Cygwin socket option SO_REUSEADDR operates unlike Linux

[Corinna Vinschen]

On Jan 13 13:39, Mark Geisert wrote:
> Corinna Vinschen wrote:
> > On Jan 13 00:36, Mark Geisert wrote:
> > > ~ ./bindtest
> > > 1st socket is 3
> > > 1st bind OK
> > > 1st connect OK
> > > 2nd socket is 3
> > > 2nd bind OK
> > > 2nd connect: Address already in use
> > > 
> > > ~ ./bindtest
> > > 1st socket is 3
> > > 1st bind OK
> > > 1st connect: Address already in use
> > > 
> > > On Fedora 27, running the same STC shows:
> > > 
> > > [mark@lux ~]$ netstat -an|grep :111
> > > tcp        0      0 0.0.0.0:111         0.0.0.0:*        LISTEN
> > > tcp6       0      0 :::111              :::*             LISTEN
> > > udp        0      0 0.0.0.0:111         0.0.0.0:*
> > > udp6       0      0 :::111              :::*
> > > [mark@lux ~]$ ./bindtest
> > > 1st socket is 3
> > > 1st bind OK
> > > 1st connect OK
> > > 2nd socket is 3
> > > 2nd bind OK
> > > 2nd connect OK
> > 
> > I can't reproduce this:
> > 
> > $ uname -sr
> > Linux 4.14.13-300.fc27.x86_64
> > $ netstat -an|grep :111
> > tcp        0      0 0.0.0.0:111             0.0.0.0:*               LISTEN
> > tcp6       0      0 :::111                  :::*                    LISTEN
> > udp        0      0 0.0.0.0:111             0.0.0.0:*
> > udp6       0      0 :::111                  :::*
> > $ ./bindtest
> > 1st socket is 3
> > 1st bind OK
> > 1st connect OK
> > 2nd socket is 3
> > 2nd bind OK
> > 2nd connect: Cannot assign requested address
> > [...]
> Rats.  I'll have to investigate a couple of directions, deeper.  It makes
> sense that connect() returns EADDRINUSE rather than bind() [...]

After some more digging it turns out that both of the above observations
on Linux are correct.  I can reproduce the 2nd connect succeeding by
simply adding a `sleep(1)' after the first close.  So it turns out that
Linux has a timing issue at socket cleanup which can be alleviated
by an extra sleep.  I opened a case about this issue.  EADDRNOTAVAIL
sounds a bit weird in this scenario, but it's kind of ok.

In terms of Cygwin, the EADDRINUSE is a completely different matter.

It turns out that the second connect fails because the first socket
connection is in TIME_WAIT state.  This is not exactly correct in POSIX
terms.  The TIME_WAIT connection should not disallow a new socket to
reuse the same local address.  That's what we observe on Linux (apart from
the timing issue).

But here's the problem:  Regardless if we actually use SO_REUSEADDR or
not, Windows sockets apparently disallows a subsequent connect to
succeed while the first socket is still in TIME_WAIT.  I tweaked Cygwin
to enforce SO_REUSEADDR before bind, but connect still fails with
EADDRINUSE as long as the first socket is in TIME_WAIT.

It seems the code path for listen/accept is different here compared to
connect.  Given that SO_REUSEADDR seems to cover mostly server side
scenarios, and given that I don't see this scenario discussed at all
in Steven's book, I wonder if bind/connect is a bit of a grey area.

Either way, the bottom line is that this is a WinSock restriction,
apparently.  As of today, I don't see any way around that.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat
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