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Firewall spotting and networks analysis with a broken CRC

Packet filters firewall are going to be deployed more and more for the sense of security the word "firewall" has got on not-technical people. Available as commercial software, embedded device or inside opensource OS they work at level 3. The support for level 4 isn't complete: they filter ports numbers, TCP flags, seq numbers, defragmentation, but ...

What about level 4 checksum? Are they checking for TCP checksum before analyze flags or port numbers?

No. Most developers say there would be too overhead and other think that the packet will be simply dropped by destination OS stack. All correct, but how could we take advantage of this "feature"? Firewall spotting and networks analysis with a broken CRC | SecGuru

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