backport patch to resolv for underqualified domain names

[yaauie]

In the Ruby stdlib that ships with JRubies < 9.0, there was a bug that prevented underqualified domain names from resolving (that is, domain names with fewer dots in them than the configured minimum, which defaults to 1).

Effectively this prevented unqualified domain names (e.g., dot-less domain names registered with a dns server like dnsmasq, or even localhost) from resolving.

If applicable to the runtime, this patch backports a patch from https://bugs.ruby-lang.org/issues/10412

---

It also clean up the resolv_patch.rb file in two ways:

• it moves it into the logstash/filter/dns directory to avoid it being loaded accidentally (and perhaps out-of-order) by another plugin
• it uses local variables to avoid setting global top-level constants, which could interfere with other plugins

[yaauie]

I have been able to validate this locally, by pointing nameserver at a dnsmasq server on 127.0.0.1 that emits a reply for fubar:

╭─{ yaauie@castrovel:~/src/elastic/logstash-plugins/logstash-filter-dns (✘ backport-underqualified-domain-fix) }
╰─● dig @127.0.0.1 fubar +nocomments

; <<>> DiG 9.8.3-P1 <<>> @127.0.0.1 fubar +nocomments
; (1 server found)
;; global options: +cmd
;fubar.				IN	A
fubar.			0	IN	A	123.45.67.89
;; Query time: 0 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Thu Feb  7 22:28:37 2019
;; MSG SIZE  rcvd: 38

[success]

And then running a config that includes nameserver => '127.0.0.1'; without the patch applied, the name fails to resolve, but with it applied, it resolves as expected.

Note: this issue does not occur when the hostname resolves in a provided hostfile, as the bug is in DNS canonicalisation.

[colinsurprenant]

LGTM
For clarity; this path applies to JRuby < 9.0 which translate to logstash < 6.0.

[yaauie]

This patch has been included in the 3.0.12 release of this plugin.

bin/logstash-plugin update logstash-filter-dns

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