BK8s datasource middleware -- PoC for A records (#153)

* Laying down kubernetes middleware foundation

* Duplicated a bunch of code form etcd middleware
* Duplicated code hacked to compile and load as a separate middleware

* Adding verbose build option to Makefile

* Removing stubzone and tls support

tls and stubzone support was carried over from base etcd middleware code.
Removing to simplify the kube middleware implementation. (For now.)

* Adding conf directory for sample conf files

* Removing stubzone support from query handler

* Remove upstream and proxy from k8s corefile.

Not sure that upstream or proxy makes sense for a k8s backed zone.

* Comment out use of singleflight serialization

* Removing parsing support for "upstream" directive from k8s

* Removing upstream directive parsing code

* Removing CNAME and TXT lookup implementation

* Create README.md

Brain-dump of DNS record name assembly and open work items.

* Adding notes about wildcard handling

* Adding basic k8s API client

* Fleshing out methods on k8s connector

* Remove PathPrefix from middleware init

* Removing incorrect plural

* Adding brute-force k8s service lookup functions

* Initializing k8s API connector during startup

* Hacking around to call k8s connector

* Parsing incoming domain name into serviceName and namespace

* Improving and simplifying k8s zone matching and label segmentation

* Removing unused functions carried over from etcd middleware

* Adding basic return of k8s data to DNS client

* updated debugging println statements to flag with "[debug]"
* removed code in kubernetes.go::Records that was a hold-over from etcd middleware.
* Removed some random exploratory hacking.

* Minior README.md updates

* Updating with demo instructions

* Updating README.md with CoreFile and removing completed TODO items

* Updating conf file and README to reflect DNS response cache works

* Disabling DNS response caching

* Adding debug statement on entry to Records()

* Changing port number in exampes to port 53.

* Misc style and clarity changes

* Removing empty function definitions

* Adding comment to track future cleanup

* Refactoring README to follow style of other middleware

* Exposing dataobject field (typo)

1 files changed, 101 insertions, 0 deletions

diff --git a/middleware/kubernetes/handler.go b/middleware/kubernetes/handler.go
new file mode 100644
index 000000000..44de8da3d
--- /dev/null
+++ b/middleware/kubernetes/handler.go
@@ -0,0 +1,101 @@
+package kubernetes
+
+import (
+	"fmt"
+
+	"github.com/miekg/coredns/middleware"
+
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+func (k Kubernetes) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+
+    fmt.Println("[debug] here entering ServeDNS: ctx:%v dnsmsg:%v", ctx, r)
+
+	state := middleware.State{W: w, Req: r}
+	if state.QClass() != dns.ClassINET {
+		return dns.RcodeServerFailure, fmt.Errorf("can only deal with ClassINET")
+	}
+
+    // Check that query matches one of the zones served by this middleware,
+    // otherwise delegate to the next in the pipeline.
+	zone := middleware.Zones(k.Zones).Matches(state.Name())
+	if zone == "" {
+		if k.Next == nil {
+			return dns.RcodeServerFailure, nil
+		}
+		return k.Next.ServeDNS(ctx, w, r)
+	}
+
+	m := new(dns.Msg)
+	m.SetReply(r)
+	m.Authoritative, m.RecursionAvailable, m.Compress = true, true, true
+
+	var (
+		records, extra []dns.RR
+		err            error
+	)
+	switch state.Type() {
+	case "A":
+		records, err = k.A(zone, state, nil)
+	case "AAAA":
+		records, err = k.AAAA(zone, state, nil)
+	case "TXT":
+		records, err = k.TXT(zone, state)
+	case "CNAME":
+		records, err = k.CNAME(zone, state)
+	case "MX":
+		records, extra, err = k.MX(zone, state)
+	case "SRV":
+		records, extra, err = k.SRV(zone, state)
+	case "SOA":
+		records = []dns.RR{k.SOA(zone, state)}
+	case "NS":
+		if state.Name() == zone {
+			records, extra, err = k.NS(zone, state)
+			break
+		}
+		fallthrough
+	default:
+		// Do a fake A lookup, so we can distinguish betwen NODATA and NXDOMAIN
+		_, err = k.A(zone, state, nil)
+	}
+	if isKubernetesNameError(err) {
+		return k.Err(zone, dns.RcodeNameError, state)
+	}
+	if err != nil {
+		return dns.RcodeServerFailure, err
+	}
+
+	if len(records) == 0 {
+		return k.Err(zone, dns.RcodeSuccess, state)
+	}
+
+	m.Answer = append(m.Answer, records...)
+	m.Extra = append(m.Extra, extra...)
+
+	m = dedup(m)
+	state.SizeAndDo(m)
+	m, _ = state.Scrub(m)
+	w.WriteMsg(m)
+	return dns.RcodeSuccess, nil
+}
+
+// NoData write a nodata response to the client.
+func (k Kubernetes) Err(zone string, rcode int, state middleware.State) (int, error) {
+	m := new(dns.Msg)
+	m.SetRcode(state.Req, rcode)
+	m.Ns = []dns.RR{k.SOA(zone, state)}
+	state.SizeAndDo(m)
+	state.W.WriteMsg(m)
+	return rcode, nil
+}
+
+func dedup(m *dns.Msg) *dns.Msg {
+	// TODO(miek): expensive!
+	m.Answer = dns.Dedup(m.Answer, nil)
+	m.Ns = dns.Dedup(m.Ns, nil)
+	m.Extra = dns.Dedup(m.Extra, nil)
+	return m
+}