Re^2: What's the best way to find an unused TCP port on the local system

I have one more question. I'm interested in this problem of finding an unused port on a local system too. It's for some client/server based IPC. Given any method of those mentioned here (I prefer yours most), how would I go to communicate it to possible clients? One way could be to write that info into a (configuration) file, as the OP suggests. But that is somewhat unsatisfactory because if I wanted to make the processes communicate through files I may have chosen to do by means e.g. of a named pipe in the first place.

Of course I may set up a predefined port, and failing that, use a workaround. On the client side, if the server doesn't respond, and only then, the client itself may check a suitable file. However how do people generally deal with this problem? (If it is a problem at all...)

Comment on Re^2: What's the best way to find an unused TCP port on the local system

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Re^3: What's the best way to find an unused TCP port on the local system by ikegami (Patriarch) on Oct 20, 2006 at 20:02 UTC
It's hard to answer without knowing what you are doing. What's the difference between the server on port A and the one on port B? Or rather, why would someone want to connect to the server on port A as opposed to one on port B? Generically speaking, the problem is the same as finding a someone's phone number. How do you that? Maybe you search a directory. Maybe you ask someone who knows. In both of these, you query an external system. Therefore, a possible solution is to have the server communicate it's vitals to a directory server. The client (user or program) then selects the server to which it wishes to connect from the directory. For example, ICQ used to work this way (before changes were made to fight spam and privacy invasion). The ICQ client would become a server and then connect to the central ICQ server. Someone wishing to send you a message would query the central ICQ server for your IP address. Another example is battle.net. People wishing to play Starcraft over the internet log into the battle.net chat server (through the game). When someone creates (hosts) a game, Starcraft contacts battle.net letting it know at which IP the game resides. battle.net would present a list of running games to users and passes on the IP address and port to people wanting to join. Update: Added examples.	[reply]
Re^3: What's the best way to find an unused TCP port on the local system by sgifford (Prior) on Oct 20, 2006 at 20:29 UTC
portmap is the traditional UNIX RPC way of doing this. It has just one service on a well-known port, which allows you to ask for a service by name and get the port it's currently running on. It may be overkill for your application, but maybe implementing something simpler with a SOAP interface would work. -- sgifford's Web page	[reply]