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Perhaps the code from my article[^] will work for you - there are people for which it doesn't work, but I think that the people for which it does work just don't usually take the effort of reporting that it works.. It works for me that's for sure 
Hole punching would be better of course but UPnP is better than nothing (right?) I'm sorry I don't know enough about hole punching, last time I looked it up it didn't seem too hard but there's probably a 'catch' somewhere..
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yes, that's maybe right, i'll test your code in this week. Well... back to UDP hole punching, i doesn't understand, how to find each others ports? because the most people have a "Port restricted Cone NAT", which means, that to a specific port from the router, only answers from a specific server from a specific port are accepted. But i can't see into the future, which port the router will use. 
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softwarejaeger wrote: But i can't see into the future, which port the router will use
I don't think that's quite right, official sites say "an external host can send a packet, with source IP address X and source port P, to the internal host only if the internal host had previously sent a packet to IP address X and port P. " I think it means that as long as you send the first packet, you can receive packets as well, but only from the IP that you sent to and only from the port that you sent to. Both ends can send what looks like the first packet to them, so this isn't a problem, just send a bogus packet first (but it must be to the same IP/port combination as what you want to receive from)
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Exactly, but how do i know from which port a data package from the other side will come? For example:
Step 1: Client 1 sends a UDP Package to the Server (IP: 1.1.1.1:1000)
Step 2: Client 2 sends a UDP Package to the Server (IP: 2.2.2.2:2000)
Step 3: Client 1 and Client 2 gets the UDP Informations from each other from the server
Step 4: Client 1 (LAN: 192.168.1.1:61000, WAN:1.1.1.1:1001)sends a UDP Package to Client 2 (2.2.2.2:2000) (!! Couldn't work! Port 2000 is reserved for the server-address and port)
Step 5: Client 2 (LAN: 192.168.1.2:62000, WAN:2.2.2.2:2001) sends a UDP Package to Client 1 (1.1.1.1:1000) (!! Same problem like in Step 4)
So... here you see the problem. I must find out from which port the Client 1 WILL send his data the same for Client 2. And there i found no solution.
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Well that's really an unrelated problem - and there is no good solution. This is why most virtual networks (in the broadest sense) have at least 1 central server, even the P2P DHT networks do because otherwise you don't have a "starting point" from which to explore the other nodes. Now you could just keep it simple and have a rule that "I will talk on port 2000" and that would work, but without such a rule you need a known place to store which port goes with which address. I know it's ugly, it's just the way it is..
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well a starting point is avaliable, that's the server, which holds the communication informations (id of the node, port from the listener, public ip, local ip and so on)...
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Ok so, what is the problem then? You want to talk to a node on a different port then it says it opened?
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Look at my previous post with the different steps. The NAT Browser doesn't use the same port for the connection to client 2 like to the connection of the server. And how can i "hit the right port?" i doesn't understand that
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Just open both ports then
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can you specify that? or write a few lines of code, that i can follow you?
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PunchHole(ip, 2000);
PunchHole(ip, 2001);
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Well... if i had now the Method/Function "PunchHole" i could imagine what you mean. That's not a C# Problem from my side, it's a logical problem, what to do to get this connection. I can't open the port and make a connection, because the other side doesn't know on which port i send this package and the other side doesn't know on which port the other side will answer and to find that out is my problem
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Sure you know, 2000 and 2001, because that's what you specified, it's not random
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How do i specifiy that? Well... you mean, that's on which port i listen right? But that would only works in a LAN, not in a WAN, my Router doesn't open a port if my PC listens on 2000/2001 for example. So i can't access Client1 from Client2 if i send a package to "Client1:2000" from Internet, because the router doesn't know what to do with Port 2000. That's right or not?
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Yes that's why you punch a hole before listening, but it's still the same port
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Well really i don't understand how that should works. OK, say following, i have the IP address of client 1 and client 2... then i make following:
Client 1 -> sends UDP package to Client 2 (which port?)
This UDP Package will get blocked by CLient 2s NAT
Client 2 -> sends UDP package to Client 1 (how do Client 2 knows which port)
Could you make a short example a short "Step-for-Step-List" (like i made above with example Ports/IPs and so on) or maybe a short code for that problem? I really get no idea  i get more confused and more confused :P
Well on the webpages and so on it sounds sooo easy and it is easy, if i know the ports but i think i need a concrete Step for step list or a short code from you to understand that. I've tested a whole week, with a lot of possibilities how that could get to work but nothing works (except the client-server connection  )
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Client1:2000 sends to Client2:2000 now Client2:2000 can send back to Client1:2000 (packet should never reach destination, but that's ok)
Client2:2000 sends to Client1:2000 so now Client1:2000 can send back to Client2:2000
And then the connection is open both ways.
If it doesn't work the router is being annoying and assigning a random other port to the connection.
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This would only work in a LAN!? Exactly that is the problem, that the Router takes another Port to open the connection, but i can't send a answer to the Client 1 if i didn't know which Port the Router from Client 1 use. Or did i misunderstood something?
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AFAIK it's supposed to work, maybe not with symmetric NAT
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That looks exactly like what I said actually
Still, it probably won't work with symmetric NAT (I hope it's rare)
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so now i had a little bit time to test that, i've found this code:
[^]
I've ported it to c# and it works fine in a LAN!! But if i test it over WAN, the Client2 want to send the UDP Package to "localaddress:16000" and that couldn't work because my router doesn't use port 16000! it uses a random port! And so how to do that now?
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Ok, well, it worked over LAN but it isn't needed over LAN - so this could mean that it did silently failed (at least in theory, it could)
But now, I don't know man, that is bad.. port forwarding would work, but it's not very reliable.. I think there has to be an other way, but I don't know it.. 
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Hey I am facing exactly same problem .
please guide me as well on this.
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hi everybody,
I use toolstrip button in windows application using c#.
i have toolstrip in mdi child form.
when i click the toolstrip button it open the related form.
The prob is here that i clicked that button again it open another form.
the related form opened again & again when i clicked.
i need only the form is opened once when i click it multiple times.
please help me.
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