When true, the underlying multicast socket drops inbound packets whose source is not on a local-link subnet (RFC 6762 §11-style hardening).
SSDP is not mDNS: RFC 6762 §11 normatively governs only the mDNS group (224.0.0.251). SSDP runs on the administratively-scoped group 239.255.255.250 and is expected to interoperate across hops bounded by TTL. The default is therefore false. Single-segment deployments that want the extra hardening can pass true.
Whether our sockets should also receive the messages that are sent
The host interface to bind the instance to