I've implemented some sort of "poor man's Docker" using LD_PRELOAD, back then in 2011 when Docker wasn't a thing. It works by overriding getaddrinfo (IIRC) and

I've implemented some sort of "poor man's Docker" using LD_PRELOAD, back then in 2011 when Docker wasn't a thing. It works by overriding getaddrinfo (IIRC) and capturing name lookups of "localhost", which are then answered by an IP address that's taken from an env variable. The intended use is the parallelization of automated testing of a distributed system: by creating lots of loopback devices with individual IPs and assigning those to test processes (via the LD_PRELOAD hack), I could suddenly test as many instances of the software system next to each other as I wanted, on the same machine (the test machine is some beefy dual-socket server with lots of CPU cores and RAM). Each instance (which consists of clients and several processes that provide server services, thus they're by default configured to bind themselves to specific ports on localhost, as it is common for dev and test purposes) would then be able to route its traffic over its own loopback device, and I was spared of having to somehow untangle the server ports of all the different services just in order to be able to parallelize them on a single machine and of the configuration hell that would have come with this. It helped that processes by default inherit the env variables from their parents that spawned them - that made it a lot easier to propagate the preload path and the env variable containing the loopback IP to use. I just had to provide it to the top-most process, basically. Today, one would use Docker for this exact purpose, putting each test run into its own container (or even multiple containers). But since the LD_PRELOAD hack worked so well, the project in which I implemented the above is still using it (although they're eyeing a switch to Docker, in part because it also makes it easier to separate non-IP-related resources such as files on the filesystem, but mostly because knowledge about Docker is more widespread than about such ancient tech as LD_PRELOAD and how to hack into name resolution of the OS).