Sourcing ICMP reply to a different ip address

[haesu at towardex.com]

For those who's a little confused in my email below sent before, here is a
similar post that describes what I mean (hopefully):

http://www.monkey.org/openbsd/archive/misc/9903/msg00109.html

Thanks in advance!
-hc

On Mon, Dec 08, 2003 at 01:53:20PM -0500, haesu at towardex.com wrote:
> Hi,
> 
> Is there anyway to source the ICMP reply (i.e. ttl-exceeded for traceroute) on
> a FreeBSD box acting as router to an IP address different than the one bound on
> the interface, in which the destination route is pointed at?
> 
> For example:
> 
> Let say we have an asymmetric routing situation here...
> 
> A client host is 1.2.3.4, and the FreeBSD box has fxp0 with 2.2.2.2, and fxp1
> with 3.3.3.3,
> 
> Client runs traceroute to a host  routed by the FreeBSD router. The packet
> arrives on FreeBSD router's FXP0 interface. But the route for 1.2.3.4 (client)
> on router's routing table points out to FXP1.
> 
> Here in this case, the icmp ttl-exceeded message from the FreeBSD router will
> be sourced from 3.3.3.3, which is the main ip address of FXP1 (hence, the 
> interface where route for 1.2.3.4(the client) is bound to)
> 
> I'm looking to make it so that if a packet arrives on FXP0, I'd like the FreeBSD
> box to respond icmp ttl-exceeded OUT via FXP0, with source address of FXP0's IP.
> So in other words, I'd like to have icmp replies go out via the interface the
> packet originally hit the box, instead of via the interface that holds the
> route for the source of the packet. This type of implementation is done on 
> some vendors (including Cisco) and sometimes can be helpful troubleshooting
> asym. routing situations..
> 
> Any suggestions would be appreciated. Thanks
> `
> -- 
> James Jun (formerly Haesu)
> Network Operations
> TowardEX Technologies, Inc.
> Consulting, colocation, web hosting, network design and implementation
> http://www.towardex.com  | james at towardex.com
> Cell: (978)394-2867      | Office: (978)263-3399 Ext. 170
> Fax: (978)263-0033       | AIM: GigabitEthernet0
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-- 
James Jun (formerly Haesu)
Network Operations
TowardEX Technologies, Inc.
Consulting, colocation, web hosting, network design and implementation
http://www.towardex.com  | james at towardex.com
Cell: (978)394-2867      | Office: (978)263-3399 Ext. 170
Fax: (978)263-0033       | AIM: GigabitEthernet0
NOC: http://www.twdx.net | POC: HAESU-ARIN, HDJ1-6BONE