knot-resolver: NXNSAttack mitigation (CVE-2020-12667)

Knot Resolver before 5.1.1 allows traffic amplification via a crafted DNS answer from an attacker-controlled server, aka an "NXNSAttack" issue. This is triggered by random subdomains in the NSDNAME in NS records.

References:

• https://nvd.nist.gov/vuln/detail/CVE-2020-12667
• https://www.openwall.com/lists/oss-security/2020/05/19/2

Patch:

https://www.openwall.com/lists/oss-security/2020/05/19/2/1

Affected branches:

• master
• 3.11-stable