netfilter query

netfilter query

[cranium2003]

hello,
        how can i use packets in userspace using
netfilter. i want some sample examples that
manipulates on packet.
        Also,which headers are added when packet
reaches to netfilter hook NF_IP_LOCAL_OUT? I found
TCP/UDP/ICMP ,IP. Is that correct?

regards,
cranium



		
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Re: netfilter query

[Henrik Nordstrom]

On Sun, 21 Nov 2004, cranium2003 wrote:

> Also,which headers are added when packet
> reaches to netfilter hook NF_IP_LOCAL_OUT? I found
> TCP/UDP/ICMP ,IP. Is that correct?

Yes.

netfilter is running at the IP layer and only reliably have access to IP 
headers and up. Lower level headers such as Ethernet MAC header is 
transport dependent and not always available, and certainly not available 
in NF_IP_LOCAL_OUT as it is not yet known the packet will be sent to an 
Ethernet.

In some netfilter hooks it is possible to rewind back to the Ethernet MAC 
header but one must be careful to verify that it really is an Ethernet 
packet one is looking at when doing this. Unfortunately there is no 
perfect solution how to detect this.. For an example of how one may try to 
look at the Ethernet MAC header see ipt_mac.c. But be warned that it is 
possible for non-Ethernet frames to pass the simple checks done there..

Regards
Henrik

RE: netfilter query

[Stuart Macdonald]

Just a parallel thought here,

A different approach is to implement the Netfilter Bridge hooks and run a
box as a bridge. This requires a kernel parameter for Bridge to be enabled
when the kernel is built and then the brctl utility to setup the bridge. In
this manner, your bridge netfilter hooks always receive packets starting at
the MAC headers. You can parse from there to derive subsequent protocols:
IP, IPX, LLC, SNAP, NETBEUI...

Stuart



-----Original Message-----
From: kernelnewbies-bounce@nl.linux.org
[mailto:kernelnewbies-bounce@nl.linux.org]On Behalf Of Henrik Nordstrom
Sent: Monday, November 22, 2004 5:03 AM
To: cranium2003
Cc: kernelnewbies@nl.linux.org; netdev@oss.sgi.com;
netfilter-devel@lists.netfilter.org; linux-kernel@vger.kernel.org
Subject: Re: netfilter query


On Sun, 21 Nov 2004, cranium2003 wrote:

> Also,which headers are added when packet
> reaches to netfilter hook NF_IP_LOCAL_OUT? I found
> TCP/UDP/ICMP ,IP. Is that correct?

Yes.

netfilter is running at the IP layer and only reliably have access to IP
headers and up. Lower level headers such as Ethernet MAC header is
transport dependent and not always available, and certainly not available
in NF_IP_LOCAL_OUT as it is not yet known the packet will be sent to an
Ethernet.

In some netfilter hooks it is possible to rewind back to the Ethernet MAC
header but one must be careful to verify that it really is an Ethernet
packet one is looking at when doing this. Unfortunately there is no
perfect solution how to detect this.. For an example of how one may try to
look at the Ethernet MAC header see ipt_mac.c. But be warned that it is
possible for non-Ethernet frames to pass the simple checks done there..

Regards
Henrik

--
Kernelnewbies: Help each other learn about the Linux kernel.
Archive:       http://mail.nl.linux.org/kernelnewbies/
FAQ:           http://kernelnewbies.org/faq/

RE: netfilter query

[cranium2003]

hello Stuart,
           Thanks for the reply. Which kernel
parameter
for Bridge to be enabled. I have RH9 with 2.4.20-8
kernel installed and i found nearly all kernel
parameters with word bridging enabled. Also i try
brctl command at console prompt but no utility is
present in my linux.
          one more thing how can i see packets to
parse them?

regards,
cranium.
--- Stuart Macdonald <stuart@ken-caryl.net> wrote:

> Just a parallel thought here,
> 
> A different approach is to implement the Netfilter
> Bridge hooks and run a
> box as a bridge. This requires a kernel parameter
> for Bridge to be enabled
> when the kernel is built and then the brctl utility
> to setup the bridge. In
> this manner, your bridge netfilter hooks always
> receive packets starting at
> the MAC headers. You can parse from there to derive
> subsequent protocols:
> IP, IPX, LLC, SNAP, NETBEUI...
> 
> Stuart
> 
> 
> 
> -----Original Message-----
> From: kernelnewbies-bounce@nl.linux.org
> [mailto:kernelnewbies-bounce@nl.linux.org]On Behalf
> Of Henrik Nordstrom
> Sent: Monday, November 22, 2004 5:03 AM
> To: cranium2003
> Cc: kernelnewbies@nl.linux.org; netdev@oss.sgi.com;
> netfilter-devel@lists.netfilter.org;
> linux-kernel@vger.kernel.org
> Subject: Re: netfilter query
> 
> 
> On Sun, 21 Nov 2004, cranium2003 wrote:
> 
> > Also,which headers are added when packet
> > reaches to netfilter hook NF_IP_LOCAL_OUT? I found
> > TCP/UDP/ICMP ,IP. Is that correct?
> 
> Yes.
> 
> netfilter is running at the IP layer and only
> reliably have access to IP
> headers and up. Lower level headers such as Ethernet
> MAC header is
> transport dependent and not always available, and
> certainly not available
> in NF_IP_LOCAL_OUT as it is not yet known the packet
> will be sent to an
> Ethernet.
> 
> In some netfilter hooks it is possible to rewind
> back to the Ethernet MAC
> header but one must be careful to verify that it
> really is an Ethernet
> packet one is looking at when doing this.
> Unfortunately there is no
> perfect solution how to detect this.. For an example
> of how one may try to
> look at the Ethernet MAC header see ipt_mac.c. But
> be warned that it is
> possible for non-Ethernet frames to pass the simple
> checks done there..
> 
> Regards
> Henrik
> 
> --
> Kernelnewbies: Help each other learn about the Linux
> kernel.
> Archive:      
> http://mail.nl.linux.org/kernelnewbies/
> FAQ:           http://kernelnewbies.org/faq/
> 
> 
> 


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