Network Working Group M. Stecher

Network Working Group M. Stecher
Request for Comments: 4496 Secure Computing
Category: Informational A. Barbir
Nortel
May 2006

Open Pluggable Edge Services (OPES) SMTP Use Cases

Status of This Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2006).

Abstract

The Open Pluggable Edge Services (OPES) framework is application
agnostic. Application-specific adaptations extend that framework.
This document describes OPES SMTP use cases and deployment scenarios
in preparation for SMTP adaptation with OPES.

Table of Contents

1. Introduction ....................................................2
2. Terminology .....................................................2
3. Brief Overview of SMTP Architecture .............................3
3.1. Operation Flow of an OPES SMTP System ......................4
3.1.1. OPES SMTP Example ...................................5
4. OPES/SMTP Use Cases .............................................6
4.1. Security Filters Applied to Email Messages .................6
4.2. Spam Filter ................................................7
4.3. Logging and Reporting Filters ..............................8
4.4. Access Control Filters .....................................8
4.5. Secure Email Handling ......................................8
4.6. Email Format Normalization .................................8
4.7. Mail Rerouting and Address Rewriting .......................9
4.8. Block Email during SMTP Dialog .............................9
4.9. Convert Attachments to HTTP Links ..........................9
5. Security Considerations ........................................10
6. References .....................................................10
6.1. Normative References ......................................10
6.2. Informative References ....................................10
Acknowledgements ..................................................11

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1. Introduction

The Open Pluggable Edge Services (OPES) architecture [1] enables
cooperative application services (OPES services) between a data
provider, a data consumer, and zero or more OPES processors. The
application services under consideration analyze and possibly
transform application-level messages exchanged between the data
provider and the data consumer. The OPES processor can distribute
the responsibility of service execution by communicating and
collaborating with one or more remote callout servers.

The execution of such services is governed by a set of rules
installed on the OPES processor. The rule evaluation can trigger the
execution of service applications local to the OPES processor or on a
remote callout server.

Use cases for OPES based on HTTP [8] are described in [2]. This work
focuses on OPES for SMTP [7] use cases, whereby additional use cases
and enhancements to the types of OPES services defined in [2] are
provided.

In SMTP, the OPES processor may be any agent participating in SMTP
exchanges, including a Mail Submission Agent (MSA), a Mail Transfer
Agent (MTA), a Mail Delivery Agent (MDA), and a Mail User Agent
(MUA). This document focuses on use cases in which the OPES
processor is a MTA.

SMTP is a store-and-forward protocol. Current email filtering
systems either operate during the SMTP exchange or on messages that
have already been received, after the SMTP connection has been closed
(for example, in an MTA's message queue).

This work focuses on SMTP-based services that want to modify command
values or want to block SMTP commands. In order to block a command,
the service will provide an error message that the MTA should use in
response to the command it received. An OPES MTA will be involved in
SMTP command modification and command satisfaction, analogous to
request modification and request satisfaction from HTTP [8].

2. Terminology

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 [6]. When used with
the normative meanings, these key words will be all uppercase.
Occurrences of these words in lowercase comprise normal prose usage,
with no normative implications.

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3. Brief Overview of SMTP Architecture

The SMTP design, taken from RFC 2821 [7], is shown in Figure 1. When
an SMTP client has a message to transmit, it establishes a two-way
transmission channel to an SMTP server. The responsibility of an
SMTP client is to transfer mail messages to one or more SMTP servers,
or report its failure to do so.

+----------+ +----------+
+------+ | | | |
| User |<-->| | SMTP | |
+------+ | Client |Commands/Replies| Server |
+------+ | SMTP |<-------------->| SMTP | +------+
| File |<-->| | and Mail | |<-->| File |
|System| | | | | |System|
+------+ +----------+ +----------+ +------+
SMTP client SMTP server

Figure 1: SMTP Design

In some cases, the domain name(s) transferred to, or determined by,
an SMTP client will identify the final destination(s) of the mail
message. In other cases, the domain name determined will identify an
intermediate destination through which those mail messages are to be
relayed.

An SMTP server may be either the ultimate destination or an
intermediate "relay" or "gateway" (that is, it may transport the
message further using some protocol other than SMTP or using again
SMTP and then acting as an SMTP client).

SMTP commands are generated by the SMTP client and sent to the SMTP
server. SMTP responses are sent from the SMTP server to the SMTP
client in response to the commands. SMTP message transfer can occur
in a single connection between the original SMTP sender and the final
SMTP recipient, or it can occur in a series of hops through
intermediary systems. SMTP clients and servers exchange commands and
responses and eventually the mail message body.

