Use a filtering proxy instead and no gateway / route to the internet.
2) You're advising security through obscurity instead of a network namespace + firewall.
or DNS stubs with filtering capabilities.
And regarding failed reverse DNS names: Little Snitch is sniffing DNS lookups. If lookups are encrypted, there is little it can do. We usually recommend DNS encryption at the systemd layer, not at app layer. This way we can see lookups on 127.0.0.53 and the actual lookup sent out is still encrypted.
Also, it's currently only sniffing UDP lookups, not TCP. The eBPF part is already very close to the complexity limits (700k instructions of allowed 1M) and adding TCP parsing would exceed this limit. It should be possible to forbid TCP port 53 with a rule, though. Some complex DNS lookups will fail, but routine things should still work.
Perhaps there should be a mode where littlesnitch just does its own lookup using the system-configured rDNS, for example from the ui or for specific processes, etc? It should be cached if it is a recent lookup, so minimal performance implications; and offloaded to the system rDNS resolver, so minimal instruction set.
We do not want the reverse lookup name. For instance, if you look up a google.com name with dig, you get an IP address. If you then do the reverse lookup with dig -x, you get a 1e100.net name. That's as good as the IP address for our purpose.
Plus: We need to respond with a DROP or ALLOW verdict to a network packet without the ability to do any blocking requests. So we can only use information already available in the kernel to decide.
[1] "Little Snitch for Linux is built for privacy, not security, and that distinction matters. The macOS version can make stronger guarantees because it can have more complexity. On Linux, the foundation is eBPF, which is powerful but bounded: it has strict limits on storage size and program complexity. Under heavy traffic, cache tables can overflow, which makes it impossible to reliably tie every network packet to a process or a DNS name. And reconstructing which hostname was originally looked up for a given IP address requires heuristics rather than certainty. The macOS version uses deep packet inspection to do this more reliably. That's not an option here." -- from https://obdev.at/products/littlesnitch-linux/index.html