Use SSH socks tunnels to bypass firewall that blocks outbound Internet

reverseSocksTunnel.sh

#!/bin/bash -eu

# Script is designed to build an SSH proxy for external Internet access on a
# host, which sits behind a NAT/firewall that doesn't allow outbound traffic.
# (Common in exchange data centers).
#
# Call this from a bridge with access to the Intrent and inbound access to the
# host. On success it will make available on host both a Socks5 proxy deamon
# (at localhost:8080) and a tunnel to the bridge machine's http proxy (at 
# Elocalhost:8118).
#
# Requires local/bridge machine to have ssh, bash, and whoami. For HTTP
# proxy functionality the bridge machine must have an HTTP proxy (e.g. privoxy)
# running at the HTTP proxy port. If not, Socks5 proxy will still work.
#
# Requires remote/destination machine to have ssh, nc, and socks5 capable 
# client. (proxychains suggested)

destAuthArg=""
localAuthArg=""

while getopts i:j:P opt ; do
    case "$opt" in
	i) destAuthArg="-i $OPTARG";;  # Must exist on local machine, if used
	j) localAuthArg="-i $OPTARG";; # Must exst on destination machine
    esac
done

shift $(($OPTIND - 1))
destAddr=$1    # Use user@host if different user on destination machine

reverseTunnelPort=19999
reverseTunnelDest=localhost:22
socksProxyPort=8080
httpProxyPort=8118

# Before each tunnel step, check if the port is already built, in which case
# we don't need to run the command again. This avoids spurious errors when
# the script is re-run.
function testRemotePort() {
    local portNum=$1
    ! isRemotePortOpen $portNum
}

function isRemotePortOpen() {
    local portNum=$1
    local portTestCmd="nc -z localhost $portNum" 
    ssh $destAuthArg $destAddr "$portTestCmd"
}

# First build a reverse tunnel to allow an outbound path on the remote.
if testRemotePort $reverseTunnelPort ; then
    ssh $destAuthArg -f -C -N -R $reverseTunnelPort:$reverseTunnelDest $destAddr
fi

# These commands get called on the remote destination...
socksProxyCmd="ssh $localAuthArg -f -D $socksProxyPort -C -N \
-p $reverseTunnelPort $(whoami)@localhost"

# ...with these remote calls
if testRemotePort $socksProxyPort ; then
   ssh $destAuthArg -f $destAddr "$socksProxyCmd"
fi

# Now do the same with the HTTP proxy
httpTunnel=$httpProxyPort:localhost:$httpProxyPort
httpProxyCmd="ssh $localAuthArg -f -L $httpTunnel -C -N \
-p $reverseTunnelPort $(whoami)@localhost"

if testRemotePort $httpProxyPort ; then
   ssh $destAuthArg -f $destAddr "$httpProxyCmd"
fi

# This spawns two SSH daemons on the localmachine and one on the remote host,
# all of which survive after the script finishes running. If you want to kill
# the daemons, use ps/pkill.

# To acccess outbound internet on destination configure proxychains config with
#    proxy_dns
#    socks5 127.0.0.1 8080
#
# Then run any outbound-accessing internet command as 
#   $ proxychains [command]
# e.g.
#   $ proxychains curl google.com
#
# HTTP proxy access will be available on the remote machine at 8081. However
# for any requests to work, it requires the bridge machine to have an actual
# HTTP proxy (like privoxy) listening at localhost on 8081