Re: tcp checksum

Re,

On Thu, May 27, 2004 at 06:19:10PM +0100, Jee J.Z. wrote:
> 
> Thank you for your reply. Yes, I didn't call enough functions before
> libnet_write(). I used to use only libnet_build_tcp() and then
> libnet_build_ipv4(), however, I missed libnet_build_tcp_options(), because
> the packets I captured and wish to reinject include 8-byte tcp header
> option. And I figured out the proper sequence of using these functions are:
> libnet_build_tcp_options()->libnet_build_tcp()->libnet_build_ipv4()->libnet_
> write(); otherwise things would happen weird.

Seems ok :)

> On the other hand, are there any relatively complete tutorials on the newest
> version libnet? I never found one, and many instructions on web are still
> for old versions and therefore are somewhat misleading. I never came across
> libnet_build_tcp_options() until looked into the source codes at
> libnet_build_tcp.c, and I never knew the sequence of using these "build"
> functions until I tried it, which is clearly hard for a new user to grasp
> libnet. Anybody has the same problem?

<adv>
http://www.security-labs.org/index.php3?page=libnet
</adv>

The build function is very logical as it is excactly the same as the
one used by your system: you start from the higher layer (eg dns), and
go down the the lower ones (eth for instance).

> Amit, what do you think it happens when a packet with a dst address of
> another machine is injected into the kernel? Will it hit the POST routing
> hook? I am asking this because my SNAT on POSTROUTING seems invalid to my
> injected packets. What do you think?

Look at a thread called "icmp redirect at layer 3" started by Alberto
Ornaghi: we explore the travel of a packet through the Linux kernel.
However, we still havent found the solution to Alberto's problem.

BTW, do I have a problem with my browser or hav the archives on
securityfocus for the latest weeks disappeared ?

	Fred