On 07/07/2016 09:07 PM, Nick Holland wrote:
hi,
Ran into a problem which I thought was an AIXism, but have since found
that it can be reproduced on Linux and MacOS. It can NOT be
reproduced on OpenBSD.
Reproduced on:
AIXv7.1 OpenSSH v6.0p1
RedHat 6.8 OpenSSH 5.4p1
Redhat 7.2 OpenSSH 6.6.1p1
MacOS 10.11 (sorry, forgot to grab the OpenSSH version)
Could not reproduce on:
OpenBSD 5.9-current, march snapshot, OpenSSH_7.2
OpenBSD 5.3-current, April snapshot, OpenSSH_6.2
Demonstration: set up an account on a system that can ssh to itself
and authenticate via keys.
Create and run this script:
=====
#!/usr/bin/ksh
mkdir -p ~/dest
for X in 1 2 3 4 5 6 7 8 9 10; do
echo $X
echo "cd dest
put $0" | sftp -b - localhost 2>&1 | tee -a outfile
done
=====
Change first line to whatever shell your system uses (that's AIX's
default), otherwise, should be pretty portable.
Run it, it should copy itself to a directory in your home directory
ten times, should take just a few seconds.
Now, add the following to ~/.ssh/config :
=====
ControlMaster auto
ControlPath ~/.ssh/control/%h:%r:%p
ControlPersist 10s
=====
re-run script.
* What I think should happen is the persistent control channel should
greatly reduce the SSH connection time, so it should run significantly
faster.
* What DOES happen on platforms with a problem: the "ControlPersist"
value becomes an SSH rate limiter -- instead of holding a connection
OPEN for ten seconds, it PREVENTS another SSH session from starting
for ten seconds! So, instead of taking maybe four seconds before, now
it takes 104 seconds (10 x 10 seconds + ssh connection overhead).
Change the ControlPersist to some other value, the overall speed goes
up or down, but never as fast as without the ControlPersist options in
place.
here's where it gets weird.
Remove the "2>&1" from the sftp line, and the problem goes away -- the
script runs much faster with the .ssh/config file than without it.
Unfortunately, I need the stderr output. :-/
It appears to be the "2>&1 | {cmd}" structure that is at fault --
doesn't matter if {cmd} is tee, wc, or a shell function (as it was in
my "real" project).
I have found that 2>error.file |tee outfile works, and then I can
append the error.file output into the end of the "outfile" and
accomplish my goals, but that's kinda ugly.
This looks like related to the bug #1988 [1]. The mux process is holding
the stderr. Could you test it with the patch from the referenced bug
applied?
Also as I see you are using RHEL, you can contact your support to
provide a test package (we plan to fix this in RHEL7.3).
[1] https://bugzilla.mindrot.org/show_bug.cgi?id=1988
--
Jakub Jelen
Associate Software Engineer
Security Technologies
Red Hat
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