Permanently modify routing table (to make connection to OBD dongle hotspot via ethernet wifi bridge working)

[gauner1986]

I would like to modify the routing table, because i have an ethernet -> wifi bridge that i use to access my OBD dongle wifi simultaneously with tethering via usb.

If i don't do this access to eth0 (the ethernet port) will be lost because upon connecting the usb tethering device. The connection introduces new routing rules that effectively block access to the ethernet network.

I know there is root access via adb with which i was able to verify if it works. Upon connecting to usb tethering i can use the following commands to fix routing and enable access to my OBD dongle wifi:

ip addr add 192.168.4.2/24 dev eth0
ip route add 192.168.4.0/24 dev eth0 table eth1

I need to find a way to automate this process (without the need for adb) and i was thinking about a cronjob just repeatedly running every few seconds.

[gauner1986]

Figured it out myself.
I just remounted the system partition with

adb root
adb remount system

installed two files

/etc/init/custom.rc

service custom /system/bin/custom.sh

    enabled

    seclabel u:r:su:s0

on property:sys.boot_completed=1
    start custom

/system/bin/custom.sh

echo "$(date): Starting program..."
watch -n2 "ip addr add 192.168.4.2/24 dev eth0 && ip route add 192.168.4.0/24 dev eth0 table eth1"

and gave the correct access rights to those files:

chown 0.0 /system/bin/custom.sh
chmod 554 /system/bin/custom.sh
chcon u:object_r:system_file:s0 /system/bin/custom.sh

chown 0.0 /etc/init/custom.rc
chmod 644 /etc/init/custom.rc
chcon u:object_r:system_file:s0 /etc/init/custom.rc

This does the job. Not sure if everything is needed as posted above. Got the solution from:

https://android.stackexchange.com/questions/214839/how-to-run-an-android-init-service-with-superuser-selinux-context/214866