plugin/forward: Allow Proxy to be used outside of forward plugin. (#5951)

* plugin/forward: Move Proxy into pkg/plugin/proxy, to allow forward.Proxy to be used outside of forward plugin.

Signed-off-by: Patrick Downey <patrick.downey@dioadconsulting.com>

plugin/pkg/proxy/connect.go

@@ -0,0 +1,152 @@
+// Package proxy 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 opening a new connection for every client. It works with UDP and TCP and uses
+// inband healthchecking.
+package proxy
+
+import (
+	"context"
+	"io"
+	"strconv"
+	"sync/atomic"
+	"time"
+
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+// limitTimeout is a utility function to auto-tune timeout values
+// average observed time is moved towards the last observed delay moderated by a weight
+// next timeout to use will be the double of the computed average, limited by min and max frame.
+func limitTimeout(currentAvg *int64, minValue time.Duration, maxValue time.Duration) time.Duration {
+	rt := time.Duration(atomic.LoadInt64(currentAvg))
+	if rt < minValue {
+		return minValue
+	}
+	if rt < maxValue/2 {
+		return 2 * rt
+	}
+	return maxValue
+}
+
+func averageTimeout(currentAvg *int64, observedDuration time.Duration, weight int64) {
+	dt := time.Duration(atomic.LoadInt64(currentAvg))
+	atomic.AddInt64(currentAvg, int64(observedDuration-dt)/weight)
+}
+
+func (t *Transport) dialTimeout() time.Duration {
+	return limitTimeout(&t.avgDialTime, minDialTimeout, maxDialTimeout)
+}
+
+func (t *Transport) updateDialTimeout(newDialTime time.Duration) {
+	averageTimeout(&t.avgDialTime, newDialTime, cumulativeAvgWeight)
+}
+
+// Dial dials the address configured in transport, potentially reusing a connection or creating a new one.
+func (t *Transport) Dial(proto string) (*persistConn, bool, error) {
+	// If tls has been configured; use it.
+	if t.tlsConfig != nil {
+		proto = "tcp-tls"
+	}
+
+	t.dial <- proto
+	pc := <-t.ret
+
+	if pc != nil {
+		ConnCacheHitsCount.WithLabelValues(t.addr, proto).Add(1)
+		return pc, true, nil
+	}
+	ConnCacheMissesCount.WithLabelValues(t.addr, proto).Add(1)
+
+	reqTime := time.Now()
+	timeout := t.dialTimeout()
+	if proto == "tcp-tls" {
+		conn, err := dns.DialTimeoutWithTLS("tcp", t.addr, t.tlsConfig, timeout)
+		t.updateDialTimeout(time.Since(reqTime))
+		return &persistConn{c: conn}, false, err
+	}
+	conn, err := dns.DialTimeout(proto, t.addr, timeout)
+	t.updateDialTimeout(time.Since(reqTime))
+	return &persistConn{c: conn}, false, err
+}
+
+// Connect selects an upstream, sends the request and waits for a response.
+func (p *Proxy) Connect(ctx context.Context, state request.Request, opts Options) (*dns.Msg, error) {
+	start := time.Now()
+
+	proto := ""
+	switch {
+	case opts.ForceTCP: // TCP flag has precedence over UDP flag
+		proto = "tcp"
+	case opts.PreferUDP:
+		proto = "udp"
+	default:
+		proto = state.Proto()
+	}
+
+	pc, cached, err := p.transport.Dial(proto)
+	if err != nil {
+		return nil, err
+	}
+
+	// Set buffer size correctly for this client.
+	pc.c.UDPSize = uint16(state.Size())
+	if pc.c.UDPSize < 512 {
+		pc.c.UDPSize = 512
+	}
+
+	pc.c.SetWriteDeadline(time.Now().Add(maxTimeout))
+	// records the origin Id before upstream.
+	originId := state.Req.Id
+	state.Req.Id = dns.Id()
+	defer func() {
+		state.Req.Id = originId
+	}()
+
+	if err := pc.c.WriteMsg(state.Req); err != nil {
+		pc.c.Close() // not giving it back
+		if err == io.EOF && cached {
+			return nil, ErrCachedClosed
+		}
+		return nil, err
+	}
+
+	var ret *dns.Msg
+	pc.c.SetReadDeadline(time.Now().Add(p.readTimeout))
+	for {
+		ret, err = pc.c.ReadMsg()
+		if err != nil {
+			pc.c.Close() // not giving it back
+			if err == io.EOF && cached {
+				return nil, ErrCachedClosed
+			}
+			// recovery the origin Id after upstream.
+			if ret != nil {
+				ret.Id = originId
+			}
+			return ret, err
+		}
+		// drop out-of-order responses
+		if state.Req.Id == ret.Id {
+			break
+		}
+	}
+	// recovery the origin Id after upstream.
+	ret.Id = originId
+
+	p.transport.Yield(pc)
+
+	rc, ok := dns.RcodeToString[ret.Rcode]
+	if !ok {
+		rc = strconv.Itoa(ret.Rcode)
+	}
+
+	RequestCount.WithLabelValues(p.addr).Add(1)
+	RcodeCount.WithLabelValues(rc, p.addr).Add(1)
+	RequestDuration.WithLabelValues(p.addr, rc).Observe(time.Since(start).Seconds())
+
+	return ret, nil
+}
+
+const cumulativeAvgWeight = 4

