . . . . . . . . . . 3
2.2 Two-Hop Example. . . . . . . . . . . . . . . . . . . . . . 4
2.3 Failed Set-Up Example. . . . . . . . . . . . . . . . . . . 5
2.4 Non-BEEP Example . . . . . . . . . . . . . . . . . . . . . 5
2.5 Profile Example. . . . . . . . . . . . . . . . . . . . . . 6
2.6 Endpoint Example . . . . . . . . . . . . . . . . . . . . . 8
3. Message Syntax. . . . . . . . . . . . . . . . . . . . . . . . 9
4. Message Semantics . . . . . . . . . . . . . . . . . . . . . . 10
5. Provisioning . . . . . . . . . . . . . . . . . . . . . . . . . 12
6. Reply Codes. . . . . . . . . . . . . . . . . . . . . . . . . . 13
7. Security Considerations. . . . . . . . . . . . . . . . . . . . 14
8. Normative References . . . . . . . . . . . . . . . . . . . . . 15
A. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
A.1 Registration: BEEP Profile . . . . . . . . . . . . . . . . 16
A.2 Registration: A System (Well-Known) TCP
port number for TUNNEL . . . . . . . . . . . . . . . . . . 16
B. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 17
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 18
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RFC 3620 The TUNNEL Profile October 2003
1. Rationale
The TUNNEL profile provides a mechanism for cooperating BEEP peers to
form an application-layer tunnel. The peers exchange "tunnel"
elements that specify a source route, with the outermost element
being stripped off and used to decide the next hop. The innermost,
empty "tunnel" element tells the final destination that it is,
indeed, the final destination. The term "proxy" is used to refer any
of the BEEP peers other than the initiator and the final destination.
In one use of this profile, a BEEP peer implementing the TUNNEL
profile is co-resident with a firewall. An initiating machine inside
the firewall makes a connection to the proxy, then ask that proxy to
make a connection to an endpoint outside the firewall. Once this
connection is established, the proxy tells the outside endpoint that
it will be tunneling. If the outside machine agrees, the proxy "gets
out of the way," simply passing octets transparently, and both the
initiating and terminating machines perform a "tuning reset," not
unlike the way starting a TLS negotiation discards cached session
state and starts anew.
Another use for this profile is to limit connections to outside
servers based on the user identity negotiated via SASL. For example,
a manager may connect to a proxy, authenticate herself with SASL,
then instruct the proxy to tunnel to an information service
restricted to managers. Since each proxy knows the identity of the
next proxy being requested, it can refuse to tunnel connections if
inadequate levels of authorization have been established. It is also
possible to use the TUNNEL profile to anonymize the true source of a
BEEP connection, in much the way a NAT translates IP addresses.
However, detailed discussion of such uses is beyond the scope of this
document.
Once both endpoint machines are connected, the tunneling proxy
machine does no further interpretation of the data. In particular,
it does not look for any BEEP framing. The two endpoint machines may
therefore negotiate TLS between them, passing certificates
appropriate to the endpoints rather than the proxy, with the
assurance that even the proxy cannot access the information
exchanged.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119 [1].
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RFC 3620 The TUNNEL Profile October 2003
2. Examples
While the semantics described in Section 4 may seem complex, the
results are actually relatively simple. A few examples will show the
operation and use of this profile. In these examples, the machine
attempting to establish the connection is named "initial", while the
intermediate proxies are "proxy1" or "proxy2", and the machine with
the service that "initial" wishes to access is called "final". The
examples also assume that the BEEP framework [2] is implemented on
top of TCP [3], or some other mapping where one transport connection
carries all channels.
