[Bug 188990] [xen] dhclient(8) does not accept DHCP Offer on FreeBSD 10 as Xen DomU Guest

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=188990

--- Comment #15 from Roger Pau Monné <royger at freebsd.org> ---
(In reply to Bhavesh Davda from comment #14)
And the mbuf(9) man page says:

"If a particular network interface just indicates success or failure of TCP or
UDP checksum validation without returning the exact value of the checksum to
the host CPU, its driver can mark CSUM_DATA_VALID and CSUM_PSEUDO_HDR in
csum_flags, and set csum_data to 0xFFFF hexadecimal to indicate a valid
checksum."

I think that's what maps best to NETRXF_data_validated/CHECKSUM_UNNECESSARY (in
fact it's the only combination of outbound flags that make sense for the use
case here AFAICT), but I'm not a network expert.

I think setting CSUM_IP_CHECKED and CSUM_IP_VALID in the netfront driver for
incoming packets is wrong, because netfront is also setting the header to
0xffff, and that's only valid with (CSUM_DATA_VALID | CSUM_PSEUDO_HDR).

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