netdev - Re: [PATCH v2] act_nat: fix the wrong checksum when addr isn't in old

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Message-ID: <AANLkTinDe-AluGZx87q3nvxCfushfeH0jS35Fav95IXk@mail.gmail.com>
Date:	Sun, 30 May 2010 21:33:22 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	jamal <hadi@...erus.ca>, "David S. Miller" <davem@...emloft.net>,
	netdev@...r.kernel.org
Subject: Re: [PATCH v2] act_nat: fix the wrong checksum when addr isn't in 
	old_addr/mask

On Sun, May 30, 2010 at 8:58 PM, Herbert Xu <herbert@...dor.apana.org.au> wrote:
> On Sun, May 30, 2010 at 08:43:41AM -0400, jamal wrote:
>>
>> Copying Herbert, taking linux-kernel off...
>
> Thanks Jamal.
>
>> On Sun, 2010-05-30 at 08:26 +0800, Changli Gao wrote:
>> > fix the wrong checksum when addr isn't in old_addr/mask
>> >
>> > For TCP and UDP packets, when addr isn't in old_addr/mask we don't do SNAT or
>> > DNAT, and we should not update layer 4 checksum.
>> >
>> > Signed-off-by: Changli Gao <xiaosuo@...il.com>
>> > ----
>> >  net/sched/act_nat.c |    4 ++++
>> >  1 file changed, 4 insertions(+)
>> > diff --git a/net/sched/act_nat.c b/net/sched/act_nat.c
>> > index d885ba3..5709494 100644
>> > --- a/net/sched/act_nat.c
>> > +++ b/net/sched/act_nat.c
>> > @@ -159,6 +159,9 @@ static int tcf_nat(struct sk_buff *skb, struct tc_action *a,
>> >                     iph->daddr = new_addr;
>> >
>> >             csum_replace4(&iph->check, addr, new_addr);
>> > +   } else if ((iph->frag_off & htons(IP_OFFSET)) ||
>> > +              iph->protocol != IPPROTO_ICMP) {
>> > +           goto out;
>> >     }
>
> Yes the patch is correct.
>
> However, the fact that you need this patch means that your act_nat
> setup isn't perfect.  Ideally all the unNATed packets should be
> filtered out before you hit act_nat.

Thinking about this topologic:

client -> DNAT -> router -> server.

DNAT is used to map a public IP to server's private IP. If a
DEST_UNREACH ICMP packet is sent out by router, in order to handle
this ICMP packet correctly, I have to pass it to act_nat.c. How can I
filter out the other packets? By inspecting the inner IP destination
address of this ICMP packet? Maybe I can use u32 with complicate
parameters.

-- 
Regards,
Changli Gao(xiaosuo@...il.com)
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