How is a client (cellphone) is located by the server in push email. Does it require an active TCP connection to be maintained by the client ? Does the client need to have a public IP for that ?
The cellphone must maintain an open connection to the server - almost all cells are behind one or more layers of NAT gateways, making it impossible for the server to reach out to the client unless the client has opened a connection and kept it open.
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VB.NET Server Does Not Receive Connection Requests from Remote Clients
A VB.NET server application does not see connection requests from another client on the network (i.e. with a different IP address). However, it does see connection requests from the client application running on the same computer as the server.
The listening socket is created with the following parameters
System.Net.Sockets.AddressFamily.InterNetwork
System.Net.Sockets.SocketType.Stream
System.Net.Sockets.ProtocolType.IP
We have experimented with the preceding without success.
The endpoint to which the listening socket is bound specifies the local IP address and a specific port number.
The wait-for-connection code is textbook asynchronous:
thelistener.Listen(10)
thelistener.BeginAccept(New System.AsyncCallback(AddressOf targetofaccept), thelistener)
If the client that attempts connection is on the same computer as the listener, then targetofaccept is run successfully. If the client that attempts connection is on some other computer on the network, then targetofaccept is not run.
The behavior occurs for any other client on the network (i.e., not just one).
Thinking that there was some firewall issue, we created VB6 servers and clients using the same addresses and ports. The VB6 server will receive connection requests regardless of the client system.
There is no other issue with communication between clients and the server, as far as we can see. The network architecture has not been modified for a number of years.
We are debugging the code as a VB.NET console application.
Any tips on how to diagnose appreciated.
Before calling Listen() you need to bind your listener socket to the address 0.0.0.0 (in .NET IPAddress.Any) so that it listens to connections from any IP address.
This can be done using the Socket.Bind() method:
Dim listenerEndpoint As New IPEndPoint(IPAddress.Any, <your port>)
thelistener.Bind(listenerEndpoint)
thelistener.Listen(10)
Thank you.
Issue WAS firewall. Fixed by finding exact location of the IDE (devenv.exe), opening "Windows Firewall" in the control panel, selecting "Allow a program or feature through firewall", selecting "Allow another program...", browsing to the exact location and selecting the executable, then ensuring "Home/Work (Private)" column is checked for that "Name".
I am developping a FTPS client in a Embedded system with LWIP and mbedTLS stacks.
For now, what I do is :
Get an IP adress with DHCP
Get IP address of the server with DNS
Start a TCP connection to the server ( the control connection)
Telling the FTP server that I want to work with the TLS protocol with the command AUTH TLS
Handshake of the TLS protocol with the server
Validate the x509 certificate that the server sends
Telling the client that I want to work in passive mode and that i want to read a file with the command RETR.
Now, I need to open a data connection to read my file. So what I wanted to know is does the data connection is secured in the same way the control connection is ? That means do I have a certificate to validate ? Is the handshake the same ?
If this isn't clear let me know i'm not an expert at all in this area.
Thank you for your future responses
In explicit FTPS connection after you send AUTH command, the data connection is secured all the way. Depending on Active/Passive connection the data connection is opened through port 20(Active) or some random port negotiated(Passive).
You don't need any extra handshakes as far as I know.
You might or might not be able to configure if you want to reuse the same session ID created in opening the control channel. This is related for server to know that data connection has been established by the same client which opened the control channel. Some server require this, some servers allow this, some servers does not support this.
I have created a simple tcp client-server application in vb.net. Everything works fine when i set local host (127.0.0.1) as the ip address. I wish to make a connection to a remote machine (keeping client at my machine and server on the other - or vice versa).
EDIT:
ok, the code of interest is as follows:
the server:
Dim server As TcpListener = New TcpListener(IPAddress.Any, port)
the client:
Dim theip As IPAddress = IPAddress.Parse(TextBox1.Text)
client = New TcpClient(theip.ToString, port)
Now, what should i enter in "TextBox1.Text" if i have my client on a remote machine (connected over internet) and server at a different computer (whose external IP address is known)?
Can i enter the external IP address of the machine running the server directly in the text box? Also, do i need to take care of things like "port forwarding" / "let firewall allow the connection" and all? If yes, can you please guide me or link a tutorial so that my application does this automatically or is there an alternative approach to achieve this and so that port forward is not required? Thanks a lot!
You say the other machine has an external ip address. This implies it also has a local ip address; ie it is behind a NAT firewall or similar. In this case, you will have to configure the firewall to allow communication to take place, eg by setting up port forwarding.
Applications such as MSN Messenger, VOIP clients etc have elaborate mechanisms to negotiate this automatically, but they're often not very successful.
Is it not possible to specify the local client port used for server calls via NetTcpBinding?
NetPeerTcpBinding support this - I find it hard to believe there's no way to specify the local port (and IP Address for that matter) on clients.
I need to be able to use port sharing and I can't do this if I don't know what port the client is listening on. I won't be able to share the port (and IP) with another client so it can connect.
The only work-around I found is something like this:
Client A opens a connection to Server
Server logs the IP and Port of Client A
Server shares this info. with
Client B
Client B should be able to
connect to Client A on specified IP
and Port (in theory)
This is known as TCP/IP NAT hole punching I believe. Anyone with experience in this?
You probably don't understand the communication pattern with NetTcpBinding. It works this way:
The client opens connection to the server
Two-way communication: The client calls the server and gets the response over the same connection.
Duplex communication: The client calls the server. The server stores reference to the channel somewhere (you must implement this). Once the server wants to notify client about anything it takes stored channel and calls the client on the connection established by the client during its first call.
The server never opens connection to the client. The client doesn't expose any address with port for incomming communication. If you want to do that you must implement service on the client in the same way you did it on the server and you must manully send the address and port to the service exposed by the server.
The difference with NetPeerTcpBinding is that there is no real server and any client must be able to get incomming request. NetTcpBinding is for client-server scenarios where the server is the only peer able to process incomming requests. Because of that only the server needs to define an address and a port.
I am creating an authentication server for some projects I'm working on. This authentication server works by receiving and transmitting data to users trying to authenticate. The user can send data to the server succesfully but when sending back it requires port forwarding. I read a way that I would not require port forwarding by using reverse connection but I have no idea how to get it working. I found a tutorial and tried to implement but when it tries to connect to read the stream it says the connection is refused.
Any ideas?
If the user is already connecting to the server, why not just keep the (existing) connection open, and use that for bidirectional communication?
I can see no reason to open a second connection from the server back to the client.