about fragment packet

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There is a question about the filtering method of fragment pachet using ip6tables.

A and B prepare two PCs, the command of ip6tables of the following [ B ] is struck, and 4000 bytes of UDP packet (address port number 1025) is sent from A to B.

(PC on B) %ip6tables -A INPUT -p udp -m frag --dport 1025 -j DROP

In this environment, since MTU was 1500 bytes, the packet was fragmentation-ized by three, and all the packets reached and carried out to B.

Then, although it is a question, since there is information on a port number in the first fragmentation-ized packet, although it thinks that it is filtered and two of succession pass, why is it?

Is it the specification which cannot use the frag option and the port option together?

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