plugin/pkg/proxy/errors.go

@@ -0,0 +1,26 @@
+package proxy
+
+import (
+	"errors"
+)
+
+var (
+	// ErrNoHealthy means no healthy proxies left.
+	ErrNoHealthy = errors.New("no healthy proxies")
+	// ErrNoForward means no forwarder defined.
+	ErrNoForward = errors.New("no forwarder defined")
+	// ErrCachedClosed means cached connection was closed by peer.
+	ErrCachedClosed = errors.New("cached connection was closed by peer")
+)
+
+// Options holds various Options that can be set.
+type Options struct {
+	// ForceTCP use TCP protocol for upstream DNS request. Has precedence over PreferUDP flag
+	ForceTCP bool
+	// PreferUDP use UDP protocol for upstream DNS request.
+	PreferUDP bool
+	// HCRecursionDesired sets recursion desired flag for Proxy healthcheck requests
+	HCRecursionDesired bool
+	// HCDomain sets domain for Proxy healthcheck requests
+	HCDomain string
+}

plugin/pkg/proxy/health.go

@@ -0,0 +1,131 @@
+package proxy
+
+import (
+	"crypto/tls"
+	"sync/atomic"
+	"time"
+
+	"github.com/coredns/coredns/plugin/pkg/log"
+	"github.com/coredns/coredns/plugin/pkg/transport"
+
+	"github.com/miekg/dns"
+)
+
+// HealthChecker checks the upstream health.
+type HealthChecker interface {
+	Check(*Proxy) error
+	SetTLSConfig(*tls.Config)
+	GetTLSConfig() *tls.Config
+	SetRecursionDesired(bool)
+	GetRecursionDesired() bool
+	SetDomain(domain string)
+	GetDomain() string
+	SetTCPTransport()
+	GetReadTimeout() time.Duration
+	SetReadTimeout(time.Duration)
+	GetWriteTimeout() time.Duration
+	SetWriteTimeout(time.Duration)
+}
+
+// dnsHc is a health checker for a DNS endpoint (DNS, and DoT).
+type dnsHc struct {
+	c                *dns.Client
+	recursionDesired bool
+	domain           string
+}
+
+// NewHealthChecker returns a new HealthChecker based on transport.
+func NewHealthChecker(trans string, recursionDesired bool, domain string) HealthChecker {
+	switch trans {
+	case transport.DNS, transport.TLS:
+		c := new(dns.Client)
+		c.Net = "udp"
+		c.ReadTimeout = 1 * time.Second
+		c.WriteTimeout = 1 * time.Second
+
+		return &dnsHc{
+			c:                c,
+			recursionDesired: recursionDesired,
+			domain:           domain,
+		}
+	}
+
+	log.Warningf("No healthchecker for transport %q", trans)
+	return nil
+}
+
+func (h *dnsHc) SetTLSConfig(cfg *tls.Config) {
+	h.c.Net = "tcp-tls"
+	h.c.TLSConfig = cfg
+}
+
+func (h *dnsHc) GetTLSConfig() *tls.Config {
+	return h.c.TLSConfig
+}
+
+func (h *dnsHc) SetRecursionDesired(recursionDesired bool) {
+	h.recursionDesired = recursionDesired
+}
+func (h *dnsHc) GetRecursionDesired() bool {
+	return h.recursionDesired
+}
+
+func (h *dnsHc) SetDomain(domain string) {
+	h.domain = domain
+}
+func (h *dnsHc) GetDomain() string {
+	return h.domain
+}
+
+func (h *dnsHc) SetTCPTransport() {
+	h.c.Net = "tcp"
+}
+
+func (h *dnsHc) GetReadTimeout() time.Duration {
+	return h.c.ReadTimeout
+}
+
+func (h *dnsHc) SetReadTimeout(t time.Duration) {
+	h.c.ReadTimeout = t
+}
+
+func (h *dnsHc) GetWriteTimeout() time.Duration {
+	return h.c.WriteTimeout
+}
+
+func (h *dnsHc) SetWriteTimeout(t time.Duration) {
+	h.c.WriteTimeout = t
+}
+
+// For HC, we send to . IN NS +[no]rec message to the upstream. Dial timeouts and empty
+// replies are considered fails, basically anything else constitutes a healthy upstream.
+
+// Check is used as the up.Func in the up.Probe.
+func (h *dnsHc) Check(p *Proxy) error {
+	err := h.send(p.addr)
+	if err != nil {
+		HealthcheckFailureCount.WithLabelValues(p.addr).Add(1)
+		atomic.AddUint32(&p.fails, 1)
+		return err
+	}
+
+	atomic.StoreUint32(&p.fails, 0)
+	return nil
+}
+
+func (h *dnsHc) send(addr string) error {
+	ping := new(dns.Msg)
+	ping.SetQuestion(h.domain, dns.TypeNS)
+	ping.MsgHdr.RecursionDesired = h.recursionDesired
+
+	m, _, err := h.c.Exchange(ping, addr)
+	// If we got a header, we're alright, basically only care about I/O errors 'n stuff.
+	if err != nil && m != nil {
+		// Silly check, something sane came back.
+		if m.Response || m.Opcode == dns.OpcodeQuery {
+			err = nil
+		}
+	}
+
+	return err
+}

