Tutorial :Multiple services from the same executable



Question:

I've written a small service (plain Win32) and I'd like to know if it's possible to run multiple instances of it when multiple users are logged on.

Basically, let's say we've got UserA and UserB for UserA the service would log on as "domain\UserA" and for UserB the service would log on as "domain\UserB" - this is from the same executable of course. I can change the logon dynamically using the ChangeServiceConfig() function, but it changes it system-wide it seems, while I'd like each user to have his own copy of the service running only for him.

Thank you in advance for any pointers.


Solution:1

Win32 services are designed to be system-wide, and start running before any user is logged in. If you want something to run on a per-user basis, it's probably better to design it as a regular application and run it from the user's Startup group.


Solution:2

Is it possible to perhaps have the service create child processes which then adopt the user credentials (or be started with them)? This way you're still limited to a single instance of the service, but it is able to do its per-user jobs all the same. IIRC the Windows Task Scheduler service does this.


Solution:3

The whole concept of a service is that it is started before any user is even logged on. so even if this was possible, you wouldn't be able to choose between userA and userB when the service starts because none of them is logged on yet.


A possible direction would be for the service to run as SYSTEM And every few minutes check if there is a user logged in, if there is- impersonate that user and do this stuff.


Solution:4

Yes, that sounds close (I'm answering comment from Greg, but comments are too short to fit my reply).

I don't know the list of users beforehand, but there's a GUI control application that would be used to enter username/password pairs for each user. So, userA would log on, run the application, enter his credentials and service would use that. At the same time (after userA has logged off, but the service is still running with userA's credentials) userB logs on, uses the app, and another copy of the service starts running as logged on userB. Thus, at the same time userA and userB services are running.

Is that possible?


Solution:5

You are probably looking to Impersonate the users. Check out some references I found with a quick Google search here:


Solution:6

It sounds as if you actually have two different, conflicting requirements, as to timing and identity.

  1. Run as each logged in user
  2. Run automatically even if no user is logged in.

No way to do this trivially, instead consider wrapping your program in a service; the program will run normally on startup for each user (either thru the startup folder or taskscheduler), and in addition create a service to run your app as a system user (or any other user you define).
Since you also need (you mention this in the comments) the app to keep running as the enduser even after he logs out, you can have the service manage this process for you.

HOWEVER this might not be the best idea, since the user is still effectively logged in. This can have numerous side effects, including security, performance (too many users logged in at once...), etc.


Solution:7

You could create an service application and a non-service(normal) application and make them communicate through IPC (Mapped File, Pipes, MailSolts ... you name it).

This way you solve all the troubles.

NOTE: The same application can behave differently - when started as a process and when started by a user, but in the end it is the same thing, you still have 2 applications (no matter if you got only one executable).


Solution:8

Running with different accounts is possible. In fact, this is common. See svchost.exe, which implements a bunch of OS services.

I just don't get how you determine which accounts. In a big company, many PCs are set up so all 100.000+ employees could use it. You don't want to run your service as the logged-in users, nor can you want to run it for all 100.000 users. So for which accounts, I have to ask?


Solution:9

A Windows process can only execute with the privileges of one single user at a time. This applies to services and other processes. With enough privileges it is possible to "switch" between different users by using impersonation. The most common pattern for what you are trying to do is to have one instance of a privileged service which registers to log in/log out events and creates children processes accordingly, each one of them impersonating the logged in user. The pattern will also simplify UI as each process runs on each separate user's Desktop, as if it were a regular application.

If you keep the privileged service's code as simple as possible this pattern has the added benefit that you are minimizing the attack surface of your code. If a user finds a security problem on the "running as user" side of your service it is a non-issue, while security problems in the privileged services could lead to privilege escalation. In fact, before Vista privileged services implementing a Windows message processing loop are vulnerable to a type of attack called Shatter attacks, which you should be aware of given what you are trying to do.


Solution:10

You want this running all the time, so you want a service.

You want something tracking each user, so you want an application which runs in the user session and communicates with the service (using named pipes or DCOM or whatever fits your requirements).


Solution:11

You don't need multiple instances of your service. From the description of your problem it looks like what you need is one service that can impersonate users and execute jobs on their behalf.

You can do this by implementing a COM object hosted in a service. Your client application (that the end user runs) will call CoCreateInstanceEx on your CLSID. This would cause new instance of your COM object to be created in your service. Then the application can use a method on one of your interfaces to pass the collected user credentials to the COM object (though I'd be wary of collecting credentials and instead see if I can pass the user token instead). The COM object which is running in the context of the service can then call LogonUser() to log on the user and impersonate it, so it can do whatever on her behalf (like finding the user local appdata folder :-)). Other answers havve good links to impersonating users using credentials or token.

If you feel comfortable with COM, I'd suggest you create your objects as multithreaded (living in the MTA), so that their execution is not serialized by COM. If not, the default single threaded model would be good enough for you.

The Visual Studio ATL wizard can generate the skeleton of a COM object living in a service. You can also read about implementing Windows Service with ATL here: http://msdn.microsoft.com/en-us/library/74y2334x(VS.80).aspx

If you don't know COM at all, you can use other communication channels to pass the credentials to your service.

In any case, once your service gets the credentials, all the work on behalf of the user will have to be executed on a background thread, so as to not block the application running as the user.


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