andrewv@melbpc.org.au writes:

Assume I have (at most), all of these entries in /etc/hosts

custom1.company.com nnn.nnn.nnn.nnn custom2.company.com nnn.nnn.nnn.nnn default.company.com nnn.nnn.nnn.nnn

That won't work. It's <address> <canonical name> [<alias> ...]

where nnn is some specific IPv4 address number

I want to lookup the hosts in say this order:

custom2.company.com custom1.company.com default.company.com

It doesn't work that way, sorry.

If the custom2 entry is not found, I want to try custom1, and if not ok, then I want to try default.

You're talking about reverse resolution (IP to FQDN)?

Importantly, I do _not_ want _any_ DNS lookup to be performed for _these_ _specific_ host lookups, because I want an immediate failure if the entry is not defined in /etc/hosts.

I don't think you can have that, at least not on a per-host basis. The way different name services (such as flat files and DNS) are used is governed by nsswitch on eglibc/glibc-using (and most other) systems. You can probably add 1.2.3.4 canthappen.invalid And if nsswitch has "hosts: files dns" then it will hit that first, and not try DNS. You can't have an absence in /etc/hosts -- hosts(5) has no support for the equivalent of an authoritative NX RR.

Note: I want an _immediate_ failure initially, _and_ for every lookup thereafter - an initial DNS lookup that fails after a DNS lookup timeout, and then perhaps caches that failure result is no good, because every lookup must respond immediately (and of course /etc/hosts and DNS service entries can change at any time so even then, cacheing is not useful).

You can't have this with hosts(5). Your best bet is probably to point resolv.conf at a local DNS resolver that is configured to return spoof results for particular domains (including, obviously, the C.B.A.in-addr.arpa for the PTR RR). You could, alternatively, disable DNS altogether and ensure that all hosts you need to reach are hard-coded in /etc/hosts.

Assume that I am using software that I cannot change, and so workarounds like actually reading the /etc/hosts file to see which entries exist, cannot be wired into the software.

LD_PRELOAD might still be a viable option, though I don't recommend it.

I am thinking of something functionally like a like /etc/disable_dns_lookup that I can also copy all 3 of the above entries to, and linux will _never_ try DNS if the entries appear in /etc/disable_dns_lookup.

AFAIK no such feature exists. This all sounds like an xyproblem; what is the ACTUAL problem you're trying to solve?