[DRAFT] JGRP 1388/dns ping

[slaskawi]

https://issues.jboss.org/browse/JGRP-2139

At this point please consider this PR as a draft. There is still some work to be done including:

• documentation
• DNS SRV entries

However, this is a running implementation of DNS PING that operates on Headless Services in Kubernetes (or OpenShift). In Kubernetes, a Headless Service is responsible for creating DNS entries for Containers. It creates multiple A entries for each host and multiple SRV entries for each port. Here's an example:

sh-4.2$ dig +search jgroups

; <<>> DiG 9.9.4-RedHat-9.9.4-38.el7_3 <<>> +search jgroups
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 38901
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;jgroups.			IN	A

;; Query time: 0 msec
;; SERVER: 192.168.0.17#53(192.168.0.17)
;; WHEN: Wed Dec 21 12:37:53 UTC 2016
;; MSG SIZE  rcvd: 25

sh-4.2$ dig +search jgroups-dns-ping.myproject.svc.cluster.local

; <<>> DiG 9.9.4-RedHat-9.9.4-38.el7_3 <<>> +search jgroups-dns-ping.myproject.svc.cluster.local
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 40178
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 5, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;jgroups-dns-ping.myproject.svc.cluster.local. IN A

;; ANSWER SECTION:
jgroups-dns-ping.myproject.svc.cluster.local. 30 IN A 172.17.0.10
jgroups-dns-ping.myproject.svc.cluster.local. 30 IN A 172.17.0.3
jgroups-dns-ping.myproject.svc.cluster.local. 30 IN A 172.17.0.5
jgroups-dns-ping.myproject.svc.cluster.local. 30 IN A 172.17.0.6
jgroups-dns-ping.myproject.svc.cluster.local. 30 IN A 172.17.0.7

;; Query time: 0 msec
;; SERVER: 192.168.0.17#53(192.168.0.17)
;; WHEN: Wed Dec 21 12:38:13 UTC 2016
;; MSG SIZE  rcvd: 142

Unfortunately DNS A entries have no port assigned, so I'm assuming that all instances have the same configuration and reusing TP's port.

A working example that uses this protocol can be found here: https://github.com/slaskawi/jgroups-dns-ping-example

[belaban]

OK, I merged this into master as the code is quite self-reliant and there were no changes to other code. I had to change the unit test slightly in order for it to pass.
Closing this for now. Please create another PR to update the code and add a section to the documentation.
Cheers