> What software actually parses /etc/hosts, at least on Linux?

glibc resolver

A good entry point for reading more about it:

$ man nsswitch.conf

If your /etc/nsswitch.conf file's "hosts" line contains the keyword "files", then it potentially uses /etc/hosts. If "files" is first (typical default config), it looks there first, before the other places listed.

This is done under the hood when programs use resolver functions like gethostbyname or getaddrinfo.

You can see this in action on musl source code, which is arguably a much more readily understandable implementation of libc:

- Function that actually parses /etc/hosts is name_from_hosts(), implemented here: http://git.musl-libc.org/cgit/musl/tree/src/network/lookup_n...

- Which is called by __lookup_name() on the same file: http://git.musl-libc.org/cgit/musl/tree/src/network/lookup_n...

- Which is, in turn, called directly from getaddrinfo() [http://git.musl-libc.org/cgit/musl/tree/src/network/getaddri...], the actual function exposed to you as libc user.