Mangle Routing (routing-mark) can be accomplished using which chains?

The ability to use mangle routing with routing marks is primarily supported in the prerouting and output chains. Mangle is used for packet manipulation, and the prerouting chain is crucial for modifying packets before they are routed, allowing you to apply changes based on certain criteria like source or destination IP addresses. The output chain is utilized for packets generated by the router itself and passed through after being created.

While the forward chain is primarily for packets that are being routed through the device (not generated by it or destined for it), it doesn’t facilitate the same level of manipulation regarding outbound routing decisions. Thus, while you may think of the forward chain as a participant in routing, it lacks the capabilities specifically tied to mangle routing (routing marks) that prerouting and output chains provide.

Understanding the roles of each chain in the mangle table clarifies why prerouting and output are the correct options. The prerouting chain allows you to mark packets as they arrive before any routing decisions are made, which is essential for influencing router behavior. The output chain enables manipulation of packets that the router generates, allowing you to apply routing marks to packets just before they leave the router. This functionality is what is needed for mangle routing with routing marks.