Possible DNS cache poisoning attack

[Rob Tanner]

Or, at least that's what it looks like.
Last nigh (Oct 28) we were barraged by thousands of emails with a return 
path of facebookmail.com.  Our MTA checks the return path of each 
incoming message so as to reject anything that can't be replied to.  
That, of course, requires a DNS lookup but every attempt to lookup 
facebookmail.com timed out and when I flushed the cache, it would 
resolve for a short while and then hang again until a again flushed my 
cache.  This effectively brought both of my email edge servers to their 
knees as all the SMTP connections were tied up while the server was 
waiting on DNS.

I upgraded back in July when the major security bug was discovered and 
my name servers all run BIND 9.5.0-P1.  I know there were a couple of 
Windows specific updates since then which I ignored because I'm running 
on Linux.  Is that version otherwise at risk and do I need to update for 
security reasons?

Thanks,
Rob