Checking a port if it's in use/open

[squidge]

There are numerous examples in the Qt installation. Have a look in the 'examples' directory.

As explained before however, don't expect your application to run at the same time as your VNC server, unless you want the pitfalls as explained above.

[prof.ebral]

I am a non-conformist when it comes to software. I agree with you that 'reinventing the wheel' is not truely innovation, but it can create innovation. I also like having the option of being able to code my own firewall.

There are numerous examples in the Qt installation. Have a look in the 'examples' directory.

Definitely a good start. The examples have a network and client that will show you something of how it works.

As explained before however, don't expect your application to run at the same time as your VNC server, unless you want the pitfalls as explained above.

I don't agree with this totally, fatjuicymole. The OP can thread the connections and when a connetion attempt is made to a port he can be notified, release the port, and allow the connection to proceed.

[wysota]

I also like having the option of being able to code my own firewall.

But do it with means meant for coding a firewall, not a userspace application.

The OP can thread the connections and when a connetion attempt is made to a port he can be notified, release the port, and allow the connection to proceed.

Hmm? Could you share a snippet of that does what you mean? Where would the connection proceed exactly? It seems you are using unix, so let's assume netcat started as

netcat -l 10001

netcat -l 10001

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... to be our server. Please write a minimal application (using whatever technology available for a standard u*ix system) that will bind a userspace application to tcp port 10001, intercept the connection, issue some debugging statement to the console and let the connection be picked up by the netcat pseudo-server.

Please also perform a (shallow) theoretical analysis of how the tcp handshake (SYN ->, SYN+ACK <-, ACK ->)would look like from the client's perspective in such a situation.

[squidge]

I don't agree with this totally, fatjuicymole. The OP can thread the connections and when a connetion attempt is made to a port he can be notified, release the port, and allow the connection to proceed.

Explain how to do this with Qt. I don't think it's possible for the simple reason that a user-level application doesn't have that kind of access (and Qt only works in user-land). Sure, you can get the connect notification and ignore it (not accept the connection request), but you then can't pass on that notification to some other program without affecting the source route. The only way I see it working is if the client automatically has multiple retries, so the notification app gets the first, releases the port, and VNC gets the next, but thats hardly a practical solution.

[wysota]

The only way I see it working is if the client automatically has multiple retries, so the notification app gets the first, releases the port, and VNC gets the next, but thats hardly a practical solution.

IMO this wouldn't work too, because of at least these facts and situations:

1. smart network stacks (or maybe it's even enforced by the standard, I don't remember) disallow binding to a just-released port to prevent stale connections/stray packets that might confuse the new daemon
2. race condition between the client and "server-switching" after an incoming connection is detected
3. a situation when the server can handle more than one connection simoultaneously - you wouldn't get any notification about new connections when there would already be a connection accepted by the real server
4. detecting when to reinstate the "peeking" daemon once the true connection is closed (+ a race condition again)

Of course I know how to do it in some special conditions in userland using Qt, i.e. on Linux you can implement a netlink device that will receive all incoming SYN packets before they are delivered to their destination but this is part of Linux firewalling mechanism, so it's kind of cheating (as you're still using kernel space means to get the data delivered to user space) and may slow down networking due to the necessity of copying the first packet of the connection back and forth between kernel and user space (fortunately only for a single packet per connection). By the way, that's probably how Windows personal firewalls work too more or less which would also explain why networking (and whole computer experience as well) is slower on Windows when a firewall is active.

[prof.ebral]

I am not assuming anything with you wysota. I am using multiple Linux distros and multiple Windows distros. To start assuming with you from your first post is just going to lead the thread further off topic, prevent me from working on the networking software I am working on right now, and create a list of assumptions that digress from a truth based reality; something I need in my life.

FYI, I am working on a software's server and client networking script at the moment and it is using threading. The code is in Python and PyQt, so I am not using pure Qt classes I am also using Python classes.

Query: Where is 'user land'?

[wysota]

I am not assuming anything with you wysota. I am using multiple Linux distros and multiple Windows distros. To start assuming with you from your first post is just going to lead the thread further off topic, prevent me from working on the networking software I am working on right now, and create a list of assumptions that digress from a truth based reality; something I need in my life.

Then I don't see why you're making opinions and defending them if you are not ready to make one more step and prove your points. It's easy to say "I don't have time to respond to your arguments, I have better things to do". Well, the truth is I'm devoting my private time to being here as well. The biggest compensation I can get is to study difficult cases such as this one and learn from them.

FYI, I am working on a software's server and client networking script at the moment and it is using threading. The code is in Python and PyQt, so I am not using pure Qt classes I am also using Python classes.

I don't care - you can write the proof of concept for your solution in pure python code even, I'm sure we'll understand the principles. The software doesn't even have to work, just show us the main idea.

Query: Where is 'user land'?

Everywhere where 'kernel land' is not.

[prof.ebral]

Then why don't you give me some time. I am rewriting the network and the current re-write is far from complete. If you want, you can download the source and see how the network uses threading.

http://www.assembla.com/wiki/show/traipse

The way the software's network is working: it uses threading to create new socks for each client, though all data is sent to one Port. It is pretty conceivable to use the same technology, only in reverse, to hand off the port request to a decision making function in the software. The function could notify the user through the UI and then the user could tell the software how to handle the request.

While the port request is being made, yes the port will be used by a service, but after the decision is completed the software can stop using the port allowing it to be accessed.

[squidge]

Then why don't you give me some time. I am rewriting the network and the current re-write is far from complete. If you want, you can download the source and see how the network uses threading.

Using threading to handle multiple connections is not a problem, but how do you pass a connection handled by a thread to another process?

[sutee84]

I will check the examples about network.
Does somebody know other way to check if somebody connect to the pc with winvnc (on windows), than check the vnc's port?

[wysota]

The application probably logs a connection to some file in which case you can monitor the log file.

[prof.ebral]

Using threading to handle multiple connections is not a problem, but how do you pass a connection handled by a thread to another process?

I don't know at the moment because that is not what I am working on.

The application probably logs a connection to some file in which case you can monitor the log file.

Please don't tell me you are talking about my application .. if you are, high rating or not check your ego at the source.

EDIT: I think this thread has been unfortunately hijacked. I was just trying to point out that the OP's idea is possible. I didn't mean to hijack his question thread.

[wysota]

I don't know at the moment because that is not what I am working on.

That's the whole point of this discussion. We say this is not possible.

Please don't tell me you are talking about my application .. if you are, high rating or not check your ego at the source.

You may use the threaded or hybrid mode of the thread view to see which post I was replying to.

EDIT: I think this thread has been unfortunately hijacked. I was just trying to point out that the OP's idea is possible. I didn't mean to hijack his question thread.

And we're trying to point out it is not possible so the discussion is not offtopic.

[squidge]

Does somebody know other way to check if somebody connect to the pc with winvnc (on windows), than check the vnc's port?

Have you taken a look at the VNC source code? The simplest solution may be to modify the code of VNC to popup a message box asking if you wish to accept the connection and show the IP address of the computer thats attempting to connect.