No more sessions available' / channels not closing issue

[Mickel Daelmans | Add to Favorites]

Hi Jakub,

Thanks for your reply. It can be reproduced with the shell as follows:

1) add the following lines to ssh_config:

ControlMaster auto
ControlPath ~/.ssh/control:%h:%p:%r

2) Open 2 sftp connections to the same machine

sftp user at host

3) exit from the first connection (this will block until you close the second one). If you exit the second with leaving the first open it will exit gracefully.

4) same happens with opening 3 channels. Closing the second blocks until the 3rd is closed. Etc. This will snowball in our case. We usually open a new channel before the last channel is completed and exited.

OpenSSH_6.6 does not have this issue. Is there a patch available for the OpenSSH_5.3p1, OpenSSL 1.0.1e-fips (still default for CentOs 6.5) or do we need to upgrade to OpenSsh 6?


Thanks,

Mickel Daelmans
Developer
 

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-----Oorspronkelijk bericht-----
Van: Jakub Jelen [mailto:jjelen at redhat.com] 
Verzonden: dinsdag 12 september 2017 9:58
Aan: Mickel Daelmans | Add to Favorites; openssh-unix-dev at mindrot.org
Onderwerp: Re: No more sessions available' / channels not closing issue

On Mon, 2017-09-11 at 10:30 +0000, Mickel Daelmans | Add to Favorites
wrote:
> Hi all,
> 
> 
> 
> We've run into a curious issue.
> 
> 
> 
> We run CentOS 6.5 with openssh installed (OpenSSH_5.3p1, OpenSSL 
> 1.0.1e-fips 11 Feb 2013 by default). We connect to this server through 
> sftp (Subsystem sftp internal-sftp) using the java JSch library. When 
> opening many sftp channels on a single ssh session OpenSsh does not 
> seem to release the sftp channels closed from the client side when 
> there are still sftp channels open that are opened later than the 
> channel that is closed..  Eventually resulting in "No more sessions 
> available".

The obvious questions: Can you reproduce it with OpenSSH client or only with the java library? Can you reproduce it with the latest OpenSSH server? In this old OpenSSH version, it can be cause by many factors.

Regards,
--
Jakub Jelen
Software Engineer
Security Technologies
Red Hat, Inc.