Re: ssh_channel_open_session only works the first time called

[Jakub Jelen <jjelen@xxxxxxxxxx>]

On 11/2/21 14:05, libssh.org_ml@xxxxxxxxxxxxxxxxxxxx wrote:
> Hello!
>
> I am trying to implement a simple "activate/deactive port" function for
> a Cisco switch in C++.

Hi,
does your Cisco switch support more channels? You can try it for example 
with openssh with connection multiplexing using ControlMaster/ControlPath.

I do not see anything wrong on the first sight, but the exception comes 
from an error writing some packet. Can you step through that part of the 
code for example with gdb to see what is failing where. I would start 
around ssh_socket_unbuffered_write() after the first call to your function.

Also, the important question is which libssh version are you using? Is 
it up to date?

Jakub

> For that I initialize a new SSH conection to the switch, which works
> great, and then I am doing the following:
>
> void activePort() {
>    ssh_channel channel = ssh_channel_new(getSession());
>    if (ssh_channel_open_session(channel) < 0)
>    {
>      // print the ssh_get_error() string
>      return;
>    }
>    ssh_channel_request_pty(channel);
> [...]
>    read/write from and to channel
> [...]
>    ssh_channel_close(channel);
>    ssh_channel_free(channel);
> }
>
> In my program I can call activatePort() excactly one times.
> The second time I call activatePort() the ssh_channel_open() method
> returns -1 and ssh_get_error() shows:
>
> "Socket error: No error"
>
> The last log message is: "channel_open: Sent a SSH_MSG_CHANNEL_OPEN type
> session for channel 44"
>
> Below is an extract of the first-run logs and from the second run.
>
> What am I doing wrong? :(
>
> =====================
>
> FIRST RUN
> =========
> [2021/11/02 13:49:10.891679, 2] channel_open:  Creating a channel 43
> with 64000 window and 32768 max packet
> [2021/11/02 13:49:10.891679, 3] ssh_packet_need_rekey:  packet:
> [data_rekey_needed=0, out_blocks=5, in_blocks=2
> [2021/11/02 13:49:10.892676, 3] ssh_socket_unbuffered_write:  Enabling
> POLLOUT for socket
> [2021/11/02 13:49:10.892676, 3] packet_send2:  packet: wrote [type=90,
> len=44, padding_size=19, comp=24, payload=24]
> [2021/11/02 13:49:10.893673, 3] channel_open:  Sent a
> SSH_MSG_CHANNEL_OPEN type session for channel 43
> [2021/11/02 13:49:11.887885, 3] ssh_packet_socket_callback:  packet:
> read type 91 [len=28,padding=10,comp=17,payload=17]
> [2021/11/02 13:49:11.888847, 3] ssh_packet_process:  Dispatching handler
> for packet type 91
> [2021/11/02 13:49:11.888847, 3] ssh_packet_channel_open_conf:  Received
> SSH2_MSG_CHANNEL_OPEN_CONFIRMATION
> [2021/11/02 13:49:11.889845, 2] ssh_packet_channel_open_conf:  Received
> a CHANNEL_OPEN_CONFIRMATION for channel 43:3
> [2021/11/02 13:49:11.890856, 2] ssh_packet_channel_open_conf:  Remote
> window : 8192, maxpacket : 4096
>
> SECOND RUN
> ==========
> [2021/11/02 13:49:12.669605, 2] channel_open:  Creating a channel 44
> with 64000 window and 32768 max packet
> [2021/11/02 13:49:12.670603, 3] ssh_packet_need_rekey:  packet:
> [data_rekey_needed=0, out_blocks=13, in_blocks=17
> [2021/11/02 13:49:12.671609, 3] ssh_socket_unbuffered_write:  Enabling
> POLLOUT for socket
> [2021/11/02 13:49:12.671609, 1] ssh_socket_exception_callback:  Socket
> exception callback: 2 (0)
> [2021/11/02 13:49:12.672598, 1] ssh_socket_exception_callback:  Socket
> error: No error
> [2021/11/02 13:49:12.673644, 3] packet_send2:  packet: wrote [type=90,
> len=44, padding_size=19, comp=24, payload=24]
> [2021/11/02 13:49:12.673644, 3] channel_open:  Sent a
> SSH_MSG_CHANNEL_OPEN type session for channel 44
> Socket error: No error
>
> ===================


--
Jakub Jelen
Crypto Team, Security Engineering
Red Hat, Inc.