CardDAV is an open-source protocol based on WebDAV, which is specially designed for syn­chron­ising contact databases. The client-server protocol enables digital address books and contact details to be stored and shared on a server. This makes it possible to access and change important addresses, telephone numbers and in­form­a­tion from all end devices. It also sim­pli­fies access to a syn­chron­ised database by multiple users.

After the Internet Engin­eer­ing Task Force (IETF) of­fi­cially released and proposed the CardDAV standard in August 2011 in RFC 6352, CardDAV was referred to as the iOS syn­chron­isa­tion protocol. Today, it’s supported by various email ap­plic­a­tions including Gmail. The protocol was updated in 2011. Among other im­prove­ments, this reduced data con­sump­tion during the syn­chron­isa­tion process and im­ple­men­ted the pos­sib­il­ity to capture images in very high res­ol­u­tion.

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How CardDAV works

CardDAV stores contact data as vCards, also known as “elec­tron­ic business cards”. The main advantage of this format is that data stored as vCards:

  • Can be embedded in HTML pages
  • Sent as an email at­tach­ment
  • Shared as a QR code
  • Inserted into an address book of an email program

To use CardDAV to manage contacts, all you need is an active con­nec­tion between the CardDAV client and the server where the data is stored. A cor­res­pond­ing client ap­plic­a­tion, therefore, has to be installed on the device used – whether that’s a desktop PC or mobile device. What’s more, the address data of the server and in­di­vidu­al access data are required for the login.

The syn­chron­isa­tion mech­an­isms es­sen­tially work like CalDAV, another protocol for calendar syn­chron­isa­tion which stores contact data as events in the iCalendar format. All changes made to the contacts are auto­mat­ic­ally adapted on the server and sub­sequently syn­chron­ised across all connected clients. This means each au­thor­ised user is kept up to date whenever new contact data are entered or existing addresses are changed. In par­tic­u­lar, this enables the central man­age­ment of contact details for pro­fes­sion­al purposes, keeping all col­leagues up to speed.

It’s easy to set up CardDAV syn­chron­isa­tion, but it often requires the in­stall­a­tion of ad­di­tion­al software. For instance, if the address book of a Gmail account needs to be syn­chron­ised via CardDAV (Gmail features a CardDAV server im­ple­ment­a­tion) to be available in Outlook, the following steps need to be taken:

  1. Install a CardDAV plug-in for Outlook, for example, CalDav Syn­chron­izer or cFOS Outlook DAV. Outlook needs to be closed during this process. Now open Outlook and start the menu for the installed plug-in.
  2. Enter the login data to sync to the Google account.
  3. Press “Sync All” or similar to syn­chron­ise the data. This can take a few minutes.
  4. The Google address book will now auto­mat­ic­ally syn­chron­ise with the Outlook account in real-time.
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Here’s what a CardDAV entry in vCard format looks like

To store contact data as a vCard on a CardDAV server, the client will send a PUT request (a standard request for uploading data). The following example shows what the content of this request looks like:

BEGIN:VCARD
VERSION:4.0
N:Jones;Anna;;Dr.;
FN:Dr. Anna Jones
ORG:Wikimedia
ROLE:Communication
TITLE:Editing & Design
PHOTO;MEDIATYPE=image/jpeg:http://commons.wikimedia.org/wiki/File:Anna_Jones_2010.jpg
TEL;TYPE=work,voice;VALUE=uri:tel:+1 555 555-1234
TEL;TYPE=home,voice;VALUE=uri:tel:+1 555 555-9876
ADR;TYPE=home;LABEL=”Main Street 1\n10001 New York\nUSA"
 :;;Main Street 1;New York;;10001;USA
EMAIL:anna@jones.example.com
REV:20140301T221110Z
END:VCARD

The contact in­form­a­tion stored in vCards is visible and ac­cess­ible to users – such as first and last name, title, address, telephone number, company and position.

How is CardDAV used and what are the ad­vant­ages of syn­chron­isa­tion?

Using CardDAV makes practical sense wherever contacts need to be centrally managed. This saves users the time and effort otherwise required to maintain address in­form­a­tion on different devices. Whether for pro­fes­sion­al or private use, the protocol means that all end devices can hold the same in­form­a­tion. Moreover, col­leagues, field employees or family members are able to access, amend or update the contact in­form­a­tion. Even if an end device is lost, stolen, broken or changed, it’s still possible to access all contact details since they’re stored sep­ar­ately on the server. This way, a new device can be quickly syn­chron­ised.

Ad­di­tion­al benefits include:

  • Having your own CardDAV server means you’re in­de­pend­ent of external services like iCloud, Google Drive and others.
  • You always have control over data security, since the data is stored on a selected server.
  • The syn­chron­isa­tion method can be used in­de­pend­ent of device type and ap­plic­a­tion.

Clients and servers with CardDAV support

The following ap­plic­a­tions provide server-side support for the CardDAV protocol:

  • Apple Contacts Server
  • Com­muniG­ate Pro
  • DAViCal (from version 0.9.9.2)
  • fruux
  • Gmail and Google Calendar
  • Office
  • Horde Groupware
  • MDaemon Email Server
  • Meishi
  • Nextcloud
  • ownCloud
  • Radicale
  • SabreDAV
  • SOGo
  • Zimbra 6
  • SYNOLOGY DSM 6.0
  • Xandikos

The following ap­plic­a­tions implement the client-side part of the CardDAV protocol:

  • Apple Contacts
  • Apple iOS
  • Black­berry
  • DAVx
  • eM Client
  • Evolution
  • KDE Software
  • Kerio Connect
  • Outlook CalDav Syn­chron­izer
  • Sailfish OS
  • Thun­der­bird
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