MARID D. Otis
Internet-Draft Mail Abuse Prevention System
Expires: January 16, 2005 D. Crocker
Brandenburg InternetWorking
J. Leslie
JLC.net
July 18, 2004
Client SMTP Authorization (CSA)
draft-ietf-marid-csv-csa-01
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
Internet operation has typically required no public mechanism for
announcing restriction or permission of particular hosts to operate
clients or servers for particular services on behalf of particular
domains. What is missing is an open, interoperable means by which a
trusted agency can announce authorization for a host to operate a
service. The current specification supports this capability for
sending SMTP clients. Specifically, is a sending SMTP client
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permitted to act as a client MTA? Has a separate authority given it
permission to perform this service? Client SMTP Authorization (CSA)
specifies a DNS-based record that states whether an associated host
has permission to operate as a client MTA.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Model . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Client SMTP Authorization SRV Record . . . . . . . . . . . . 5
6. Domain administrator advice . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . 8
9. Working Group Evaluation . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1 References - Normative . . . . . . . . . . . . . . . . . . 9
10.2 References - Informative . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . 12
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1. Introduction
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 turned into unwilling participants in large networks
of hostile MTAs that send spam and worms, and contribute to denial of
service attacks. Enhancing the Internet mail transfer service to deal
with these issues requires identification, authentication,
authorization and accreditation capabilities about the sending SMTP
client, as per [ID-Marid-CSV]. The current specification addresses
the requirement for explicit authorization.
It is important to distinguish this security function from
authentication. Authentication establishes that a name is being used
legitimately. Authorization establishes that the name is permitted to
perform a particular service. The relationship between these two
functions is that once a client of an exchange is authenticated, then
it is possible to query the permission of that client to perform
specific services.
This specification defines a mechanism to permit session-time
verification that a connecting SMTP client is authorized to request
service as a mail transfer client. The mechanism uses a DNS SRV
[RFC2782] record as a basis for verifying that the associated domain
name is authorized to act as an SMTP client. The mechanism is small,
simple and useful. Separate mechanisms provide the means of
authenticating that the domain name is associated with the connecting
host, and accrediting the agency that is authorizing the sending
host's operation as an SMTP client.
Use of the mechanism specified here MAY also satisfy the
authentication requirement. This can occur as a side-effect of the
DNS server response optimization that returns IP Address mappings in
the Additional Information portion of a response.
Terminology: Terminology conforms to [ID-email-arch].
2. Definitions
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 [RFC2119].
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3. Model
The SMTP [RFC2821], [RFC0821] protocol permits a client to declare
its affiliation, by asserting a domain name in the HELO or EHLO
announcement.
The current proposal has a receiving SMTP server take the domain name
associated with an SMTP client and do a forward query of the DNS. The
returned DNS information indicates whether that domain name is
authorized by the domain administrator to be an SMTP client.
For efficiency, the DNS response MAY also return authentication
information, as per [ID-Marid-CSV]. However the authentication
functionality is outside the scope of this specification.
4. Mechanism
The receiving SMTP server's authorization procedure is:
1. Obtain a domain name that is associated with the sending SMTP
client.
2. Perform a DNS lookup of:
QNAME = _client._smtp.<name>
QCLASS = IN
QTYPE = SRV
where <name> is associated with the host attempting to obtain
service as an SMTP client.
3. If there is no SRV RR matching this QNAME, the CSA information is
Unknown; otherwise at least one CSA record exists.
4. If there is a matching QNAME:
Target addresses MAY be returned in the Additional Data
section, or a query for address records of the target name may
be needed to determine the associated address(es). This MAY be
used to satisfy the authentication function specified in
Client SMTP Validation [ID-Marid-CSV].
Examine Priority, Weight, and Port, to assess whether the
client address is authorized as an SMTP client.
