Hello everybody,

I've coded a multi-client server based on internet sockets using the scheme
listen on port X-accept-fork, exactly like beej's guide

At some point I would like to establish a secondary connection between a client and the server-child serving him.

I was considering the server-child bind() a socket to port X+1 listen() to it and then accept() a connect()ion by the client.But that can't be done, because if 2 server-children do that simultaneously, one would get "address already in use" error.

So then I've considered the client listen()ing on the Y+1 port (Y is the port assigned to the client's socket automatically by the primary connect() ).But my problem here is how can I find this port ? getpeername() on client's socket gives me the server's address & port.Is there any function giving the local address & port ?

I know that some problems remain. I am not guaranteed that port Y+1 is available.

A possible solution is that the client could send a free port number to the server through the primary connection and then the server-child connect()s to that port.

Or another one: The server-child knows the client's Y port when server-parent accepted() the connection.So server-child could send it back to the client.

But these aren't elegant solutions.Have you any ideas or suggestions?

Thank you

well, to me that sounds all a bit complicated. why not accept new connections on one specified port,then if a client wants another connection, it just asks at that same port again. if a connection is accepted it gets it own socket. there is no need to chose a port for every connection the server gets in. if you have to choose ports within the client, just do it randomly above 1024 and retry if one wont open.

Have a look at how the FTP protocol does this, then look at the problems this causes and why it's not simple how FTP traverses firewalls.

For what reason do you need a secondary connection when you already have a pefectly good established connection between the client and server?

Another couple of examples

(a) X's DISPLAY environment variable, where the host and port are encoded in a single string (port = display+6000)

(b) CORBA, where an IOR contains hostname and port

Both of these have problems when it comes to firewalls, the best solution for X for instance is using Xnest to proxy the display on the far side of the firewall.

CORBA introduced bidirectional-IOP to help solve the firewall-traversal problem by attempting to reuse existing connections.