[Xorp-users] XORP + User Click

[Pavlin Radoslavov]

> On Friday 14 January 2005 22:02, Pavlin Radoslavov wrote:
> ...
> > Myself I am not familiar enough with Click configurations to say
> > whether it is possible to create user-level Click configuration that
> > can pick-up the packets originated from the local host so they can
> > be forwarded by user-level Click.
> 
> 
> KernelTun() element could help in such scenarios, and should work for 
> both receiving and sending packets from Click to the OS.  Here's an 
> example config (pretty much useless, though):
> 
> rt :: LinearIPLookup(
>                 192.168.200.200/30 2,
>                 161.53.0.0/16 0,
>                 10.0.0.0/8 1
>                 );
> 
> kt :: KernelTun(10.1.2.3/24);
> 
> kt -> rt;
> 
> rt[0] -> Print("0") -> Discard;
> rt[1] -> Print("1") -> Discard;
> rt[2] -> Print("2") -> kt;
> 
> 
> At least on FreeBSD, this will automatically instantiate a "tun" network 
> interface in the kernel, and any packets sent down this interface will 
> then show up in Click.

Thanks, that is useful to know it.
Though, my question was slightly different.

If the user uses the ping command on that host (as Julian did),
then the ICMP packets originated by the ping command will be
sent-down the IP stack of the OS, will be looked-up against the
kernel forwarding table (and eventually throwed away if there is no
matching forwarding entry).

With your example above I understand that if somehow the user
explicitly specifies that the outgoing ICMP ping packets are sent on
the "tun" interface, then the outgoing ICMP ping packets will be
forwarded properly by user-level Click.

However, it doesn't seem that you can tell ping (at least the
FreeBSD ping) to send the packets on a specific interface.
Hence, even with the help of KernelTun, it looks like it is not
possible to intercept all packets sent down to the IP stack (unless
they are explicitly sent via the tun interface).
If someone wants to do that, I believe the answer would be
"use kernel-level Click".

Thanks,
Pavlin