help: Disable GRE connection tracking

# GRE shouldn't be disabled when PPTP is enabled because PPTP depends on it
create: cli-shell-api exists system conntrack modules pptp disable
        if [ $? == 0 ]; then
          # Unload PPTP modules if they are loaded
          lsmod | grep -e "^nf_nat_pptp" 2>&1 >/dev/null
          if [ $? == 0 ]; then
            sudo rmmod nf_nat_pptp
            sudo rmmod nf_conntrack_pptp
          fi
          # And GRE modules then
          sudo rmmod nf_nat_proto_gre
          sudo rmmod nf_conntrack_proto_gre
        else
          echo "Error: can not disable GRE connection tracking when PPTP connection tracking is enabled!"
          exit 1
        fi

delete: sudo modprobe nf_conntrack_proto_gre
        sudo modprobe nf_nat_proto_gre