Support A record for multile IPs for a headless services.

[toshipp]

Non statefulset pods associating to a headless service have different
IPs, but have a same hostname. In this case, external-dns registered
only one A records due to attempting to register multiple A records for
a same hostname for each IP.
This patch now registers one A record having multiple IPs.

fixes #549

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[alfredkrohmer]

I see that this is fixing a bug, but in general, shouldn't it be a configurable option if you want
a) one A record that has all pod IPs or
b) one A record per pod (where the record contains the pod name)
?

[toshipp]

In AWS, the case b is not possible.
For example, A record hostname foo associating to 1.1.1.1 is registered, then try to register A record same hostname foo associating to 1.1.1.2, the second attempt is failed because hostname foo is duplicated.
It seems a restriction of route53.

[alfredkrohmer]

I meant for case b) that the pod name (which has usually either a sequential number in it for static sets or a random string for replica sets) could be part of the hostname. E.g. a pod is named my-pod-2uzxy2-d826sh9a, so it would create a DNS record my-pod-2uzxy2-d826sh9a.example.com.

If I understand you pull request correctly, it will completely remove the ability to do that.

[alfredkrohmer]

More specifically I'm talking about the host port use case:
https://github.com/Arttii/external-dns/blob/master/docs/tutorials/hostport.md

Nevertheless, even the usecase you mentioned is possible. You can simply created weighted or multi value records in Route53 which would allow to have multiple instances of the same record name (e.g. my-service.example.com) with a different IP each. (Multi value would return up to 8 IPs per record name, weighted would return one IP via weighted round robin.)

[toshipp]

Ok, I got it.
The PR does only aggregate target IPs associating SAME hostname to one record.
So, if pods have unique hostname, external-dns registers records for each hostname.
This does not breaks current behavior.

In replica set case, pod names are unique, but the hostnames are same because it is given by the
pod resource definition.
see https://kubernetes.io/docs/concepts/services-networking/dns-pod-service/#pods

Currently when a pod is created, its hostname is the Pod’s metadata.name value.

The Pod spec has an optional hostname field, which can be used to specify the Pod’s hostname. When specified, it takes precedence over the Pod’s name to be the hostname of the pod. For example, given a Pod with hostname set to “my-host”, the Pod will have its hostname set to “my-host”.

My motivation is exactly creating multi value records for headless service with replica set.
Current implementation registers only one IP for the service due to a bug.
The PR registers all IPs for the service.

[dene14]

Any chance to get that merged?

[artsiukhou]

Struggling with the same problem, it will be extremely helpful to have a fix for the issue.

[alfredkrohmer]

This would be really helpful, indeed. Can we have that merged please?

[xdrus]

Thanks, this PR works very well in our env. Our use case is that we use AWS CNI plugin that gives real VPC IPs to pods we want to expose via R53.

Look forward to having this merged!

[Raffo]

@njuettner @linki let's review this one, do you have capacity? It looks like it was open for a long time.

[njuettner]

/lgtm

[njuettner]

/approve

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