2.1 One-Hop Example
A simple one-hop connection through a single proxy is illustrated
first.
initial proxy1 final
----- xport connect ----->
<------- greeting -------->
--- start TUNNEL [1] ---->
----- xport connect ------>
<-------- greeting -------->
---- start TUNNEL [2] ---->
<---------- ok ------------
<------- ok -------------- [3]
<------------- greeting [4]-------------------------->
Notes:
[1] The TUNNEL element looks like this:
[2] The TUNNEL element looks like this:
[2] The TUNNEL element looks like this:
[3] The TUNNEL element looks like this:
-------
--- xport close ---->
<---- ------
--- xport close ---> [3]
Notes:
[1] The TUNNEL element looks like this:
[2] The TUNNEL element looks like this:
[3] This close is optional. "Initial" may also send another
element, attempting to contact a different server, for example.
2.4 Non-BEEP Example
This example shows the initiator connecting through two proxys, the
second proxy attempting to connect to the specified service and
accepting that the destination is not a BEEP server. The difference
at the protocol level is two-fold: The "initial" machine does not
include the innermost "tunnel" element, and the final proxy
("proxy2") therefore does not expect a BEEP greeting.
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RFC 3620 The TUNNEL Profile October 2003
initial proxy1 proxy2 final
--- xport connect -->
<---- greeting ------>
--start TUNNEL [1]-->
--- xport connect -->
<----- greeting ----->
--start TUNNEL [2]-->
---- xport connect --->
<------- login: -------
<------ ------- [3]
<----- login: ------ [4]
<------ --------- [3]
<----- login: -------- [4] [5]
Notes:
[1] The TUNNEL element looks like this:
Note the lack of an innermost no-attribute element.
[2] The TUNNEL element looks like this:
Note the lack of an innermost no-attribute element.
[3] Each proxy starts transparently forwarding octets after this
.
[4] Each proxy forwards any data it received from the final host,
even if that data arrived before the was sent.
[5] After receiving the "ok" message, the "initial" peer can expect
raw, non-BEEP data to be sent to and received from the "final"
machine.
2.5 Profile Example
This example shows the initiator connecting through two proxys. The
initial machine knows there is a server offering the SEP2 profile
somewhere beyond proxy1, but it need not know where. Proxy1 has been
locally configured to know that all SEP2 servers are beyond proxy2.
Proxy2 has been locally configured to chose "final" as the server of
choice for SEP2 services. Note that "final" does not necessarily
need to offer the requested profile in its initial greeting.
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initial proxy1 proxy2 final
--- xport connect -->
<---- greeting ------>
--start TUNNEL [1]-->
-- xport connect --->
<----- greeting ----->
--start TUNNEL [2]-->
--- xport connect --->
<------- greeting ----->
---start TUNNEL [3]--->
<-------- ok ----------
<------- ok --------- [4]
<------- ok --------- [5]
<-------------------------- greeting ---------------------------->
Notes:
[1] The TUNNEL element looks like this:
element.
[2] Proxy1 maps this to
based on local configuration, then processes the new
element, stripping off the outer element and routing
Note the presence of an innermost no-attribute element.
Proxy2 then strips the outermost element, looking up the
appropriate address and port, and forwards the .
[5] Proxy1 starts transparently forwarding octets after this .
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RFC 3620 The TUNNEL Profile October 2003
2.6 Endpoint Example
This example shows the initiator connecting through two proxys. The
initial machine knows there is a server known as "operator console"
somewhere beyond proxy1, but it needs not know where. Proxy1 has
been locally configured to know that "operator console" is beyond
proxy2. Proxy2 has been locally configured to use "final" as
"operator console". This example is almost identical to the previous
example, except that "endpoint" is intended to route to a particular
server, while "profile" is intended to route to a particular service.
Otherwise, these two attributes are very similar.
initial proxy1 proxy2 final
--- xport connect -->
<---- greeting ------>
--start TUNNEL [1]-->
-- xport connect --->
<----- greeting ----->
--start TUNNEL [2]-->
--- xport connect --->
<------- greeting ----->
---start TUNNEL [3]--->
<-------- ok ----------
<------- ok --------- [4]
<------- ok --------- [5]
<-------------------------- greeting ---------------------------->
Notes:
[1] The TUNNEL element looks like this:
Note the lack of an innermost no-attribute element.