plugin/pkg/proxy/health_test.go

@@ -0,0 +1,153 @@
+package proxy
+
+import (
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/coredns/coredns/plugin/pkg/dnstest"
+	"github.com/coredns/coredns/plugin/pkg/transport"
+
+	"github.com/miekg/dns"
+)
+
+func TestHealth(t *testing.T) {
+	i := uint32(0)
+	s := dnstest.NewServer(func(w dns.ResponseWriter, r *dns.Msg) {
+		if r.Question[0].Name == "." && r.RecursionDesired == true {
+			atomic.AddUint32(&i, 1)
+		}
+		ret := new(dns.Msg)
+		ret.SetReply(r)
+		w.WriteMsg(ret)
+	})
+	defer s.Close()
+
+	hc := NewHealthChecker(transport.DNS, true, "")
+	hc.SetReadTimeout(10 * time.Millisecond)
+	hc.SetWriteTimeout(10 * time.Millisecond)
+
+	p := NewProxy(s.Addr, transport.DNS)
+	p.readTimeout = 10 * time.Millisecond
+	err := hc.Check(p)
+	if err != nil {
+		t.Errorf("check failed: %v", err)
+	}
+
+	time.Sleep(20 * time.Millisecond)
+	i1 := atomic.LoadUint32(&i)
+	if i1 != 1 {
+		t.Errorf("Expected number of health checks with RecursionDesired==true to be %d, got %d", 1, i1)
+	}
+}
+
+func TestHealthTCP(t *testing.T) {
+	i := uint32(0)
+	s := dnstest.NewServer(func(w dns.ResponseWriter, r *dns.Msg) {
+		if r.Question[0].Name == "." && r.RecursionDesired == true {
+			atomic.AddUint32(&i, 1)
+		}
+		ret := new(dns.Msg)
+		ret.SetReply(r)
+		w.WriteMsg(ret)
+	})
+	defer s.Close()
+
+	hc := NewHealthChecker(transport.DNS, true, "")
+	hc.SetTCPTransport()
+	hc.SetReadTimeout(10 * time.Millisecond)
+	hc.SetWriteTimeout(10 * time.Millisecond)
+
+	p := NewProxy(s.Addr, transport.DNS)
+	p.readTimeout = 10 * time.Millisecond
+	err := hc.Check(p)
+	if err != nil {
+		t.Errorf("check failed: %v", err)
+	}
+
+	time.Sleep(20 * time.Millisecond)
+	i1 := atomic.LoadUint32(&i)
+	if i1 != 1 {
+		t.Errorf("Expected number of health checks with RecursionDesired==true to be %d, got %d", 1, i1)
+	}
+}
+
+func TestHealthNoRecursion(t *testing.T) {
+	i := uint32(0)
+	s := dnstest.NewServer(func(w dns.ResponseWriter, r *dns.Msg) {
