Mangling problem

Mangling problem

[Regina Reine]

Hi,
I am doing an experiment with iptables. I use a hub to connect two
computers. Computer A captures the video packet (or any packets) and
computer B runs iptables and my code in Red Hat 8. Basically, with hub,
computer B will be able to see the packets coming to computer A. I would
like computer B to preroute this packet and put these packets in
userspace where I run my code. 

I uses :

** Iptables -t mangle -I PREROUTING -p udp -I eth0 -j QUEUE

However, seems like I only be able to capture broadcast packets. (But
TCPDUMP could see the udp packets). I have checked my code (just a
simple code that scan what kind of packets and it worked with udp
generator such as iperf or hping). 
I have tried a few other similar ways but still only could capture
broadcast packets. Does anybody spots my mistake or could give me advice
for better solution? Thank you

Best regards,
Regina Reine

RE: Mangling problem

[Chris Hoeschen]

I maybe wrong but I don't believe netfilter will process any packets not
intended for that box.  TCPDump will see it as in a hubbed environment
all nodes receive all packets but only those that are destined for that
box will netfilter process.

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-----Original Message-----
From: netfilter-admin@lists.netfilter.org
[mailto:netfilter-admin@lists.netfilter.org] On Behalf Of Regina Reine
Sent: Wednesday, May 21, 2003 6:50 AM
To: netfilter@lists.netfilter.org
Subject: Mangling problem



Hi,
I am doing an experiment with iptables. I use a hub to connect two
computers. Computer A captures the video packet (or any packets) and
computer B runs iptables and my code in Red Hat 8. Basically, with hub,
computer B will be able to see the packets coming to computer A. I would
like computer B to preroute this packet and put these packets in
userspace where I run my code. 

I uses :

** Iptables -t mangle -I PREROUTING -p udp -I eth0 -j QUEUE

However, seems like I only be able to capture broadcast packets. (But
TCPDUMP could see the udp packets). I have checked my code (just a
simple code that scan what kind of packets and it worked with udp
generator such as iperf or hping). 
I have tried a few other similar ways but still only could capture
broadcast packets. Does anybody spots my mistake or could give me advice
for better solution? Thank you

Best regards,
Regina Reine

RE: Mangling problem

[Scott MacKay]

My believe too.  iptables is not a promiscious app; it
handles packets which come in/go out of the device. 
You see the broadcast because it is 'intended' for
Computer B (and any other computer).  
The only way B would get it is if Computer A
transmitted the packets to computer B (either directly
or because B was a router).

One question:  How does computer A get the video? 
internal capture or is the data being sent to it? 

-Scott

--- Chris Hoeschen <chris.hoeschen@primevest.com>
wrote:
> I maybe wrong but I don't believe netfilter will
> process any packets not
> intended for that box.  TCPDump will see it as in a
> hubbed environment
> all nodes receive all packets but only those that
> are destined for that
> box will netfilter process.
> 


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RE: Mangling problem

[Regina Reine]

Chris,
I think you are right. Thanks

Regina

Re: Mangling problem

[Arnt Karlsen]

On Wed, 21 May 2003 12:49:54 +0100, 
"Regina Reine" <regina@erg.abdn.ac.uk> wrote in message 
<000001c31f8f$1866f1d0$88cc858b@TOSHIBAFIRE>:

> 
> Hi,
> I am doing an experiment with iptables. I use a hub to connect two

..hub?  And being visible wont hurt your experiment?

> computers. Computer A captures the video packet (or any packets) and
> computer B runs iptables and my code in Red Hat 8. Basically, with
> hub, computer B will be able to see the packets coming to computer A.
> I would like computer B to preroute this packet and put these packets
> in userspace where I run my code. 
> 
> I uses :
> 
> ** Iptables -t mangle -I PREROUTING -p udp -I eth0 -j QUEUE
> 
> However, seems like I only be able to capture broadcast packets. (But
> TCPDUMP could see the udp packets). I have checked my code (just a
> simple code that scan what kind of packets and it worked with udp
> generator such as iperf or hping). 
> I have tried a few other similar ways but still only could capture
> broadcast packets. Does anybody spots my mistake or could give me
> advice for better solution? Thank you
> 
> Best regards,
> Regina Reine
> 

..you want tcpdump running in promiscous mode, 'info tcpdump' 
for the gory details, it is also possible to pipe it.

..you could also set up an ip-less sniff bridge, it works nicely 
for bandwidth throttling too.  ;-)

-- 
..med vennlig hilsen = with Kind Regards from Arnt... ;-)
...with a number of polar bear hunters in his ancestry...
  Scenarios always come in sets of three: 
  best case, worst case, and just in case.