How to Keep Remote SSH Sessions Running after Disconnection

SSH (Secure Shell) is the end to end encrypted networking system
that allows users to get access remotely from client to server or
system. Because of its asymmetric cryptography security system,
it’s pretty much safe and secure to access the server even from an
unsafe client network. But sometimes due to inactivity or bad
network signal, SSH remote access can get disconnected.
Whatever the reason is, as a Linux administrator, it’s a vital job
to keep SSH sessions and processes running after disconnection.
^[1][2]

Reasons Why SSH Sessions Get
Disconnected

In a nutshell, SSH tunnel proxy error, network timeout, using
the wrong network port, or even not logged into your system as
root user can also get you disconnected
from the SSH remote administration. In this post, we are going to
discuss the most frequently asked question about SSH; how to keep
SSH sessions and processes running after disconnection is
occurred.^[3]

1. Keep Running SSH Sessions Using
the screen Command

The screen command is the most used and useful
command for SSH administration. The screen command can
resolve hostname problems for both IPv4 and IPv6 addresses. With
root functionalities, the screen command can log out
or dismiss a session from the client end to the server end. If
you’re a very newbie at Linux SSH, here are some CLI that can help
you to install and find manuals of the screen
command.

For Ubuntu:

$ sudo apt install screen

For OpenSUSE:

$ sudo zypper install screen

For Arch Linux:

$ sudo pacman -S screen

For Red Hat Enterprise Linux:

$ sudo yum install screen

After you’re done installing, start monitoring your system start
screen.

$ screen
$ screen --help

Once you have the access, SSH into the server, you want to get
access remotely. If you’re planning a long night work from your
station, but you don’t want to get your SSH client down, you can
use the screen command. This can detach your screen
session, but no worries, the tasks you initiated will be
completed.

To detach your screen press Ctrl-A and then
Ctrl-D from your keyboard. You can log in to monitor
the work progress anytime from your terminal. To re-connect or
connect with the existing one with the session, use the following
CLI.

$ screen -r
screen -D -r

2. Keep Running SSH Sessions Using
the tmux Tool

The tmux^[4]
or terminal multiplexer is a very popular and useful tool for
programmers and system administrators that allows users to switch
between program to program in the same terminal. To keep SSH
sessions running in the background, tmux can be a very
quick solution. It can detach and reattach applications where the
processes are kept running in the background. Here is the
installation instruction of tmux for Linux
distros.

For Arch Linux:

$ sudo pacman -S tmux

For Red Hat Enterprise Linux:

$ sudo yum install tmux

For Ubuntu:

$ sudo apt-get install tmux

For OpenSUSE:

$ sudo zypper install tmux

After installing tmux, start the tmux
session using terminal commands. It won’t take much time to
get installed. Once you’ve done installing you can now get started
with tmux. To start tmux, just type tmux
in the terminal.

$ tmux

If you need to run more than one terminal multiplexer to keep
all SSH sessions running in the background, in that case, you may
find difficulties to switch between tmux to
tmux. Here are the command lines to switch one form
another.

$ tmux detach
$ tmux attach
$ tmux attach -t 2

To check all the tmux screen use the tmux
list command.

$ tmux ls

3. Keep Running SSH Sessions Using
the byobu Tool

The byobu^[5]
tool is mostly used for remote SSH administration and on-screen
monitoring on Linux. It is an open source-sourced software and can
be used alongside the tmux or the screen
tool. You can see the current status, notifications, and messages
with the byobu tool. Though the
byobu comes installed by default with Ubuntu. But if
it doesn’t, you can install it inside your Linux machine. To check
whether byobu is installed or not in your system by
checking the other versions of byobu.

$ byobu --version

Then enable byobu, it will help to keep your SSH sessions
running in the background after even disconnection.

$ byobu-enable

Here the installation processes are explained with CLI.

For Ubuntu:

$ sudo apt install byobu

For OpenSUSE:

$ sudo zypper addrepo https://download.opensuse.org/repositories/utilities/openSUSE_Leap_42.3/utilities.repo

$ sudo zypper refresh
$ sudo zypper install byobu

For Arch Linux:

$ yaourt -S byobu
$ packer -S byobu

For Red Hat Enterprise Linux:

$ sudo yum install byobu

For Fedora:

$ sudo dnf install byobu

To get started with byobu, just type byobu in the
terminal and hit Enter.

$ byobu

Now, if you’re using the tmux or the
screen, you can choose and select between any of them
for backend collaborated.

