In my Qt application I am using a peer to peer DBus connection. Server runs on computer A, client on B, connected via DBus TCP/IP connection. Works fine.
I wonder if I can somehow find out whether the server is running and what its IP address is? So far I need to provide the correct address/port in the client.
Both, server and client run in a local network. Of course, I can use a trial and error approach and ping all machines in my network. Is there something better, something like a broadcast asking for the server, and the server responding appropriately? Is Qt providing something for this?
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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.
Is there any server-client like inter-process-communication method which allows:
the server to reliably get the process-ID of a connected client
any client-application to connect to the server
works in C/C++
Optionally: Works also on Linux
Since your server runs with root rights, you can work with task_for_pid() and a two way connection. First, the client will send its pid to the server via some special bootstrap port on the server, the server will then resolve the pid via task_for_pid to a task port and then use mach_port_insert_right to insert a send right to a new port into the client. The new port is then exclusively to the client and you know what security level the client has.
I have client server application that works with Firebird server. Everytime when clients connect to the server they(client apps) don't check if there is a network connection to the server so at this time my application sometimes freezes when the server computer is switched off or service has stopped, so first of all I need to check connection if remote host is switched on or at some port anything listening....
Before establishing the connection I need to check it and make sure server and service is running using Indy components.
Any ideas? also I can use IcmpClient to ping remote host and then establish connection but which is the most optimal way ?
If you just want to check if the server computer can be reached, you could do a "ping" to check that. However, if you want to check if a specific TCP port is open, then the only way to find that out is to actually do a proper connect, which leads to the "freezing" program while the connection times out if there is no-one listening on that port.
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.
I have a C++ Program listening for incoming socket connections on port 2222.
I have an Adobe AIR/Flex application that attempts to connect to it, when I click a button.
When I Connect To My Socket Over My Intranet, My C++ program hears and accepts the incoming socket connection and Everything Works Fine:
var Sock:Socket=new Socket("192.168.1.100",2222);
But When I Try To Connect Using My Real IP, I Get Error #2031:
var Sock:Socket=new Socket("76.18.24.118",2222);
Both programs are running on my laptop computer. I am behind a router, but have configured that router with port forwarding so that port 2222 maps to my laptop.
What Am I Missing? I am very new to this, so perhaps I'm missing something obvious to you smarties.
What does the error code mean (see your docs for socket)
Are you sure the receiving server process is alive?
The traditional starting point is Beej's guide
Port forwarding does not mean that you can use an external IP address, your laptop is still on the private 192.168 network. What it means is that when someone tries to connect on 78.18.24.118:2222 the router converts the IP address to 192.168.1.100:2222. This effectively allows you to run a server inside your network but allows an outside client to connect to you.
Look up DNAT for more information.