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When I login to my Debian server over SSH, it "freezes": in interactive mode, I don't get the prompt, if I say ssh server 'ls', nothing happens. I can, hovewer, use Bash built-ins, for example ssh server 'su' will reply "su: must be run from a terminal" and then the session ends. (Only the first session I tried gave me my home directory listing.)

I can also connect to other services (SMTP, IMAP, HTTP) over the network, but get no response.

When I reboot, everything works again.

This happens from time to time and I do not have physical access to the machine at the moment. What should I look for in the logs when I have the chance? Is there any way reboot the server remotely?

Thanks.

Edit: It turned to be a kernel panic caused by a misbehaving firmware. I will open a new thread I will answer to let others know how I fixed it. Thanks for all help.

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  • Check for disk errors in particular. If you have no other way to control the server, e.g. IPMI or a networked PDU, you can't reboot the server if SSH etc. don't work. Having something like this is a mandatory requirement for a remote server you can't access, IMHO.
    – Sven
    May 11, 2014 at 8:56
  • Check MTUs on the network pathway between client and server, too: the "I can get little packets between these two endpoints but not big ones" often indicates MTU negotiation failure.
    – MadHatter
    May 11, 2014 at 8:56
  • @MadHatter Unfortunately, I cannot get any packets whatsoever.
    – David
    May 11, 2014 at 9:04
  • You said ssh server 'su' worked. That required quite a lot of (small) packets to flow.
    – MadHatter
    May 11, 2014 at 9:13
  • Oh I see. I thought that when ls worked once (for my first session), it would work again. Is this assumption incorrect? I do not think this is the case; echo x wouldn't work either :(.
    – David
    May 11, 2014 at 9:18

1 Answer 1

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Check MTU or "Fragmented Packets" on the intermediate devices between you and the destination server.

I see this most often with poorly-configured Sonicwall firewall devices.

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  • I will, but how could this be solved by server reboot?
    – David
    May 11, 2014 at 9:19
  • I have checked MTU setup using SpeedGuide TCP/IP Analyzer. As I haven't had to deal with MTUs till now, I would be grateful for your opinion: [MSS: 1460, MTU: 1500 , TCP Window: 29312 (NOT multiple of MSS), RWIN Scaling: 7 bits (2^7=128), Unscaled RWIN: 229, Recommended RWINs: 64240, 128480, 256960, 513920, 1027840, BDP limit (200ms): 1172kbps (147KBytes/s), BDP limit (500ms): 469kbps (59KBytes/s), MTU Discovery: ON]
    – David
    May 11, 2014 at 10:01

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