FWD: Truncated non-interactive sessions

[sys-soft@ida.ing.tu-bs.de]

I'm resending the following posting without attachments - as it didn't 
reach the mailing lists until now.

---------- Forwarded message ----------
From: sys-soft at ida.ing.tu-bs.de
To: dropbear at ucc.asn.au
Date: Tue, 2 Aug 2005 12:33:31 +0200 (CEST)
Subject: Truncated non-interactive sessions


following problem:

have two computers here - one running dropbear compiled with uclibc and a 
stock suse 9.2/9.3 & 9.0. when transferring files a la:

cat /etc/motd | ssh root at server "cat - > /etc/motd"

they occasionally get truncated by the dropbear server. please don't focus 
on the method to transfer files - it's just a testcase for this bug.
with the default RECV_MAXWINDOW 8000 (see channel.h) files between 8000 
and 12096 are affected (8000+4096=12096). To be more specific the problem 
occurs with files n*8000+i where 0 < i <= 4096. With the probability 
decreasing with increasing i.


The problem is that dropbear fails to flush the writebuffer before sending
the a received EOF.

transcript of a sample session:

#~> dd if=/dev/zero bs=1 count=4321 | ssh root at server "wc"
0 0 4321

#~> dd if=/dev/zero bs=1 count=8000 | ssh root at server "wc"
0 0 8000

#~> dd if=/dev/zero bs=1 count=8090 | ssh root at server "wc"
0 0 8000
      ~~ <- truncated 90 bytes

#~> dd if=/dev/zero bs=1 count=8090 | ssh root at server "wc"
0 0 8090
      ~~ <- occassionally correct... not deterministic

#~> dd if=/dev/zero bs=1 count=12095 | ssh root at server "wc"
0 0 8000

#~> dd if=/dev/zero bs=1 count=12097 | ssh root at server "wc"
0 0 12097


I appended four tracefiles of failed and successful dropbear sessions. 
Dropbear was compiled with RECV_MAXWINDOW 10000 and RECV_MAXPACKET 8000 
for this test. Affected files had a size between 10000 and 14095 as well 
as 20000-23090 (couldn't reproduce with higher values).

In all four traced sessions a 12k file was transferred. In two cases only 
10k were transferred.

The problem was also reproducible with dropbear compiled on a suse 9.2 
with glibc.

Best Regards

	Matt