MIR-5: DNS Discovery

Abstract

Mailio needs to distinguish between ordinary SMTP able server and Mailio able server. It also needs to exchange public keys to prevent possible security issues (mail spoofing), alternate allowed message senders and verifiability of the exchanged messages.

Motivation

Mailio needs to recognize other Mailio systems in order to successfully exchange messages.

Specification

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 RFC-2119.

Verifiability

Mailio senders must have a way to verify/authenticate the receiving messages.

Mailio server MUST have a DNS TXT record that MUST adhere to the format defined in this section.

mailio public exchange key

mailio._mailiokey.mail.io 	TXT	"v=MAILIO1; k=ed25519; p=5uW7anEGF1nIjGfp5pS2kiN0cn2mGYkuSa+TCBoFIbQ="

The structure of a MAILIO DNS TXT record MUST be <selector>._mailiokey.<domain>, where the selector MUST be mailio.

p=5uW7anEGF1nIjGfp5pS2kiN0cn2mGYkuSa+TCBoFIbQ= MUST be the public Mailio Ed25519 signature key in base64 format. The Mailio public key MUST be 32 bytes in length.

The DNS TXT record MUST always start with verision: v=MAILIO1.

The structure is insipired by the DKIM DNS TXT record. Currently only version MAILIO1 will be supported.

Security Considerations

• Sender/Receiver signature verification

The current specification does not support routing messages via alternate servers.

Copyright

Copyright and related rights waived via CC0.

Citation

Please cite this document as:

Igor Rendulic, "MIR-5: DNS Discovery," Mailio Improvement Proposals, no. 5, June 2022. [Online serial]. Available: https://mirs.mail.io/MIRS/mir-5.