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.
- 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.