Figure 2 expands on the mail flow in an SMTP system. Further
information about the architecture of email in the Internet may be
found in [9].

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+-------+ +---------+ +---------+ +--------+ +-------+
|mail M| |M mail M| SMTP |M mail M| SMTP |M mail M| |M mail |
|clnt U|--|S srvr T|------|T gway T|------|T srvr D|--|U clnt |
| A| |A A| |A A| |A A| |A |
+-------+ +---------+ +---------+ +--------+ +-------+

Figure 2: Expanded SMTP Flow

In this work, the OPES processor may be any agent that is
participating in SMTP exchanges, including an MSA, MTA, MDA, and MUA.
However, this document focuses on use cases in which the OPES
processor uses the SMTP protocol or one of the protocols derived from
SMTP Message Submission (SUBMIT) [10] and the Local Mail Transfer
Protocol (LMTP) [11]).

3.1. Operation Flow of an OPES SMTP System

In this work, an MTA is the OPES processor device that sits in the
data stream of the SMTP protocol. The OPES processor gets enhanced
by an OCP (OPES callout protocol) [3] client that allows it to vector
out data to the callout server. The filtering functionality is on
the callout server.

A client (a mail user) starts with an email client program (MUA).
The user sends email to an outgoing email server. In the email
server, there is an MSA (mail submission agent) that is waiting to
receive email from the user. The MSA uses an MTA within the same
server to forward the user email to other domains. (Communication
between the MUA and MSA may be via SMTP, SUBMIT [10], or something
else such as MAPI).

The MTA in the user email server may directly contact the email
server of the recipient or may use other intermediate email gateways.
The sending email server and all intermediate gateway MTAs usually
communicate using SMTP. Communication with the destination email
server usually uses SMTP or its derivative, LMTP [11].

In the destination email server, a mail delivery agent (MDA) may
deliver the email to the recipient's mailbox. The email client
program of the recipient might use a different protocol (such as the
Post Office Protocol version 3 (POP3) or IMAP) to access the mailbox
and retrieve/read the messages.

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+-------+ +---------+ +---------+ +--------+ +-------+
|mail M| |M mail M| SMTP |M mail M| SMTP |M mail M| |M mail |
|clnt U|--|S srvr T|------|T gway T|------|T srvr D|--|U clnt |
| A| |A A| |A A| |A A| |A |
+-------+ +---------+ +---------+ +--------+ +-------+
| | |
| OCP | OCP | OCP
| | |
+----------+ +----------+ +----------+
| callout | | callout | | callout |
| server | | server | | server |
+----------+ +----------+ +----------+

Figure 3: OPES SMTP Flow

From Figure 3, the MTA (the OPES processor) is either receiving or
sending an email (or both) within an email server/gateway. An OPES
processor might be the sender's SMTP server, the destination SMTP
server, or any intermediate SMTP gateway. (Which building block
belongs to which authoritative domain is an important question but
different from deployment to deployment.) Note that this figure
shows multiple OPES deployment options in a typical chain of mail
servers and gateways with different roles as MSA, MTA, and MDA; the
OPES standard case, however, will only have a single OPES processor
and a single callout server in the message flow.

3.1.1. OPES SMTP Example

A typical (minimum) SMTP dialog between two OPES SMTP processors
(MTA) will consist of the following (C: means client, S: means
server):

S: 220 mail.example.com Sample ESMTP MAIL Service, Version: 1.2
ready at Thu, 20 Jan 2005 11:24:40+0100
C: HELO [192.0.2.138]
S: 250 mail.example.com Hello [192.0.2.138]
C: MAIL FROM:<[email protected]>
S: 250 2.1.0 [email protected] OK
C: RCPT TO:<[email protected]>
S: 250 2.1.5 [email protected]
C: DATA
S: 354 Start mail input; end with "CRLF"."CRLF"
C: From: [email protected]
C: To: [email protected]
C: Subject: Test
C:
C: Hi, this is a test!
C: .

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S: 250 2.6.0 "[email protected]" Queued mail for
delivery
C: QUIT
S: 221 2.0.0 mail.example.com Service closing transmission channel

The client (C:) is issuing SMTP commands and the server (S:) is
generating responses. All responses start with a status code and
then some text. At minimum, 4 commands are needed to send an email.
Together, all commands and responses to send a single email message
form "the dialog". The mail message body is the data sent after the
"DATA" command. An OPES processor could see that as command
modification.

