Allow hostnames in the config about g3

[zh-jq]

If there's a concern about resolving multiple ips, perhaps we could add a check to count the number of ips resolved and return an error if it's more than one?

A warning message is needed to tell the users that the domain support is only restricted to one IP address and no dynamic update will be available.

The UpstreamAddr type can be used for proper domain support. And we should resolve the domain just before we try to connect to the peer.

We may also use the resolved IP addresses to do load balancing, but this should be decided case by case. For the udpdump case, this will cause problem if we send FTP control connection and data connection to different peers.

[mspublic]

Thanks @jeffreydwalter. I think this would be useful for more than just docker - many production environments use hostnames not IPs even without docker.

[zh-jq-b]

Is it possible to use environment variables? You can set the value to $SOME _RESOLVED_ADDR to read from env var SOME_RESOLVED_ADDR.

Domains can be dynamically updated and may resolve to many IP addresses, it's not easy to add proper support.

[mspublic]

In my opinion this patch doesn’t prevent someone from doing that or using an IP. It just gives the option of using the domain instead.

Some environments (such as ones using envoy/istio) need the domain info to do routing. Without it they can’t find the target.

[jeffreydwalter]

If there's a concern about resolving multiple ips, perhaps we could add a check to count the number of ips resolved and return an error if it's more than one?