When calling mbuf_outbound_finalize() to compute the incomplete
TCP/UDP checksums on the mbuf from my iff_output_func (interface
filter output function), I kept getting messages from the kernel
"cksum: out of data".
Looking at the xnu-792.13.8 source, it looks like the underlying
in_cksum_skip, called from in_delayed_cksum_offset, expects the ip_len
field in the IP header to be in host-byte-order, while in reality the
mbuf handed to my iff_output_func had the ip_len field in the IP
header correctly populated in network-byte-order.
What is my iff_output_func to do in such a situation?
This is a known bug and a fix is already in order. Assuming that
you call this from an interface filter, and that only the transport
checksum is to be recomputed, you should be able to byte-swap the
IP length, call mbuf_outbound_finalize() and restore the IP length
back to the network-byte order. If the driver offloads IP header
checksumming, and if this is an IPv4 packet, then you would need
to recompute the IP header checksum yourself prior to sending the
packet downstream. Once the fix is made available, you will no
longer need to do any of the above.
(Also note that this issue does not exist for PPC.)
Adi
Thanks!
--
Bhavesh P. Davda
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