+		if r.Question[0].Name == "." && r.RecursionDesired == false {
+			atomic.AddUint32(&i, 1)
+		}
+		ret := new(dns.Msg)
+		ret.SetReply(r)
+		w.WriteMsg(ret)
+	})
+	defer s.Close()
+
+	hc := NewHealthChecker(transport.DNS, false, "")
+	hc.SetReadTimeout(10 * time.Millisecond)
+	hc.SetWriteTimeout(10 * time.Millisecond)
+
+	p := NewProxy(s.Addr, transport.DNS)
+	p.readTimeout = 10 * time.Millisecond
+	err := hc.Check(p)
+	if err != nil {
+		t.Errorf("check failed: %v", err)
+	}
+
+	time.Sleep(20 * time.Millisecond)
+	i1 := atomic.LoadUint32(&i)
+	if i1 != 1 {
+		t.Errorf("Expected number of health checks with RecursionDesired==false to be %d, got %d", 1, i1)
+	}
+}
+
+func TestHealthTimeout(t *testing.T) {
+	s := dnstest.NewServer(func(w dns.ResponseWriter, r *dns.Msg) {
+		// timeout
+	})
+	defer s.Close()
+
+	hc := NewHealthChecker(transport.DNS, false, "")
+	hc.SetReadTimeout(10 * time.Millisecond)
+	hc.SetWriteTimeout(10 * time.Millisecond)
+
+	p := NewProxy(s.Addr, transport.DNS)
+	p.readTimeout = 10 * time.Millisecond
+	err := hc.Check(p)
+	if err == nil {
+		t.Errorf("expected error")
+	}
+}
+
+func TestHealthDomain(t *testing.T) {
+	hcDomain := "example.org."
+
+	i := uint32(0)
+	s := dnstest.NewServer(func(w dns.ResponseWriter, r *dns.Msg) {
+		if r.Question[0].Name == hcDomain && r.RecursionDesired == true {
+			atomic.AddUint32(&i, 1)
+		}
+		ret := new(dns.Msg)
+		ret.SetReply(r)
+		w.WriteMsg(ret)
+	})
+	defer s.Close()
+
+	hc := NewHealthChecker(transport.DNS, true, hcDomain)
+	hc.SetReadTimeout(10 * time.Millisecond)
+	hc.SetWriteTimeout(10 * time.Millisecond)
+
+	p := NewProxy(s.Addr, transport.DNS)
+	p.readTimeout = 10 * time.Millisecond
+	err := hc.Check(p)
+	if err != nil {
+		t.Errorf("check failed: %v", err)
+	}
+
+	time.Sleep(12 * time.Millisecond)
+	i1 := atomic.LoadUint32(&i)
+	if i1 != 1 {
+		t.Errorf("Expected number of health checks with Domain==%s to be %d, got %d", hcDomain, 1, i1)
+	}
+}

