| Internet-Draft | Relay Flow Identifier | January 2023 |
| Chuang | Expires 21 July 2023 | [Page] |
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, base64 identifier token 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.¶
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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 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.¶
Acronyms¶
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.¶
This specification defines an identifier for authenticated relay. Typically it is password authenticated such as methods provided for in [RFC4954] but other methods MAY be possible. This identifier MAY be used for other authenticated forwarding flows such as mailing lists and with other authentication methods such DKIM or SPF that verifies who the sender is. Because some relays may be originated at the relay which traditionally will be DKIM signed, this document provides a specification for DKIM ([RFC6376]). In other instances, relays act as forwarders, and these are sometimes not DKIM signed by the relay so instead this document proposes a specification using ARC ([RFC8617]).¶
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. If a message is associated with more than one flow, the relay SHOULD select the more specific flow based on local policy. This specification defines the relay flow identifier as a base64 ([RFC4648]) token. That name MUST not be any internal name of the relay though MAY be a secure cryptographic hash of such.¶
This proposes a new DKIM [RFC8617] DKIM-Signature tag-value that identifies the presence of a relay flow and a flow identifier as a token. The tag is defined "rfid", while the token value is in base64 ([RFC4648]). The token parsers MUST ignore a reserved comma 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=a+to+z/A+to+Z/0123456789,¶
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 flow identifier as a token. 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 "id" property i.e "policy.id" that takes a single token value. That token value is in base64 ([RFC4648]). The token parser MUST ignore a reserved comma that may be further specified in the future.¶
Example: \¶
ARC-Authentication-Results: i=1; auth.example.com; relay=pass (comments) policy.id=a+to+z/A+to+Z/0123456789,¶
This document has no IANA actions yet.¶
Thanks goes to Emil Gustafsson for suggesting a DKIM specification.¶