Prevent Routing External Traffic to Local Loopback on All IPv4 Interfaces

Description

To set the runtime status of the net.ipv4.conf.all.route_localnet kernel parameter, run the following command:

$ sudo sysctl -w net.ipv4.conf.all.route_localnet=0

To make sure that the setting is persistent, add the following line to a file in the directory /etc/sysctl.d:

net.ipv4.conf.all.route_localnet = 0

Rationale

Refuse the routing of packets whose source or destination address is the local loopback. This prohibits the use of network 127/8 for local routing purposes. Enabling route_localnet can expose applications listening on localhost to external traffic.