MARID D. Crocker
Internet-Draft Brandenburg InternetWorking
Expires: December 16, 2004 J. Leslie
JLC.net
D. Otis
Mail Abuse Prevention System
June 17, 2004
Client SMTP Validation (CSV)
draft-ietf-marid-csv-intro-00
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Abstract
Internet mail relies on exchanges between systems that have made no
prior arrangement with each other. The current service fails to
provied an adequate level of accountability for participating hosts.
Client SMTP Validation (CSV) provides an economical service that
permits an SMTP server to decide whether messages sent by the client
SMTP are likely to be well-behaved, or at least to decide whether the
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client is sufficiently accountable for its actions. CSV provides a
small, simple and useful improvement to Internet mail service
accountability. It builds upon the existing practise of service
providers that accredit the networks from which sending systems are
connecting.
Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Design Goals . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1 Identification . . . . . . . . . . . . . . . . . . . . . . 6
4.2 Authentication . . . . . . . . . . . . . . . . . . . . . . 6
4.3 Authorization . . . . . . . . . . . . . . . . . . . . . . 7
4.4 Accreditation . . . . . . . . . . . . . . . . . . . . . . 7
5. Client SMTP Validation Details . . . . . . . . . . . . . . . . 7
5.1 Identification . . . . . . . . . . . . . . . . . . . . . . 8
5.2 Authentication . . . . . . . . . . . . . . . . . . . . . . 8
5.3 Authorization . . . . . . . . . . . . . . . . . . . . . . 8
5.4 Accreditation . . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1 References - Normative . . . . . . . . . . . . . . . . . . . 9
7.2 References - Informative . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
B. Host Name Authentication . . . . . . . . . . . . . . . . . . . 11
B.1 DNS-based Mapping . . . . . . . . . . . . . . . . . . . . 11
B.2 Reverse DNS . . . . . . . . . . . . . . . . . . . . . . . 12
B.3 Forward DNS Lookup . . . . . . . . . . . . . . . . . . . . 12
B.4 Encryption-Based Authentication . . . . . . . . . . . . . 13
B.4.1 StartTLS . . . . . . . . . . . . . . . . . . . . . . . 13
B.4.2 SMTP Auth . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . 14
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1. Overview
To validate an SMTP session from an unknown sending SMTP client using
CSV, the receiving SMTP server SHOULD:
1. Obtain the source IP address of the TCP connection
2. Extract the EHLO domain name
3. Query a chosen Accreditation Service for the EHLO domain name
(see Domain Name Accreditation (DNA) [ID-Marid-CSVDNA])
4. Query DNS for a SRV record under the EHLO domain name (see Client
SMTP Authorization (CSA) [ID-Marid-CSVCSA])
5. Check the flags returned and check for a match in the list of
returned IP addresses
6. Based on the results of (3) and (5), determine the level of trust
to give to the sending SMTP client.
If the level of trust is high enough, process all email from that
session in the traditional manner, delivering or forwarding without
the need for further validation.
If the level of trust is too low, return an error showing the reason
for not trusting the sending SMTP client.
If the level of trust is in between, document the result in a header
in each email delivered or forwarded, and/or perform additional
checks
2. Background
Internet mail suffers from the operation of hosts acting as mail
transfer agents (MTA) without any meaningful cross-net
accountability. This makes it impossible to vet MTAs or find
recourse when their operations cause problems. Many of these hosts
have been compromised and have been turned into unwilling
participants in large networks of hostile MTAs that send spam and
worms, and contribute to denial of service attacks.
When a server MTA receives a connection, it must decide whether to
accept the message traffic that is being sent to it, trusting that
its delivery will not be problematic to the operation of the provider
or their users. How can it do this, when operating in the open
Internet? Client SMTP Validation (CSV) defines a service that permits
the receiving SMTP server to decide whether messages sent by the
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sending SMTP client are likely to be well-behaved, or at least to
decide whether the client is sufficiently accountable for its
actions.
The process of deciding on this trust of the client requires
performing a series of conceptually discrete steps:
Identification:
What is the "name" of the client to be trusted? How is it
referenced?
CSV uses the domain name supplied by a client in the SMTP HELO/
EHLO.
Authentication:
Is the client MTA legitimately associated with that name? Can we
prove that the client is who it purports to be?
CSV documents a range of existing techniques that are appropriate
for use with CSV.
Authorization:
Is the sending SMTP client permitted to act as a client MTA? Has a
separate authority given it permission to perform this service?
CSV specifies a DNS-based record that states whether an associated
host has permission to operate as a client MTA.