plugin/pkg/proxy/metrics.go

@@ -0,0 +1,49 @@
+package proxy
+
+import (
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+// Variables declared for monitoring.
+var (
+	RequestCount = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: "proxy",
+		Name:      "requests_total",
+		Help:      "Counter of requests made per upstream.",
+	}, []string{"to"})
+	RcodeCount = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: "proxy",
+		Name:      "responses_total",
+		Help:      "Counter of responses received per upstream.",
+	}, []string{"rcode", "to"})
+	RequestDuration = promauto.NewHistogramVec(prometheus.HistogramOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: "proxy",
+		Name:      "request_duration_seconds",
+		Buckets:   plugin.TimeBuckets,
+		Help:      "Histogram of the time each request took.",
+	}, []string{"to", "rcode"})
+	HealthcheckFailureCount = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: "proxy",
+		Name:      "healthcheck_failures_total",
+		Help:      "Counter of the number of failed healthchecks.",
+	}, []string{"to"})
+	ConnCacheHitsCount = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: "proxy",
+		Name:      "conn_cache_hits_total",
+		Help:      "Counter of connection cache hits per upstream and protocol.",
+	}, []string{"to", "proto"})
+	ConnCacheMissesCount = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: "proxy",
+		Name:      "conn_cache_misses_total",
+		Help:      "Counter of connection cache misses per upstream and protocol.",
+	}, []string{"to", "proto"})
+)

