system choose one for me on the connect() call? Should I bind()
system choose one for me on the connect() call? Should I bind()
From Andrew Gierth (andrew@erlenstar.demon.co.uk):
** Let the system choose your client's port number **
The exception to this, is if the server has been written to be picky
about what client ports it will allow connections from. Rlogind and
rshd are the classic examples. This is usually part of a Unix-specific
(and rather weak) authentication scheme; the intent is that the server
allows connections only from processes with root privilege. (The
weakness in the scheme is that many O/Ss (e.g. MS-DOS) allow anyone to
bind any port.)
The rresvport() routine exists to help out clients that are using this
scheme. It basically does the equivalent of socket() + bind(),
choosing a port number in the range 512..1023.
If the server is not fussy about the client's port number, then don't
try and assign it yourself in the client, just let connect() pick it
for you.
If, in a client, you use the naive scheme of starting at a fixed port
number and calling bind() on consecutive values until it works, then
you buy yourself a whole lot of trouble:
The problem is if the server end of your connection does an active
close. (E.G. client sends 'QUIT' command to server, server responds
by closing the connection). That leaves the client end of the
connection in CLOSED state, and the server end in TIME_WAIT state. So
after the client exits, there is no trace of the connection on the
client end.
Now run the client again. It will pick the same port number, since as
far as it can see, it's free. But as soon as it calls connect(), the
server finds that you are trying to duplicate an existing connection
(although one in TIME_WAIT). It is perfectly entitled to refuse to do
this, so you get, I suspect, ECONNREFUSED from connect(). (Some
systems may sometimes allow the connection anyway, but you can't rely
on it.)
This problem is especially dangerous because it doesn't show up unless
the client and server are on different machines. (If they are the same
machine, then the client won't pick the same port number as before).
So you can get bitten well into the development cycle (if you do what
I suspect most people do, and test client & server on the same box
initially).
Even if your protocol has the client closing first, there are still
ways to produce this problem (e.g. kill the server).
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