Hi!
Suppose that I have a license server that goes with my program.
The client installs both of them on his computers so when he tries to use a floating license, I can tell whether or not it is already in use.

The license server listens on a UDP port and the clients use it to communicate with the server and receive a TCP port number to which they should
connect in order to continue their conversation with the server.

Since the server is also installed at the clients, I cannot know which other processes run on the same computer while my server does and they might use the same UDP port. If they use the same UDP port, I'd either be blocked from using it or I'd have to share the port with the other process, losing messages since they go to the other process instead of going to my server.

How can I avoid this?
Should I use a configurable UDP port? The problem is that if I do it, the client will be able to run several license servers, each on a different port, unaware of each other while the clients will be able to use the same license on different instances of the server... I'll miss the whole point...

What should I do?
thanks

I don't have a website yet, so the license server can't be there...
This also will allow computers which are disconnected from the internet to use it too.
(I think that Matlab, BoundsChecker, etc. do it this way too)

Yes, FlexLM (the license server used by many professional CAD/CAE tools and Matlab) is similar. Maybe you should make the UDP port used part of the license? FlexLM uses an Ethernet ID (~MAC address) and the servers name.

But that won't solve it because if that license-dependent port is taken by another process, the problem will remain.

Hello Idov,

are you sure, you need to check the license all the time?
Let the client "Check-out" a license for a number of days (or weeks) and after the time limit it has to have access the internet to connect a master license authority and renew its license. Like a certificate that expires.
Of course you would still have the problem, that if someone cloned a valid license over multiple VMs with exactly the same charactaristics you wouldn't be able to catch that. Someone might even reverse engineer your server to circumvent this all together.
Do you really expect that to be a major problem? Are we talking about a thousands of dollars per license software that has an extremly small market? If yes, you might need to check into professional solutions or even track every registers site of use.
But if you are talking about a 5 Dollar Game, than invest that time into the game and learn to live with the illigal copies out there.

--GWDev

I don't understand your use case, why does the client have access to the license server? Doesn't that defeat the point?

Actually, not at all. As already mentioned, especially in the CAD/CAE/CFD field many license models work like that (e.g. StarCCM+, Flowtech Shipflow, NUMECA tools). You get a license issued to a server and several clients can connect to it. FlexLM is a great (and most commonly) used program to achieve that.

Back to the OP...
I assume FlexLM is no option for you. If we are talking about a software where each license is worth a couple of thousands dollars (or euros or the equivalent value in any other currency), then I'd say looking into professional solutions for this problem is definitely worth it. Again (as I am working in the CAD/CFD field and have good experience with it) FlexLM would be a very good option in that case.

Now, if you really want/need to roll your own:
Having a server application with non-configurable ports is a no-go imho.

First of all, (of course) the floating license needs to be node locked to the server it runs on. I assume that's a given, but I'd still like to stress it. HDD-serials and MAC-addresses are good candidates for that. They are easy to get from the OS and rarely change. Limiting the license to the OS is also a good way to go, to avoid "cracking" your license using parallels, for example.

To circumvent the problem of having multiple servers on the same machine I would take a shared memory approach. I don't know what language your server is in, but for C++ the boost interprocess library offers a ncie API.
I'd generate a signature based on the hardware infos stored in the license (which are neccessary and would prevent the license from working on that particular machine if they were changed) and put that into the shared memory. Then have every instance of the server access that shared memory and look for that signature. If it's there, the license is in use and cannot be used again. If it isn't, store the signature in the shared memory which locks it for other instances of the server.

About the VM thing... Sure if the original license was issued for a VM, it is possible to clone it which would cause the same hardware IDs to be generated and then multiple VMs could run on the same license.
However, for most VMs (VMWare, Virtual Box, for example) there are pretty good methods to detect them. It usually involves using some inline assembler, but google it, and then it's only copy paste ;)
So I would actually put a check in the server (which we do for our software, btw.) to prevent it from running on a VM. You might even put a switch in your license to allow customers that can explain why they want to run the server in a VM to do so (which we do).

The problem with multiple servers is when they run on different computers...
Servers running on the same computer be dealt with by not allowing 2 instances of the server to run on the same computer

Why does a floating license need to be node-locked? isn't it supposed to be not limited to a specific computer?

BTW: My program is a performance profiler, I don't know what the price is going to be yet...

are you sure, you need to check the license all the time?
Let the client "Check-out" a license for a number of days (or weeks) and after the time limit it has to have access the internet to connect a master license authority and renew its license. Like a certificate that expires.
--GWDev

I want to give other user the option to use the license if it was idle for a period of time, so every time a user wants to use it I check if it's idle or not.

You can also check for other copies of your license server running on the same machine.

Edit: Somehow this post got delayed by a few hours. Kinda irrelevant now...