If a callout service wants to adapt the email message body, it is
mainly interested in this part of the dialog:

From: [email protected]
To: [email protected]
Subject: Test

Hi, this is a test!

The callout service may need to examine values of previous commands
of the same dialog. For example, the callout service needs to
examine the value of the RCPT command (it is "[email protected]"),
which is different from the "[email protected]" that the email
client displays in the visible "To" field. That might be an
important fact for some filters such as spam filters (Section 4.2).

4. OPES/SMTP Use Cases

In principle, all filtering that is deployed at SMTP gateways today
and tomorrow defines use cases for OPES callout filtering. An
OCP/SMTP callout protocol will enable an SMTP gateway to vector out
(parts of) an SMTP message or parts of the SMTP dialog to a callout
server that is then performing actions on behalf of the gateway.
(OCP/SMTP would be a profile defined for OCP analogous to the
OCP/HTTP profile [4] that has been defined earlier.)

Here is a collection of some typical use cases describing different
filtering areas and different actions caused by those filters.

4.1. Security Filters Applied to Email Messages

These filters concentrate on the email message body and usually
filter the email sections one by one. Email sections (attachments)
that violate the security policy (e.g., because they contain a virus

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or contain an unwanted mime type) define an event that can cause a
combination of different actions to be performed:

o The attachment is replaced by an error message.

o The email is marked by inserting a warning into the subject or the
email body.

o An additional header is added for post-processing steps.

o The email storage is advised to put the email into quarantine.

o Notifications are sent to sender, recipients, and/or
administrators.

o The incident is reported to other tools such as intrusion
detection applications.

These kinds of filters usually do not require working with elements
of the SMTP dialog other than the email message body. An exception
to this is the need to map email senders and recipients to different
security sub-policies that are used for a particular message. A
security filter may therefore require receiving the information of
the RCPT TO and MAIL FROM commands as meta data with the email
message body it examines.

4.2. Spam Filter

Next to security filters, spam filters are probably the most wanted
filtering application today. Spam filters use several methods. They
concentrate most on the email message body (that also includes the
email headers), but many of these filters are also interested in the
values of the other SMTP commands in order to compare the SMTP
sender/recipients with the visible From/To fields. They may even
want to get the source IP of the connected SMTP client as meta
information to verify this against lists of open relays, known
spammers, etc.

These are typical actions that could be performed when a message has
been classified as spam:

o Add a mark to the subject of the email.

o Add an additional header for post-processing steps.

o The email storage is advised to put the email into a spam queue.

o The email is rejected or returned to the sender.

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4.3. Logging and Reporting Filters

The nature of these kinds of filters is not to modify the email
message. Depending on what is being logged or reported on, the
filter may need access to any part of the SMTP dialog. Most wanted
is the sender and recipient information. Depending on the ability of
the OPES processor to pre-calculate and transfer information about
the message body, the callout filter may want to see the email
message body itself or just that meta info; an example is the email
size. This information would be typical logging and reporting
information that is easy for the SMTP gateway to calculate although
not a direct parameter of the SMTP dialog. Transferring the complete
email message body only because the callout server wants to calculate
its size would be a waste of network resources.

4.4. Access Control Filters

These filters operate on the values of the MAIL FROM and RCPT TO
commands of the SMTP dialog. They run an access control policy to
determine whether a sender is currently allowed to send a message to
the given recipients. The values of HELO/EHLO, AUTH, and STARTTLS
commands may also be applied. The result of this filter has a direct
influence on the SMTP response that the OPES processor has to send to
its peer for the filtered SMTP command.

4.5. Secure Email Handling

Filters of this kind can support an email gateway to centrally encode
and decode email, and to set and to verify email signatures. They
will therefore modify the email message body to encrypt, decrypt,
verify, or sign the message, or use an action as specified in the
"Security Filter" (Section 4.1) section if the decryption or
signature verification fails.

Sending the SMTP sender and recipient information as meta data to
these filters is mission critical because these filters may not trust
the information found in the header section of the email message
body.

4.6. Email Format Normalization

SMTP messages may be sent with an illegal or uncommon format; this
may have happened by a buggy SMTP application or on purpose in order
to exploit vulnerabilities of other products. A normalization filter
can correct the email format. The format correction can be done for
the email body and for the value of other SMTP commands. An example
for the email body format correction would be a strange length of
UUencoded lines or unusual names of MIME sections. Command values

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may be analysed against buffer overflow exploits; a rewrite will not
always be possible in this case (cannot simply rewrite an email
address that is very long) but will require that the callout server
tells the OPES processor to send an error response in reply to such a
command.

