[c-nsp] Strict multihoming supported on a Cisco?

[Stephen J. Wilcox]

Hi Marcel,
 this looks normal behaviour to me, what are you trying to stop it from doing?

Steve

On Wed, 28 Jul 2004, Marcel Lammerse wrote:

> an incoming ip packet, addressed to a Cisco router, will be forwarded 
> to the destination address even if that address is not the ip address 
> of the directly connected interface:
> 
> spectrum2#sh ip int brief
> Interface              IP-Address      OK? Method Status Protocol
> Ethernet0              192.168.2.1     YES NVRAM  up                    
> up
> Serial0                unassigned      YES NVRAM  up                    
> up
> Serial0.1              172.16.1.1      YES NVRAM  up                    
> up
> Serial0.100            212.189.28.2    YES NVRAM  up                    
> up
> Serial1                unassigned      YES NVRAM  administratively down 
> down
> spectrum2#
> 
> Testmachine has an ip of 192.168.2.2 and has its default gateway 
> pointing to 192.168.2.1
> 
> [test at testmachine]$ ping 212.189.28.2
> PING 212.189.28.2 (212.189.28.2) from 192.168.2.2 : 56(84) bytes of 
> data.
> 64 bytes from 212.189.28.2: icmp_seq=1 ttl=255 time=6.20 ms
> 64 bytes from 212.189.28.2: icmp_seq=2 ttl=255 time=2.08 ms
> 64 bytes from 212.189.28.2: icmp_seq=3 ttl=255 time=2.09 ms
> 
> I've heard some security concerns about this. Is there a way of 
> enforcing what is known as the Strong End-System Model (RFC1122) or 
> strict multihoming behavior on a Cisco router? Or would that break 
> routing functionality (and thus would explain why I haven't seen it 
> anywhere in the manuals)?
> 
>  From the rfc:
> 
> There are two key requirement issues related to multihoming:
> 
>              (A)  A host MAY silently discard an incoming datagram whose
>                   destination address does not correspond to the physical
>                   interface through which it is received.
> 
>              (B)  A host MAY restrict itself to sending (non-source-
>                   routed) IP datagrams only through the physical
>                   interface that corresponds to the IP source address of
>                   the datagrams.
> 
> 
>              DISCUSSION:
>                   Internet host implementors have used two different
>                   conceptual models for multihoming, briefly summarized
>                   in the following discussion.  This document takes no
>                   stand on which model is preferred; each seems to have a
>                   place.  This ambivalence is reflected in the issues (A)
>                   and (B) being optional.
> 
>                   o    Strong ES Model
> 
>                        The Strong ES (End System, i.e., host) model
>                        emphasizes the host/gateway (ES/IS) distinction,
>                        and would therefore substitute MUST for MAY in
>                        issues (A) and (B) above.  It tends to model a
>                        multihomed host as a set of logical hosts within
>                        the same physical host.
> 
> 			 o    Weak ES Model
> 
>                        This view de-emphasizes the ES/IS distinction, and
>                        would therefore substitute MUST NOT for MAY in
>                        issues (A) and (B).  This model may be the more
>                        natural one for hosts that wiretap gateway routing
>                        protocols, and is necessary for hosts that have
>                        embedded gateway functionality.
> 
> Regards,
> 
> Marcel
>