Check Avahi Responses' TTL Field

Description

To make Avahi ignore packets unless the TTL field is 255, edit /etc/avahi/avahi-daemon.conf and ensure the following line appears in the [server] section:

check-response-ttl=yes

Rationale

This helps to ensure that only mDNS responses from the local network are processed, because the TTL field in a packet is decremented from its initial value of 255 whenever it is routed from one network to another. Although a properly-configured router or firewall should not allow mDNS packets into the local network at all, this option provides another check to ensure they are not permitted.