Accreditation:
What is the trust that is to be extended to the entity that
authorized the client MTA? Does the server MTA have a basis for
deciding that the entity providing authorization for the client
MTA can, itself, be trusted to make valid authorizations?
CSV discusses existing techniques for accrediting authorizing
systems. It also defines a DNS record that permits such systems
to announce the accreditation services in which they are listed.
It defines another DNS record that permits accreditation services
to publish their assessments of client MTAs.
A proposal or its implementation well might combine these steps.
However it is important to consider them independently, in order to
ensure that the proposal specifies that they are performed in a valid
manner, or at least that the constraints of the proposal are clear
for each of these conceptual functions. This specification
distinguishes each of these logical steps and defines their operation
separately. It is based on validation of the EHLO domain name. The
proposed mechanism is small, simple and useful. In particular it
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permits detecting machines that are prohibited from acting as Client
MTAs and those that are permitted. The mechanism is designed to be
useful between peer MTAs and only requires use of well-established
mechanisms.
Address-based Authentication:
Currently, service providers often maintain lists of remote
networks that are known to be trustworthy or untrustworthy as
sending SMTP clients. Typically, these lists are based on the use
of IP Addresses of the clients. The IP Addresses serve as
identifiers. The list specifies positive or negative
authorization, and the source of the list is an organization the
service provider deems worthy to assess other sites.
When used in this way, IP Addresses are authenticated by relying
on their use in the IP routing infrastructure. Packets are routed
to the specified IP Address, over the open Internet. A repeated
exchange using that IP Address is therefore presumed to be an
interaction with the host legitimately associated with that IP
Address.
Increased topological, transfer and access complexities on the
Internet are making IP Addresses increasingly problematic for use
as persistent identifiers. Instead they are viewed as appropriate
only for the most transient task of delivering individual packets.
CSV builds upon this popular model. Besides the considerable benefit
of having operational practice, the model can be extremely efficient.
It permits the service provider to assess the source of an entire
message stream, rather than having to evaluate each message. Also,
CSV makes its assessment before messages cross the Internet, thereby
saving bandwidth and reducing the impact of a distributed denial of
service attack.
3. Design Goals
CSV will verify that a host is authorized to act as an SMTP client
and that the client is likely to be operated acceptably. CSV will
enhance current practice with:
o Identification by persistent domain name rather than transient IP
Address
o Alternative authentication techniques
o Explicit listing of client service authorization
o A standardized method of referencing accreditation services
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o A standardized method of querying an accreditation service.
Terminology: Terminology conforms to [ID-email-arch].
Discussion: The venue for discussing this proposal is the
<http://ietf.org/html.charters/marid-charter.html>.
NOTE: The current draft makes reference to quite a few underlying
services that need citations (ed.)
4. Requirements
4.1 Identification
The means of identifying a remote host or service requires uniqueness
and is aided by persistence. The identifier must not be ambiguous
and its use is made far more efficient if it is stable over time.
The two usual choices are IP Addresses and Domain Names.
An IP Address typically refers to a single host and can change
relatively frequently, as the host's connection to the Internet
changes. IP Addresses are reported by the Internet infrastructure
and for simple security requirements, transactional use of an IP
Address through the Internet's routing fabric is taken as validation
of the Address. Domain Names are longer-lived but require new
administrative effort. They can be used to refer to multiple hosts
simultaneously. The administrator of a Domain Name may list any IP
Address they wish to associate with the name, independent of the
administrator and the Domain Name having a valid relationship to that
Address. Therefore, authentication of a domain name's reference to a
particular IP Address requires an explicit authentication step.
4.2 Authentication
If the sending SMTP client of an connection can be authenticated,
then it is possible to develop an accountability mechanism based on
that authentication. MUA-MSA exchanges have a substantial number of
useful authentication mechanisms available. These are often very
strong, and involve significant prior arrangement. The same holds
true for MDA-MUA exchanges, and often for MSA-MTA and MTA-MDA
exchanges, such as within an organizations local network.
What is missing is a useful means of authenticating MTA-MTA exchanges
over the open Internet. Prior arrangement between such a pair of
MTAs is antithetical to the history and operation of Internet mail.
Spontaneous communications are at the core of Internet design and
operation. So the challenge is to develop an authentication
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mechanism that permits the necessary amount of accountability,
without imposing undue overhead or restrictions.
4.3 Authorization
Internet operation has typically required no public mechanism for
restricting or permitting particular hosts to operate clients or
servers for particular services on behalf of particular domains. The
DNS MX record states where to route mail that is destined for a
specific domain; this implies a degree of authorization for the host
referenced in the MX. However the record is really for routing and
there is no equivalent means of specifying prohibition of other hosts
that might act as intermediaries. Similarly there is no means for
checking the authorization of World Wide Web servers, DNS servers,
telnet clients or other Internet applications.
