Information
The net.ipv4.ip_forward and net.ipv6.conf.all.forwarding flags are used to tell the system whether it can forward packets or not.
Rationale:
Setting the flags to 0 ensures that a system with multiple interfaces (for example, a hard proxy), will never be able to forward packets, and therefore, never serve as a router.
Solution
Run the following command to restore the default parameter and set the active kernel parameter:
# grep -Els '^s*net.ipv4.ip_forwards*=s*1' /etc/sysctl.conf /etc/sysctl.d/*.conf /usr/lib/sysctl.d/*.conf /run/sysctl.d/*.conf | while read filename; do sed -ri 's/^s*(net.ipv4.ip_forwards*)(=)(s*S+b).*$/# *REMOVED* 1/' $filename; done; sysctl -w net.ipv4.ip_forward=0; sysctl -w net.ipv4.route.flush=1
IF IPv6 is enabled:
Run the following command to restore the default parameter and set the active kernel parameter:
# grep -Els '^s*net.ipv6.conf.all.forwardings*=s*1' /etc/sysctl.conf /etc/sysctl.d/*.conf /usr/lib/sysctl.d/*.conf /run/sysctl.d/*.conf | while read filename; do sed -ri 's/^s*(net.ipv6.conf.all.forwardings*)(=)(s*S+b).*$/# *REMOVED* 1/' $filename; done; sysctl -w net.ipv6.conf.all.forwarding=0; sysctl -w net.ipv6.route.flush=1