sysctl.d: switch net.ipv4.conf.all.rp_filter from 1 to 2

This switches the RFC3704 Reverse Path filtering from Strict mode to Loose
mode. The Strict mode breaks some pretty common and reasonable use cases,
such as keeping connections via one default route alive after another one
appears (e.g. plugging an Ethernet cable when connected via Wi-Fi).

The strict filter also makes it impossible for NetworkManager to do
connectivity check on a newly arriving default route (it starts with a
higher metric and is bumped lower if there's connectivity).

Kernel's default is 0 (no filter), but a Loose filter is good enough. The
few use cases where a Strict mode could make sense can easily override
this.

The distributions that don't care about the client use cases and prefer a
strict filter could just ship a custom configuration in
/usr/lib/sysctl.d/ to override this.

NEWS

@@ -70,6 +70,15 @@ CHANGES WITH 240 in spe:
           glibc is going to make it available too. This locale enables UTF-8
           mode by default, which appears appropriate for 2018.
 
+        * The "net.ipv4.conf.all.rp_filter" sysctl will now be set to 2 by
+          default. This effectively switches the RFC3704 Reverse Path filtering
+          from Strict mode to Loose mode. This is more appropriate for hosts
+          that have multiple links with routes to the same networks (e.g.
+          a client with a Wi-Fi and Ethernet both connected to the internet).
+
+          Consult the kernel documetnation for details on this sysctl:
+          https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt
+
 CHANGES WITH 239:
 
         * NETWORK INTERFACE DEVICE NAMING CHANGES: systemd-udevd's "net_id"

sysctl.d/50-default.conf

@@ -22,7 +22,7 @@ kernel.sysrq = 16
 kernel.core_uses_pid = 1
 
 # Source route verification
-net.ipv4.conf.all.rp_filter = 1
+net.ipv4.conf.all.rp_filter = 2
 
 # Do not accept source routing
 net.ipv4.conf.all.accept_source_route = 0