Add coredns proposal for kubernetes/enhancements#427

contributors/design-proposals/network/coredns.md

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+# <Title>
+
+Status: Pending
+
+Version: Alpha
+
+Implementation Owner: TBD
+
+## Motivation
+
+CoreDNS is another CNCF project and is the successor to SkyDNS, which kube-dns is based on. It is a flexible, extensible
+authoritative DNS server and we have built a direct integration to the Kubernetes API. It can serve as cluster DNS,
+complying with the [dns spec](https://github.com/kubernetes/dns/blob/master/docs/specification.md). 
+
+CoreDNS has fewer moving parts than kube-dns, since it is a single executable and single process. It is written in Go so
+it is memory-safe (kube-dns includes dnsmasq which is not). It supports a number of use cases that kube-dns does not
+(see below). As a general-purpose authoritative DNS server it has a lot of functionality that kube-dns could not reasonably
+be expected to add. See, for example, the [intro](https://docs.google.com/presentation/d/1v6Coq1JRlqZ8rQ6bv0Tg0usSictmnN9U80g8WKxiOjQ/edit#slide=id.g249092e088_0_181) or [coredns.io](https://coredns.io) or the [CNCF webinar](https://youtu.be/dz9S7R8r5gw).
+
+## Proposal
+
+The proposed solution is to enable the selection of CoreDNS as an alternate to Kube-DNS during cluster deployment, with the
+intent to make it the default in the future.
+
+## User Experience
+
+### Use Cases
+
+ * Standard DNS-based service discovery
+ * Federation records
+ * Stub zone support
+ * Adding custom DNS entries
+   * Making an alias for an external name [#39792](https://github.com/kubernetes/kubernetes/issues/39792)
+   * Dynamically adding services to another domain, without running another server [#55](https://github.com/kubernetes/dns/issues/55)
+   * Adding an arbitrary entry inside the cluster domain
+ * Verified pod DNS entries (ensure pod exists in specified namespace)
+ * Experimental server-side search path to address latency issues [#33554](https://github.com/kubernetes/kubernetes/issues/33554)
+ * Limit PTR replies to the cluster CIDR [#125](https://github.com/kubernetes/dns/issues/125)
+
+By default, the user experience would be unchanged. For more advanced uses, existing users would need to modify the
+ConfigMap that contains the CoreDNS configuration file.
+
+## Implementation
+
+Each distribution project (kubeadm, minikube, kubespray, and others) will implement CoreDNS as an optional
+add-on as appropriate for that project.
+
+### Client/Server Backwards/Forwards compatibility
+
+Fully compatible.
+
+## Alternatives considered
+
+Maintain existing kube-dns, and add functionality to meet the use cases above and fix underlying issues. To ensure
+the user of memory-safe code, this would require replacing dnsmasq with another caching DNS server, or implementing
+caching within kube-dns.
+