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From: Changli Gao on 30 Jul 2010 06:20
On Fri, Jul 30, 2010 at 5:09 PM, Herbert Xu <herbert(a)gondor.apana.org.au> wrote:
> On Fri, Jul 30, 2010 at 08:04:18AM +0800, Changli Gao wrote:
>> after updating the value of the ICMP payload, inet_proto_csum_replace4() should
>> be called with zero pseudohdr.
>>
>> Signed-off-by: Changli Gao <xiaosuo(a)gmail.com>
>
> No, the code is correct as is. �We need to update the checksum
> even if the checksum is partial, which is what the 1 is for.
>

Is it really necessary, and I have noticed that netfilter doesn't call
inet_proto_csum_replace4 in this way.

static bool
icmp_manip_pkt(struct sk_buff *skb,
unsigned int iphdroff,
const struct nf_conntrack_tuple *tuple,
enum nf_nat_manip_type maniptype)
{
const struct iphdr *iph = (struct iphdr *)(skb->data + iphdroff);
struct icmphdr *hdr;
unsigned int hdroff = iphdroff + iph->ihl*4;

if (!skb_make_writable(skb, hdroff + sizeof(*hdr)))
return false;

hdr = (struct icmphdr *)(skb->data + hdroff);
inet_proto_csum_replace2(&hdr->checksum, skb,
hdr->un.echo.id, tuple->src.u.icmp.id, 0);
hdr->un.echo.id = tuple->src.u.icmp.id;
return true;
}

Thanks.

--
Regards,
Changli Gao(xiaosuo(a)gmail.com)
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From: Changli Gao on 30 Jul 2010 10:20
On Fri, Jul 30, 2010 at 6:24 PM, Herbert Xu <herbert(a)gondor.apana.org.au> wrote:
>
> The checksum update is for the inner IP header. �netfilter does
> of course update the checksum, it just doesn't do it here which is
> for the outer IP header.
>

I know we need to update the ICMP checksum if we alter the payload(the
inner IP header here) of ICMP. But I doubt if the update is really
necessary if the checksum is partial, as the checksum will be done
later(by ether skb_checksum_help() or NIC hardware). In fact, as there
isn't any pseudo header, the icmph->checksum should be always ZERO,
otherwise skb_checksum_help() or NIC will give the wrong checksums,
when the checksum is partial.

--
Regards,
Changli Gao(xiaosuo(a)gmail.com)
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