CODEWITHSUNDEEP
cnetwork-programming
user899604
user899604

Reputation:

Create Random Usable Port

I'm writing a program in C in which the server listens on a well known port, waits for client to connect, and then creates a random port for the client to use and send this port number back to the client. My main difficulty is how to create a "random" port. Should I just be using srand and create a random 4 digit port is the usable range? Or is there a better way to do this? I know that if I use port 0 a port will be chosen for me but the problem here is the fact that I don't think i can "get/see" the actual value of the port so that I can send this port number back to the client.

Thanks...

Upvotes: 2

Views: 3018

Answers (4)

ugoren
ugoren

Reputation: 16441

Binding port 0 is the solution. It gives you an arbitrary port, not a random port, but this is what many applications do (e.g. FTP etc).

After binding, you can use getsockname to figure out which port you got.

Upvotes: 3

Ben Jackson
Ben Jackson

Reputation: 93720

What you do is bind() with port set to 0. The system will assign one. Then use getsockname() to discover what port the system assigned. Send that back to the client. That way there is no race condition and you follow any system rules for port assignment.

Upvotes: 3

Ioan
Ioan

Reputation: 2412

I'm guessing TCP considering your description of listening and automatic port assignment by the OS. In this case, you don't need to worry about it. Once you accept the TCP connection, the OS on both sides takes care of all that you're trying to do and you're left with a working connection, ready for use. Unless you have a particular reason for doing any of this yourself, it's already done for you.

Upvotes: 1

David
David

Reputation: 87

A random 4-digit port checked to make sure it's not in use is OK for that purpose..

Technically speaking, it sounds like you're trying to implement this for added security (some kind of primitive port knocking routine)? It might be worth mentioning that this approach is generally not considered too secure. It also imposes some artificial constraints on how many clients you can serve at a time and actually adds unnecessary load on the server. Why not just listen on the single well-known port for all clients?

Upvotes: -1

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