Currently netfront is setting the flags of inbound packets with the checksum not present (offloaded) to (CSUM_IP_CHECKED | CSUM_IP_VALID | CSUM_DATA_VALID | CSUM_PSEUDO_HDR). According to the mbuf(9) man page this is not the correct combination of flags, it should instead be (CSUM_DATA_VALID | CSUM_PSEUDO_HDR).
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