4.7. Mail Rerouting and Address Rewriting

A corporation with multiple locations may want to deploy a central
gateway that receives all email messages for all employees and then
determines at which location the mail storage of the employee
resides. The callout server will then need the RCPT TO command value
and it will look up the location in the corporate directory service.
It then tells either the OPES processor where the next SMTP server is
(i.e., the next SMTP server to connect to) or it rewrites the
recipient address; in the first case, the SMTP servers at the
different locations accept emails of the same domain as the central
gateway does; in the second case, the other locations will probably
use the sublocation of the original domain ([email protected] ->
[email protected] or [email protected]).

4.8. Block Email during SMTP Dialog

In a first step, the callout server will check the sender and
recipient information that was transmitted in the SMTP dialog; that
information again maps to a policy that will deny all messages either
from that sender or to that recipient, or it checks the body of the
email and classifies it (maybe just by looking for some words in the
subject or by doing in-depth content analysis), which can then also
lead to the decision to deny the message.

Unlike previous examples, this use case wants to deny the email while
the SMTP dialog is still active, i.e., before the OPES processor
finally accepted the message. Depending on the exact policy, the
error response should then be sent in reply to the MAIL FROM, RCPT
TO, or DATA command.

4.9. Convert Attachments to HTTP Links

This use case will only modify the email message body without any
other influence on the SMTP dialogs, mail routing, etc. Larger
sections (attachments) are removed from the email, and the content is
stored on a web server. A link to that new URL is then added into
the text of a first section that is likely to be displayed by an
email client. Storing the attachments onto the web server is not in
the scope of the OPES/SMTP scenario and needs to be implemented by
the callout server.

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5. Security Considerations

Application-independent security considerations are documented in
application-agnostic OPES specifications [5]. This document contains
only use cases and defines no protocol operations. Security
considerations for protocols that appear in these use cases are
documented in the corresponding protocol specifications.

Use case "Secure Email Handling" (Section 4.5) is special in this
regard because it requires the extension of the end-to-end encryption
model and a secure handling of private cryptographic keys when
creating digital signatures or when decrypting messages. Both are
out of scope of OPES protocol specifications. An implementation of
such a service raises security issues (such as availability and
storage of cryptographic keys) that must be addressed regardless of
whether the implementation happens within an MTA or within an OPES
callout server.

6. References

6.1. Normative References

[1] Barbir, A., Penno, R., Chen, R., Hofmann, M., and H. Orman, "An
Architecture for Open Pluggable Edge Services (OPES)", RFC 3835,
August 2004.

[2] Barbir, A., Burger, E., Chen, R., McHenry, S., Orman, H., and R.
Penno, "Open Pluggable Edge Services (OPES) Use Cases and
Deployment Scenarios", RFC 3752, April 2004.

[3] Rousskov, A., "Open Pluggable Edge Services (OPES) Callout
Protocol (OCP) Core", RFC 4037, March 2005.

[4] Rousskov, A. and M. Stecher, "HTTP Adaptation with Open
Pluggable Edge Services (OPES)", RFC 4236, November 2005.

[5] Barbir, A., Batuner, O., Srinivas, B., Hofmann, M., and H.
Orman, "Security Threats and Risks for Open Pluggable Edge
Services (OPES)", RFC 3837, August 2004.

[6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

6.2. Informative References

[7] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April
2001.

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[8] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.

[9] Crocker, D., "Internet Mail Architecture", Work in Progress,
March 2005.

[10] Gellens, R. and J. Klensin, "Message Submission", RFC 2476,
December 1998.

[11] Myers, J., "Local Mail Transfer Protocol", RFC 2033, October
1996.

Acknowledgements

Many thanks to everybody who provided input for the use case
examples, namely, jfc and Markus Hofmann. Thanks also for the
discussion and feedback given on the OPES mailing list.

Authors' Addresses

Martin Stecher
Secure Computing Corporation
Vattmannstr. 3
33100 Paderborn
Germany

EMail: [email protected]
URI: http://www.securecomputing.com/

Abbie Barbir
Nortel
3500 Carling Avenue
Ottawa, Ontario
CA

Phone: +1 613 763 5229
EMail: [email protected]

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