plugin/pkg/proxy/persistent.go

@@ -0,0 +1,156 @@
+package proxy
+
+import (
+	"crypto/tls"
+	"sort"
+	"time"
+
+	"github.com/miekg/dns"
+)
+
+// a persistConn hold the dns.Conn and the last used time.
+type persistConn struct {
+	c    *dns.Conn
+	used time.Time
+}
+
+// Transport hold the persistent cache.
+type Transport struct {
+	avgDialTime int64                          // kind of average time of dial time
+	conns       [typeTotalCount][]*persistConn // Buckets for udp, tcp and tcp-tls.
+	expire      time.Duration                  // After this duration a connection is expired.
+	addr        string
+	tlsConfig   *tls.Config
+
+	dial  chan string
+	yield chan *persistConn
+	ret   chan *persistConn
+	stop  chan bool
+}
+
+func newTransport(addr string) *Transport {
+	t := &Transport{
+		avgDialTime: int64(maxDialTimeout / 2),
+		conns:       [typeTotalCount][]*persistConn{},
+		expire:      defaultExpire,
+		addr:        addr,
+		dial:        make(chan string),
+		yield:       make(chan *persistConn),
+		ret:         make(chan *persistConn),
+		stop:        make(chan bool),
+	}
+	return t
+}
+
+// connManagers manages the persistent connection cache for UDP and TCP.
+func (t *Transport) connManager() {
+	ticker := time.NewTicker(defaultExpire)
+	defer ticker.Stop()
+Wait:
+	for {
+		select {
+		case proto := <-t.dial:
+			transtype := stringToTransportType(proto)
+			// take the last used conn - complexity O(1)
+			if stack := t.conns[transtype]; len(stack) > 0 {
+				pc := stack[len(stack)-1]
+				if time.Since(pc.used) < t.expire {
+					// Found one, remove from pool and return this conn.
+					t.conns[transtype] = stack[:len(stack)-1]
+					t.ret <- pc
+					continue Wait
+				}
+				// clear entire cache if the last conn is expired
+				t.conns[transtype] = nil
+				// now, the connections being passed to closeConns() are not reachable from
+				// transport methods anymore. So, it's safe to close them in a separate goroutine
+				go closeConns(stack)
+			}
+			t.ret <- nil
+
+		case pc := <-t.yield:
+			transtype := t.transportTypeFromConn(pc)
+			t.conns[transtype] = append(t.conns[transtype], pc)
+
+		case <-ticker.C:
+			t.cleanup(false)
+
+		case <-t.stop:
+			t.cleanup(true)
+			close(t.ret)
+			return
+		}
+	}
+}
+
+// closeConns closes connections.
+func closeConns(conns []*persistConn) {
+	for _, pc := range conns {
+		pc.c.Close()
+	}
+}
+
+// cleanup removes connections from cache.
+func (t *Transport) cleanup(all bool) {
+	staleTime := time.Now().Add(-t.expire)
+	for transtype, stack := range t.conns {
+		if len(stack) == 0 {
+			continue
+		}
+		if all {
+			t.conns[transtype] = nil
+			// now, the connections being passed to closeConns() are not reachable from
+			// transport methods anymore. So, it's safe to close them in a separate goroutine
+			go closeConns(stack)
+			continue
+		}
+		if stack[0].used.After(staleTime) {
+			continue
+		}
+
+		// connections in stack are sorted by "used"
+		good := sort.Search(len(stack), func(i int) bool {
+			return stack[i].used.After(staleTime)
+		})
+		t.conns[transtype] = stack[good:]
+		// now, the connections being passed to closeConns() are not reachable from
+		// transport methods anymore. So, it's safe to close them in a separate goroutine
+		go closeConns(stack[:good])
+	}
+}
+
+// It is hard to pin a value to this, the import thing is to no block forever, losing at cached connection is not terrible.
+const yieldTimeout = 25 * time.Millisecond
+
+// Yield returns the connection to transport for reuse.
+func (t *Transport) Yield(pc *persistConn) {
+	pc.used = time.Now() // update used time
+
+	// Make this non-blocking, because in the case of a very busy forwarder we will *block* on this yield. This
+	// blocks the outer go-routine and stuff will just pile up.  We timeout when the send fails to as returning
+	// these connection is an optimization anyway.
+	select {
+	case t.yield <- pc:
+		return
+	case <-time.After(yieldTimeout):
+		return
+	}
+}
+
+// Start starts the transport's connection manager.
+func (t *Transport) Start() { go t.connManager() }
+
+// Stop stops the transport's connection manager.
+func (t *Transport) Stop() { close(t.stop) }
+
+// SetExpire sets the connection expire time in transport.
+func (t *Transport) SetExpire(expire time.Duration) { t.expire = expire }
+
+// SetTLSConfig sets the TLS config in transport.
+func (t *Transport) SetTLSConfig(cfg *tls.Config) { t.tlsConfig = cfg }
+
+const (
+	defaultExpire  = 10 * time.Second
+	minDialTimeout = 1 * time.Second
+	maxDialTimeout = 30 * time.Second
+)

