Restore  SSH session after network breakdown (not using SCREEN)

Posted on 2007-04-10
Medium Priority
Last Modified: 2008-01-09
This question bothers me for so long - is there any way to restore ssh (pts) session after client's disconnetion?

Here is an algorythm:
1. # ssh username@some.remotehost.com
2. open several interactive programms (mc, vim etc)
3. plug of network cable from client's computer :-)
4. open ssh connection from another computer and run:
    # who
    username   pts/3        Apr 10 14:04 (   <------ this session must be restored
    root    pts/6        Apr  2 11:04 (
    root    pts/7        Apr  5 15:39 (
5. huh? what to do?
Question by:OLEshiy
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 5
  • 2
  • 2
  • +3
LVL 34

Accepted Solution

PsiCop earned 500 total points
ID: 18886168
No, because the cryptographic state has been broken.

If what you suggest were possible, then that would open SSH up to session hijacking, where an attacker impersonated the client of another session.

Security is balancing functionality/convenience with good defenses. You're looking for a convenience from a tool designed to provide good defenses.
LVL 27

Expert Comment

ID: 18886672
OLEshiy, hi.
I'm completely agree with PsiCop, you cannot connect to already dead session.
However _theoretical_ small possibility exists, on modified system.
OpenSSH uses openpty() library call (libutil) to allocate a pseudo terminal device. This call is implemented differently on different systems, but on Linux it opens '/dev/ptmx' and /dev/pts/* devices (by name) to find first free pseudo terminal. So, it's theoretically possible to fake /dev/ptmx and create a node with the same name, but your own pseudo terminal multiplexor driver with some  controlling process in a user space (much like screen). Then you may write program that can 'reconnect' such disconnected process and view the last screen. However you cannot fake 'ssh' to think that this tcp session is still alive (as correctly said by PsiCop) and any input will disconenect stale ssh session and kill your sshd.

Expert Comment

ID: 18887124
Please enlighten me.

 I believe to have heard before that someone say that this is possible. On the server side you would have to make the TTL of the session to last longer and that the client has a fix ip. In order for SSH to work, you would require a 3 factor authentication which is the IP address, Application ID and also the SSL cert.

One can spoof the ID and maybe the Application ID but not the same cert right? The user will still need to relogin username and password to continue its session. I've seen it working but not the actual steps on how its being done. The reason behind it was that dial-up users which has slow connectivity find it a hassle if the connection drops half way and their password gets locked.
Comprehensive Backup Solutions for Microsoft

Acronis protects the complete Microsoft technology stack: Windows Server, Windows PC, laptop and Surface data; Microsoft business applications; Microsoft Hyper-V; Azure VMs; Microsoft Windows Server 2016; Microsoft Exchange 2016 and SQL Server 2016.

LVL 27

Expert Comment

ID: 18887147
> dr_shivan
It seems you have missed some stuff from the original post

You say:
> On the server side you would have to make the TTL of the session to last longer *and that the client has a fix ip*

And what OLEshiy asked for:
> 4. open ssh connection from *another* computer and run:
LVL 27

Expert Comment

ID: 18887153
dr_shivan, really I tested such 'session restoration' what you are talking about, but from the same host (FreeBSD with some minor TCP tuning)
LVL 38

Expert Comment

ID: 18887647
to restore/resume a session, you can use VNC or freenx (NX client).
VNC is packaged in most of Linux distribution.
freeNX and NX client you can download from

Basically, those two tools allow you to resume/restore the remote terminal session, doesn't matter via telnet or SSH.
It allows you to resume/restore from other machine as well.

Author Comment

ID: 18889428
Ok, let me paraphrase my question: is there any way to connect to existing pseudo-terminal (analogue to screen -x)?
As you've said above, every ssh-connection opens (or creates?) pseudo terminal. When netwotk connection breaks down, that pts remains alive for some time (according to such sshd configuration as "timeout", "keepalive" etc). I have root privelegies on the server.
LVL 38

Expert Comment

ID: 18891482
VNC or NX can fit your need just fine.
You can resume the VNC or NX session from any other machine if the network connection breaks down.
Those tools basically keep the session alive, unless you kill then or reboot, even thouth the network connection breaks down.
ssh or SSL for VNC or NX is used to encrypted the connection from client to server. After connection, it looks for the existing session first.
So you can easy to resume your session without deal with restoring ssh session.

I use NX/VNC at work and suspend it when I leave for home. Then resume the session at home when I need it.

Author Comment

ID: 18896235
Thanks for all advices, but I still haven't heard an answer to my last question: is there any way to connect to existing pseudo-terminal?
LVL 27

Expert Comment

ID: 18896606
> is there any way to connect to existing pseudo-terminal?

LVL 27

Expert Comment

ID: 19061418
sirbounty, hi.

The correct answer is here, was first suggested by PsiCop. There is _no_ possible way to connect to dead ssh session and it _is_ correct answer.

Expert Comment

ID: 20294537
Forced accept.

EE Admin

Featured Post

Portable, direct connect server access

The ATEN CV211 connects a laptop directly to any server allowing you instant access to perform data maintenance and local operations, for quick troubleshooting, updating, service and repair.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Linux users are sometimes dumbfounded by the severe lack of documentation on a topic. Sometimes, the documentation is copious, but other times, you end up with some obscure "it varies depending on your distribution" over and over when searching for …
Note: for this to work properly you need to use a Cross-Over network cable. 1. Connect both servers S1 and S2 on the second network slots respectively. Note that you can use the 1st slots but usually these would be occupied by the Service Provide…
Connecting to an Amazon Linux EC2 Instance from Windows Using PuTTY.
Viewers will learn how to properly install and use Secure Shell (SSH) to work on projects or homework remotely. Download Secure Shell: Follow basic installation instructions: Open Secure Shell and use "Quick Connect" to enter credentials includi…
Suggested Courses
Course of the Month15 days, 14 hours left to enroll

741 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question