systemd.network(5) получить значение из файла или среды?

Согласно книге Policy Routing Using Linux , пакеты, исходящие с локальной машины и предназначенные для внешних систем, попадают в базу данных политик маршрутизации после прохождения выходных цепочек, и именно здесь находится ваш селектор from 10.10.10.73. будет соответствовать исходящему эхо-ответному пакету из 10.10.10.73, предназначенному для 172.16.1.1.

Изhttp://www.policyrouting.org/PolicyRoutingBook/ONLINE/CH03.web.html:

Consider the path for an externally sourced packet destined for an internal service. It enters the system and is processed by the entrance packet mangling and tagging stage, Pre-Route(1). This stage is where you would apply packet mangling operations such as fwmark and TOS/QoS tagging or perhaps the NetFilter NAT. The packet then enters the RPDB to obtain routing and is routed to the Input(2) chain. The Input chain provides the firewalling functions for packets destined to the Local Machine services.

The reverse scenario is the packet path for an internal service sourced packet destined for an external system, such as the reply packet to the one described in the previous paragraph. It exits the Local Machine and enters the Output(4) chains, which provides the firewalling functions. It then enters the RPDB for route processing and exits the system via the exit packet mangling and tagging stage, Post-Route(5).