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am using a debian based linux system. There is a console access to it via Lantronix SLC8000 Advanced Console Manager. When I login it asks for username and password (Both Console manager and server )

[user.USER-G9SW5Z1] ➤ ssh [email protected]
Password:
X11 forwarding request failed on channel 0

Welcome to the Lantronix SLC8000 Advanced Console Manager
Model Number: SLC8048
For a list of commands, type 'help'.

[DCE-Console-01]> connect direct deviceport 46
Connecting to Device Port 46.
Connected to port 46. Escape sequence is ESC A


Open Network Linux OS ONL-3.1.1, 2019-11-19.15:58-3bcc913
localhost login: root
Password:
Last login: Thu Jan  6 21:28:51 UTC 2000 from 172.27.173.173 on pts/3
Linux localhost 4.14.109-OpenNetworkLinux #1 SMP Tue Nov 19 16:21:40 UTC 2019 x86_64

But after that I just close the session abruptly ( No 'exit' or 'logout' )

Next when I try to login it does not ask me for the login info (username/password)

[user.USER] ➤ ssh [email protected]
Password:
X11 forwarding request failed on channel 0

Welcome to the Lantronix SLC8000 Advanced Console Manager
Model Number: SLC8048
For a list of commands, type 'help'.

[DCE-Console-01]> connect direct deviceport 46
Connecting to Device Port 46.
Connected to port 46. Escape sequence is ESC A

root@localhost:~#
root@localhost:~#

Again it should ask me for the username and password for the server. Where is the issue here and how to fix this ?

PS : It looks like the system is storing the screen for some time. But is there a way to stop this?

1 Answer 1

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You are just about to learn something that, in decades past, many people had to learn, sometimes from bitter experience: Switching off a local terminal does not log one off.

Serial terminals, in the world of Unices and Linux operating systems, come in two flavours: dial-in and local. Dial-in serial terminals are marked as dialin in the /etc/ttys table on the BSDs, for example. Local serial terminals are unmarked. (Linux operating systems have the /etc/ttys table, but a wide variation in whether applications respect it. Marking a terminal as dial-in or local is somewhat more complex, less standardized, and depends from what softwares one has chosen to run.)

A dial-in terminal has modems and whatnot between it and the host. Modems deal in serial control lines such as Carrier Detect and Data Terminal Ready. Hanging up the remote end causes the Carrier Detect signal to drop, which for a dial-in terminal causes the line discipline to send a HUP (hangup) signal to the session leader process of the login session, which in turn handles reflecting it to all of its children. Dial-in terminals have the -clocal flag (to use the stty terminology) in their line disciplines.

A local terminal is directly connected to the host. Local terminals have the clocal flag in their line disciplines. There is no carrier to detect, and turning off the terminal does not equate to a hangup of a telephone call.

Your SLC8000 is just a glorified serial terminal, as far as the host is concerned. It has all of the behaviour of a local terminal unless it is marked as dialin on the host. Connecting to and disconnecting from different deviceports does not signal the hosts on those ports. Mark it as dialin on the host, and your next problem is configuring your SLC8000 to signal the host when you disconnect, with Carrier Detect as a modem would, so that the host knows to perform terminal hangup on the login session. A brief skim of the SLC8000 user manual indicates that this may not even be possible. There's not a separate CD pin on its serial sockets.

Which leads to advice that you can find in over four decades of security books for multi-user operating systems with terminals, from Unix to VMS: Always log off when leaving your terminal.

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