This doesn't mesh with my memory. I recall building BIND from source and having to rebuild network programs (e.g. on 4.3 on the RT or VAXen) to pick up the new version of libresolv.a, and hacking the resolver library into libc.so on Suns. I remember using resolv.conf fairly early on, but my memory is that nsswitch.conf came later (Solaris 2.x era?). Ultrix did have a configuration file for where to do host lookups, but I think the set of sources was fixed: files, NIS or DNS. This would have been in the Ultrix 4.4 or 4.5 era on MIPS. I remember seeing some description of a configuration file accompanied by an editorialized comment saying something like, "this is an idea that's time has come: Ultrix has had it for several years." The dig on uglix was, well, kind of funny (I had a DECstation at home at the time).