North American Network Operators Group

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Re: That pesky AS path corruption bug...

  • From: Peter T. Whiting
  • Date: Tue May 23 13:40:12 2000

looking for some more details about what the malformed aspath looks
like.  I took a router in my lab and started sending it some bogus 
as_paths.  It seemed to accept everything I would send.  Here is one 
aspath which doesn't include the remote-as and loops all over the 
place.  It was happily accepted.

BGP routing table entry for, version 414648
Paths: (1 available, best #1)
  Advertised to non peer-group peers: 
  1 2 3 4 3 2 1 2 3 4 3 2 from (
      Origin EGP, localpref 100, valid, external, best
      Dampinfo: penalty 980, flapped 2 times in 00:00:57


On Tue, May 23, 2000 at 12:40:00PM -0400, Blaine Christian wrote:
> Hello all,
> After observing a recent issue regarding a router that sent corrupted AS
> path (all names are witheld to protect the guilty).  I took a look at the
> path information that was being received and have a possible solution.
> Since the corrupted AS-path does not include the AS that the route is
> coming from (at least in the corruption that I saw) it seems to me that a
> simple solution for all is to filter on AS i.e. only allow routes that
> have the AS of your EBGP neighbor prepended to them.  I realize this does
> not cover all cases of wacky AS corruption problems but it may fix some of
> them.  I would suggest that those of you running mixed vendor EBGP (again
> names witheld) should implement a version of this strategy for your own
> self protection.  It can certainly be implemented as part of an overall
> customer access functionality.  This may be obvious to some of you but I
> do not believe that everyone is at this level yet.
> Of course the tirade part of this email is for all vendors involved in
> this travesty.  If you do not understand or dislike a route that you have
> received don't just CRASH.  Anyone ever thought of checking the route and
> throwing it out with an error message if you don't like it?  I, of course,
> have heard and seen that several vendors have fixed this in the more
> recent releases.  This type of bug is something that everyone who writes
> software has to deal with.  If you raise an exception for bad input it is
> bad form to crash or reset your application.
> BTW, I am sure all have heard this argument before.  I just wanted to get
> this topic renewed.
> Regards,
> Blaine