how can know ssh is disconected and retry with bash script - bash

Question

I'm using reverse ssh for connecting to remote client , Operator run reverse one time and leave client system
how can i write bash script , when reverse ssh disconnected from server retry to connect to server (ssh)

Answer 1

Use autossh. Autossh "automatically restart[s] SSH sessions and tunnels"
sudo apt-get install autossh
I use autossh to to keep open reverse tunnel that I depend on. It works very well, even with long periods of lost connection.
Here is the script I use to create the tunnel:
#!/bin/bash
AUTOSSH_GATETIME=0
export AUTOSSH_GATETIME
autossh -f -N -R 8022:localhost:22 username#host -o "ServerAliveInterval 45" -o "ServerAliveCountMax 2"
I execute this script at boot with this cronjob:
#reboot /home/scripts/./persistent-tunnel.sh

Answer 2

If you simply want to retry a command until it succeeds, you can use this pattern:
while ! ssh [...]
do
echo "Command failed, retrying..." >&2
done

Answer 3

I have a slightly different method.
My method always tries to reconnect you if you have a dirty disconnection: '~.' or 'Connection closed by remote host.'
But if you disconnect with 'CRTL+D' or with 'exit' it just disconnects and show you some info of the connections.
I have a slightly different method.
My method always tries to reconnect you if you have a dirty disconnection: '~.' or 'Connection closed by remote host.'
But if you disconnect with 'CRTL+D' or with 'exit' it just disconnects and show you some info of the connections.
#/bin/bash
if [ -z "$1" ]
then
echo '''
Please also provide ssh connection details.
'''
exit 1
fi
retries=0
repeat=true
today=$(date)
while "$repeat"
do
((retries+=1)) &&
echo "Try number $retries..." &&
today=$(date) &&
ssh "$#" &&
repeat=false
sleep 5
done
echo """
Disconnected sshx after a successful login.
Total number of tries = $retries
Connected at:
$today
"""

Answer 4

You might want to take a look into ssh options ServerAliveInterval, ServerAliveCountMax and TCPKeepAlive because sometimes your line dies without making this obvious, let me demonstrate:
#!/bin/sh
while true; do
ssh -T user#host \
-o IdentityFile=~/.ssh/tunnel \
-o UserKnownHostsFile=~/.ssh/known_hosts.tunnel \
pkill -f "^sshd:\ user\ \ \ \ $" # needs to be edited for nearly every case
sleep 2
ssh -T -N user#host \
-o IdentityFile=~/.ssh/tunnel \
-o UserKnownHostsFile=~/.ssh/known_hosts.tunnel \
-o Batchmode=yes \
-o ExitOnForwardFailure=yes \
-o ServerAliveCountMax=1 \
-o ServerAliveInterval=60 \
-o LocalForward=127.0.0.1:2501=127.0.0.1:25 \
-o RemoteForward=127.0.0.1:2501=127.0.0.1:25
sleep 60
done

Answer 5

You can use netstat -ntp | grep ":22" or ss -ntp | grep ":22" to see established connections to ssh port, then use grep to filter the ip address you're looking for. If you don't find a connection then reconnect the tunnel.

Answer 6

Use autossh if it works on your version of Linux. It did not on mine as it was an outdated Linux distribution for a custom NAS box.
The alternative is a simple bash script in crontab like this:
maintain_reverse_ssh_tunnel.sh
if ! netstat -planet |grep myserver_ip_or_name |grep ESTABLISHED > /dev/null; then
echo "REVERSE SSH DOWN - Restarting the tunnels"
ssh -fN -R 32999:localhost:22 -R 28080:localhost:80 myusername#myserver_ip_or_name
fi
Replace myusername and myserver_ip_or_name with those of your user and server.
Then add an entry to crontab by typing crontab -e and adding the following line:
1 * * * * /path_to_my_script/maintain_reverse_ssh_tunnel.sh
Make sure to have the execute permissions on the script:
chmod 755 maintain_reverse_ssh_tunnel.sh