Support multiple k8s api servers specification and load balance among api servers (#820)

* Support multiple k8s api servers specification and load balance among api servers

This fix adds supports for multiple k8s api servers specification,
load balance among api servers.

When two or more api servers are specified in kubernetes block (endpoint ...),
a proxy is created locally (with randomly generately port). The coredns
will points to the generated proxy so that load balancing could be achieved.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Setup initial healthcheck at the beginning

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Update README.md for kubernetes middleware and remove whitespaces.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Use middleware/pkg/healthcheck in middleware/kubernetes

for api proxy

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

1 files changed, 76 insertions, 0 deletions

diff --git a/middleware/kubernetes/apiproxy.go b/middleware/kubernetes/apiproxy.go
new file mode 100644
index 000000000..966e5753b
--- /dev/null
+++ b/middleware/kubernetes/apiproxy.go
@@ -0,0 +1,76 @@
+package kubernetes
+
+import (
+	"fmt"
+	"io"
+	"log"
+	"net"
+	"net/http"
+
+	"github.com/coredns/coredns/middleware/pkg/healthcheck"
+)
+
+type proxyHandler struct {
+	healthcheck.HealthCheck
+}
+
+type apiProxy struct {
+	http.Server
+	listener net.Listener
+	handler  proxyHandler
+}
+
+func (p *proxyHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	upstream := p.Select()
+	network := "tcp"
+	if upstream.Network != "" {
+		network = upstream.Network
+	}
+	address := upstream.Name
+	d, err := net.Dial(network, address)
+	if err != nil {
+		log.Printf("[ERROR] Unable to establish connection to upstream %s://%s: %s", network, address, err)
+		http.Error(w, fmt.Sprintf("Unable to establish connection to upstream %s://%s: %s", network, address, err), 500)
+		return
+	}
+	hj, ok := w.(http.Hijacker)
+	if !ok {
+		log.Printf("[ERROR] Unable to establish connection: no hijacker")
+		http.Error(w, "Unable to establish connection: no hijacker", 500)
+		return
+	}
+	nc, _, err := hj.Hijack()
+	if err != nil {
+		log.Printf("[ERROR] Unable to hijack connection: %s", err)
+		http.Error(w, fmt.Sprintf("Unable to hijack connection: %s", err), 500)
+		return
+	}
+	defer nc.Close()
+	defer d.Close()
+
+	err = r.Write(d)
+	if err != nil {
+		log.Printf("[ERROR] Unable to copy connection to upstream %s://%s: %s", network, address, err)
+		http.Error(w, fmt.Sprintf("Unable to copy connection to upstream %s://%s: %s", network, address, err), 500)
+		return
+	}
+
+	errChan := make(chan error, 2)
+	cp := func(dst io.Writer, src io.Reader) {
+		_, err := io.Copy(dst, src)
+		errChan <- err
+	}
+	go cp(d, nc)
+	go cp(nc, d)
+	<-errChan
+}
+
+func (p *apiProxy) Run() {
+	p.handler.Start()
+	p.Serve(p.listener)
+}
+
+func (p *apiProxy) Stop() {
+	p.handler.Stop()
+	p.Close()
+}