Re: [LARTC] netmask 255.255.255.255 vs ip route add via ... (bug?)

[Alexandru Dragoi <alex@zoomnet.ro>]

Martin A. Brown wrote:
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> Greetings Andrew McGill,
>
>  : I want to use the netmask 255.255.255.255 to insulate (not quite 
>  : isolate) machines on a shared subnet from each other.  This works 
>  : just fine on win XP, but Linux iproute will not acccept the 
>  : gateway address in one step -- neither on the command line nor 
>  : via DHCP:
>
> Try using the onlink nexthop flag for your route:
>
>   # ip route add onlink default via 192.168.1.17
>   
shouldn't be

# ip route add onlink default via 192.168.1.17 dev $DEV
?

Because from the point of view of the kernel, 192.168.1.17 is unreachable, it must know the interface.


> This marks the route for entry even though the local routing table 
> may not have a route to the nexthop destination.  In your case, this 
> is a valid parameter, and should prevent the need for you to add the 
> host route only to remove it.
>
>  : So why did we need that host route?
>
> You need the host route to the destination as a simple sanity check.  
> - From the perspective of the kernel, there's no route to 192.168.1.17 
> if the IP bound to your interface is a /32.  When you add the route, 
> the sanity check succeeds.
>
> Essentially, you are suppressing this sanity check by using the 
> onlink parameter, which says "Yes, I know there's no route to IP 
> 192.168.1.17 out this interface, but I know the IP is there on this 
> link layer anyway, so set the route anyway and stop griping."*
>
> Good luck,
>
> - -Martin
>
>  * RTNETLINK answers: Network is unreachable
>
> - -- 
> Martin A. Brown
> http://linux-ip.net/
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