[bug] fix issues with dns resolution with systemd-resolved
In ubuntu 17.10 some changes with systemd-resolved broke our firewall, blocking all DNS queries. The masquerade rules in the firewall, that are used to rewrite the source IP address of the DNS queries, were wrongly modifying the queries to systemd-resolved.
Let's apply masquerade only to the packets addressed to the nameserver.
- Resolves: #9137 (closed)