General question on rfc1918

Robert Bonomi bonomi at
Tue Nov 13 15:35:34 UTC 2007

> From owner-nanog at  Tue Nov 13 09:12:04 2007
> Cc: "nanog at" <nanog at>
> From: Joe Abley <jabley at>
> To: Drew Weaver <drew.weaver at>
> Subject: Re: General question on rfc1918
> Date: Tue, 13 Nov 2007 10:10:26 -0500
> On 13-Nov-2007, at 10:08, Drew Weaver wrote:
> >        Hi there, I just had a real quick question. I hope this is  
> > found to be on topic.
> >
> > Is it to be expected to see rfc1918 src'd packets coming from  
> > transit carriers?
> You should not send packets with RFC1918 source or destination  
> addresses to the Internet. Everybody should follow this advice. If  
> everybody did follow that advice, you wouldn't see the packets you are  
> seeing.

Really?  What do you do if a 'network internal' device -- a legitimate
use of RFC1918 addresses -- discovers 'host/network unreachable' for an 
external-origin packet transitinng that device?   <evil grin>

Your comment _is_ "generally correct", but there are some significant
'corner cases' that do complicate life.

Packets that could conceivably generate a reply/response and have an
RFC 1918 address (source -or- dest) should be ingress *and* egress
filtered -- unless there is specific agreement with the adjacent network
with regard to coordinated use of specific portions of that space.

Packets which are strictly error/status reporting -- e.g. IMP 'unreachable',
'ttl exceeded', 'redirect', etc. -- should *NOT* be filtered at network
boundaries  _solely_ because of an RFC1918 source address.

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