What is missing is an open, interoperable means by which a trusted
agency can announce its authorization of a particular host to operate
a particular service.
4.4 Accreditation
In non-Internet environments the basis for deciding that an
authorizing agency is, itself, to be trusted, is highly varied and
often is not well-understood. It is expected that this portion of an
Internet mail validation service will therefore need to support be a
variety of accreditation service styles.
What is needed is a means of announcing performance of accreditation
and a means of querying a service to obtain information about the
host it is accrediting.
5. Client SMTP Validation Details
CSV defines a mechanism for session-time, domain-based validation of
a sending SMTP client. It is useful across the open Internet,
between MTAs that have made no prior arrangement with each other.
Validation establishes that the operation of the MTA is authorized by
an accredited administrator of the declared domain name.
The validation requirements are modest, because the system does not
seek to provide long-term vetting of the client host, nor does it
assess the actual content being exchanged. Techniques that would be
wholly inadequate for classic, strong authentication and validation
can be entirely sufficient for CSV's needs.
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5.1 Identification
The sending SMTP client host is identified by a Domain Name, supplied
by that host as the parameter to an opening SMTP HELO or EHLO. The
domain name serves as a unique, topologically-independent, persistent
identifier that is registered in the Domain Name Service.
For CSV, a sending SMTP client places the domain name into the
<Domain> field specified for a SMTP HELO or EHLO [RFC2821]. The
domain name is any name under which it is claiming authorization to
act as a sending SMTP client. A receiving SMTP server will extract
this name and use it as the identification for the client seeking to
send email.
5.2 Authentication
There is no universal method to authenticate that a host is correctly
identifying itself. For most email purposes, it will be sufficient
to show that the EHLO domain name forward-resolves to the IP address.
CSV usually returns the list of IP addresses in the reply to the SRV
query. The Host Name Authentication appendix gives advice on how to
proceed if no list is returned.
If the list is returned and the actual IP address is in it, the
receiving SMTP server SHOULD consider the EHLO domain name to be
authenticated. Conversely, if the list is returned and the actual IP
address is not in it, the assertion of the EHLO domain name SHOULD be
considered incorrect, and an error returned.
5.3 Authorization
The purpose of authorization, in CSV, is to establish that an
accountable authority has given permission for the sending SMTP
client host to operate in that role.
CSV participants SHOULD use the method defined in Client SMTP
Authorization (CSA) [ID-Marid-CSVCSA]. It specifies a DNS record
that is associated with the domain name offered by the sending SMTP
client host.
5.4 Accreditation
The utility of any service like CSV is entirely dependent upon the
relevance, reliability and accuracy of the service that accredits the
authorizing agent. It is expected that there will be numerous
services that provide accreditation. CSV is intended to support use
of any service that gains credibility among operators of SMTP
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servers.
An initial set of capabilities for specifying CSV-related
accreditation services is specified in Domain Name Accreditation
[ID-Marid-CSVDNA]. CSV participants SHOULD use the methods defined
there.
6. Security Considerations
This entire proposal pertains to security, namely authentication and
authorization of peer MTAs.
The proposal relies on the integrity and authenticity of DNS data.
7. References
7.1 References - Normative
[ID-Marid-CSVCSA]
Otis, D., Crocker, D. and J. Leslie, "sending SMTP client
Authorization (CSA)", June 2004.
[ID-Marid-CSVDNA]
Leslie, J., Crocker, D. and D. Otis, "Domain Name
Accreditation (DNA)", June 2004.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
821, August 1982.
[RFC0822] Crocker, D., "Standard for the format of ARPA Internet
text messages", STD 11, RFC 822, August 1982.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC2554] Myers, J., "SMTP Service Extension for Authentication",
RFC 2554, March 1999.
[RFC2782] 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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[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April
2001.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
7.2 References - Informative
[ID-brand-drip]
Brand, R. and L. Sherzer, "Designated Relays Inquiry
Protocol (DRIP)", draft-brand-drip-02 (work in progress),
October 2003.
[ID-email-arch]
Crocker, D., "Internet Mail Architecture", May 2004.