The results of this mechanism will provide the following
authorization levels about sending SMTP clients:
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+--------------------+----------+-----------------------------------+
| CLIENT | WEIGHT | SERVER ACTION |
| AUTHORIZATION | | |
+--------------------+----------+-----------------------------------+
| Not Authorized | 1 | Generate session error with "550 |
| | | Domain not authorized." |
| Authorized | 2 | Continue with CSV processing. |
| Authorized with | 3 | Continue according to local |
| Target Not Valid | | policy; if session error is |
| | | generated, use "550 |
| | | Authentication not resolved." |
| Unknown | (no RR) | Continue according to local |
| | | policy, as if CSV had not been |
| | | invoked; if session error is |
| | | generated, use "550 Client |
| | | Unknown." |
+--------------------+----------+-----------------------------------+
5. Client SMTP Authorization SRV Record
The SRV CSA Record has the following contents:
_Service._Proto.Name:TTL:Class:SRV:Priority:Weight:Port:Target
Service:
_client
Protocol:
_smtp
Name:
Domain name asserted in SMTP EHLO announcements.
(These first three fields become the QNAME _client._smtp.Name.)
TTL:
Standard DNS meaning [RFC1035].
Class:
Standard DNS meaning [RFC1035]. SRV-CSA records are only defined
for the IN Class.
Priority:
The intended use of [RFC2782] SRV records was to aid discovery and
selection of servers by prospective clients. Implementing this
client authentication mechanism for the server, the Priority,
Weight, and Port fields are no longer used for either discovery or
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selection. Thus only one SRV-CSA record is needed and these three
fields are assigned different meanings. Priority defines the
revision level of this mechanism starting at 1.
Weight:
Weight is a group of bit-fields, as follows:
+--------------+----------------------------------------------------+
| Bit Value | Meaning |
+--------------+----------------------------------------------------+
| 1 | Ignore Target: The domain name in the Target field |
| | is a placeholder, and any IP addresses it resolves |
| | to MUST NOT be used for authentication. |
| 2 | Authorized: Any host with a valid claim to this |
| | name is authorized to send mail. |
| - | Other bit values are reserved for expansion and |
| | must be set to zero. |
+--------------+----------------------------------------------------+
The resulting unsigned integer values for weight are:
+--------------+----------------------------------------------------+
| Summed Value | Meaning |
+--------------+----------------------------------------------------+
| 0 | Should not be used, but MAY be interpreted as the |
| | summed value 1. |
| 1 | No mail should be coming from clients with this |
| | name. |
| 2 | Clients with this name are authorized to send |
| | mail. |
| 3 | Clients with this name are authorized to send |
| | mail, but IP addresses associated with the Target |
| | field MUST NOT be used for authentication. |
+--------------+----------------------------------------------------+
Port:
The meaning of Port is reserved for future use. This field must be
set to zero.
Target:
A domain name (typically the same as the EHLO domain) that
resolves to the correct list of IP addresses. If this record is
defined with the "Ignore Target" bit value, this field should be
set to the Name portion of the QNAME, rather than the "."
mentioned in [RFC2782], as a means to prevent excessive traffic on
root DNS servers by errant implementations.
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6. Domain administrator advice
Although a conceptual framework might list the accreditation step as
logically following the authorization step, these steps MAY run in
parallel. Thus, those responsible for maintaining CSV DNS records
should make allowance for the fact that the response of the
accreditation service (which depends only on the EHLO string or the
client address) is likely to arrive at the receiving MTA before the
response to the DNS SRV query detailed here. As a result, the
receiving SMTP server may not follow-up partial or truncated UDP
responses for expediency. Regardless of what is specified, this
receiving SMTP server may decide to refuse the client if their chosen
accreditation service returns "Unknown". The following
recommendations explain how to ensure that the complete list of IP
addresses reaches the receiving SMTP server in the response to its
SRV query.
Currently UDP has a limit of 512 octets. Replies requiring more than
512 octets may create UDP fragmentation and, depending upon the
connection and handling, in addition to a higher rate of packet loss,
may also cause truncated or partial replies. Furthermore, delivery
and resolver handling of truncated and partial responses varies,
leading to additional delays and queries. Domain administrators are
strongly advised to keep DNS replies below 512 octets for these
reasons.