plugin/pkg/proxy/persistent_test.go

@@ -0,0 +1,109 @@
+package proxy
+
+import (
+	"testing"
+	"time"
+
+	"github.com/coredns/coredns/plugin/pkg/dnstest"
+
+	"github.com/miekg/dns"
+)
+
+func TestCached(t *testing.T) {
+	s := dnstest.NewServer(func(w dns.ResponseWriter, r *dns.Msg) {
+		ret := new(dns.Msg)
+		ret.SetReply(r)
+		w.WriteMsg(ret)
+	})
+	defer s.Close()
+
+	tr := newTransport(s.Addr)
+	tr.Start()
+	defer tr.Stop()
+
+	c1, cache1, _ := tr.Dial("udp")
+	c2, cache2, _ := tr.Dial("udp")
+
+	if cache1 || cache2 {
+		t.Errorf("Expected non-cached connection")
+	}
+
+	tr.Yield(c1)
+	tr.Yield(c2)
+	c3, cached3, _ := tr.Dial("udp")
+	if !cached3 {
+		t.Error("Expected cached connection (c3)")
+	}
+	if c2 != c3 {
+		t.Error("Expected c2 == c3")
+	}
+
+	tr.Yield(c3)
+
+	// dial another protocol
+	c4, cached4, _ := tr.Dial("tcp")
+	if cached4 {
+		t.Errorf("Expected non-cached connection (c4)")
+	}
+	tr.Yield(c4)
+}
+
+func TestCleanupByTimer(t *testing.T) {
+	s := dnstest.NewServer(func(w dns.ResponseWriter, r *dns.Msg) {
+		ret := new(dns.Msg)
+		ret.SetReply(r)
+		w.WriteMsg(ret)
+	})
+	defer s.Close()
+
+	tr := newTransport(s.Addr)
+	tr.SetExpire(100 * time.Millisecond)
+	tr.Start()
+	defer tr.Stop()
+
+	c1, _, _ := tr.Dial("udp")
+	c2, _, _ := tr.Dial("udp")
+	tr.Yield(c1)
+	time.Sleep(10 * time.Millisecond)
+	tr.Yield(c2)
+
+	time.Sleep(120 * time.Millisecond)
+	c3, cached, _ := tr.Dial("udp")
+	if cached {
+		t.Error("Expected non-cached connection (c3)")
+	}
+	tr.Yield(c3)
+
+	time.Sleep(120 * time.Millisecond)
+	c4, cached, _ := tr.Dial("udp")
+	if cached {
+		t.Error("Expected non-cached connection (c4)")
+	}
+	tr.Yield(c4)
+}
+
+func TestCleanupAll(t *testing.T) {
+	s := dnstest.NewServer(func(w dns.ResponseWriter, r *dns.Msg) {
+		ret := new(dns.Msg)
+		ret.SetReply(r)
+		w.WriteMsg(ret)
+	})
+	defer s.Close()
+
+	tr := newTransport(s.Addr)
+
+	c1, _ := dns.DialTimeout("udp", tr.addr, maxDialTimeout)
+	c2, _ := dns.DialTimeout("udp", tr.addr, maxDialTimeout)
+	c3, _ := dns.DialTimeout("udp", tr.addr, maxDialTimeout)
+
+	tr.conns[typeUDP] = []*persistConn{{c1, time.Now()}, {c2, time.Now()}, {c3, time.Now()}}
+
+	if len(tr.conns[typeUDP]) != 3 {
+		t.Error("Expected 3 connections")
+	}
+	tr.cleanup(true)
+
+	if len(tr.conns[typeUDP]) > 0 {
+		t.Error("Expected no cached connections")
+	}
+}

