Date: Wed, 19 Jan 2000 11:00:57 +0100 (CET) From: Oliver Fromme <olli@dorifer.heim3.tu-clausthal.de> To: freebsd-questions@FreeBSD.ORG Subject: Re: Finding an open port. Message-ID: <200001191000.LAA02225@dorifer.heim3.tu-clausthal.de> In-Reply-To: <86387u$25h2$1@atlantis.rz.tu-clausthal.de>
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Laurence Berland <stuyman@confusion.net> wrote in list.freebsd-questions:
> As some may be aware (from the post that started the rather long
> volatile variable thread), I am attempting to write an ftp server for a
> computer science project. I've gotten most of the code done, only the
> actual data module needs to be written (the Server-DTP for those
> familiar with the protocol). I'm trying to figure out the best way to
> find a spare port for PASV connections, and was hoping there's a
> slightly more elegant solution than calling bind and checking for
> EAADDRINUSE on errno.
That's easy: If you set the port number to 0, the system
will find a free one for you. Here's some code for you to
experiment. It's from an FTP implementation of mine (sorry
for using C++ style comments).
int socke; // socket descriptor
struct sockaddr_in datalisten;
int llen; // size of "datalisten"
//
// Try to open a socket for listening to the incoming
// data connection.
// First get a file descriptor for a TCP socket.
//
if ((socke = socket(AF_INET, SOCK_STREAM, 6)) < 0)
err (EX_OSERR, "socket()");
//
// Bind a local address to our socket for listening.
// To make this easier, we just copy the properties of
// the local part of of the existing FTP command/control
// connection (this is cmd->fd here).
// Setting the port number to zero lets bind() choose
// an appropriate port number for us.
//
llen = sizeof(datalisten);
if (getsockname(->fd, (struct sockaddr *) &datalisten, &llen) < 0) {
close (socke);
err (EX_OSERR, "getsockname()");
}
datalisten.sin_port = 0; // <-- !!!
llen = sizeof(datalisten);
if (bind(socke, (struct sockaddr *) &datalisten, llen) < 0) {
close (socke);
err (EX_OSERR, "bind()");
}
//
// Find out which port number was assigned to our
// socket by bind().
//
if (getsockname(socke, (struct sockaddr *) &datalisten, &llen) < 0) {
close (socke);
err (EX_OSERR, "getsockname()");
}
//
// Our portnumber is now in datalisten.sin_port.
// Now call listen() etc...
//
> I'm expecting my copy of the Stevens book any day
> now, so hopefully that'll stop my incessant babbling on these lists...
That book will help you a lot. :)
Regards
Oliver
--
Oliver Fromme, Leibnizstr. 18/61, 38678 Clausthal, Germany
(Info: finger userinfo:olli@dorifer.heim3.tu-clausthal.de)
"In jedem Stück Kohle wartet ein Diamant auf seine Geburt"
(Terry Pratchett)
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