How do I "reset" a frozen/unresponding tty (mingetty)
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How do I "reset" a frozen/unresponding tty (mingetty)
I have 3 servers attached to a cheap 4-port Belkin KVM switch. For some reason, when I am switching between computers, there are times when I lose my TTY on my Linux machine - it freezes and won't respond to keyboard input. I can switch to another TTY, but the TTY I had is lost. After a while, I lose all of my TTYs and have to initiate a reboot via SSH, or press Ctrl+Alt+Del.
When I go back to the "frozen" tty, I can still see everything (I don't get a blank screen)... I just can't type anything. It won't accept keyboard input of any kind.
Yes, it's a cheap switch, but I can't afford to get anything better at the moment. Money is tight.
What I've done is ps -A | grep tty and tried to kill -9 the processes attached to that tty, thinking that would do it. But it doesn't.
I don't have much experience with mingetty... is there a way for me to reset mingetty and bash on tty1 via tty2 or tty3?
This has been a nagging problem for several years, believe it or not, so any help would be very, very appreciated.
Last edited by utahnix; 01-30-2008 at 03:06 PM.
I have had this problem occasionally for a long time when mistyping control codes in a tty and finally found the solution. A ^S (XOFF) sent to the terminal will halt -any- further transmission of data until a corresponding ^Q (XON) is recieved. Your keyboard switch is probably sending inadvertent ^S's. Typing Ctrl-Q in the frozen TTY may resolve the problem.
See 'Software flow control' in Wikipedia for more information. Seems I'm not allowed to post links on first post.
Oddly, I seem(ed) to be having this same problem exactly right now.
I logged into my box locally on tty1 and opened up a screen session. I stopped gdm (`/etc/init.d/gdm stop`) and one of my Xorg processes went through the roof with CPU usage. So, I killed the X process with SIGTERM as root, and immediately, the screen locked up.
I was able to log in through ssh and reconnect to my screen session; everything else remained running just like normal.
Finally, exactly 24 hours later, it occurred to me to run startx as root (`sudo -s`). $DISPLAY was set to :0.0 as normal. Success! My default X session started up and I was able to logout, which (since gdm wasn't running) sent me back to the console on tty7. Ctrl+Alt+F1 brought me back to tty1 and I was able to login successfully.
Problem: TTY is frozen / locked / hung and will not display.
Solution1: If root (UID 0) cannot login directly to SSH:
user@workingbox:~$ ssh user@frozenbox
# SSH might ask for a password if you're not using
#+ public key authentication.
user@frozenbox:~$ sudo -s
# See `man sudo` for details on the '-s' argument.
# You will be prompted for your password if user isn't
#+ in your /etc/sudoers on "frozenbox".
# See `man 5 sudoers` for more information.
root@frozenbox:~# ( [ -z $DISPLAY ] || [ "$DISPLAY" -ne $':0.0' ] ) && export DISPLAY=':0.0'
# In English: if $DISPLAY isn't set OR it's not equal to ':0.0',
#+ export the value of $DISPLAY to be ':0.0' inside this shell.
# For more, see <http://wooledge.org:8000/BashGuide>.
# This should start an X session on your local machine,
#+ allowing you to logout normally and type Ctrl+Alt+F1
#+ (or F2 through F6) to get back to a working console.
Solution2: Requires that `startx` can be executed by root && that ssh allows root to login directly:
does not show any process associated to the frozen terminal and the command
ps ax | grep getty
does not show anything except the command line itself.
If I switch to the frozen terminal (ALT+Fx) I can see what follows:
The cursor is NOT blinking.
The terminal seems to be just frozen in such a state, not responding to any input from the keyboard.
I'm sure there MUST be a simple command line which root can use to reset any terminal, but I haven't found it yet. Any help would be highly appreciated.
Of course I'd also like to know WHY an ordinary "exit" command produces such an effect, but that's another question...
@frieza thanks! my tty just froze up and i came here via google. the kvm i use switches screens with a couple hits of the scroll lock, and i'm so used to it i didn't even think of that as a culprit, but, sure enough, hitting the scroll lock again on that tty thawed it right out.