Re: [PATCH] route: add more relaxed option for secure_redirects

[David Miller <davem@davemloft.net>]

From: Flavio Leitner <fbl@redhat.com>
Date: Wed, 16 Nov 2011 23:40:42 -0200

> To make sure we are in the same page, this simple setup reproduces
> the issue.
> 
> IP: 10.0.0.1
> gw: 10.0.0.100
> +--------+          +-----+ primary: 10.0.0.2
> | client |----+-----| GW1 |   alias: 10.0.0.100
> +--------+    |     +-----+      gw: 10.0.0.254
>            +--+--+
>            | GW2 |---> internet
>            +-----+
>          10.0.0.254
> 
> 1. Client sends TCP SYN to an internet host using
>    GW1 alias address as default gw address
> 
> 2. Then GW1 sends the ICMP redirect back to client
>    using the primary address as source address.
> 
> 3. GW1 forwards the original packet to GW2
> 
> 4. client ignores the ICMP redirect because
>    client.gw != gw1.primary.

GW1 must respond using a source address matching 'alias', ie.
10.0.0.100 and I would accept a mechinsm to make sure that happens,
if not by default then via a sysctl or similar control.