[ixpmanager] bird 2.0.8 malformed attributes

[Nick Hilliard (INEX)]

There's been some reports over the last couple of days about a malformed 
BGP update causing route server sessions to be reset on BGP sessions to 
BIRD2 route servers.

Fix

If any IXP is running bird 2.0.8 or older, an upgrade to something more 
recent would be advisable.

The version of BIRD on Ubuntu 24.04 is not affected by this problem, but 
older versions of Ubuntu and Debian can be upgraded to the most recent 
build using the instructions here:

https://pkg.labs.nic.cz/doc/?project=bird

Select "bird2", then the version of Ubuntu you're running, and it will 
provide the correct entry for your APT file.

Details

A prefix with BGP attribute 35 (Only to Customer (OTC) - 
https://www.iana.org/assignments/bgp-parameters#bgp-parameters-2) was 
observed on Oct 24 at around 06:00 UTC, and carried by a network with 
connections to a large number of IXPs. This update was mishandled by 
BIRD which rebuilt the attribute list and forwarded it to bgp peers with 
an incorrect path length of 1024. On more recent BGP stacks which 
implement RFC7606 error handling, the prefix was handled using 
"treat-as-withdraw". However some older stacks reverted the behaviour 
described in rfc4271, section 6.3, and reset the session with a 
malformed update error notification. I.e. the BGP session dropped.

The problem was fixed in bird shortly after 2.0.8 was released.

There's nothing about this behaviour which is specific to IXP route 
servers, however it's more noticeable because a route server typically 
has a large number of external BGP sessions.

Nick
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