Depends on what kind of security boundary. You could only allow zerotier traffic, in which case the container only lives on the virtual net and no "real" traffi

Depends on what kind of security boundary. You could only allow zerotier traffic, in which case the container only lives on the virtual net and no "real" traffic flows. You could also have the preloaded intercept library forbid any other traffic, isolating the process network wise. It's a total socket API takeover. And yes there are other mechanisms than LD PRELOAD. It can also be explicitly linked into a binary if you do want to rebuild, or linked into libc, or linked into everything via the dynamic linker configuration files. The latter us how an entire container as opposed to a process can be placed into a virtual net.