Forwarding host port to service in container

 #!/bin/sh
 set -e


usage() {
    cat <<EOF
    $(basename $0) [options] <in-interface> <out-interface> <port> <destination>

    --clear           Clear the previous rules before inserting new
                      ones

    --protocol        Protocol for the rules to use.

    in-interface      Interface on which incoming traffic is expected
    out-interface     Interface to which incoming traffic is to be
                      forwarded.

    port              Port to be forwarded. Can be integer or string
                      from /etc/services.
    destination       IP and port of the destination system to which
                      traffic needs to be forwarded. This should be in
                      form <destination_ip:port>

(C) 2015 Vasudev Kamath - This program comes with ABSOLUTELY NO
WARRANTY. This is free software, and you are welcome to redistribute
it under the GNU GPL Version 3 (or later) License

EOF
}

setup_portforwarding () {
    local protocol="$1"
    iptables -t nat -A PREROUTING -i $IN_INTERFACE -p "$protocol" --dport $PORT \
           -j DNAT --to $DESTINATION
    iptables -A FORWARD -p "$protocol" -d ${DESTINATION%%\:*} --dport $PORT -j ACCEPT

    # Returning packet should have gateway IP
    iptables -t nat -A POSTROUTING -s ${DESTINATION%%\:*} -o \
           $IN_INTERFACE -j SNAT --to ${IN_IP%%\/*}

}

if [ $(id -u) -ne 0 ]; then
    echo "You need to be root to run this script"
    exit 1
fi

while true; do
    case "$1" in
      --clear)
          CLEAR_RULES=1
          shift
          ;;
      --protocol|--protocol=*?)
      if [ "$1" = "--protocol" -a -n "$2" ];then
          PROTOCOL="$2"
          shift 2
      elif [ "${1#--protocol=}" != "$1" ]; then
          PROTOCOL="${1$--protocol=}"
          shift 1
      else
          echo "You need to specify protocl (tcp|udp)"
          exit 2
      fi
      ;;

      *)
          break
          ;;
    esac
done

if [ $# -ne 4 ]; then
    usage $0
    exit 2
fi

IN_INTERFACE="$1"
OUT_INTERFACE="$2"
PORT="$3"
DESTINATION="$4"

# Get the incoming interface IP. This is used for SNAT.
IN_IP=$(ip addr show $IN_INTERFACE|\
             perl -nE '/inet\s(.*)\sbrd/ and print $1')


if [ -n "$CLEAR_RULES" ]; then
    iptables -t nat -X
    iptables -t nat -F
    iptables -F
fi

if [ -n "$PROTOCOL" ]; then
    setup_portforwarding $PROTOCOL
else
    setup_portforwarding tcp
    setup_portforwarding udp
fi