With a complete response to an SRV-CSA query, SMTP server is able to
employ Right Hand Side Black List (RHSBL) services based upon the
domain name rather than address alone and as well as the
accreditation services detailed in [ID-Marid-CSVDNA]. These
domain-based services will not suffer from the same outdated-record
problems as the IP-Address-based services widely used at the time of
this writing. Also, of course, domain-based services will be able to
accredit those domains which must periodically change their IP
address. Reliance on the HELO/EHLO response allows isolation of
domains which may share common address space as with virtual hosting
or allow detection of domains for which there is insufficient history
which may invoke a go-slow approach as example.
In some cases, domains advertising SRV records will benefit by
reassigning some EHLO strings so as to limit the number of IP
addresses to be reported in SRV responses. Owing to the efficient
nature of the SRV record, the mechanism discussed here calls for a
single DNS query per SMTP session (not counting an out-of-band
accreditation query), which is substantially less network traffic
than per-message methods.
To help ensure complete answers are obtained from cached records, TTL
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values of the SRV-CSA and related address records should be the same.
Beware some DNS server implementation consider the SOA TTL as a
default rather than a minimum.
7. Security Considerations
This proposal pertains to security, namely authentication and
authorization of peer MTAs.
The proposal also relies on security of the underlying IP network and
on the integrity of DNS data. It performs a basic authentication of
the peer MTA, based on domain name registration of the peer's IP
Address. As such, the mechanism provides a basic building block to a
larger repertoire of email security services.
There is no way a site can keep its hosts from being referenced as
servers. This could lead to denial of service.
With SRV, DNS spoofers can supply false addresses. Because this
vulnerability exists already with names and addresses, this is not a
new vulnerability, merely a slightly extended one. However, as
SRV-CSA records are used in an authorization context, the DNS servers
can be protected by DNSSEC [RFC3008] should this vulnerability become
intractable.
8. IANA Considerations
The tokens "_client" as _Service and "_smtp" as _Proto labels needs
to be registered as used with DNS SRV records [RFC2782].
9. Working Group Evaluation
This section contains responses to the issues put forward by the
MARID working group chairs.
1. Amount of change in software components
DNS administration, servers and clients MUST support SRV queries.
Client MTA's MUST put their registered domain name in EHLO
announcements.
Server MTA's MUST implement the validation procedure described in
this specification.
2. Configuration complexity
Requires registering each IP Addresses of an authorized Client
MTA, whenever the set of Addresses changes. No other
configuration is required.
3. Current use cases that will no longer be viable
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All current use cases will still be viable. This mechanism is
only enabled by the explicit presence of the defined SRV record
for the domain name in the EHLO announcement.
4. Needed infrastructure changes
Explicit registration of Client MTAs.
Considerations for use in both IPv4 and IPv6
Validation mechanism is based on IP Addresses and requires the
usual query and handling of address types that will be
encountered from the IP module and the DNS.
10. References
10.1 References - Normative
[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.
[RFC1034] Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES",
RFC 1034, November 1987.
[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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
2671, August 1999.
[RFC2782] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
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April 2001.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April
2001.
[RFC3008] Wellington, B., "Domain Name System Security (DNSSEC)
Signing Authority", RFC 3008, November 2000.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
10.2 References - Informative
[ID-Marid-CSV]
Crocker, D., Otis, D. and J. Leslie, "Client SMTP
Validation (CSV)", July 2004.
[ID-Marid-CSVDNA]
Leslie, J., Crocker, D. and D. Otis, "Domain Name
Accreditation (DNA)", July 2004.
[ID-email-arch]
Crocker, D., "Internet Mail Architecture", May 2004.
Authors' Addresses
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
Dave Crocker
Brandenburg InternetWorking
675 Spruce Drive
Sunnyvale, CA 94086
USA
Phone: +1.408.246.8253
EMail: dcrocker@brandenburg.com
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John Leslie
JLC.net
10 Souhegan Street
Milford, NH 03055
USA
Phone: +1.603.673.6132
EMail: john@jlc.net
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