I've created a server-client communicate program in .NET (c# or vb.net) using TCPListener - Socket on port 8080. In simple words, the program work like a chat software, client connect to server, and both wait for message from each other and then process it.
To retrieve packet from client, i using are using a "While" method like this :
While true
Dim Buffer(4096) As Byte
s.Receive(Buffer)
Dim strDataReceived As String = System.Text.Encoding.ASCII.GetString(Buffer)
ProcessData(strDataReceived) 'Process data received...........
End while
When testing both server.exe-client.exe in local, the software work fine for several hours without any problem.
But when i start to run the server.exe in my real server, the connection between server-client usually become lost each other when client connected after a few dozen minutes. The symptom is client send packet to server but server does not receive the packet from client when server is still standing in 'sck.receive(Buffer)' command. I have tested many times but i still have no lucky to keep the connection run over 1 hour.
I have investigated about this problem but it still very strange :
The server did not installed any firewall software.
The client did not using any proxy and antivirus, firewall software
I using a bandwidth logging software on server to make sure the internet in my server is stable.
I make a 'ping -t' from my client computer to the server and keep looking on it to sure there are no connection lost between client and server . The ping command indicate that the ping time is usually range from 5ms to 50ms and no connection time out occur.
Even I try to unplug the network cable in the client computer for a few seconds, and then replug again to simulation the disconnect event. I've awesome that my connection between server-client is still maintain and it's not the problem that cause my symptom.
I was thinking to write a code for auto reconnect if received timeout. But it could make my software usually delay when reconnecting if the above symptom still there. I really want to know what wrong with my code and which is the solution for me to fix the above symptom?
Likely the server is behind some sort of firewall (Cisco ASA, etc.) which has idle connection timeouts. When you "punch" through a firewall / NAT device, a "session" is created within the firewall kernel. There is an associated resource that has to be reclaimed, so firewalls do not usually allow unlimited connection timeout, but firewalls do support things like dead connection detection.
Adding a keepalive packet / activity every 5 minutes, or disconnecting / reconnecting is the only way around that. Few network admins are going to change their configs to accomodate this. It is pretty simple to implement a "ping" or "keepalive" command in custom TCP application protocols. Just send the string and consume it, you don't even have to respond to the packet to accomplish resetting the idle timer within the firewall, though that would probably be best practice.
When I say keepalive, I don't mean the TCP keepalive socket option. That is a zero-length packet, and is detectable by a good firewall, like Cisco. Cisco admins can setup rules to quietly deny your keepalive packet, so the solution is to implement it above the TCP layer, in the Application layer, by sending a small string of data like "KEEPALIVE\r\n".
Related
I have a distribution system for doing file operation with and running shell commands on target client machines on windows. and I use a custom TCP endpoint for connecting to the windows service which is resident on the server.
Now I've created this tool to create numerous instances of that agent(client) on one machine and run a certain job-set against all of them from the server. The problem is that all outgoing TCP ports on the client machine gets blocked after launching more than a few hundred agents. each agent is using a dynamic port and is listening to a single server port.
Say, i got 2000 agents running on ports 2000-3999 and all are listening to port 5111 on the server.
The error message i'm receiving in windows event log goes like this:
TCP/IP failed to establish an outgoing connection because the selected
local endpoint was recently used to connect to the same remote
endpoint. This error typically occurs when outgoing connections are
opened and closed at a high rate, causing all available local ports to
be used and forcing TCP/IP to reuse a local port for an outgoing
connection. To minimize the risk of data corruption, the TCP/IP
standard requires a minimum time period to elapse between successive
connections from a given local endpoint to a given remote endpoint.
When this occurs this machine cannot use any TCP port anymore. I did try changing some of the TCP default behavior in registry but to no avail. The interval between opening connections is between 1 to 5 seconds.
Any workaround for managing the optimal delay and/or somehow make windows trust the application irrespective of the aggressive network activity required for the test?
Turns out if you open connections without a proper gap in between, all ports on the client will get blocked due to aggressive behavior. Finally i got to connect all my agents by increasing the delay between each connection from 1000 ms to 3000 ms. I am still to figure out the dynamics of this though, and probably opening agents in parallel threads rather than processes would be a better solution after all. High number of processes with the same name seemingly doesn't appeal to the OS.
I have a server that i use to run game servers on for my friends and me, and some of the servers are "attack-able" (monsters can destroy our base) so i want the server to be shut down when not in use. Then i was wondering if there was a way to detect if there was an incoming signal (trying to connect to the server) on the given port, so the server can be turned on?
Raw question:
Is there a way to detect, if someone is trying to send a message (or connect) through a specific port in c# (or another language better suited for this action)?
