[email protected] 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?
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