I have one main PC and 3 other client PC that I am trying to send message to. All of them are in the same network, and I like to send a message to them, one by one, so that they do something simple like printing and sending a success msg to the main computer (server). I am new to MPI, so I don't know how to make this connections? How does the client computers listen to the msg from the server? I have written a program that separates the functions of clients and server. If it is the client it would connect to that with the specified port number and ip address and send a msg to the client. However when I try to do MPI_Open_port(info, port1); it gives me an error that if fails to open the port. Should I have this program on all client computers? If someone can give a general overview of how it works I would appreciate it.
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I want to write a simple program in c++ that use tcp socket to communicate with the same program on another computer in lan.
To create the tcp socket I could make the user write the ip and the port to make the connection. But I also need to be able to autodetect in the local area network if there is any computer also running the program.
My idea was:
when the program is autodetecting for available connection in lan, it will send all ips a message via udp to a specific port, meanwhile it will also keep listening to a port waiting to eventual answer.
when the program on the other computer is opened for lan connection, it will keep listening to the a port in case another computer is trying to detect, then it will send also via udp the response messagee notifying the possibility of connection.
All the security system is another problem for which I don't need answer now.
// Client 1:
// Search for all ips in local network
// create udp socket
// send check message
// thread function listening for answers
// if device found than show to menu
// continue searching process
// Client 2 (host) :
// user enable lan connection
// create udp socket
// thread function listening for detection requests
// if request structure is right send back identification message
// continue listening for request
My question - Is there a more efficient or standard way to do something like that?
Testing whether another computer is listening on a given port is what hackers do all day to try to take over the world...
When writing a software like you describe, though, you want to specify the IP and port information. A reason to search and automatically find a device would be if you are implementing a printer, for example. In that case, as suggested by Hero, you could use broadcasting. However, in that case, you use UDP (because TCP does not support that feature).
The software on one side must have a server, which in TCP parlance means a listen() call followed by an accept() until a connection materialized.
The client on the other side can then attempt a connect(). If the connect works, then the software on the other side is up and running.
If you need both to be able to attempt a connection, then both must implement the client and server (which is doable if you use ppoll() [or the old select()] you know which event is happening and can act on it, no need for threads or fork()).
On my end, I wrote the eventdispatcher library to do all those things under the hood. I also want many computers to communicate between each others, so I have a form of RPC service I call communicatord. This service is at the same time a client and a server. It listens on a port and tries to connect to other systems. If the other system has a lower IP address, it is considered a server. Otherwise, it is viewed as a client and I disconnect after sending a GOSSIP message. That way the client (larger IP address) can in turn connect to the server. This communicator service allows all my other services to communicate without having to re-implement the communication layer between computer over and over again.
Writing a UDP client-server app in C++ (done that lots of times before in many languages in the past 15 years), but somehow this one is not working correctly.
I cannot post actual code nor minimal reproducible app at the moment but I am willing to pay for live help if anyone is available to help solve this quickly with screensharing.
I think this is a particularity with C++ sockets and the way I am using them in this specific app which is quite complex.
Basically the issue is that the packets sent from the server to the client are not received by the client, only when said client is on a separate nat.
When both in same local networking and using their local IP, everything works as expected.
Here is what I am doing :
Client sendto(...) packets through UDP to the server using a specific server host and port 12345 (and keeps sending these non-stop)
On another thread, client bind(...) on port 12345 and "0.0.0.0" and tries to poll() and recvfrom() in a loop (poll always returns 0 here when client is on a separate nat)
Server bind() on port 12345 and "0.0.0.0" then poll() and recvfrom() in a loop
Upon receiving the first UDP message from a client, it starts a thread for sending
UDP messages back to the client on a new socket, using the
sockaddr_in that it got from the recvfrom() to pass in the sendto() commands.
Result : Server perfectly receives ALL messages from all clients, and sends all messages back to all clients, but any client that is not on the same NAT will never receive any messages (poll() always returns 0).
As far as I understand it, when the client sends a UDP message to the server on a specific remote port (12345 in this case), it will punch a hole in its NAT so that it can receive messages back from the remote server on that port...
