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SRV records usage for email and calendar service discovery

I did a quick investigation of the adaptation of DNS SRV records to advertise email, calendar and address book service availability and client auto-configuration among popular service providers around the globe. It would seem that auto-configuration is important enough for the majority of services to adopt it.

SRV records are used for client auto-configuration as defined in RFC 6186 “Use of SRV Records for Locating Email Submission/Access Services” and RFC 6764 “Locating Services for Calendaring Extensions to WebDAV (CalDAV) and vCard Extensions to WebDAV (CardDAV)”. I also included RFC 6120 “Extensible Messaging and Presence Protocol (XMPP): Core” as it’s a similar mechanism and is provided by the same type of service provider.

The below table shows service providers who have service discovery and auto-configuration setup for their main domains. The percentage values at the bottom of the table show how many of the service providers who offer the service also provides an SRV record for it.

IMAP SMTP CalDAV CardDAV XMPP
AOL Mail No SRV No SRV SRV SRV N/A
Comcast SRV SRV N/A N/A N/A
FastMail SRV SRV SRV SRV N/A
Google SRV SRV SRV No SRV SRV
GMX No SRV No SRV SRV SRV N/A
iCloud/MobileMe SRV SRV SRV SRV N/A
Mail.ru SRV SRV No SRV No SRV No SRV
Outlook/Hotmail No SRV No SRV N/A N/A N/A
Runbox No SRV No SRV SRV N/A N/A
Tencent QQ No SRV No SRV SRV SRV N/A
Yahoo! No SRV No SRV SRV SRV N/A
Yandex SRV SRV SRV SRV SRV
ZoHo No SRV No SRV No SRV No SRV SRV
45,45% 45,45% 88,89% 75,00% 75,00%

“SRV” means the provider offer the service with an SRV record, and “NO SRV” means it’s offered but without SRV records. “N/A” means the provider doesn’t offer the service.

I’ve contacted several of the above providers that offer services but don’t advertise them in SRV records for details about why they’re don’t use SRV records for all the services they offer. Except from Runbox—who’ve said they’re likely add the missing records soon—none of the others bothered replying to my request for details.

The popularity of personal information management software (PIM) seem to be waning on the desktop in favor of web browser based information services. On mobile, however, PIM software is still included by default and still popular. Email,calendar, and address book apps are tightly integrated with the platforms and widely used. iOS and OS X are heavy consumers of auto-configuration records, but few other widespread clients support it.

I was surprised to learn just how widespread CalDAV and CardDAV services have become since I last looked at this. I think this is a good trend and that use of open standards should be encouraged. To that effect, I’ve submitted a patch to GNOME two weeks ago that adds support for CalDAV and CardDAV services in the default GNOME applications. This patch didn’t include service auto-discovery capabilities, but based on the above data I’m gonna have to work on that for a future update.

FastMail discontinued their XMPP service last month, and Runbox did the same sometime last year. Both providers cited very few active users and little interest as the reason why they discontinued the messaging service.

The conclusion is clear for anyone making a new email, calendar, or address book client: Service auto-discovery is the way to go to give users an easier time with software–provider configuration.

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