NIS, LDAP etc should have used a unix socket interface to talk system daemons using well-defined API. This is what happens in practice in any case, but it is do

NIS, LDAP etc should have used a unix socket interface to talk system daemons using well-defined API. This is what happens in practice in any case, but it is done over hidden protocols unusable outside glibc. As for the resolver, then consider that on Ubuntu/Fedora etc. there is hack to set the nameserver in /etc/resolv.conf to 127.0.0.1 which runs a local caching resolver. However this is broken for local name resolution with multiple interfaces as DNS replies do not include interface name. So in a better world glibc simply talks to a system daemon using well-defined protocol. Instead right now to expose, say mdns into containers, one has to spend too much efforts with various hacks even if one does use glibc in a container application.