Re: RPC clnt_create() adress already in use

["PAULUS, Raimund, TI-ABN" <Raimund.Paulus@dillinger.biz>]

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Hello Corinna,

with the snapshot of cygwin1.dll and using bindresvport() from Cygwin for libtirpc (instead of the original bindresvport() from libtirpc) all my testcases work without error.

Many thanks
Raimund



-----Ursprüngliche Nachricht-----
Von: cygwin-owner@cygwin.com [mailto:cygwin-owner@cygwin.com] Im Auftrag von Corinna Vinschen
Gesendet: Montag, 5. Februar 2018 21:15
An: cygwin@cygwin.com
Betreff: Re: RPC clnt_create() adress already in use

On Feb  5 15:06, Corinna Vinschen wrote:
> On Feb  5 14:34, Corinna Vinschen wrote:
> > On Feb  5 12:26, Corinna Vinschen wrote:
> > > To reiterate the problem we observe:
> > > 
> > > - socket()
> > > - setsockopt (SO_REUSEADDR)
> > > - bind() succeeds
> > > - connect() fails with EADDRINUSE while socket is still in 
> > > TIME_WAIT
> > > 
> > > using bindresvport in place of bind only marginally changes the 
> > > situation, in particular if the second parameter is set and 
> > > requests a port number != 0.  What happens in that case is that 
> > > bindresvport calls bind with this port number and checks if bind returns EADDRINUSE.
> > > 
> > > Only then it tries to bind other port numbers in the reserved range.
> > > But we now know that bind will never return EADDINUSE if the 
> > > SO_REUSEADDR socket option has been set.
> > > 
> > > Even assuming the process calls bindresvport(sock, NULL) we may 
> > > end up returning a port number already in use if the process is 
> > > the only Cygwin process on the system.  The reason is that Cygwin 
> > > uses a round robin approach which relies on having a globally 
> > > shared value called last_used_bindresvport.  If the process is the 
> > > only Cygwin process on the system, this information is lost after 
> > > exiting the process, so the next process will start with the same 
> > > start port number and bind will again fail to notice the client with EADDRINUSE.
> > > 
> > > What potential solutions to this problem do we have?
> > > 
> > > - bindresvport could enforce SO_EXCLUSIVEADDRUSE temporarily to make
> > >   sure bind fails.
> > 
> > Nope, no way.  Even enforcing SO_EXCLUSIVEADDRUSE results in the 
> > second bind succeeding and the subsequent connect failing.  The 
> > entire SO_REUSEADDR/SO_EXCLUSIVEADDRUSE semantics only works as 
> > desired on the server side apparently
> > 
> > > - bindresvport could check every local address for being free prior
> > >   to calling bind.  However, there's a potential race here.
> > > 
> > > - DisconnectEx?  Never tried this Winsock extension but it might be
> > >   worth a shot.
> 
> I think I have a very simple solution for the scenario which calls 
> bindresvport with port number.  Still looking for a solution for the 
> second problem...

I've pushed a few patches and uploaded new developer snapshots to https://cygwin.com/snapshots.  Please give them a try.


Thanks,
Corinna

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