On 11/3/21 11:23, Thorsten R. wrote:
On 11/2/21 15:46, Jakub Jelen wrote:
Hi,
does your Cisco switch support more channels? You can try it for
example with openssh with connection multiplexing using
ControlMaster/ControlPath.
That's an interesting point. Did not know that multiplexing is possible.
But this is what I got:
First terminal
==============
tr@LinuxMint19:~/$ ssh -o ControlMaster=yes -o
ControlPath=~/.ssh/control-%h_%p_%r admin@192.168.4.219
Password: ************
gwswitch1#
Second terminal
===============
tr@LinuxMint19:~/$ ssh -o ControlPath=~/.ssh/control-%h_%p_%r
admin@192.168.4.219
mux_client_request_session: session request failed: Session open refused
by peer
This is the failure to open a session channel.
Password: ************
gwswitch1#
OpenSSH implements fallback opening a new connection if the remote does
not allow this.
Output on first terminal after second terminal connects:
gwswitch1#channel 3: open failed: resource shortage:
gwswitch1#
It seems that multiplexing is not possible on Cisco switches.
But now I am a little bit confused. I thought that calling
ssh_channel_close(channel);
ssh_channel_free(channel);
closes the shell and the session in a way that I can open a new one if
needed.
Right. This closes the session channel, but connection is still open.
With other servers, what you do might work just fine. With OpenSSH
server, you can probably emulate this with "MaxSessions 1" in sshd_config.
What is the right way to open a SSH connection, open a terminal/shell,
execute a few commands in it with
ssh_channel_write()
ssh_channel_read_nonblocking()
and close the shell without losing the connection?
This is not the most common use case. Generally, you want to execute
command and exit or open a shell, execute commands, collect results and
again exit the connection.
If you want to send more commands in some longer timespan, you will
probably have to open a shell and handle the IO yourself as described in
the following tutorial chapter:
https://api.libssh.org/stable/libssh_tutor_shell.html
Regards,
--
Jakub Jelen
Crypto Team, Security Engineering
Red Hat, Inc.
