dnscache in Daniel J. Bernstein djbdns 1.05 does not prevent simultaneous identical outbound DNS queries, which makes it easier for remote attackers to spoof DNS responses, as demonstrated by a spoofed A record in the Additional section of a response to a Start of Authority (SOA) query.
https://exchange.xforce.ibmcloud.com/vulnerabilities/48807
http://www.your.org/dnscache/djbdns.pdf