Internet DRAFT - draft-chuang-relay-flow-identifier
draft-chuang-relay-flow-identifier
Independent Stream W. Chuang
Internet-Draft Google, Inc.
Intended status: Experimental 20 January 2023
Expires: 24 July 2023
Relay Flow Identifier
draft-chuang-relay-flow-identifier-03
Abstract
To prevent spammers from using relay forwarding, we propose to
identify relay flows. We do this by having relays categorize their
authenticated traffic flows and publish to receivers identifiers
associated with those flows. This is a unique, persistent over time,
relay flow identifier name that is secured by some digital signature.
Receivers can use this identifier to help categorize traffic through
the relay and use that identifier to apply fine-grain anti-abuse
policies instead of on the entire traffic through the relay. This
document provides a specification for DKIM (RFC6376) for originating
traffic and ARC (RFC8617) for forwarded traffic.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 24 July 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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and restrictions with respect to this document. Code Components
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology and Definitions . . . . . . . . . . . . . . . 2
2. Relay Flow Identifier Specification . . . . . . . . . . . . . 3
2.1. Flow Identification . . . . . . . . . . . . . . . . . . . 4
2.2. DKIM . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. ARC . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Privacy Considerations . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
6. Normative References . . . . . . . . . . . . . . . . . . . . 5
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
Spammers utilize relays to obfuscate their identities and often to
spoof some other identity with email receivers. For example a
spammer may exploit the shared tenancy vulnerability of SPF [RFC7208]
to spoof some identity as follows. It finds a relay that hosts many
different enterprise customers who include the relay's IPs in their
SPF policies. The spammer then sends traffic through the relay
assuming the identity of one of those customers i.e. it spoofs the
MAIL FROM identity of the victim domain. While the SPF validation
(if done) of the initial send by the spammer to the relay fails, a
subsequent SPF validation when forwarded to some other victim
receiver from the relay will pass SPF because the IPs are contained
in the victim's SPF policy. At some point, the receiver notices the
spam via the relay and wants to apply anti-abuse counter measures.
With existing authentication methods, this policy would impact all
mail flows through that relay, both innocent and malicious. A better
approach would be to selectively apply anti-abuse counter measures to
the spammer's flow which is what this proposal enables. Better yet,
the receiver is enabled to cluster traffic to a spammer controlled
domain and apply policies scoped to what the spammers have control
over.
1.1. Terminology and 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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Acronyms
* Authenticated Received Chain (ARC) [RFC8617] - is a protocol that
is meant to resolve some of the issues for DMARC [RFC7489] to fix
the problems that DMARC policy rejects caused by mail forwarding,
and is based on DKIM, but suffers from similar issues as DKIM
replay. ARC defines digital signatures and Authentication-Results
by ADMD and a versioning mechanism for them.
* Authentication-Results [RFC8601]- A header containing a list of
validation results and comments.
* DomainKeys Identified Mail (DKIM) [RFC6376]- IETF standard for the
cryptographic protocol to authenticate email at the domain level
and protect the integrity of messages during transit.
- DKIM replay- [RFC6376] section 8.6 defines a vulnerability
called DKIM Replay as a spam message sent through a SMTP MTA
DKIM signer, that then is sent to many more recipients,
leveraging the reputation of the signer.
* Sender Policy Framework (SPF) [RFC7208]- IETF standard for
authenticating sending servers typically based on IP address.
2. Relay Flow Identifier Specification
This specification defines an identifier name for mail traversing a
relay. Typically the relay uses password authentication such as
methods provided for in [RFC4954] but other methods MAY be possible.
This identifier MAY also be used for authenticated forwarding flows
such as mailing lists and with other authentication methods such DKIM
or SPF that verify who the sender is. Because some traffic may have
originated at the relay, which traditionally may be DKIM signed, this
document provides a specification for DKIM [RFC6376]. In other
instances, the relay forwards traffic originated elsewhere, and these
are typically not DKIM signed by the relay, so instead this document
provides a specification using ARC [RFC8617].
Email Service Providers can delegate relay and forwarding services to
enterprise customers, typically associated with some customer domain.
