Re: [LARTC] How to bypass the local routing table, to simulate a

[Darryl Miles <lartc-list@the-morg.org>]

Antony,

I have not completely read / understood your situation, so forgive me 
for jumping in.

I don't know much about tap0 device so maybe you can eligthen me.  But 
from what I can see you can setup a NETWORK on a tapX interface and the 
kernel will route all traffic to the interface as-if its a hardware 
device, except its not, its a software device that passes packets 
between userspace and the kernel interface through the use of a file 
descriptor / socket, created by opening a device like file.

I use the word NETWORK since it behaves like a normal interface, that is 
you give it an IP address that represents the kernels interface facing 
the userspace software/virtual network.  Therefore you should have setup 
the interface with a netmask to allow other IPs to exist on that 
network, it is those IPs that your application program should assume and 
use.

To do this means the tap0 interface is configured along the lines of 
192.168.1.8/30, using a netmask of 255.255.255.252, using 192.168.1.9 as 
the kernel interface IP, 192.168.1.10 as your applications IP.   Then 
run your "ping -I tap0" again.


Now if on the other hand you are wanting to sniff all traffic to a 
particular IP address from your ethernet segment, but not have the 
kernel stack treat the packets as local, then down all the tapX 
interfaces, then checkout AF_RAW, NETLINK, whatever the current scheme 
is for injecting / sniffing packets and setup a proxy ARP addresses of 
the IPs you want to assume on the LAN.  However if the box this is all 
hosted on has a lot of other traffic destined for the kernel stack you 
will have to receive, filter, discard all those packets through into 
userspace.  Maybe socket filters can help in the kernel, but the point 
is the tapX interfaces when used as a NETWORK would be a better solution.

Darryl



Antony Lesuisse wrote:

>Imagine, on a linux box, having 3 different network interfaces on the
>same switch.  It is for simulating a network, using virtual tun/tap
>interface:
>
>
>----------------+-----+------------------------
>Hub    simulator|Linux| Userspaces apps
>                |     |
>             ?--|-----|--- tap0 192.168.1.1
>             ?  |     |
>/dev/net/tun ?--|-----|--- tap1 192.168.1.2
>             ?  |     |
>             ?--|-----|--- tap2 192.168.1.3
>                |     |
>----------------+-----+-----------------------
>


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