Re: Resend TCP segment modified to the sender

["Jee J.Z." <jz105@york.ac.uk>]

Hi Amit,

A few questions inline...

> hi,
>
>    I am trying to do something similar, if anyone wants i can send the
code
> snippets. I start a telnet session between two hosts and then capture a
> packet in between, and try to send back an RST segment, I do checksum
> calculation and all my self. (both ip and tcp)
> 1. NF_IP_LOCAL_OUT queues the packet to user space
> 2. user space reads it using ipq_read, modifies packet, then sets a
verdict
> of NF_ACCEPT
> 3. This packet is now caught at NF_IP_POST_ROUTING, I call netif_rx for
> that skb from here and return NF_STOLEN from hook call back function of
> NF_IP_POST_ROUTING.

Hm, I don't know much about the kernel routine, but just wonder whether
calling netif_rx means you grab the packet directly from POST ROUTING to the
first step of receiving a packet from the interface? And NF_STOLEN means you
never want the packet going to the interface?

> 4. what happens after that is not clear to me, surely the RST i sent
doesnt
> reach the tcp connection it was intended for and hence the packet is
> dropped  somewhere by the kernel. when and where is a problem for me, any
> idea how do I fgiure that out.

So your RST is supposed to go to the local machine itself?

> Also Sven, The problem I talked about yesterday, POST_ROUTING not getting
> the packet after LOCAL_OUT queued it to the user space. I overcame that
> problem, but only after I started setting the mark to a particular value
in
> the NF_IP_LOCAL_OUT hooks function itself, before queuing it to the user
> space. Earlier I was setting ipq_packet_msg->mark = THAT VALUE inside the
> user level program.

Then it should be my problem of using libnet...

> Hence, these are my doubts :
>
>    ipq_packet_msg-> mark i.e all the fields in ipq_packet msg can we modiy
>    them in user space, and inject the "modified" packet back into the
>    kernel ? I dont think this is possible with ipq_packet_msg->mark field,
>    that is just for reading.

Agree.

>    Also, if i want to modify the packet before
>    setting a verdict of NF_ACCEPT, how do I do it, the ip header starts
>    from (unsigned char *)(packet+1), am I right ?

if
ipq_packet_msg *m = ipq_get_packet(buf)
packet = (unsigned char *)m + sizeof(*m)
then
ip header starts at *packet, not *(packet+1).

>    Once i modify contents
>    here, say interchange the src , dest ip, and then set verdict to
>    NF_ACCEPT, the packet that is actually injected has the changed values.

Yes, just figure out the modified packet length and ipq_set_verdict(handle,
m->packet_id, packet_len, modified_packet).

Jee

> regards
> Amit
>
>
>
>
> "Kotatsu" <naughtydog@virgilio.it>@lists.netfilter.org on 06/02/2004
> 12:09:44 AM
>
> Sent by:    netfilter-admin@lists.netfilter.org
>
>
> To:    <netfilter@lists.netfilter.org>
> cc:
>
> Subject:    Resend TCP segment modified to the sender
>
>
>
> Hi guys,
> I have a problem. I have a  client (192.168.9.2) that send TCP segment
> to a server (192.168.11.2).  Between this PC there is a Linux router that
> captures all the segment  and do forwarding. I want that it takes a client
> packet (the 10th TCP data  packet sended, for example), modify it as i
> want, and then resend it to the  client with ipq_set_verdict.
> Is this operation possible? Or netfilter  can only send the modified
packet
> at the server? If it's possible, which  field must i modify to do this
> (i've tried to modify some flag but it doesn't  work)?
>
> Thanks for your help
> Best regards
>
>
>
>
>
>