$ byobu-select-backend

Now, you can manage and keep your SSH sessions running in the
background after disconnection, use the system keyboard function
keys. To get started with SSH alongside byobu, press
CTRL+SHIFT+F2 from your keyboard to enable the SSH
environment. To move your SSH sessions forward and backward you can
use ALT+UP and ALT+DOWN.

To disconnect your SSH sessions press F6 from the
keyboard. Now, if you’re looking for detaching but not disconnect
the session, here you go. To detach SSH sessions and still get
connected, press SHIFT+F6. Here is one more extra
keyboard command for you. To keep only current the screen session
active and close all other windows press ALT+F6.

4. Keep Running SSH Sessions Using
the nohup Command

The nohup or ‘no hangup’ command is a very useful
alternative tool for the screen or the tmux. It also allows users
to keep the SSH sessions running even after they got disconnected.
The nohup command tells the system to run all
processes in the background by avoiding the signal hang up
(SIGHUP).

To check the nohup manuals and options form your
Linux terminal, type the following command in the terminal and hit
Enter.

$ nohup options

To check the currently active job lists, use the -l
command from your terminal.

$ jobs -l

Now, for SSH connection, to avoid connection lost and keep your
sessions running after disconnection, use the nohup command lines
following by your job. You can also get the output list of your
jobs in a text file by using the cat command alongside the nohup
command.

$ nohup ./hello.sh
$ cat nohup.out

To run a process in the background, you can use the
& symbol just after the nohup command.
Like, if I want to test ping for wordpress-408970-1286763.cloudwaysapps.com
in the background, the command line will be just like the following
line below. And when you think your job is done, to see the
outcome, use the pgrep command. ^[6]

$ nohup ping ubuntupit.com &
$ pgrep -a ping

5. Keep Running SSH Sessions Using
the disown Command

If you don’t have the system root privileges, maybe this one is
going to solve your problem. The disown command can
make any task unlisted from your running system log. Thus, it can
hide an undergoing process to avoid all the auto log out errors or
the signal hang up (SIGHUP). You can actually use the diswon
process management command to keep your SSH sessions running by
hiding your task.

To remove or hide your task from the task manager, use the
diswon command in the terminal.

$ disown <task>

To check the current status of the task, use the current shell
command.

$ current shell

You can also set a list of tasks at a single line terminal
command.

$ disown jobs1
$ disown jobs1 jobs2 ... jobsn

And to check the task list, use the -l command.

$ jobs -l

To remove all current jobs from the window, type the
-a syntax following by the disown command.

$ disown -a

To remove only one or the running job from the window, use the
-r syntax following by the diswon command.

$ disown -r

Now for SSH, to run a job in the background after the session is
connected, use the -h syntax. This process can keep
your SSH sessions running even after disconnection.

$ disown -h jobID
$ disown -h %2

Fixing the timeout
Error of SSH in RHEL

When you’re connected to the SSH on Red Hat Enterprise Linux
(RHEL), you may get a frequent timeout problem and get your
connection lost. This problem occurs due to the invalid response
time form either the client end or the host end. In your Red Hat
root directory, you have to find and edit the
sshd_config.txt file to solve these timeout problem.
Once you’re done, you can get your SSH connection back even after
getting disconnected.

There inside the sshd_config.txt, file you will
find two options named as ClientAliveInterval and
ClientAliveCountMax, you need to edit these log files
to maximize the server to client response time. The timeout
interval is calculated by multiplying the values of
ClientAliveInterval and ClientAliveCountMax.

All you need is to edit the time duration of this two values
according to your system and network responding duration. Let you
want to maximize the time 10 minutes for the client interval end
and 5 times for the client count, then your text registry inside
the sshd_config.txt file will be like below. And then,
restart the SSH.

ClientAliveInterval 10m
ClientAliveCountMax 5

Final Thoughts

For a system administrator, keep SSH sessions and processes
running is a crucial job to satisfy the client and to fulfill the
task. Getting disconnected and frequent sessions lost is very much
annoying and bothersome. So in this post, we have tried to discuss
and illustrate the reasons why SSH sessions get disconnected and
also described all the possible methods on how to keep SSH sessions
running after disconnection.

If you have ever gone through the hassle of SSH session
disconnection and know how annoying it could be, please share your
experience with us. And also do comment if you have anything to add
or ask anything related to this post in the comment section. Don’t
forget to share this post with your friends on social media.