DMARC ViolationAS21299 - 46.42.196.0/24 ASN prepending 255 times

Amir Herzberg amir.lists at gmail.com
Sat Mar 26 01:19:09 UTC 2022


Hi Matthew and NANOG,

I don't want to defend prepending 255 times, and can understand filtering
of extra-prepended-announcements, but I think Matthew may not be correct
here:

> Anyone that is prepending to do traffic engineering is
> doing *differential* prepending; that is, a longer number
> of prepends along one path, with a shorter set of prepends
> along a different path.
>
> So, dropping the inbound announcement with 255 prepends
> merely means your router will look for the advertisement with
> a shorter number of prepends on it.
>

Right. But let's consider the (typical) case where someone is prepending
for traffic engineering. Now, if you're not very near to the origin of the
prepended announcement, and still received it (and not the shorter
alternative), then it is quite likely that you received it since the
alternate path failed - and the backup path was announced, instead (by
upstreams of the origin). So your router is quite likely not to receive the
shorter announcement.

After all, if your router received both short and long announcements (from
same relationship, e.g., both from providers), then your router would
probably select the shorter path anyway, without need to filter out the
long one, right?

So, filtering announcements with many prepends may cause you to lose
connectivity to these networks. Of course, you may not mind losing
connectivity to Kazakhstan :) ...

best, Amir

>
>
> --
Amir Herzberg

Comcast professor of Security Innovations, Computer Science and
Engineering, University of Connecticut
Homepage: https://sites.google.com/site/amirherzberg/home
`Applied Introduction to Cryptography' textbook and lectures:
 https://sites.google.com/site/amirherzberg/applied-crypto-textbook
<https://sites.google.com/site/amirherzberg/applied-crypto-textbook>




On Fri, Mar 25, 2022 at 8:19 PM Matthew Petach <mpetach at netflight.com>
wrote:

>
>
> On Fri, Mar 25, 2022 at 2:59 PM Adam Thompson <athompson at merlin.mb.ca>
> wrote:
>
>> Tom, how exactly does someone “ride the 0/0” train in the DFZ?
>>
>
> It's not so much "ride the 0/0 train" as much as it is
> "treat excessive prepends as network-unreachable"
>
> Think of prepends beyond say 10 prepends as a way
> to signal "infinite" distance--essentially, "unreachable"
> for that prefix along that path.
>
> Anyone that is prepending to do traffic engineering is
> doing *differential* prepending; that is, a longer number
> of prepends along one path, with a shorter set of prepends
> along a different path.
>
> So, dropping the inbound announcement with 255 prepends
> merely means your router will look for the advertisement with
> a shorter number of prepends on it.
>
> If you're only announcing one path for your prefix, and it is
> prepended 255 times, you're fundamentally not understanding
> how BGP works, and the only way to get a clue-by-four might
> be to discover you've made your prefix invisible to a significant
> portion of the internet.
>
>
>>
>>
>> I’m connected to both commercial internet and NREN, and
>> unfortunately-long paths are not uncommon in this scenario, in order to do
>> traffic steering.  If there’s another solution that affects global
>> *inbound* traffic distributions, I’d love to hear about it (and so would
>> a lot of my peers in edu).
>>
>>
>>
>> If there were a usable way to “dump” the excessively-long path only as
>> long as a better path was already known by at least one edge router, that
>> might be workable, but you’d have to keep track of it somewhere to
>> reinstall it if the primary route went away… at which point you may as well
>> have not dropped it in the first place.
>>
>>
> You dump the excessively-long path based on the assumption that
> the only reason for a long set of prepends out one path is to shift
> traffic
> away from that path to one that you're advertising out with a *shorter*
> set of prepends.
>
> The router doesn't need to 'look' for or 'keep track' of the different
> path; the human makes the decision that any sane BGP speaker
> would only prepend 255 times on a path if there was a shorter
> as-path advertisement they wanted people to use instead.
>
> So, drop the excessively long prepended path, and make use
> of the 'should be in the table somewhere' advertisement of the
> prefix with fewer prepends.
>
> Easy-peasy.
>
>
>>
>>
>> -Adam
>>
>>
>>
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