How can I detect another instance of the same Win32 application running on another workstation?

Question

I have a small application, which is free for personal use, but requires a paid license for corporate use.

It is most likely that in a corporate environment my application will run on multiple workstations. If it is the freeware version, I want to show an unobtrusive message. (and continue)

It doesn't have to be bulletproof, if it is not possible (i.e. firewall) then the application should just continue. And I don't want to make the user set up some kind of central service to track the instances. I don't want to annoy my users (especially not the paying ones *g*)

Is there any way to achive this kind of functionality?

I remember an older version of Dreamweaver had this kind of feature. You couldn't run it more than once in the same network.

Answer 1

There are two primary ways to achieve this:

First, you can set up a small server application on each workstation that communicates with other workstations on the network (personally I would use Bonjour for discovery, but there are other options). The drawback here is that you're going to write quite a bit more code to make this work than option #2.

Second (probably simpler) would be to use WMI to enumerate processes on other workstations (again, probably use a Bonjour-like system for discovery), and find your process running on other machines. The drawback to this is that your enumeration code will require privileges on all machines to conduct the search.

Answer 2

One way: Listen for UDP broadcast on specific ports. Let each instance send broadcast UDP packet on this port to local network. If application receives such packet, and recognizes its structure, it knows that other instance is running.

You can include license details to avoid messages if two valid licenses are used.

Broadcasts usually aren't routed, so this works on local network only. (And user can disable it completely via firewall too... but if you will use some standard port like 53 (DNS), it won't be blocked).

Other way is to use custom server, which is informed about all running instances around the world ;-)

Answer 3

When the application starts, it sends out a UDP broadcast on a specific port. This will be restricted to the local subnet, and might not make it through firewalls. This is the "is anyone else running, or can I start?" query.

If there are no responses, the application starts as normal, listening for this UDP broadcast. If it sees one, it responds with an "I'm already running; you can't start" packet.

The application that's just started receives this response packet and then refuses to start or (if you don't want to be that strict) displays a warning to the user.

You'd want to include the product ID and license key (or a hash) in the initial request, so that you can have more than one license on the same network. The response probably wants the machine name in it, so that the second user can go and find the first user and ask if they really need to use the application.

Answer 4

Evil corporation solution:
Have the application call home every time it starts. If more than one application for a license wakes up, tell it not to. If there is no internet connection, don't start at all.