I tested five different client network configurations :
Local network with the server, using local IP addresses (WORKS)
Local network with the server while client is using a VPN thus going through a remote NAT (DOES NOT WORK)
Local network with the server but client is using the WAN ip address to connect to the server (DOES NOT WORK)
Client at an actual remote network from a friend's connection, behind a router (DOES NOT WORK)
Client going through a wifi hotspot created using my phone (DOES NOT WORK)
For all tests above, the server was correctly receiving all communications from clients.
I also tried forcing the port to 12345 for the sendto() instead of using the sockaddr_in as set from recvfrom(), same issue.
Am I doing anything wrong ?
If you want to help but need to see actual code, I can do that live with screen sharing and I will pay for the help.
Thanks.
Also, if anyone can point me to a great site where I can pay for VERY QUICK help, please let me know, I don't even bother searching google because I really want actual advice from people who tried these services, not ads trying to rip me off...
Only the original receiver socket is allowed to reply to the client, because it's the client request that opens the port in the NAT. So either use the same socket in the server to receive and reply, or get the port that the second server socket was bound to and transfer it with an initial message through the original server port, so that A can send to it and punch the hole.
It looks so strange to create two half duplex sockets when a socket is a full duplex communication object that I'd go with the first option.
I am having problems trying to get UDP to work with SFML in C++.
I have made a client class and a server class.
The server can send text messages and the client can receive and print them.
Server and client runs in the seperate threads.
The problem is, it is not working on LAN.
If I have the client and server running in seperate windows on the same computer I can send messages just fine, but if the server runs on one computer and the client on another on the same LAN, the messages aren't received.
I hope someone can help.
Thank you in advance.
I am pretty new to socket programming - so this might be a simple question but I'd really like to clarify.
I have a multiple-client to single server program: the individual clients send messages to the server which processes them, and then passes it on the destination i.e. the server is an intermediary.
On the server side, there is one thread for each client which is meant to 'listen' for messages from the clients (which will be placed in a buffer). At the moment all the clients are sending messages to the same port (as far as I can tell).
I am thinking of setting up another thread on which the server will process the messages before transmitting them on. Does it make sense to use another port on the server to send those messages?
I don't mean this to be a discussion, but I don't know what is common or more logical to do - any advice?
On the client-side, I am planning for it to have one thread for sending messages to the server, and another thread for receiving. Please let me know if any other information is required!
edit
At the moment it is a 1-server-to-multiple(tens now, hundreds later)-client program - I seem to have problems with the client receiving messages from my server (I am troubleshooting so I thought that using the same ports might be the problem), but I will try it with the same ports again and see. I thought it might be a matter of the receiving port being too busy to send messages as well.
At the moment all the clients are sending messages to the same port (as far as I can tell).
What do you mean 'as far as I can tell'? You must know whether you are opening more than one port at the server.
Does it make sense to use another port on the server to send those messages?
No it doesn't. If you're using TCP, send the messages back down the same socket. If you're using UDP you don't need more than one UDP socket, and it simplifies the client and the application protocol if replies come from the same ip:port the request was sent from.
I'm writing client-server application and I need my server to find all clients in some network. I've already found some info here: Discovering clients on a wifi network, but I still don't understand how to implement this. Maybe somebody can say where I can find some code examples.
Thanks in advance.
PS. Working on c++, windows.
Generally TCP/IP is used as a communication protocol between client and server. For Windows platform Winsock library is used to implement TCP/IP. The server binds and listens on a port for incoming connections. Just like a webserver like stackoverflow listens by default on port 80 and then client (browsers) connects to it.
Here is a link to start. Here is sample
Normally all the client connects to server which listens on a well defined port. The server is only one hence the IP address and port is well know to all the client and hence they can connect to it.
In you case you want your server to have ablity to discover all the clients in the network. To achieve this the server needs to broadcast to network some message. The client will receive this message and will respond to the server that they are available on such IP and they can connect to server or provide additional information to server. Normally instead of broadcast, multicast is used which is limited broadcast. All the clients and server will subscribe to the multicast group which is a special kind of IP address. When server send a message to this multicast address all the client, which are subscribers of this address will receive this message and can respond back. Here is a sample
Edit: you can also use boost lib to implement multicast: sender eg., receiver eg.