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readme.eng : mediatek badsum limitation

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@ -182,6 +182,9 @@ add tcp option **MD5 signature**. All of them have their own disadvantages :
System never verifies checksums of locally generated packets so nfqws will always work on the router itself.
If you are behind another NAT, such as a ISP, and it does not pass invalid packages, there is nothing you can do about it.
But usually ISPs pass badsum.
Some adapters/switches/drivers enable hardware filtering of rx badsum not allowing it to pass to the OS.
This behavior was observed on a Mediatek MT7621 based device.
Tried to modify mediatek ethernet driver with no luck, likely hardware enforced limitation.
* badsum doesn't work if your device is behind NAT which does not pass invalid packets.
Linux NAT by default does not pass them without special setting `sysctl -w net.netfilter.nf_conntrack_checksum=0`
Openwrt sets it from the box, other routers in most cases don't, and its not always possible to change it.