Looking for small Confirmation about skb_pull & NF_ACCEPT !!!

[Kesava Srinivas]

Thanks Guys for the Reply. Yes., it's Tunneling . I will be getting a
Packet with an Extra IP+UDP Header Over the Original IP header. I need
to strip this Extra 28 bytes & Route the Packet based on the Original
Inner IP header. This became bit Confusing to me.

@Santosh: Well; I am successful while adding Extra Header & Routing
Packet. In that case; I have used below Mentioned  steps & it worked
Fine. After looking at the Part of TCP./IP Stack code for kernel
2.6.25.x; I see only some Functions are Exported &
__ip_route_output_key() is one important function who is giving me the
Cached Routing Table Entry if the Corresponding Flow Matches. But the
Problem was Inner IP header's Source IP was not my Machine IP & it is
some other Machine & Functions Returns NO Routing key for me. It seems
to be this Function to be best used only for the Packets Generated
(OUTPUT CHAIN ) by my Machine. What can be the best Function to play
in my use case ??? Many are NOT EXPORTED to test in my code ;( .

Friends,
 Any  Pointer/Document for me to play with the Exported Kernel
Functions ??? I am playing with many but always ending with Kernel
Panic.

#################################################
  fi.fl4_dst=iph->daddr;
  fi.fl4_src=iph->saddr;
  fi.oif=0;
  Var1 =  __ip_route_output_key(&init_net,&rt,&fi);
  dst_release (sock_buff->dst);
  sock_buff->dst = &rt->u.dst;
  Var2 =  dst_output(sock_buff);
##################################################

-Thanks in Advance,
VKS


On Fri, Apr 6, 2012 at 9:56 AM, SaNtosh kuLkarni
<santosh.yesoptus at gmail.com> wrote:
> are u doin some sort of tunneling ?
>
>
> On Fri, Apr 6, 2012 at 6:03 AM, Sri Ram Vemulpali <sri.ram.gmu06 at gmail.com>
> wrote:
>>
>> Whenever you manipulate the packet before PRE_ROUTING point in the
>> stack, the kernel will decide whether to route the packet to LOCAL or
>> FORWARD. If you have pointer pointing to right IP header kernel will
>> use it to route. You really do not have to write routing code.
>>
>> Also, when you say inner and outer header what do you mean by it. I
>> did not encounter this.
>>
>> Thanks,
>> Sri.
>>
>> On Thu, Apr 5, 2012 at 5:00 AM, SaNtosh kuLkarni
>> <santosh.yesoptus at gmail.com> wrote:
>> > i had a similar problem where in i was using SKB_PUSH to add extra
>> > header,,,, i used this... structure called flowi.... which can be used
>> > to
>> > define a sort of traffic class...based on some combination of fields
>> >
>> >
>> > iph->daddr =htonl(xxxxxx);
>> > {
>> > struct rtable *rt;
>> > struct flowi fl;
>> > memset(&fl, 0x0, sizeof(struct flowi));
>> > fl.fl4_dst  = htonl(xxxxxxx);
>> > fl.proto = IPPROTO_TCP;
>> > if (!ip_route_output_key(&init_net, &rt, &fl))
>> > {
>> > iph->saddr= htonl(ntohl(rt->rt_src));
>> >
>> > skb_dst_set(skb2, &rt->u.dst);
>> >
>> > }
>> >
>> > }
>> >
>> >
>> > On Mon, Apr 2, 2012 at 2:12 PM, Kesava Srinivas
>> > <vunnavafuture at gmail.com>
>> > wrote:
>> >>
>> >> HI Friends,
>> >> Looking for a Confirmation on my analysis.
>> >>
>> >> Once after Capturing the Socket Buffer in PRE_ROUTING Hook; Manipulated
>> >> the Socket Buffer by using the "skb_pull" Kernel Function. Using
>> >> skb_pull;
>> >> stripped 28 bytes (IP+UDP) which are the Part of outer UDP/IP Header.
>> >> Now;
>> >> My intention was to route the skb based on the Inner IP Header which is
>> >> sitting after stripping 28 bytes. At the END; returned NF_ACCEPT.
>> >>
>> >> Even though; skb_pull worked Fine., Kernel's Stack is still looking in
>> >> to
>> >> Outer Header only for Routing the Packet.I expected ;Kernel will look
>> >> the
>> >> Inner Header (As data Pointer was incremented by 28 bytes via skb_pull)
>> >> and
>> >> Take decision based on the Inner one. But; that didn't  happened. It
>> >> looks
>> >> to me like; we need to always use NF_STOLEN & should write our own code
>> >> to
>> >> route based on the INNER HEADER. Was my conclusion correct ??
>> >>
>> >> -Thanks in Advance,
>> >> VKS
>> >>
>> >>
>> >>
>> >> _______________________________________________
>> >> Kernelnewbies mailing list
>> >> Kernelnewbies at kernelnewbies.org
>> >> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>> >>
>> >
>> >
>> > _______________________________________________
>> > Kernelnewbies mailing list
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>> >
>>
>>
>>
>> --
>> Regards,
>> Sri.
>
>



-- 
-VKS
Masters,Computer Science&Engineering.