Authors' Addresses
Dave Crocker
Brandenburg InternetWorking
675 Spruce Drive
Sunnyvale, CA 94086
USA
Phone: +1.408.246.8253
EMail: dcrocker@brandenburg.com
John Leslie
JLC.net
10 Souhegan Street
Milford, NH 03055
USA
Phone: +1.603.673.6132
EMail: john@jlc.net
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Douglas Otis
Mail Abuse Prevention System
1737 North First Street, Suite 680
San Jose, CA 94043
USA
Phone: +1.408.453.6277
EMail: dotis@mail-abuse.org
Appendix A. Acknowledgements
This proposal is similar to DRIP [ID-brand-drip], however it uses a
different DNS [RFC1035] record.
Appendix B. Host Name Authentication
The routing infrastructure of the Internet distinguishes hosts by
their topological attachment, noted as its IP Address. Because IP
Addresses change periodically and users prefer references that can be
mnemonic, hosts on the Internet generally have one or more Domain
Names (DNS) [RFC1035] assigned to them. Domain Name are globally
unique. The core function of the DNS is to map from a name supplied
by the user, to an IP Address associated with that name. Internet
protocols often permit a host to identify itself with its domain
name.
But what if a host is programmed incorrectly, or even maliciously.
We need a way to authenticate that a host is reporting its name
correctly. Establishing this authentication is separate from
determining its authorization to perform any particular service.
Until the relationship is authenticated, we cannot apply policies
associated with the name.
This appendix lists a number of methods for authenticating the
relationship between the host and its reported name. Other equally
valid methods are possible. The purpose of this document is simply
to demonstrate that more than one valid technique is already in
common use.
B.1 DNS-based Mapping
The Domain Name System has a common mapping mechanism that can be
used in a variety of ways, based on the schema for assigning names
and the types of data listed under those names. The two most popular
schemas are forward mapping and Reverse-DNS. Forward looks up a
"regular" domain name and receive information about it, such as a
list of IP Addresses associated with that name. Reverse DNS starts
with an IP Address and maps it to a pointer to a "regular" domain
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name.
Often when contacted by a remote host, a host uses a reverse-DNS
query to get the name of the remote host. This can be followed by a
forward-DNS query to see if the name reported by the reverse-DNS
query matches an IP address reported by the forward-DNS query. If
so, this is generally considered authentication of the relationship
of the name to the host. This method is often used by receiving SMTP
servers to decide whether to trust the sending SMTP client.
Closing the circle in this manner permits verifying both that the
domain assigning the name and the service provider assigning IP
addresses agree that this is the appropriate name for that remote
host. Although this process has known limitations, it is considered
sufficient for many basic uses.
Use of an IP Address returned by the DNS is sufficient for
CSV-related authentication requirements of this service. However it
MUST NOT be considered a strong form of authentication as to allow
otherwise privileged access. The use of this mechanism is to aid
selection of accreditation services, such as whether to query using
the domain name or the client address. Other measures may be taken
intended to limit exposure to unknown clients but are beyond the
scope of this specification.
B.2 Reverse DNS
Reverse DNS can be used by itself to associate a domain name with an
IP address. It indicates that the entity responsible for allocating
that block of IP addresses has designated an IP address to be used by
the domain name. Unfortunately, the reverse-IP branch of the DNS has
a long history of being poorly maintained, and often does not match
the forward-DNS information even when the relationship of host to
name is genuine.
Reverse DNS by itself should not be considered sufficient
authentication.
B.3 Forward DNS Lookup
It is possible that an isolated forward lookup will be sufficient for
simple sending SMTP client authentication, if an IP Address returned
for that name matches the IP Address reported by the underlying IP
service for that remote host. This indicates that the domain in
question currently designates that IP Address as an IP address
entitled to respond for that domain name.
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B.4 Encryption-Based Authentication
The use of encryption-based authentication supports a process that is
entirely independent of the underlying data delivery service. For
example, it does not use the IP Address.
B.4.1 StartTLS
A common certificate method as used with StartTLS [RFC3207] can
authenticate an unknown server after an investment in signed periodic
digital certificates, encryption capabilities, and services of a
Certificate Authority. This investment creates a barrier for
large-scale use over the open Internet. Reliance on the certificate
signature also adds a need to vet Certificate Authorities in addition
to the confirmed domains.
Spontaneous communications are at the core of Internet design and
operation. So omission of a Certificate Authority is typically
allowed of clients. When this is allowed, StartTLS loses any ability
to authenticate the relationship of the client to its claimed domain
name
B.4.2 SMTP Auth
Another technique is SMTP Auth [RFC2554]. StartTLS invokes a
TLS-based security mechanisms for the entire life of the connection.
SMTP Auth provides an additional authentication command to an
otherwise normal SMTP session, where the client must respond
correctly to a challenge by the server. Thus, it it generally useful
only for clients and servers which have a pre-existing trust
relationship.
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