netdev - Re: [PATCH] act_nat: the checksum of ICMP doesn't have pseudo header

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Message-ID: <AANLkTimKXsgmOSVmw-cP6Zw6UoK5CSshK38Oo5tLuAeA@mail.gmail.com>
Date:	Fri, 30 Jul 2010 22:16:05 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Jamal Hadi Salim <hadi@...erus.ca>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, Patrick McHardy <kaber@...sh.net>
Subject: Re: [PATCH] act_nat: the checksum of ICMP doesn't have pseudo header

On Fri, Jul 30, 2010 at 6:24 PM, Herbert Xu <herbert@...dor.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@...il.com)
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