Utility Script for Reverse SSH Tunneling When Collaborating with Developers Behind Firewalls

# Use Case:
# - Collaboration with peers that allow them to directly 
#   interface with your local development TCP/IP Ports

#########################################################################################
# For further documentation scroll to the bottom of this script where all the variables #
# are described in detail how they work                                                 #
#########################################################################################

# Scenario 1:
# - As a developer, I want to be able to expose my local development address/port for usage 
#   by another developer who is attempting to assist me, 
# - My local firewalls are preventing direct connection on the host and port so I will 
#   connect to a remote server and expose a tunnel that will allow the developer to connect 
#   in this alternative and secure manner

VARIABLE_REMOTE_SSH_SERVER="myRemoteSshServerForCollaboration.com"
VARIABLE_REMOTE_SSH_PORT="22"
VARIABLE_REMOTE_SSH_USER="myRemoteSshUser"

VARIABLE_REMOTE_BIND_ADDRESS="[::]" #Bind on all remote addresses
VARIABLE_REMOTE_BIND_PORT="1234"

VARIABLE_LOCAL_ADDRESS_TO_FORWARD="localhost"
VARIABLE_LOCAL_PORT_TO_FORWARD="1993"

echo ""
echo "########################################################"
echo "## Give this to the user you are trying to connect to ##"
echo "########################################################"
echo ""
echo ssh -fN -o StrictHostKeyChecking=no -R "$VARIABLE_REMOTE_BIND_ADDRESS:$VARIABLE_REMOTE_BIND_PORT:$VARIABLE_LOCAL_ADDRESS_TO_FORWARD:$VARIABLE_LOCAL_PORT_TO_FORWARD" -l $VARIABLE_REMOTE_SSH_USER $VARIABLE_REMOTE_SSH_SERVER -p $VARIABLE_REMOTE_SSH_PORT

# Scenario 2:
# - As a developer, I want to be able to assist my fellow developer who has exposed their connection
#   on a remote server using my local computer 
# - The remote firewall on the SSH server does not permit me to connect directly to this exposed port 
#   that the developer setup so I will establish a port forwarded tunnel that redirects a specified port
#   on my local machine to the remote server and remote port that I am attempting to hit - therefore allowing
#   me to establish the connection to the end developers reverse proxy

VARIABLE_REMOTE_SSH_SERVER="myRemoteSshServerForCollaboration.com"
VARIABLE_REMOTE_SSH_PORT="22"
VARIABLE_REMOTE_SSH_USER="myRemoteSshUser"

VARIABLE_LOCAL_PORT_FOR_RELAY="1993"

VARIABLE_REMOTE_ADDRESS_FOR_RELAY="localhost"
VARIABLE_REMOTE_PORT_FOR_RELAY="1234"

echo ""
echo "########################################################"
echo "## Execute this locally to connect to the user you    ##"
echo "## who is attempting to expose their local dev ports  ##"
echo "########################################################"
echo ""
echo ssh -fN -o StrictHostKeyChecking=no -L $VARIABLE_LOCAL_PORT_FOR_RELAY:$VARIABLE_REMOTE_ADDRESS_FOR_RELAY:$VARIABLE_REMOTE_PORT_FOR_RELAY -l $VARIABLE_REMOTE_SSH_USER $VARIABLE_REMOTE_SSH_SERVER -p $VARIABLE_REMOTE_SSH_PORT

#
# - VARIABLE_REMOTE_SERVER - specifies what remote intermediary 
#   server you will be exposing your local ports to for another 
#   user to connect with/through
# 
# - VARIABLE_REMOTE_USER - specifies what remote intermediary 
#   server user account you will be using in conjunction with 
#   VARIABLE_REMOTE_SERVER variable
#
# - Parameter -l on SSH Command:
#     - login_name - Specifies the user to log in as on the 
#       remote machine. This also may be specified on a per-host 
#       basis in the configuration file.
#
# - Parameter -N on SSH Command:
#     - Do not execute a remote command. This is useful for 
#       just forwarding ports (protocol version 2 only).
#
# - Parameter -f on SSH Command:
#     - Requests ssh to go to background just before command execution. 
#       This is useful if ssh is going to ask for passwords or passphrases, 
#       but the user wants it in the background. This implies -n. The 
#       recommended way to start X11 programs at a remote site 
#       is with something like ssh -f host xterm.
#
# - Parameter -L on SSH Command:
#     - [bind_address:]port:host:hostport
#     - Specifies that the given port on the local (client) host is to be 
#       forwarded to the given host and port on the remote side. This works 
#       by allocating a socket to listen to port on the local side, optionally 
#       bound to the specified bind_address. Whenever a connection is made to 
#       this port, the connection is forwarded over the secure channel, and a 
#       connection is made to host port hostport from the remote machine. 
#       Port forwardings can also be specified in the configuration file. 
#
# - Parameter -R on SSH Command:
#     - Specifies that the given port on the remote (server) host is to be 
#       forwarded to the given host and port on the local side. This works by 
#       allocating a socket to listen to port on the remote side, and whenever 
#       a connection is made to this port, the connection is forwarded over the 
#       secure channel, and a connection is made to host port hostport from the 
#       local machine.
#
# - Parameter -o on SSH Command:
#     - option - Can be used to give options in the format used in the 
#       configuration file. This is useful for specifying options for which 
#       there is no separate command-line flag. For full details of the options 
#       listed below, and their possible values, see ssh_config(5).
#
# - Option StrictHostKeyChecking on SSH Option Command:
#     - StrictHostKeyChecking can be used to control logins to machines whose 
#       host key is not known or has changed. The keyword is described in StrictHostKeyChecking.
#     - Useful with AWS EC2 Instances whose host keys constantly change when destroyed/recreated.
#
# - VARIABLE_REMOTE_BIND_ADDRESS - specifies what hostnames or addresses the reverse
#   connection will be available on for usage by other user later on
#
# - VARIABLE_REMOTE_BIND_PORT - specifies the port the reverse connection will be available
#   on for usage by other user later on
#
# - VARIABLE_LOCAL_ADDRESS_TO_FORWARD - Specifies the local hostname to forward to the remote
#   server that will be exposed on the VARIABLE_REMOTE_BIND_ADDRESS and VARIABLE_REMOTE_BIND_PORT
#
# - VARIABLE_LOCAL_PORT_TO_FORWARD - Specifies the local port to forward to the remote
#   server that will be exposed on the VARIABLE_REMOTE_BIND_ADDRESS and VARIABLE_REMOTE_BIND_PORT
#
# - VARIABLE_REMOTE_PORT_FOR_RELAY - Specifies the remote port we want to connect to and make 
#   available locally via the VARIABLE_LOCAL_ADDRESS_FOR_RELAY and VARIABLE_LOCAL_PORT_FOR_RELAY
# 
# - VARIABLE_LOCAL_ADDRESS_FOR_RELAY - Specifies the local address we want to listen on for 
#   connection to the VARIABLE_REMOTE_PORT_FOR_RELAY on the remote ssh server
#
# - VARIABLE_LOCAL_PORT_FOR_RELAY - Specifies the local port we want to listen on for 
#   connection to the VARIABLE_REMOTE_PORT_FOR_RELAY on the remote ssh server
#
# - VARIABLE_REMOTE_SSH_SERVER/VARIABLE_REMOTE_SSH_PORT/VARIABLE_REMOTE_SSH_USER - Specifies
#   the remote ssh server that we will be connecting to where both scenarios are exposed and 
#   accessible for connectivity by both end users