OpenBGPd problems relating to misuse of RESERVED bits in BGP Attribute Flags field

Thu Nov 29 08:44:32 UTC 2012

I had two downstream BGP customers experience problem with an OpenBGPd bug
tonight.  Before diving into detail, I would like to link this mailing list
thread, because this is not a new issue and a patch is available:
http://www.mail-archive.com/[email protected]/msg115071.html

For the following DFZ routes, I see wrong use of the fifth bit in the
Attribute Flags field:
          Aggregator (7), length: 8, Flags [OT+8]:  AS #68, origin
192.65.95.253
            0x0000:  0000 0044 c041 5ffd
          Updated routes:
            128.165.0.0/16
            141.111.0.0/16
            192.65.95.0/24
            192.12.184.0/24
            204.121.0.0/16

According to RFC 4271 page 17, "the low-order four bits of the Attribute
Flags octet are unused.  They MUST be zero when sent and MUST be ignored
when received."  I read "ignored" to mean, don't tear down the BGP session
and print a cryptic error that the user probably will be unable to debug.
 The OpenBGPd guys clearly agree and have supplied a patch, so affected
users should visit the above mailing list link, and install it.

Here are my notes for this RFC page and a small diagram of the packet
header, because surprisingly, there isn't one in the RFC already
http://inconcepts.biz/~jsw/img/1121129aa-rfc4271pg17scan.jpg  Sorry about
the poor quality of this, but it is past 3am here, and I know of several
operators (besides my downstream customers) who are experiencing this
problem right now.

If I were someone who is broken by this right now, I would either patch my
OpenBGPd or ask your eBGP neighbors not to send you the above five routes
(filtering it on your own OpenBGPd router probably won't help.)

Thanks, I hope this is helpful
-- 
Jeff S Wheeler <jsw at inconcepts.biz>
Sr Network Operator  /  Innovative Network Concepts