DNS Based Service Discovery

Axigen Auto-discovery

As an alternative Auto-discovery method, RFC 6186 defines a method for DNS-based service discovery for the SMTP, IMAP, and POP3 services, to enable simple auto-configuration of MUAs (aka mail clients: Outlook, Mail.app, Thunderbird, etc). For the CalDAV and CardDAV services, the DNS-based discovery is covered by RFC 6764.

Email submission

In order to publish the SMTP details that your mail clients could use to automatically configure an account, the _submission SRV service label should be configured to look like:

Note that this covers connections both with and without Transport Layer Security (TLS) [RFC5246] as defined for SMTP in [RFC3207]. The difference is made by the indicated port (587) in our example here. See Why 3 different ports (25, 465 and_587) for SMTP Receiving services for an explanation on what are the SMTP ports.

 

For example, a dig srv _submission._tcp.yourDomain.com on your domain would output:

IMAP

For IMAP there are two SRV service labels:

  • _imap
  • _imaps

For example, a dig srv _imap._tcp.yourDomain.com might look like this:

POP3

For POP3 there are two SRV service labels:

  • _pop3
  • _pop3s

For example, a dig srv _pop3s._tcp.yourDomain.com might look like this:

Additional information

It is possible to prioritize services, for example: service records for both IMAP and POP3, with IMAP having a lower-numbered priority value (0) than POP3 (10), indicating to the MUA that IMAP is preferred over POP3, when the MUA can support either service.

In addition, with SRV RRs it is possible to indicate that a particular service is not supported at all at a particular domain by setting the target of an SRV RR to ".". If such records are present, clients MUST assume that the specified service is not available, and instead make use of the other SRV RRs for the purposes of determining the domain preference.

Example: service records for IMAP and POP3 with both TLS and non-TLS service types are present. Both IMAP and POP3 non-TLS service types are marked as not available. IMAP (with TLS) has a lower-numbered priority value 0 than POP3 (with TLS) at priority 10, indicating to the MUA that IMAP is preferred over POP3, when the MUA can support either service, and only the TLS versions of the services are available.

WebDAV calendaring and vCard extensions

For CalDAV, there are two SRV service labels:

  • _caldav
  • _caldavs

For example, a dig srv _caldavs._tcp.yourDomain.com might look like this:

For CardDAV, there are two SRV service labels:

  • _carddav
  • _carddavs

For example, a dig srv _carddavs._tcp.yourDomain.com might look like this:

autodiscover and autoconfig DNS records

In order to enable ActiveSync / Outlook / eMClient (autodiscover) and Thunderbird (autoconfig) autodiscovery, the following DNS records as to be configured on a per-domain basis:

autodiscover          IN      A       193.230.245.23
autoconfig            IN      A       193.230.245.23