Re: [PATCH 1/1] sysctl configurable icmperror sourceaddress

[Lennert Buytenhek <buytenh@xxxxxxxxxxxxxx>]

On Wed, Jun 08, 2005 at 02:49:41PM -0700, David S. Miller wrote:

> > Can this patch go in, pretty please?
> > 
> > Here's the patch again for reference:
> 
> We have a similar sysctl for ARP handling, why don't
> we make use of it?

Which one do you mean, the arp_{filter,announce,ignore} ones?  IMHO
this is a very different issue, this patch just selects which source
address we use when we reply to a packet with an ICMP.

In the case below, if all routers are linux routers, a traceroute
from source to dest will show something like this:
        1 10.0.0.254 x.xxx ms x.xxx ms x.xxx ms
        2 12.0.0.100 x.xxx ms x.xxx ms x.xxx ms
        3 12.0.0.1 x.xxx ms x.xxx ms x.xxx ms

Whereas we'd prefer seeing this, partly because it makes more sense,
partly because a very large fraction of networking hardware does it
this way too:
        1 10.0.0.254 x.xxx ms x.xxx ms x.xxx ms
        2 11.0.0.100 x.xxx ms x.xxx ms x.xxx ms   <===
        3 12.0.0.1 x.xxx ms x.xxx ms x.xxx ms

I used to work at an ISP and there are a number of practical cases
where the linux behavior is rather confusing.


cheers,
Lennert


        +------------+
        |   source   |
        |  10.0.0.1  |
        +-----+------+
              |
              V
              |
              |
        +-----+------+
        | 10.0.0.254 |
        |            |
        |   router1  +---------------------+
        |            |                     |
        | 11.0.0.254 |                     |
        +-----+------+                     |
              |                            |
              V                            ^ asymmetric route
              |                            |  back to source
              |                            |
        +-----+------+               +-----+------+
        | 11.0.0.100 |               | 13.0.0.13  |
        |            |               |            |
        |   router2  |               |   router3  |
        |            |               |            |
        | 12.0.0.100 |               | 12.0.0.101 |
        +-----+------+               +-----+------+
              |                            |
              +------------->--------------+
              V
              |
        +-----+------+
        |    dest    |
        |  12.0.0.1  |
        +------------+