Spammers utilize these features either by acting as an enterprise
customer or by hijacked accounts. This specification proposes naming
flows by enterprise customers to help the email receiver with
categorization and application of anti-abuse counter measures. As
some mechanisms for mail forwarding such as mailing lists are often
opaque after being sent and problematic for debug and abuse
protection, this offers a naming scheme to help identify those
mechanisms.
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2.1. Flow Identification
The relaying service choosing to use this specification MUST
categorize and name relayed traffic flows such that receivers can do
anti-abuse analysis upon them if necessary. In order for the
identifier to be effective, it SHOULD be persistent in time and
uniquely named across all flows through the relay. As relayed
traffic flow is often associated with a delegated domain, the first
part of the identifier MUST either include a domain associated url-
safe base64 [RFC4648] token, or be empty if no such delegated domain
is present. It MAY include a local part url-safe base64 [RFC4648]
token after the domain token and separated by a period '.'. This
local part token can help describe the mail forwarding mechanism.
Combined the domain token and the optional local token form the relay
flow identifier name. If a message is associated with more than one
flow, the relay SHOULD select the more specific flow based on local
policy. That name MUST NOT be any relay internal name though MAY be
a secure cryptographic hash of such. Also that name MUST NOT contain
or be associated with any Personally Identifiable Information (PII).
The parser should ignore commas '+' whose use may be specified in the
future.
Example valid names:
0123456789
0123456789.abcdwxyz
.abcdwxyz
<empty>
2.2. DKIM
This proposes a new DKIM [RFC8617] DKIM-Signature tag-value that
identifies the presence of a relay flow and a relay flow identifier
name. The tag is defined "rfid", while the value name consists of a
domain url-safe base64 token and the optional local url-safe base64
token separated by a period. The token parsers MUST ignore a
reserved plus that may be further specified in the future.
Example:
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=example.com;
s=20230116; h=...; bh=...; b....=; rfid=0123456789.abcdwxyz
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2.3. ARC
This proposes a new ARC [RFC8617] ARC-Authentication-Result defined
method [RFC8601] that identifies the presence of a relay flow and its
property that identifies a relay flow identifier name. The defined
method is "relay", which when present, takes a single result value of
"pass" that indicates the relay was authenticated. The relay method
will have a propspec tag-value with a policy ptype with a "rfid"
property i.e "policy.rfid" that takes a single token value. That
token value consists of a domain url-safe base64 token and the
optional local url-safe base64 token separated by a period. The
token parsers MUST ignore a reserved plus that may be further
specified in the future.
Example:
ARC-Authentication-Results: i=1; auth.example.com;
relay=pass (comments) policy.rfid=0123456789.abcdwxyz
3. Privacy Considerations
4. Security Considerations
5. IANA Considerations
This document has no IANA actions yet.
6. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/rfc/rfc4648>.
[RFC4954] Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service
Extension for Authentication", RFC 4954,
DOI 10.17487/RFC4954, July 2007,
<https://www.rfc-editor.org/rfc/rfc4954>.
[RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", STD 76,
RFC 6376, DOI 10.17487/RFC6376, September 2011,
<https://www.rfc-editor.org/rfc/rfc6376>.
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[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208,
DOI 10.17487/RFC7208, April 2014,
<https://www.rfc-editor.org/rfc/rfc7208>.
[RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<https://www.rfc-editor.org/rfc/rfc7489>.
[RFC8601] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 8601,
DOI 10.17487/RFC8601, May 2019,
<https://www.rfc-editor.org/rfc/rfc8601>.
[RFC8617] Andersen, K., Long, B., Ed., Blank, S., Ed., and M.
Kucherawy, Ed., "The Authenticated Received Chain (ARC)
Protocol", RFC 8617, DOI 10.17487/RFC8617, July 2019,
<https://www.rfc-editor.org/rfc/rfc8617>.
Appendix A. Acknowledgments
Thanks goes to Emil Gustafsson for suggesting a DKIM specification.
Author's Address
Weihaw Chuang
Google, Inc.
Email: weihaw@google.com
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