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If make DF to 1, can I confirm that fragmented packet with DF=1 will go through routers to the dest IP without any reassembly and re-fragmentation?

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Interesting question. The DF bit is usually only set by the source, but it's technically possible for any hop to set it, even after fragmentation.

RFC 791 states (emphasis mine)

An internet datagram can be marked "don't fragment." Any internet datagram so marked is not to be internet fragmented under any circumstances. If internet datagram marked don't fragment cannot be delivered to its destination without fragmenting it, it is to be discarded instead.

and

If the Don't Fragment flag (DF) bit is set, then internet fragmentation of this datagram is NOT permitted, although it may be discarded.

That very closely makes fragments (MF bit set or fragment offset > 0, and identification filled) with a set DF bit illegal and may cause them to be dropped right away.

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  • I suppose you could. It's an edge case no one has thought of. Of course, with the most common internet security default of "drop all fragments", there's not much point.
    – Ricky
    Jan 26 at 15:47
  • If so, "{pktFrag_16, pktFrag_16} ---> someRouter(reassemble & re-fragment) ---> {pktFrag_24, pktFrag_8}" can be possible? Where pktFrag_16 means fragmented packet with 16 bits size and DF=0.
    – lee yk
    Jan 26 at 22:14

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