Is AS information useful for security?

Patrick Sumby patrick.sumby at
Fri Dec 16 17:40:32 UTC 2011

On 15/12/2011 16:28, Drew Weaver wrote:
> -----Original Message-----
> From: Justin M. Streiner [mailto:streiner at]
> Sent: Thursday, December 15, 2011 9:45 AM
> To: nanog at
> 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?

> Thanks,
> -Drew

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