[2] Proxy1 maps this to
based on local configuration, then processes the new
element, stripping off the outer element and routing
to proxy2.
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RFC 3620 The TUNNEL Profile October 2003
[3] Proxy2 receives the TUNNEL element with simply the endpoint
specified. Local provisioning maps this to
Note the presence of an innermost no-attribute element.
Proxy2 then strips the outermost element, looking up the
appropriate address and port, and forwards the .
[5] Proxy1 starts transparently forwarding octets after this .
3. Message Syntax
The only element defined in this profile is the "tunnel" element. It
is described in the following DTD, with additional limitations as
described afterwards.
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RFC 3620 The TUNNEL Profile October 2003
The format of the "fqdn" attribute is a fully qualified domain name,
such as "proxy.example.com". The format of the "ip4" attribute is
four sets of decimal numbers separated by periods, such as
"10.23.34.45". The format of the "ip6" attribute is as specified in
RFC2373 [4]. The format of the "port" attribute is a decimal number
between one and 65535, inclusive. The format of the "srv" attribute
is a pair of identifiers each starting with an underline and
separated by a period, such as "_sep._tcp". The format of the
"profile" attribute is a URI [5]. The format of the "endpoint"
attribute is any string that may appear as an attribute value.
The only allowable combinations of attributes are as follows:
o fqdn + port;
o fqdn + srv;
o fqdn + srv + port;
o ip4 + port;
o ip6 + port;
o profile, but only on the innermost element;
o endpoint, but only on the innermost element; or,
o no attributes, but only on the innermost element.
4. Message Semantics
When a TUNNEL channel is started, the listener expects a "tunnel"
element from the initiator, either in the "start" element on channel
zero or on the new channel created. As usual, if it arrives on
channel zero, it is processed before the reply is returned.
In either case, the outermost "tunnel" element is examined. If it
has no attributes, then this peer is hosting the BEEP service that
the initiator wishes to use. In this case, the listener performs a
tuning reset:
o All channels, including channel zero, are implicitly closed.
o Any previously cached information about the BEEP session is
discarded.
o A new plaintext greeting is sent.
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RFC 3620 The TUNNEL Profile October 2003
If the outermost element has a "port" attribute and an "fqdn"
attribute but no "srv" attribute, then "fqdn" is looked up as an A
record via DNS for translation to an IP number. An "ip4" attribute
is interpreted as the dotted-quad representation of an IPv4 address.
An "ip6" attribute is interpreted as a text representation of an IPv6
address. In each of these cases, a transport connection is
established to the so-identified server. If the outermost element
has a "srv" attribute, the concatenation of the "srv" attribute and
the "fqdn" attribute (with a period between) is looked up in the DNS
for a SRV record [6], and the appropriate server is contacted; if
that lookup fails and a "port" attribute is present, the connection
is attempted as if the "srv" attribute were not specified.
Alternately, if the outermost element has a "profile" attribute, then
it must have no nested elements. The proxy processing this element
is responsible for determining the appropriate routing to reach a
peer serving the BEEP profile indicated by the URI in the attribute's
value. Rather than source routing, this provides a hop-by-hop
routing mechanism to a desired service.
Similarly, if the outermost element has an "endpoint" attribute, then
it must have no nested elements. The proxy processing this element
is responsible for determining the appropriate routing to reach a
peer indicated by the value of the "endpoint" attribute. Rather than
source routing, this provides a hop-by-hop routing mechanism to a
desired machine. There are no restrictions on how machines are
identified.
Then, if the outermost element has no nested elements, but it does
have attributes other than "profile" or "endpoint", then this peer is
the final BEEP hop. (This corresponds to "proxy2" in the "Non-BEEP"
example above.) In this case, as soon as the final underlying
transport connection is established, an "ok" element is returned over
the listening session, and the tunneling of data starts. No BEEP
greeting (or indeed any data) from the final hop is expected.
