mjhjmtu

I have a Raspberry with eth0 (192.168.02 / static) connected to a router (Fritz Box @ 192.168.0.1). The Router has internet connection. The Raspberry has a mobile usb stick with ppp connect to the internet. Actually if ppp is not connected the internet access from the Raspberry is through the Router over eth0. If ppp is connected the internet access is via ppp. if i del the default route for eth0 there is no internet connection if ppp is not connected and if ppp is connected internet connection is through ppp. so far everything fine. but if restart the raspberry it again has internet connection through eth0. But i only want to allow the raspberry internet connection only via ppp. eth0 just for internal communication like ssh. any ideas how to configure that?

/etc/network/interfaces looks like:
source-directory /etc/network/interfaces.d

auto lo

iface lo inet loopback

auto eth0

iface eth0 inet static

address 192.168.178.2

netmask 255.255.255.0

allow-hotplug wlan0

iface wlan0 inet manual

wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf

allow-hotplug wlan1

iface wlan1 inet manual

wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf

Sounds like you want to get rid of a static route. Check your /etc/network/interfaces file for any route configuration. Comment these lines (add a # at the beginning of the line), save the file, reboot, and try again.
But your Pi is more than likely getting this info via DHCP, I would suggest configuring the interface with static details (and without a route).

If you don't want the Raspberry to have an connection when PPP is disconnected, remove the gateway from your network configurations. Probably you're using the file /etc/network/interfaces, so open it on a text editor and change it from

auto eth0
inet iface eth0 static
 address 192.168.0.1
 netmask 255.255.255.0
 gateway 192.168.0.1

to

auto eth0
inet iface eth0 static
 address 192.168.0.1
 netmask 255.255.255.0
 #gateway 192.168.0.1

You'll still able to access the raspberry on local network (192.168.0.0/24) but it won't have an internet connection if PPP is down.