[ILUG] networking query

[Paul Jakma]

o ye of little faith...

On Wed, 21 Jun 2000, kevin lyda wrote:

> i am of the opinion that no packet can come in on the public interface
> and either get received by the private address or go out on the private
> network.  paul disagrees.
> 

[paulj at rossi paulj]$ telnet gateway-private
Trying 192.168.7.high...
Connected to gateway-private.itg.ie.
Escape character is '^]'.

login: mylogin
Password: 
Last login: Wed Jun 21 14:40:50 from xxxx
[mylogin at gateway]$

logout and look at arp cache on my laptop:

[paulj at rossi paulj]$ arp
Address			HWtype	HWaddress	    Flags Mask Iface
gateway         	ether   00:08:C7:F3:2A:47   C 	eth0
gateway-private         ether   00:08:C7:F3:2A:47   C 	eth0


hey look... the public interface on our server is claiming to be both
gateway and gateway-private!!! (if you believe ifconfig then
gateway-private should at least have a different MAC addr).

Moral of the story:

there is no connection between IP addresses and interfaces in
linux (2.2+). Absolutely none...... 

if you think there is then you have been sucked in by ifconfig. (use 'ip'
instead for a view more consistent with how linux actually works). All the
IP's that are local to a linux box are accessible from /any/ interface.
(unless you setup INPUT rules with ipchains to prune out packets according
to source/interface).

IP's are not bound to interfaces in any way....

also, in this setup, packets would go across from public to private (if
ip_forward was enabled). I can't demonstrate with telnet because the
routing means there will be no replies going back - but that doesn't
matter too much for UDP app's.

> which is it?
> 

kev, you know i'm right.. :)

> kevin
> 

--paulj