If you create a TCP client socket with port 0 instead of a non-zero port, then the operating system chooses any free ephemeral port for you. Most OSes choose ephemeral ports from the IANA dynamic port range of 49152-65535. However in Windows Server 2003 and earlier (including XP) Microsoft used ports 1025-5000 as the ephemeral range, according to their bind() documentation.
I run multiple Java services on the same hardware. On rare occasions, this range collides with well-known ports that I use for other services (e.g. port 4160 for Jini discovery). While rare, this has caused real problems. Is there any easy way to tell Windows or Java to use a different port range for client sockets? Microsoft’s docs indicate that I can change the high end of that range via the MaxUserPort TcpIP registry setting, but I see no way to change the low end.
Update: I’ve made some progress on this. It looks like Microsoft has a concept of reserved ports that are exceptions to the ephemeral port range. There’s a registry setting that lets you change this permanently and apparently there must be an API to do the same thing because there’s a data structure that holds high/low values for reserved port ranges, but I can’t find the actual function call anywhere… The registry solution may work, but now I’m fixated on this API.
Update2: I accepted a solution on ServerFault for how to do this via the Windows registry. I’d still like a way to do it via API, but I guess I’m satisfied for now.
It’s not as elegant as using OS support for ephemeral ports, but the docs show that you should be able to specify a port for your socket to bind to. Specify a port at the base of the range you want and if it is used an exception will be thrown, in which case increment the port and try again. Given that windows isn’t using the port range that you want, there shouldn’t be many collisions.