Re: [RFC] ipv4: add link_filter sysctl

[David Miller <davem@davemloft.net>]

From: Stephen Hemminger <shemminger@vyatta.com>
Date: Fri, 13 Mar 2009 16:12:53 -0700

> Add a new parameter that controls how kernel responds to packets
> when interface is down. This is done to solve the problem of:
> 
> Assume topology of:
>       A <----------->       Router             X--- down link
>  10.1.1.2/24       10.1.1.1/24    10.2.1.1/24 
>                      eth0          eth1
> 
> If A pings 10.2.1.1 then with normal Linux semantics Router would
> respond even if eth1 link on 10.2.1.1 was down. This causes some network
> management tools (that work with other router OS's) to falsely
> report that link is okay.
> 
> The problem is that a Linux router does not respond the way
> other systems do. This is the router equivalent of "Strong ES"
> model, it is not the same as "Strong ES" as defined in Host
> Requirements.
> 
> The new parameter adds an additional check on slow input packet
> path, and causes route cache flush if enabled and carrier is
> lost.
> 
> Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>

There is nothing "router" about this situation.

When 10.2.1.1 is being pinged, it is in the role of an end-system in
that transaction.

The "router" is reachable by "A", and as a consequence so is that IP
address 10.2.1.1, and therefore the ping should succeed.