config: hints for using v4 dns to avoid 503 (#2478)

[winguse]

Add a YAML comment to the config file that might help someone who encounters v4/v6 dns problem in the future.

Risk Level

Low, fixing sample config

Testing

manual testing.

Environment: a machine don't have ipv6 network but can resolve v6 address of google

before

$ curl -i http://127.0.0.1:10000
HTTP/1.1 503 Service Unavailable
content-length: 57
content-type: text/plain
date: Wed, 31 Jan 2018 03:23:39 GMT
server: envoy

upstream connect error or disconnect/reset before headers

uncomment dns_lookup_family: V4_ONLY

$ curl -i http://127.0.0.1:10000
HTTP/1.1 200 OK
date: Wed, 31 Jan 2018 03:27:02 GMT
expires: -1
cache-control: private, max-age=0
content-type: text/html; charset=ISO-8859-1
...

fix

#2478

[htuch]

Can you merge master to pickup #2490?

[alyssawilk]

LGTM pending a fresh merge (I'd rather not force-merge anything but other folks are welcome to :-) )

[mattklein123]

nit: (As long as you have to merge master):

"Uncomment the following line if your environment doesn't have v6 network access or Envoy may try to talk to a v6 endpoint which will result in a 503 error."

[winguse]

thanks!

[mattklein123]

Thanks!