Network Working Group B. Haberman
Internet-Draft JHU APL
Intended status: Standards Track J. Levine
Expires: July 20, 2017 Taughannock Networks
January 16, 2017

Using a DNS SRV Record to Locate an X.509 Certificate Store


This document describes a method to allow parties to locate X.509 certificate stores with Domain Name System Service records in order to retrieve certificates and certificate revocation lists. The primary purpose of such retrievals is to facilitate the association of X.509 and PGP public keys with e-mail addresses to allow for encrypted e-mail exchanges.

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Table of Contents

1. Introduction

X.509 and PGP public keys can be used to encrypt or sign e-mail messages. In order to verify a sender's signature or encrypt an e-mail, the e-mail client needs to locate the appropriate public key. The X.509-based Public Key Infrastructure (PKI) [RFC5280] provides the necessary services to allow for the retrieval of certificates and certificate revocation lists, but lacks the discovery mechanism needed to associate e-mail domains with specific PKI servers.

This document specifies an approach that uses a Domain Name System (DNS) Service Record (SRV) that allows mail service providers to advertise the X.509 or PGP certificate store [RFC4387] that contains certificates and certificate revocation lists for their e-mail users. Additionally, this document specifies the appropriate query strings to use when accessing the certificate store.

The capitalized 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].

2. Service Record Format

The general format of a DNS SRV record is documented in [RFC2782] as:

     _Service._Proto.Name TTL Class SRV Priority Weight Port Target


To support the advertisement of an X.509 certificate store, service providers publish an SRV record for the certificates service with the appropriate parameters, as described in [RFC4387], section 3.2. An example of such an SRV record is:

     _certificates._tcp 86400 IN SRV 0 0 443


The parameters of the DNS SRV record are set based on the operational needs of the service provider. The DNS SRV record SHOULD be signed via DNSSEC [RFC4033][RFC4034]. The server MUST be an https server and will typically use port 443. The certificate of the https server SHOULD be validated by a DNSSEC signed TLSA record, and MAY also be validated by a certificate authority.

3. Certificate Store Queries

To retrieve an X.509 S/MIME certificate, the attribute type is "uri", and the URI is constructed using the path described in [RFC4387], Section 3.3, specifically "/certificates/search.cgi". Using the SRV record above to look up a certificate for, the URI would be:


X.509 certificate stores MUST support the uri attribute and MAY support other attributes.

To retrieve a PGP certificate, the attribute type is "email", and the URI is constructed using the path described in [RFC4387], Section 3.3, specifically "/pgpkeys/search.cgi". Using the SRV record above to look up a certificate for, the URI would be:


PGP certificate stores MUST support the email attribute and MAY support other attributes.

4. Name Matching

SMTP [RFC5321] specifies that the local part of a mailbox is interpreted only by the mailbox domain itself. This document does not update or modify that document.

If a certificate store has no certificate with an e-mail address that matches the uri or email attribute in a retrieval request, but it does have a certificate with an e-mail address that the mailbox domain treats similarly to the requested address, the server MAY return that certificate. The definition of what is sufficiently similar is a matter of local policy, but the intention is that a human correspondent would consider the same person or entity to receive mail at the two addresses.

5. Certificate Validation

The certificate is returned as a blob of binary data. If multiple certificates are returned, the response is encoded as multipart/mixed as described in [RFC4387] section 2.

X509 S/MIME certificates are validated by checking for a signature by a Certificate Authority (CA) that is acceptable to the validating party. This specification defines an additional validation technique. The domain MAY publish validation certificates using TLSA records at the name _smimeca._tcp. The TLSA records MUST have PKIX-TA or DANE-TA usage[RFC7218]. A validation certificate published by a domain MUST NOT be used to validate certificates other than those with e-mail addresses in that domain.

Since the relationship between a domain and its mailbox users is in general unknown to correspondents, a client applies a local policy to decide whether to use a S/MIME certificate validated only by a signing certificate published by the domain.

PGP certificates are validated by the PGP web of trust. A domain can endorse the certificates it publishes by signing them with a signature of postmaster@<domain>. Since the relationship between a domain and its mailbox users is in general unknown to correspondents. a client applies a local policy to decide whether to use a PGP certificate retrieved from a certificate server. This policy would typically be the same one used to decide whether to use a certificate retrieved from a traditional PGP key server.

6. Certificate use and cacheing

Clients SHOULD cache responses to queries as advised by http cache headers. This includes both returned certificates, and 404 failures saying that an address (or other search key) has no certificate.

S/MIME keys retrieved from the certificate store SHOULD NOT be used for validation of signatures on incoming mail without further validation of the certificate. S/MIME signed mail includes a copy of the signing certificate which, if it can be validated, typically would be used instead.

7. Security Considerations

Certificate queries could be used to try to validate lists of e-mail addresses. This is essentially the same problem that mail servers face with VRFY, EXPN, and RCPT TO probes, and the same countermeasures would apply, such as rate limiting, blacklisting abusive clients, and returning fake results for non-existent addresses.

DNSSEC signatures on the SRV record and the https server certificate ensure that any keys retrieved by the technique described in this document are the ones published by the domain's management. But since correspondents often do not know the relationship between a domain and its mailbox users, it would be imprudent to assume that such certificates are in fact ones issued to or used by mailbox recipients or to assume that mail encrypted using the certificates will be readable only by the intended recipient.

A domain could publish man-in-the-middle certificates that allowed it to decode and read mail, and perhaps re-encrypt it using different certificates used by the recipients. In some cases this would be entirely legitimate, e.g., a financial institution that is required to log all of its employees' correspondence. In other cases, it could be intrusive or improper surveillance of the contents of users' mail. Identifying or describing the relationship between a domain and its mail users is beyond the scope of this document.

8. IANA Considerations

IANA is requested to update two entries in the Service Name and Transport Protocol Port Number Registry.

8.1. Certificates service

Service Name: certificates

Transport Protocol(s): tcp

Assignee: IESG

Contact: <>

Description: Server for S/MIME and PGP certificates

Reference: [this document]

Port Number: none

Service Code: none

Known Unauthorized Uses: none

8.2. Smimeca service

Service Name: simeca

Transport Protocol(s): tcp

Assignee: IESG

Contact: <>

Description: Per-domain authority certificate for S/MIME certificates

Reference: [this document]

Port Number: none

Service Code: none

Known Unauthorized Uses: none

9. Acknowledgements

We thank Wei Chuang, Nicolas Lidzborski, and Andreas Schulze for comments and suggestions.

10. 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.
[RFC2782] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, DOI 10.17487/RFC2782, February 2000.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D. and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, DOI 10.17487/RFC4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D. and S. Rose, "Resource Records for the DNS Security Extensions", RFC 4034, DOI 10.17487/RFC4034, March 2005.
[RFC4387] Gutmann, P., "Internet X.509 Public Key Infrastructure Operational Protocols: Certificate Store Access via HTTP", RFC 4387, DOI 10.17487/RFC4387, February 2006.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R. and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, DOI 10.17487/RFC5321, October 2008.
[RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify Conversations about DNS-Based Authentication of Named Entities (DANE)", RFC 7218, DOI 10.17487/RFC7218, April 2014.

Authors' Addresses

Brian Haberman Johns Hopkins University Applied Physics Lab EMail:
John Levine Taughannock Networks PO Box 727 Trumansburg, NY 14886 Phone: +1 831 480 2300 EMail: