BBN’s TCP implementation contained something akin to the hosts file, called hostmap
there:
https://www.tuhs.org/cgi-bin/utree.pl?file=BBN-Vax-TCP/doc
I have not looked at the code for a while, but if I remember correctly the BBN kernel code
also read in this file (pre-processed into a binary form) to build its internal routing
table.
I do not recall having seen an equivalent file with UoI's NCP Unix in any of the
surviving docs or sources - but that does not exclude a library having existed to do
lookups in a local copy the SRI-NIC host file. In fact there is some evidence for that in
the 2.9 BSD source.
The only surviving copy of the 4.1a (network) source code that I know is in the back-port
of this code to 2.8/2.9 BSD. This code includes #ifdef’ed code for accessing the SRI-NIC
online host table via NCP:
https://www.tuhs.org/cgi-bin/utree.pl?file=2.9BSD/usr/net/local
This source also contains tools to convert the SRI-NIC data into - inter alia - a hosts
file:
https://www.tuhs.org/cgi-bin/utree.pl?file=2.9BSD/usr/net/man/man8/htable.8
https://www.tuhs.org/cgi-bin/utree.pl?file=2.9BSD/usr/net/man/man8/gettable…
It would seem that the modern host.txt on Unix evolved late ’81 (BBN code) to early ’82
(4.1a BSD). Possibly NCP Unix has prior work.
Paul