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/connect.go

@@ -0,0 +1,66 @@
+// 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 (
+	"strconv"
+	"time"
+
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+func (p *Proxy) connect(ctx context.Context, state request.Request, forceTCP, metric bool) (*dns.Msg, error) {
+	start := time.Now()
+
+	proto := state.Proto()
+	if forceTCP {
+		proto = "tcp"
+	}
+	if p.host.tlsConfig != nil {
+		proto = "tcp-tls"
+	}
+
+	conn, err := p.Dial(proto)
+	if err != nil {
+		return nil, err
+	}
+
+	// Set buffer size correctly for this client.
+	conn.UDPSize = uint16(state.Size())
+	if conn.UDPSize < 512 {
+		conn.UDPSize = 512
+	}
+
+	conn.SetWriteDeadline(time.Now().Add(timeout))
+	if err := conn.WriteMsg(state.Req); err != nil {
+		conn.Close() // not giving it back
+		return nil, err
+	}
+
+	conn.SetReadDeadline(time.Now().Add(timeout))
+	ret, err := conn.ReadMsg()
+	if err != nil {
+		conn.Close() // not giving it back
+		return nil, err
+	}
+
+	p.Yield(conn)
+
+	if metric {
+		rc, ok := dns.RcodeToString[ret.Rcode]
+		if !ok {
+			rc = strconv.Itoa(ret.Rcode)
+		}
+
+		RequestCount.WithLabelValues(p.host.addr).Add(1)
+		RcodeCount.WithLabelValues(rc, p.host.addr).Add(1)
+		RequestDuration.WithLabelValues(p.host.addr).Observe(time.Since(start).Seconds())
+	}
+
+	return ret, nil
+}