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This is meant to replace /proc/sys/net/ipv4/conf/*/rp_filter, which only works for ipv4. Furthermore, it's nicer to handle this kind of filtering in the firewall. There are some more subtle differences, please see: https://home.regit.org/netfilter-en/secure-use-of-helpers/ I chose to enable this by default (when the firewall is enabled) as it's a good idea in general. Only people with advanced routing needs might not want this, but I guess they don't use the nixos firewall anyway and use a custom solution. Furthermore, the option only becomes available in kernel 3.3+, so conservative nixos users that just stick to the default kernel will not need to act now just yet.
*** NixOS *** NixOS is a Linux distribution based on the purely functional package management system Nix. More information can be found at http://nixos.org/nixos and in the manual in doc/manual.