Re: Loopback security...

[Pascal Hambourg <pascal.mail@plouf.fr.eu.org>]

Hello,

Grant Taylor a écrit :
> 
> No, you mis-understood me.  What I meant by "Linux considers it secure" 
> is that (by default) it will not let any traffic in to our out of the 
> loopback interface from / to a different interface.

There is no such traffic forwarded between the loopback interface and 
another interface, because it just makes no sense. The loopback is 
designed for local host communications : all that is sent through it is 
received back by the host, and all that is received through it was sent 
by the host.

> +---+                  +---+
> | A +-- - - -  - - - --+ B |
> +---+ .1 (10.0.0) .254 +---+
> 
> Suppose I bind 192.0.2.1 to A's loop back interface and add a route to 
> 192.0.2/24 to B via 10.0.0.1.  If I try to ping 192.0.2.1 from B, the 
> traffic will leave B and go down the wire just like it should.  However 
> my experience shows that A will not forward the traffic in to the 
> loopback interface and destination IP.

Of course not. Why would it ? The destination is local (see 'ip route 
show table local'), and is treated just as any other local destination 
like 10.0.0.1. Traffic is forwarded only when the destination is remote.

> Said another way, Linux will not allow foreign traffic (non localhost) 
> on the loopback interface for security reasons.  I believe this to be a 
> design decision based on security.

I believe it is rather based on common sense.