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/forward.go

@@ -0,0 +1,154 @@
+// 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"
+	"errors"
+	"time"
+
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+	ot "github.com/opentracing/opentracing-go"
+	"golang.org/x/net/context"
+)
+
+// Forward represents a plugin instance that can proxy requests to another (DNS) server. It has a list
+// of proxies each representing one upstream proxy.
+type Forward struct {
+	proxies []*Proxy
+	p       Policy
+
+	from    string
+	ignored []string
+
+	tlsConfig     *tls.Config
+	tlsServerName string
+	maxfails      uint32
+	expire        time.Duration
+
+	forceTCP   bool          // also here for testing
+	hcInterval time.Duration // also here for testing
+
+	Next plugin.Handler
+}
+
+// New returns a new Forward.
+func New() *Forward {
+	f := &Forward{maxfails: 2, tlsConfig: new(tls.Config), expire: defaultExpire, hcInterval: hcDuration, p: new(random)}
+	return f
+}
+
+// SetProxy appends p to the proxy list and starts healthchecking.
+func (f *Forward) SetProxy(p *Proxy) {
+	f.proxies = append(f.proxies, p)
+	go p.healthCheck()
+}
+
+// Len returns the number of configured proxies.
+func (f *Forward) Len() int { return len(f.proxies) }
+
+// Name implements plugin.Handler.
+func (f *Forward) Name() string { return "forward" }
+
+// ServeDNS implements plugin.Handler.
+func (f *Forward) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+
+	state := request.Request{W: w, Req: r}
+	if !f.match(state) {
+		return plugin.NextOrFailure(f.Name(), f.Next, ctx, w, r)
+	}
+
+	fails := 0
+	var span, child ot.Span
+	span = ot.SpanFromContext(ctx)
+
+	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 completely broken and randomly
+			// select an upstream to connect to.
+			r := new(random)
+			proxy = r.List(f.proxies)[0]
+
+			HealthcheckBrokenCount.Add(1)
+		}
+
+		if span != nil {
+			child = span.Tracer().StartSpan("connect", ot.ChildOf(span.Context()))
+			ctx = ot.ContextWithSpan(ctx, child)
+		}
+
+		ret, err := proxy.connect(ctx, state, f.forceTCP, true)
+
+		if child != nil {
+			child.Finish()
+		}
+
+		if err != nil {
+			if fails < len(f.proxies) {
+				continue
+			}
+			break
+		}
+
+		ret.Compress = true
+		// When using force_tcp the upstream can send a message that is too big for
+		// the udp buffer, hence we need to truncate the message to at least make it
+		// fit the udp buffer.
+		ret, _ = state.Scrub(ret)
+
+		w.WriteMsg(ret)
+
+		return 0, nil
+	}
+
+	return dns.RcodeServerFailure, errNoHealthy
+}
+
+func (f *Forward) match(state request.Request) bool {
+	from := f.from
+
+	if !plugin.Name(from).Matches(state.Name()) || !f.isAllowedDomain(state.Name()) {
+		return false
+	}
+
+	return true
+}
+
+func (f *Forward) isAllowedDomain(name string) bool {
+	if dns.Name(name) == dns.Name(f.from) {
+		return true
+	}
+
+	for _, ignore := range f.ignored {
+		if plugin.Name(ignore).Matches(name) {
+			return false
+		}
+	}
+	return true
+}
+
+// List returns a set of proxies to be used for this client depending on the policy in f.
+func (f *Forward) list() []*Proxy { return f.p.List(f.proxies) }
+
+var (
+	errInvalidDomain = errors.New("invalid domain for proxy")
+	errNoHealthy     = errors.New("no healthy proxies")
+	errNoForward     = errors.New("no forwarder defined")
+)
+
+// policy tells forward what policy for selecting upstream it uses.
+type policy int
+
+const (
+	randomPolicy policy = iota
+	roundRobinPolicy
+)