Sourcing ICMP reply to a different ip address

[Andre Oppermann]

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..

Yes, this can be done.  Nice feature for debugging as you say.  I've got
a couple of other things in the priority queue first.  It'll be after
christmas/new-year until I can do it.

-- 
Andre