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winnet: use SO_EXCLUSIVEADDRUSE for listening sockets.

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Thanks to Patrick Stekovic for pointing out that, unlike sensible IP
stacks, Windows requires a non-default socket option to prevent a
second application from binding to a port you were already listening
on, causing some of your incoming connections to be diverted.

This replaces the previous setsockopt that enabled SO_REUSEADDR, which
I put there a long time ago in order to fix an annoying behaviour if
you used the same listening socket twice in rapid succession (e.g. for
successive PuTTYs forwarding the same port) and the second one failed
to bind the listening port because a left-over connection from the
first one was still in TIME_WAIT and causing the port number to be
marked as used.

As far as I can see, SO_EXCLUSIVEADDRUSE and SO_REUSEADDR are mutually
exclusive - if I try to set both, either way round, then setsockopt
returns failure on the second one - so if I have to set the former
then I _can't_ set the latter. And fortunately, re-testing on Windows
10, the TIME_WAIT problem that SO_REUSEADDR was supposed to solve
doesn't seem to exist any more: I deliberately tried listening on a
port that had a TIME_WAIT connection sitting on it, and it worked for
me even without SO_REUSEADDR.

(I can't remember now whether I definitely confirmed the TIME_WAIT
problem on a previous version of Windows, or whether I just assumed it
would happen on Windows in the same way as Linux, where I definitely
do remember observing it.)

While I'm changing that setsockopt call, I've also fixed its 'on'
parameter so that it's a BOOL rather than an int, in accordance with
the docs for WinSock setsockopt.
This commit is contained in:
Simon Tatham 2019-08-03 18:22:08 +01:00
parent 2c279283cc
commit 15653f67e8
1 changed files with 5 additions and 2 deletions
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7
windows/winnet.c
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@ -1140,7 +1140,6 @@ Socket *sk_newlistener(const char *srcaddr, int port, Plug *plug,
char *errstr;
NetSocket *ret;
int retcode;
int on = 1;
int address_family;
@ -1197,7 +1196,11 @@ Socket *sk_newlistener(const char *srcaddr, int port, Plug *plug,
ret->oobinline = false;
p_setsockopt(s, SOL_SOCKET, SO_REUSEADDR, (const char *)&on, sizeof(on));
{
BOOL on = true;
p_setsockopt(s, SOL_SOCKET, SO_EXCLUSIVEADDRUSE,
(const char *)&on, sizeof(on));
}
#ifndef NO_IPV6
if (address_family == AF_INET6) {