Re: Fwd: SSHD not connecting from outside(real) IP

[Oleg Volkov]

04.01.2016 22:05, João Paulo writes:

Hello all,
I have a mystery and I need help. I have openssh(Cygwin) installed and
working just fine in here.
I can connect normally from any of my machines to the server using local
IPs. Ex: ssh localhost or ssh 192.168.0.2. BUT, when using my external
IP(over internet->real IP) it times out.
I can validate that the service is up and running on
http://www.canyouseeme.org/ on the port FW I did for this service.
Port 22 is correctly FW to my SSHD server. On windows, I can only see an
entry on Security Events(attached) and nothing else after that. (Means
it is reaching my machine)
I have tried to reinstall Cygwin, change ports. sshd_config is in
default, disable firewall, AV. I ran out of ideas..
I had this working fine on previous window 7, now I am on Windows 10.
Thanks.


Try to issue the `traceroute' command to your real IP over the Internet 
from a Linux computer and specify packet length 1452. If it fails, then 
maybe a misconfigured router (e.g. CISCO) on the way has a lesser MTU 
set and large packets sent by SSH do not get through. If your Internet 
service provider indeed has this problem, then "MSS FIX 1452" command 
issued on a CISCO router may fix it.


Oleg

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Re: Cygwin speed

[Oleg Volkov]

I had come across with the following problem: after upgrading Cygwin from
version 1.5.19-4 to 1.5.24-2 my application (relational database)
began to function several times slower. The reason was a slowdown of
a function, which performs rebalancing of table index tree; this function
calls write() function very many times, at each call 4 bytes are updated
in index file (row number in tree node). To test performance of write()
function I created the following test program:

#include fcntl.h
#include unistd.h

int main(int argc, char **argv)
{
char chunk[64]=;
int i, fd;
if ((fd=open(tst_chunks.bin,
 O_CREAT|O_WRONLY|O_TRUNC,
 0666))0) return 1;
for (i=0; i100; i++)
if (write(fd,chunk,sizeof(chunk))!=sizeof(chunk)) return 1;
close(fd);
return 0;
}

When launched on Celeron 1.3MHz via time -p, it works:

  on 1.5.24-2 : 48 seconds;
  on 1.5.19-4 : 18 seconds.

After investigating differences between 1.5.24-2 and 1.5.19-4 I have found
out, that the problem is in function sig_dispatch_pending(), which is
called in the beginning of writev() function, which is called from write().
In function sig_dispatch_pending() the following has been changed:

void __stdcall
sig_dispatch_pending (bool fast)
{
 if (exit_state || _my_tls == _sig_tls || !sigq.start.next)  // 
version 1.5.19-4
  // if (exit_state || _my_tls == _sig_tls)  // 
version 1.5.24-2
{
  //...
  return;
}

  //...
  sig_send (myself, fast ? __SIGFLUSHFAST : __SIGFLUSH);
}

When make this modification in sources for 1.5.24-2 and rebuild cygwin1.dll,
my test program begins to work as fast as on 1.5.19-4. In message

  http://cygwin.com/ml/cygwin-developers/2006-07/msg00034.html

Brian Ford pointed to the following description of a change between
1.5.19-4 and 1.5.24-2:

  2006-02-24  Christopher Faylor  cgf at timesys dot com

* sigproc.cc (sigheld): Define new variable.
-  (sig_dispatch_pending): Don't check sigq since that's racy.
(sig_send): Set sigheld flag if __SIGHOLD is specified, reset it if
__SIGNOHOLD is specified.  Ignore flush signals if we're holding
signals.

I think, that maybe checking of sigq is a little bit racy, but it turns,
that getting rid of such a cheap check results in a great slowdown of
sig_dispatch_pending() function for most calls, when there are no pending
signals.

Maybe introducing a critical section or some other synchronization
mechanism would be a solution.

Oleg Volkov



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