Internet-Draft Relay Flow Identifier January 2023
Chuang Expires 24 July 2023 [Page]
Independent Stream
Intended Status:
W. Chuang
Google, Inc.

Relay Flow Identifier


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.

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This Internet-Draft will expire on 24 July 2023.

Table of Contents

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].


  • 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.

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:


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.


DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;;
    s=20230116; h=...; bh=...; b....=; rfid=0123456789.abcdwxyz

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.


ARC-Authentication-Results: i=1;;
    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

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, , <>.
Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service Extension for Authentication", RFC 4954, DOI 10.17487/RFC4954, , <>.
Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, DOI 10.17487/RFC6376, , <>.
Kitterman, S., "Sender Policy Framework (SPF) for Authorizing Use of Domains in Email, Version 1", RFC 7208, DOI 10.17487/RFC7208, , <>.
Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based Message Authentication, Reporting, and Conformance (DMARC)", RFC 7489, DOI 10.17487/RFC7489, , <>.
Kucherawy, M., "Message Header Field for Indicating Message Authentication Status", RFC 8601, DOI 10.17487/RFC8601, , <>.
Andersen, K., Long, B., Ed., Blank, S., Ed., and M. Kucherawy, Ed., "The Authenticated Received Chain (ARC) Protocol", RFC 8617, DOI 10.17487/RFC8617, , <>.

Appendix A. Acknowledgments

Thanks goes to Emil Gustafsson for suggesting a DKIM specification.

Author's Address

Weihaw Chuang
Google, Inc.