Yes, you have to create a server to listen on that port. The problem you will face is that the server you create to detect incoming connections will need to be shut down so the game server can be turned on. They can't listen on the same port unless they're coded to work together and that likely isn't going to be the case with your game server.
If you want to see if there is any connections in use you can try to list all current TCP connections (assuming server using TCP) and find if there is any alive connection to specific port.
Resmon does this in his "Network" tab, so there must be a way to access it programmatically.
Here is answer describing how to get active TCP connections.
How can I get all the the active TCP connections using .NET Framework (no unmanaged PE import!)?
You probably should monitor server with some intervals because player might lose and reestablish connection, so sample it every 10 seconds or so and if there is no connection for more than few samples - shut down the server.
I want to accept about 5000 tcp client that are trying to connect exactly at one time.
when i test the program many of client can connect succusfully but many of them cant by giving "No connection could be made because the target machine actively refused it" error.
i increased backlog parameter of listen method of my socket but it didn't help
the code i used is the example of msdn with this link. can anybody help me?
It is ok for underlying stack to refuse connections while busy accepting other connections(nothing is really parallel inside). If you really need to connect that many clients at a time, you can change client logic a bit: reconnect on failing (like proposed in comments). Or you can start multiple listeners on different threads on different ports and choose which port to connect by fair dice on clientside.
You may want to look into using the SocketAsyncEventArgs object. Then you can have accept sockets to handle the initial connections and once the connection is established the accept socket will hand it off to a worker SocketAsyncEventArgs object.
Check out this project to get started with it. http://www.codeproject.com/Articles/83102/C-SocketAsyncEventArgs-High-Performance-Socket-Cod
What I have found is that using this technique works really well, but you will run up against the limitations of the OS and hardware. I tested my tcp server (running on windows server 2008 R2), which uses SocketAsyncEventArgs object, with a few thousand connections and it worked successfully without the clients getting rejected (Had to increase the backlog for this). The problem was that the time between the client and server establishing the connection, and the client getting a response, grew as the number of simultaneous connection requests grew.
I am working on a Peer-to-Peer chat program but have ran across an issue: Running the client and server simultaneously. I do not want a dedicated server to manage connections. I believe the solution may be asynchronous direct connections, but I am not sure.
What I am trying to accomplish is to be able to run the program between two hosts, the program will be started and begin trying to connect to an ip address specified by a text box. At the same time, it will also start listening for incoming connections on the localhost ip address.
***I am using tcp, because on the off chance something is corrupted the message will not be able to be read (it is encrypted)
Issues:
1) It is conceivable a client could be waiting for a period of time before the other program tries to connect. So should some form of a loop must be utilized? If so, how?
2) I assume I need to use multi-threading, with one thread for the server part and one thread for the client part, but an issue is keeping them from hanging. Since both programs are identical there way be a way to listen and simultaneously attempt to connect to the other host.
3) I am also having trouble with making my server listen for connections to it, and do not know how to automatically have it pull the ip address from my computer.
Thanks for any help.
EDIT: This is on a LAN only.
Everything you need to know is in Microsoft's docs.
http://msdn.microsoft.com/en-us/library/w89fhyex.aspx
I have a chat site (http://www.pitput.com) that connects user via socket connections.
I have in the client side a flash object that opens a connection to a port in my server.
In the server i have a service that is listening to that port in an async matter.
All is working fine except when i talk to someone after an unknown period of time(about couple of minutes) the server is closing my connection and i get an error in the server :
" A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond".
I dont know how exactly the tcp socket works. does it checking for "live" connection every couple of seconds? how does it decide when to close the connection? Im pretty sure that the close operation is not coming from the client side.
Thanks.
Sounds like the server is handling the connection but not responding. This is the point where I usually pull out WireShark to find out what's going on.
TCP/IP does have an option for checking for live connections; it's called "keepalive." Keepalives are hardly ever used. They're not enabled by default. They can be enabled on a system-wide basis by tweaking the Registry, but IIRC the lowest timeout is 1 hour. They can also be enabled on a single socket (with a timeout in minutes), but you would know if your application does that.
If you are using a web service and your client is connecting to an HTTP/HTTPS port, then it may be getting closed by the HTTP server (which usually close their connections after a couple minutes of idle time). It is also possible that an intermediate router may be closing it on your behalf after an amount of idle time (this is not default behavior, but corporate routers are sometimes configured with such "helpful" settings).
If you are using a Win32 service, then it does in fact sound like the client side is dropping the connection or losing their network (e.g., moving outside the range of a wireless router). In the latter case, it's possible that the client remains oblivious to the fact that the connection has been closed (this situation is called "half-open"); the server sees the close but the client thinks the connection is still there.
Is this an ASP web service hosted with some company? If so, the server generally recycles apps every 10 to 20 minutes. You cannot have a web service running indefinitely, unless it's your own server (I believe).