||This article possibly contains original research. (August 2008)|
Phoning home, in computing, refers to an act of client to server communication which is undesirable to the user and/or proprietor of the device or software. It is often used to refer to the behavior of security systems which report network location, username, or other such data to another computer.
Phoning home may be useful for the proprietor in tracking a missing or stolen computer. This type of phoning home is frequently used on mobile computers at corporations. It typically involves a software agent which is difficult to detect or remove. However, there are malicious types of phoning home such as surreptitious communication between applications or hardware installed at end-user sites and their manufacturers or developers. The traffic may be encrypted to make it difficult or impractical for the end-user to ascertain what data are being transmitted.
Legal phoning home
There are some uses for the phoning home practice that are legal in some countries, for example:
Phoning home could be for purposes of access restriction, such as transmitting an authorization key. This is done with the Adobe Creative Suite. Each time one of the programs is opened, it phones home with the serial number. If the serial number is listed as being already in use, or a fake, then the program will present the user with the option of inputting the correct serial number. If the user refuses, the next time the program loads, it will operate in trial mode until a valid serial number has been input. However, the method can be thwarted by either disabling the internet connection when starting the program or adding a firewall or Hosts file rule to prevent the program from communicating with the verification server.
Phoning home could also be for marketing purposes, such as the "Sony BMG Rootkit", which transmits a hash of the currently playing CD back to Sony, or a digital video recorder (DVR) reporting on viewing habits. High-end computing systems such as mainframes have had 'phone home' capabilities for many years, to alert the manufacturer of hardware problems with the mainframes or disk storage subsystems (this enables repair or maintenance to be performed quickly and even proactively under the maintenance contract). Similarly, high-volume copy machines have long been equipped with phone-home capabilities, both for billing and for preventive/predictive-maintenance purposes.
Aside from malicious software phoning home, phoning home may be done to track computer assets—especially mobile computers. One of the most well-known software applications that leverages phoning home for tracking is Absolute Software's CompuTrace. This software employs an agent which calls into an Absolute-managed server on regular intervals with information companies or the police can use to locate a missing computer.
More phone-home uses
Other than phoning to the home (website) of the applications' authors, applications can allow their documents to do the same thing, thus allowing the documents' authors to trigger (essentially anonymous) tracking by setting up a connection which is intended to be logged. Such behavior, for example, caused v7.0.5 of Adobe Reader to add an interactive notification whenever a PDF file tries phoning (to its author's) home. 
HTML e-mail messages can easily implement a form of "phoning home". Images and other files required by the e-mail body may generate extra requests to a remote web server before they can be viewed. The IP address of the user's own computer is sent to the web server (an unavoidable process if a reply is required), and further details embedded in request URLs can further identify the user by e-mail address, marketing campaign etc. Such extra page resources have been referred to as "web bugs" and they can also be used to track off-line viewing and other uses of ordinary web pages. So as to prevent the activation of these requests, many e-mail clients do not load images or other web resources when HTML e-mails are first viewed, giving users the option to load the images only if the e-mail is from a trusted source.
Malicious phoning home
There are many malware applications that "phone home" to gather and store information about a person's machine. For example, the Pushdo Trojan shows the new complexity of modern malware applications and the phoning home capabilities of these systems. Pushdo has 421 executables available to be sent to an infected Windows client.
- Technology Meetings Website - http://technologymeetings.com/ar/meetings_catch_laptop_thief/index.htm
- ZoneAlarm phones home, Apple throws Intel a bone http://www.infoworld.com/article/06/01/13/73792_03OPcringley_1.html
- IBM EServer and ZSeries and S/390 Servers - http://books.google.com/books?id=NtHhpIjIFMEC&pg=PA9&lpg=PA9&dq=%22phone+home%22+mainframes&source=bl&ots=iXj04sMVQ6&sig=L3_rOl6Xsik0Bi4DTigwnNb2DwI&hl=en&sa=X&oi=book_result&resnum=4&ct=result#PPA9,M1
- Xerox Model 1090 Copier/Duplicator User Guide. Xerox Corporation, Stamford, Connecticut and North York, Ontario, Canada, August 1990.
- Absolute Software's website: http://www.absolute.com/
- "New features and issues addressed in the Acrobat 7.0.5 Update (Acrobat and Adobe Reader for Windows and Mac OS)". Adobe Systems. 2008-05-02. Archived from the original on 2008-05-29. Retrieved 2008-08-14. "New features: 4. "Phone home" notification enhancements, meaning that when a PDF document attempts to contact an external server for any reason, the end user will be notified via a dialogue box that the author of the file is auditing usage of the file, and be offered the option of continuing."
- Pushdo Trojan - http://www.eweek.com/c/a/Security/Inside-a-Modern-Malware-Distribution-System/