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/proxy.go

@@ -0,0 +1,77 @@
+package forward
+
+import (
+	"crypto/tls"
+	"sync"
+	"time"
+
+	"github.com/miekg/dns"
+)
+
+// Proxy defines an upstream host.
+type Proxy struct {
+	host *host
+
+	transport *transport
+
+	// copied from Forward.
+	hcInterval time.Duration
+	forceTCP   bool
+
+	stop chan bool
+
+	sync.RWMutex
+}
+
+// NewProxy returns a new proxy.
+func NewProxy(addr string) *Proxy {
+	host := newHost(addr)
+
+	p := &Proxy{
+		host:       host,
+		hcInterval: hcDuration,
+		stop:       make(chan bool),
+		transport:  newTransport(host),
+	}
+	return p
+}
+
+// SetTLSConfig sets the TLS config in the lower p.host.
+func (p *Proxy) SetTLSConfig(cfg *tls.Config) { p.host.tlsConfig = cfg }
+
+// SetExpire sets the expire duration in the lower p.host.
+func (p *Proxy) SetExpire(expire time.Duration) { p.host.expire = expire }
+
+func (p *Proxy) close() { p.stop <- true }
+
+// Dial connects to the host in p with the configured transport.
+func (p *Proxy) Dial(proto string) (*dns.Conn, error) { return p.transport.Dial(proto) }
+
+// Yield returns the connection to the pool.
+func (p *Proxy) Yield(c *dns.Conn) { p.transport.Yield(c) }
+
+// Down returns if this proxy is up or down.
+func (p *Proxy) Down(maxfails uint32) bool { return p.host.down(maxfails) }
+
+func (p *Proxy) healthCheck() {
+
+	// stop channel
+	p.host.SetClient()
+
+	p.host.Check()
+	tick := time.NewTicker(p.hcInterval)
+	for {
+		select {
+		case <-tick.C:
+			p.host.Check()
+		case <-p.stop:
+			return
+		}
+	}
+}
+
+const (
+	dialTimeout = 4 * time.Second
+	timeout     = 2 * time.Second
+	hcDuration  = 500 * time.Millisecond
+)