Starting with the octet following the END(CR)(LF) trailer of the
frame with the completion flag set (more=".") of the RPY carrying the
"ok" element, the proxy begins copying octets directly and without
any interpretation between the two underlying transport connections.
If the identified server cannot be contacted, an "error" element is
returned over the listening channel and any connection established as
an initiator is closed. If there is a nested "tunnel" element, and
the server that has been contacted does not offer a BEEP greeting, or
the BEEP greeting offered does not include the TUNNEL profile, then
this too is treated as an error: the initiating transport connection
is closed, and an error is returned.
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RFC 3620 The TUNNEL Profile October 2003
If there is a nested "tunnel" element, and the identified server is
contacted and offers a BEEP greeting including the TUNNEL profile,
then the outermost element from the "tunnel" element received is
stripped off, a new TUNNEL channel is started on the initiating
session, and the stripped (inner) element is sent to start the next
hop. In this case, the peer is considered a "proxy" (meaning that
the next paragraph is applicable).
Once the proxy has passed the "tunnel" element on the TUNNEL channel,
it awaits an "error" or an "ok" element in response. If it receives
an "error" element, it closes the initiated session and its
underlying transport connection. It then passes the "error" element
unchanged back on the listening session. If, on the other hand, it
receives an "ok" element, it passes the "ok" element back on the
listening session. Starting with the octet following the END(CR)(LF)
trailer of the frame with the completion flag set (more=".") of the
RPY carrying the "ok" element, the proxy begins copying octets
directly and without any interpretation between the two underlying
transport connections.
5. Provisioning
While the BEEP Framework [2] is used, the attributes described are
sufficient for the TCP mapping [3] of BEEP. The attributes on the
"tunnel" element may need to be extended to handle other transport
layers.
In a mapping where multiple underlying transport connections are
used, once the "ok" element is passed, all channels are closed,
including channel zero. Thus, only the underlying transport
connection initially established remains, and all other underlying
transport connections for the session should be closed as well.
If a transport security layer (such as TLS) has been negotiated over
the session, the semantics for the TUNNEL profile are ill-defined.
The TUNNEL profile MUST NOT be advertised in any greetings after
transport security has been negotiated.
An SRV identifier of "_tunnel" is reserved by IANA for use with this
profile. Hence, the "srv" attribute "_tunnel._tcp" MAY be used as a
default for finding the appropriate address for tunneling into a
particular domain.
System port number 604 has been allocated by the IANA for TUNNEL.
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6. Reply Codes
This section lists the three-digit error codes the TUNNEL profile may
generate.
code meaning
==== =======
421 Service not available
(E.g., the proxy does not have sufficient resources.)
450 Requested action not taken
(E.g., DNS lookup failed or connection could not
be established. See too 550.)
500 General syntax error (E.g., poorly-formed XML)
501 Syntax error in parameters
(E.g., non-valid XML, letters in "ip4" attribute, etc.)
504 Parameter not implemented
530 Authentication required
534 Authentication mechanism insufficient
(E.g., too weak, sequence exhausted, etc.)
537 Action not authorized for user
538 Encryption already enabled
(E.g., TLS already negotiated, or a SASL that
provides encryption already negotiated.)
550 Requested action not taken
(E.g., next hop could be contacted, but
malformed greeting or no TUNNEL profile advertised.)
553 Parameter invalid
554 Transaction failed (E.g., policy violation)
Note that the 450 error code is appropriate when the destination
machine could not be contacted, while the 550 error code is
appropriate when the destination machine could be contacted but the
next phase of the protocol could not be negotiated. It is suggested
that the beginning of any reply from the destination machine be
included as part of the CDATA text of the error element, for
debugging purposes.
