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Re: Is AS information useful for security?
From: Patrick Sumby <patrick.sumby () sohonet co uk>
Date: Fri, 16 Dec 2011 17:40:32 +0000

On 15/12/2011 16:28, Drew Weaver wrote:

-----Original Message-----
From: Justin M. Streiner [mailto:streiner () cluebyfour org]
Sent: Thursday, December 15, 2011 9:45 AM
To: nanog () nanog org
Subject: Re: Is AS information useful for security?

origin-AS could be another story.  If you know of an AS that is being used by the bad guys for bad purposes, you can write 
a routing policy to dump all traffic to/from that AS into the bit bucket or take some other action that could be dictated 
by your security policy.  In that case, a routing policy could be>considered an extension of a security policy.

I could be wrong here but I believe origin-AS uses a lookup from the routing table to figure out what the originAS for 
the source IP should be (and not what it explicitly IS) which means the information is unreliable.

For example if someone is sending spoofed packets towards you the origin AS will always show up as the originator of 
the real route instead of the origin AS of the actual traffic.

This is why it would be useful to have the originAS (from the actual origin) in the packet header.

How would you determine and enforce this?

Ok so a packet leaves my network that I know originated from my network based on some factor (IGP route existing or matched prefix list) and the origin AS is put into a new field in the packet header...

Whats to stop the spoofer putting that origin AS into their spoofed packet headers?

This means that another level of checking then needs to be put into inter AS BGP sessions to make sure that all traffic passing across the link would need to be checked to make sure origin ASs are matched.

Couldn't most of the same protection be solved by more people running BCP38 and RPKI?


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