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If a machine sends a UDP packet to some destination over the internet, and that destination sends a packet to the source address in a protocol that is not UDP, will the router forward that packet to the machine or drop it?

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  • Ports are per protocol. TCP port 9876 is not the same port as UDP port 9876. Some layer-4 protocols do not use port numbers.
    – Ron Maupin
    Commented Sep 23, 2017 at 18:51

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A "router" always forwards a packet when it can, ie. has a routing entry to apply. It doesn't care about the L4 protocol used nor its state.

It depends on what your understanding of "router" is. A simple router just forwards but with firewall rules, NAT and other functions applied, forwarding may or may not occur.

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