When one does a DNS scan for the nameserver of su2.ff.avast one gets bad zone: Could not get name servers for ‘pns.avast.com’.
See: http://toolbar.netcraft.com/site_report?url=su2.ff.avast.com
See: http://who.is/nameserver/pns.avast.com/
Delegation not found at parent: http://dnscheck.pingdom.com/?domain=pns.avast.com&timestamp=1442139880&view=1
Not enough nameserver information was found to test the zone pns.avast.com, but an IP address lookup succeeded in spite of that.
Re: http://www.tcpiputils.com/browse/ip-address/91.213.143.1
So is there something misconfigured?
Re: https://www.metricsbot.com/nameserver/pns.avast.com/
Also consider my posting here: https://forum.avast.com/index.php?topic=154511.0

Name Server
pns.avast.com is a known Domain Name Server. This server provides name services (DNS) for the following domains:
avast.comhttp://www.dnsinspect.com/avast.com/1442140070
Warning: WARNING: Found stealth name servers:
ns6.avast.com.
sns.avast.com.
This should not be: WARNING: Name servers software versions are exposed:
91.213.143.1: “9.3.6-P1-RedHat-9.3.6-25.P1.el5_11.2” hostname: pns.avast.com
domain ISC BIND 9.3.6-25.P1.el5_11.2
Service Info: OS: Red Hat Enterprise Linux; CPE: cpe:/o:redhat:enterprise_linux
Should be updated: https://rhn.redhat.com/errata/RHSA-2014-1984.html
SRPMS:
bind-9.3.6-25.P1.el5_11.2.src.rpm
File outdated by: RHSA-2015:1706 MD5: 219d9fcc20de4b8ebe01a9014fe8a52b
SHA-256: c6c46ab655778236a30e364d10d4766f69f2858f2da37aa296dfde8a79cb8d38

polonus