Network Traffic Generation

[Shachar Shemesh]

Daniel Feiglin wrote:

> For what it's worth, they can both be pinged
> from the host and the ifconfig output looks fine.
>   
I am sorry. "For what it's worth" is exactly nothing. The setup you are 
trying to achieve is not possible as is.

The problem is convincing the TCP/IP stack to route packets to an IP it 
knows to be local through a physical network card. Truth be told, I'm 
faced with a similar problem for a project I'm working on, and don't 
have a great solution either.

Do the following experiment. Connect the 100 card to a network, and from 
another computer ping the 101 address while a sniffer is running. You 
will see an ARP reply going out from the 100 card, carrying the 100 MAC 
address, and the ping will succeed despite the 101 card being 
disconnected. Why? Because the computer says "I know how to reach 101 - 
it's my IP, so if the ARP reached the 100 card, it's probably a good MAC 
address to respond with".


> Later, the crossed cable will be replaced by a radio link (which relates
> to the purpose of all this).
>
> To carry out the packet generation, I downloaded and built the iperf
> program (http://sourceforge.net/forum/forum.php?forum_id=808751).
>   
 From the quick look I gave it, it seems that iperf does not force the 
sending of the packets from a network interface (it can bind to address, 
but that still goes through the machine's TCP/IP stack and routing 
tables, so no solutions there).

What you need is to partition the TCP/IP stack for the machine. You need 
to have the 100 and 101 addresses belong to separate routing tables. The 
most immediate solution is to run a virtual machine, and give it 
exclusive access to one of the NICs. This way you can configure, for 
example, the 100 address in the real machine and the 101 address in the 
virtual one.

There is also a solution I've heard of once, but never had chance to 
either try it or even look at it. Basically, it does this partitioning 
inside one running kernel. It came as a patch to the Linux kernel. I 
don't recall how its called or what are the details, so it may even not 
be a solution at all.
> Has anyone on the list done this (using iperf or something similar)
> using a single box with two NICs?
>
>   
Never done it, but maybe a packet based traffic generator, instead of a 
traffic based one, will be able to be told "issue the packets through 
this NIC". The problem is, of course, that I don't think a packet 
generator checks what you are trying to check - TCP throughput. Either 
way, give tcpreplay a go and see whether it is capable of testing what 
you want tested (I doubt it).

Shachar

-- 
Shachar Shemesh
Lingnu Open Source Consulting Ltd.
http://www.lingnu.com

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