Incorrect setting of BGP Path Attribute Extended Length flag breaks MRT output

[Ondrej Zajicek]

On Sat, Mar 27, 2021 at 03:20:05PM +0000, Simon Marsh wrote:
> Hi,
> 
> i run bird as a route collector in DN42 and so get a variety of different
> BGP daemons peering with and passing routes to the collector.
> We recently ran in to an issue where MRT dumps from bird were not readable
> by any of the common MRT parsers and, on investigation, found a bug in how
> bird handles the BGP Path Attribute Extended Length flag.
> 
> The issue occurred because peers using the FRR daemon set the Extended
> Length flag on the AS PATH attribute by default, regardless of the actual
> data length. When bird re-encodes the AS Path (for example as part of the
> MRT dump) it correctly re-calculates whether to use an 8 bit or 16 bit
> length field for the attribute, but simply copies the flags and does not
> reset the Extended Length flag in the 8 bit case. This results in an AS
> PATH attribute being output that has the Extended Length flag set but
> with the actual length field being only 8 bits. Of course, any
> parsers trying read this get very confused.

Hi

You are right. Will merge that. I wondered why this issue did not
manifest during regular route propagation and that is because attributes
are normalized during route export to BGP (but not during MRT table dump).

-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
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"To err is human -- to blame it on a computer is even more so."