strange session failure on linux

Tom Pollard pollard at
Sun Apr 23 13:10:58 EST 2000


We're running an openssh server on a linux box here to give developers
access to our machines from outside the firewall.  A number of us, using
various ssh clients (Mac, Windows and Linux) regularly experience 
hung sessions.  I'm hoping that someone can give me some insight into
what's happening here or, at least, some tips for characterizing the
problem well enough for it to get fixed.

Let me try to describe what happens.  I usually have two or three ssh
sessions active from my home machine to this server.  A few minutes after
these are established (sometimes it takes much longer), one of them will
suddenly stop responding while the others remain active.  If I kill the
dead session, it's always possible to open another session immediately.  
I've discovered that these dead seesions are only dead in one direction.  
That is, things I type into my dead ssh window continue to get transmitted
to the server, but the response from the server isn't transmitted back to
my home machine.  I know this because (a) I can tell that commands typed
blindly into the dead session are properly executed, and (b) running
netstat on the server machine reveals that the Send-Q starts growing
dramatically for the dead connection.  So, both the server and the client
consider the connection to be active ("ESTABLISHED"), but nothing is
transmitted from the server to the client.

Any idea what could possibly be going on here?  As I mentioned, people
using various different clients have experienced this problem, so it seems 
clear that it's a server problem of some sort.  The Linux system sshd is
running on is, unfortunately, a non-standard version based on the 2.0.28
kernel.  We're running OpenSSH-1.2.1, protocol version 1.5.



W. Thomas Pollard                                  Schrodinger, Inc.
pollard at             

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