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RFC 3620 The TUNNEL Profile October 2003
7. Security Considerations
The TUNNEL profile is a profile of BEEP. In BEEP, transport
security, user authentication, and data exchange are orthogonal.
Refer to Section 8 of [2] for a discussion of this.
However, the intent of the TUNNEL profile is to allow bidirectional
contact between two machines normally separated by a firewall. Since
TUNNEL allows this connection between BEEP peers, and BEEP peers can
offer a range of services with appropriate greetings, the TUNNEL
profile should be configured with care. It is reasonable to strictly
limit the hosts and services that a proxy is allowed to contact. It
is also reasonable to limit the use of the TUNNEL profile to
authorized users, as identified by a SASL profile.
Negotiation of a TLS profile in an end-to-end manner after a TUNNEL
has been established will prevent intermediate proxies from observing
or modifying the cleartext information exchanged, but only if TLS
certificates are properly configured during the negotiation. The
proxy could mount a "man in the middle" attack if public key
infrastructure is not deployed.
In some environments, it is undesirable to expose the names of
machines on one side of a firewall in unencrypted messages on the
other side of that firewall. In this case, source routing (using the
"fqdn", "ip4", "ip6", "port" and "srv" attributes) can route a
connection to the firewall proxy, with an innermost "profile" or
"endpoint" attribute which the firewall proxy understands. Local
provisioning can allow a proxy to translate a particular "profile"
or "endpoint" element into a new source route to reach the desired
service. This can prevents two attacks:
o Attackers sniffing packets on one side of the firewall cannot see
IP addresses or FQDNs of machines on the other side of the
firewall; and,
o Attackers cannot exhaustively attempt to connect to many FQDNs or
IP addresses via source routing and use the error messages as an
indication of whether the queried machine exists. For this attack
to be prevented, the proxy must allow only "profile" or "endpoint"
connections, always refusing to even attempt source-routed
connections. This latter attack can also be thwarted by requiring
a SASL identification before allowing a TUNNEL channel to be
started, but this can have higher overhead.
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8. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Rose, M., "The Blocks Extensible Exchange Protocol Core", RFC
3080, March 2001.
[3] Rose, M., "Mapping the BEEP Core onto TCP", RFC 3081, March
2001.
[4] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[5] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
Identifiers (URI): Generic Syntax", RFC 2396, August 1998.
[6] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
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Appendix A. IANA Considerations
A.1 Registration: BEEP Profile
The IANA has registered the profiles specified in this section and
has selected an IANA-specific URI: "http://iana.org/beep/TUNNEL".
Profile identification: http://iana.org/beep/TUNNEL
Message exchanged during channel creation: "tunnel"
Messages starting one-to-one exchanges: "tunnel"
Messages in positive replies: "ok"
Messages in negative replies: "error"
Messages in one-to-many exchanges: None.
Message syntax: See Section 3 of this document.
Message semantics: See Section 4 of this document.
Contact information: See the Author's Address appendix of this
document.
Any extensions to this protocol MUST be documented in a Standards
track RFC.
A.2 Registration: The System (Well-Known) TCP port number for TUNNEL
A single well-known port, 604, is allocated by the IANA to the TUNNEL
profile.
Protocol Number: TCP
Message Formats, Types, Opcodes, and Sequences: See Section 3.
Functions: See Section 4.
Use of Broadcast/Multicast: none
Proposed Name: TUNNEL Profile
Short name: tunnel
Contact Information: See the "Authors' Addresses" section of this
memo
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Appendix B. Acknowledgements
The author gratefully acknowledges the contributions of Marshall
Rose, Greg Matthews, and Ben Feinstein.
Inspiration for this profile comes from the Intrusion Detection
Working Group of the IETF.
Author's Address
Darren New
5390 Caminito Exquisito
San Diego, CA 92130
US
Phone: +1 858 350 9733
EMail: dnew@san.rr.com
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Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
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