tcp checksum after port translation

Good morning, i calculate tcp checksum after changing a destination port
(nat)

For example: A (192.168.0.2) sends ssh to 192.168.0.1 : 222 (B) and packets
are redirected to 192.168.0.1 : 22

After changing port, i recalculate checksum in prerouting on B.

B responds to A, sending response from port 22, so I change in postrouting
the source port to 222, the one
A expects from.

Then I recalculate checksum.
In post routing I printk the checksum recalculated by B: it is correct,
according to the checksum expected
by A (ethereal confirms that).

The fact is that A receives back always the same checksum different from the
right one.

Why do i read the correct checksum in B postrouting and then A receives a
bad value?
Has it to do with HW_CHECKSUM?? (and what is that?)

This happens to machine B (amd athlon, realtek 8139, via motherboard),
towards A
(intel P4, natsemi net card, intel chipset).

If I invert roles, it works!!
(P4 machine responds ok to this kind of redirect).

If I insert module iptable_nat, it works!!

Without, checksum seems to be set correctly in postrouting skb, but arrives
wrong and always the same (0x81a5).
Someone changes checksum after postrouting netfilter hook??

the packet flow is

PREROUTING ->(redirect) -> INPUT ->SSH-> OUTPUT->POSTROUTING (de-dport
translation)

If I redirect port and IP all works fine (PRE->FWD->POST)

Thanks for any idea of what's going wrong
GIacomo-