plugin/forward: add it (#1447)

* plugin/forward: add it

This moves coredns/forward into CoreDNS. Fixes as a few bugs, adds a
policy option and more tests to the plugin.

Update the documentation, test IPv6 address and add persistent tests.

* Always use random policy when spraying

* include scrub fix here as well

* use correct var name

* Code review

* go vet

* Move logging to metrcs

* Small readme updates

* Fix readme

plugin/forward/lookup.go

@@ -0,0 +1,78 @@
+// Package forward implements a forwarding proxy. It caches an upstream net.Conn for some time, so if the same
+// client returns the upstream's Conn will be precached. Depending on how you benchmark this looks to be
+// 50% faster than just openening a new connection for every client. It works with UDP and TCP and uses
+// inband healthchecking.
+package forward
+
+import (
+	"crypto/tls"
+	"log"
+	"time"
+
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+// Forward forward the request in state as-is. Unlike Lookup that adds EDNS0 suffix to the message.
+// Forward may be called with a nil f, an error is returned in that case.
+func (f *Forward) Forward(state request.Request) (*dns.Msg, error) {
+	if f == nil {
+		return nil, errNoForward
+	}
+
+	fails := 0
+	for _, proxy := range f.list() {
+		if proxy.Down(f.maxfails) {
+			fails++
+			if fails < len(f.proxies) {
+				continue
+			}
+			// All upstream proxies are dead, assume healtcheck is complete broken and randomly
+			// select an upstream to connect to.
+			proxy = f.list()[0]
+			log.Printf("[WARNING] All upstreams down, picking random one to connect to %s", proxy.host.addr)
+		}
+
+		ret, err := proxy.connect(context.Background(), state, f.forceTCP, true)
+		if err != nil {
+			log.Printf("[WARNING] Failed to connect to %s: %s", proxy.host.addr, err)
+			if fails < len(f.proxies) {
+				continue
+			}
+			break
+
+		}
+
+		return ret, nil
+	}
+	return nil, errNoHealthy
+}
+
+// Lookup will use name and type to forge a new message and will send that upstream. It will
+// set any EDNS0 options correctly so that downstream will be able to process the reply.
+// Lookup may be called with a nil f, an error is returned in that case.
+func (f *Forward) Lookup(state request.Request, name string, typ uint16) (*dns.Msg, error) {
+	if f == nil {
+		return nil, errNoForward
+	}
+
+	req := new(dns.Msg)
+	req.SetQuestion(name, typ)
+	state.SizeAndDo(req)
+
+	state2 := request.Request{W: state.W, Req: req}
+
+	return f.Forward(state2)
+}
+
+// NewLookup returns a Forward that can be used for plugin that need an upstream to resolve external names.
+func NewLookup(addr []string) *Forward {
+	f := &Forward{maxfails: 2, tlsConfig: new(tls.Config), expire: defaultExpire, hcInterval: 2 * time.Second}
+	for i := range addr {
+		p := NewProxy(addr[i])
+		f.SetProxy(p)
+	}
+	return f
+}