New router feature - icmp error source-interface [was: icmp rpf]

Patrick W. Gilmore patrick at
Mon Sep 25 23:58:25 UTC 2006

On Sep 25, 2006, at 5:40 PM, Richard A Steenbergen wrote:
> On Mon, Sep 25, 2006 at 09:22:34AM -0400, Patrick W. Gilmore wrote:
>> On Sep 25, 2006, at 9:06 AM, Ian Mason wrote:
>>> ICMP packets will, by design, originate from the incoming interface
>>> used by the packet that triggers the ICMP packet. Thus giving an
>>> interface an address is implicitly giving that interface the
>>> ability to source packets with that address to potential anywhere
>>> in the Internet. If you don't legitimately announce address space
>>> then sourcing packets with addresses in that space is (one
>>> definition of) spoofing.
>> Who thinks it would be a "good idea" to have a knob such that ICMP
>> error messages are always source from a certain IP address on a  
>> router?
> You know I was just having this discussion with someone else a  
> couple days
> ago. It turns out, much to my surprise, that the RFC actually calls  
> for
> the ICMP error-message packet (as you said, the things that aren't  
> ping
> etc which require a specific source-address) to originate from the
> OUTGOING interface used to return the ICMP message to the original  
> sender.
> After much googling, I can't find any document where this has ever  
> been
> officially updated either. The defacto industry standard on the  
> other hand
> has been to use the primary address of the inbound interface, which  
> serves
> exactly one function: it makes traceroute work.

I have not read the RFC in full, but after chatting with Daniel  
offline (see, some people actually do talk without posting! :), I  
believe this only applies to packets addressed to the router.

Since packets going -through- the router have absolutely no guarantee  
what source will be used coming back, I don't seen an issue here.   
Just change the idea such that it only is used for error messages to  
packets where the dest addy is not an interface on the router.

Also, this makes traceroute -easier- to use.  Suddenly all interfaces  
on the same router have the same IP address, thereby making it easy  
to tell if two traceroutes intersect, even if they use different  

Oh, and who said RFCs can't be updated? :-)

>> (Unless, of course, I get 726384 "you are off-topic" replies, in
>> which case I withdraw the suggestion.)
> Please stop talking about networking on NANOG, you're confusing  
> people. :)

I knew someone would flame me for it. :)


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