The rp_filter kernel setting.

The rp_filter – INTEGER

0 – No source validation.

1 – Strict mode as defined in RFC3704 Strict Reverse Path. Each incoming packet is tested against the FIB and if the interface is not the best reverse path the packet check will fail. By default failed packets are discarded.

2 – Loose mode as defined in RFC3704 Loose Reverse Path. Each incoming packet’s source address is also tested against the FIB and if the source address is not reachable via any interface the packet check will fail.

Current recommended practice in RFC3704 is to enable strict mode to prevent IP spoofing from DDos attacks. If using asymmetric routing or other complicated routing, then loose mode is recommended.

The max value from conf/{all,interface}/rp_filter is used when doing source validation on the {interface}. (Which means it can be set
on each individual interface, or globally. The global setting will override the individual interface settings).

Debian packages – list and install on another server.

To list all currently installed packages, type “dpkg –get-selections”. You can save this data to a file by running “dpkg --get-selections > ~/packages.txt“.
And then place this file on another system and install the listed packages by running “cat packages.txt > sudo dpkg --set-selections && sudo apt-get dselect-upgrade“.
Also, the list of packages can be loaded for installation by running “dpkg --clear-selections && sudo dpkg --set-selections < packages.txt".
This could be used also for recovery. This trick is helpful when a system must be reinstalled, then all of the same packages can be installed all at once.
Sometimes, the output of "dpkg --get-selections" may list packages marked to be uninstalled or some other status. If so happens, use the following "dpkg --get-selections | grep -w install$ > ~/packages.txt". I mean, "| grep -w install$"