James Carlson wrote:
> Garrett D'Amore writes:
>
>> Andrew Gallatin wrote:
>>
>>> Why don't you just fix the checksum by "subtracting" off the last 4
>>> bytes of the expanded vlan header (bytes 14..17 of the frame).
>>>
>>>
>> I had considered that, but done it mostly because I am not entirely sure
>> that this is universally "safe". (This is due to ignorance on my part,
>> and I'd love to be better informed.)
>>
>> There are two complexities in the checksum calculation that I'm
>> concerned about. The first is the inclusion of carry bits. The second
>> is the negation (~). I'm not entirely sure after these are done, that
>> the operation is reversible enough to safely subtract those bytes.
>>
>
> That shouldn't be an issue. It's just 1's complement math, not
> something exotic like a Galios field.
>
> The hard part is knowing which bytes are overhead and need to be
> removed. If the sender doesn't tell you, then you're probably sunk.
>
Okay Jim. Knowing that have to subtract the VLAN tag (which is bytes 14
thru 17 of the frame), can you take a quick look at uts/sun/io/hme.c
line 6049, and tell me how I can adjust the value of cksum. Is it safe
to simply subtract from cksum? (Or perhaps I need to negate the bits
first.)
I can see that I'll have to add a check for ETHERTYPE_VLAN or somesuch,
and then also check for ETHERTYPE_IP/IPV6 at bytes 18/19. Its the
subtraction/math that I want help with. I'm *not* a cyborg, and I don't
want to screw it up. :-)
-- Garrett
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