plugin/pkg/proxy/proxy.go

@@ -0,0 +1,98 @@
+package proxy
+
+import (
+	"crypto/tls"
+	"runtime"
+	"sync/atomic"
+	"time"
+
+	"github.com/coredns/coredns/plugin/pkg/log"
+	"github.com/coredns/coredns/plugin/pkg/up"
+)
+
+// Proxy defines an upstream host.
+type Proxy struct {
+	fails uint32
+	addr  string
+
+	transport *Transport
+
+	readTimeout time.Duration
+
+	// health checking
+	probe  *up.Probe
+	health HealthChecker
+}
+
+// NewProxy returns a new proxy.
+func NewProxy(addr, trans string) *Proxy {
+	p := &Proxy{
+		addr:        addr,
+		fails:       0,
+		probe:       up.New(),
+		readTimeout: 2 * time.Second,
+		transport:   newTransport(addr),
+	}
+	p.health = NewHealthChecker(trans, true, ".")
+	runtime.SetFinalizer(p, (*Proxy).finalizer)
+	return p
+}
+
+func (p *Proxy) Addr() string { return p.addr }
+
+// SetTLSConfig sets the TLS config in the lower p.transport and in the healthchecking client.
+func (p *Proxy) SetTLSConfig(cfg *tls.Config) {
+	p.transport.SetTLSConfig(cfg)
+	p.health.SetTLSConfig(cfg)
+}
+
+// SetExpire sets the expire duration in the lower p.transport.
+func (p *Proxy) SetExpire(expire time.Duration) { p.transport.SetExpire(expire) }
+
+func (p *Proxy) GetHealthchecker() HealthChecker {
+	return p.health
+}
+
+func (p *Proxy) Fails() uint32 {
+	return atomic.LoadUint32(&p.fails)
+}
+
+// Healthcheck kicks of a round of health checks for this proxy.
+func (p *Proxy) Healthcheck() {
+	if p.health == nil {
+		log.Warning("No healthchecker")
+		return
+	}
+
+	p.probe.Do(func() error {
+		return p.health.Check(p)
+	})
+}
+
+// Down returns true if this proxy is down, i.e. has *more* fails than maxfails.
+func (p *Proxy) Down(maxfails uint32) bool {
+	if maxfails == 0 {
+		return false
+	}
+
+	fails := atomic.LoadUint32(&p.fails)
+	return fails > maxfails
+}
+
+// Stop close stops the health checking goroutine.
+func (p *Proxy) Stop()      { p.probe.Stop() }
+func (p *Proxy) finalizer() { p.transport.Stop() }
+
+// Start starts the proxy's healthchecking.
+func (p *Proxy) Start(duration time.Duration) {
+	p.probe.Start(duration)
+	p.transport.Start()
+}
+
+func (p *Proxy) SetReadTimeout(duration time.Duration) {
+	p.readTimeout = duration
+}
+
+const (
+	maxTimeout = 2 * time.Second
+)

plugin/pkg/proxy/proxy_test.go

@@ -0,0 +1,99 @@
+package proxy
+
+import (
+	"context"
+	"crypto/tls"
+	"testing"
+	"time"
+
+	"github.com/coredns/coredns/plugin/pkg/dnstest"
+	"github.com/coredns/coredns/plugin/pkg/transport"
+	"github.com/coredns/coredns/plugin/test"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+func TestProxy(t *testing.T) {
+	s := dnstest.NewServer(func(w dns.ResponseWriter, r *dns.Msg) {
+		ret := new(dns.Msg)
+		ret.SetReply(r)
+		ret.Answer = append(ret.Answer, test.A("example.org. IN A 127.0.0.1"))
+		w.WriteMsg(ret)
+	})
+	defer s.Close()
+
+	p := NewProxy(s.Addr, transport.DNS)
+	p.readTimeout = 10 * time.Millisecond
+	p.Start(5 * time.Second)
+	m := new(dns.Msg)
+
+	m.SetQuestion("example.org.", dns.TypeA)
+
+	rec := dnstest.NewRecorder(&test.ResponseWriter{})
+	req := request.Request{Req: m, W: rec}
+
+	resp, err := p.Connect(context.Background(), req, Options{PreferUDP: true})
+	if err != nil {
+		t.Errorf("Failed to connect to testdnsserver: %s", err)
+	}
+
+	if x := resp.Answer[0].Header().Name; x != "example.org." {
+		t.Errorf("Expected %s, got %s", "example.org.", x)
+	}
+}
+
+func TestProxyTLSFail(t *testing.T) {
+	// This is an udp/tcp test server, so we shouldn't reach it with TLS.
+	s := dnstest.NewServer(func(w dns.ResponseWriter, r *dns.Msg) {
+		ret := new(dns.Msg)
+		ret.SetReply(r)
+		ret.Answer = append(ret.Answer, test.A("example.org. IN A 127.0.0.1"))
+		w.WriteMsg(ret)
+	})
+	defer s.Close()
+
+	p := NewProxy(s.Addr, transport.TLS)
+	p.readTimeout = 10 * time.Millisecond
+	p.SetTLSConfig(&tls.Config{})
+	p.Start(5 * time.Second)
+	m := new(dns.Msg)
+
+	m.SetQuestion("example.org.", dns.TypeA)
+
+	rec := dnstest.NewRecorder(&test.ResponseWriter{})
+	req := request.Request{Req: m, W: rec}
+
+	_, err := p.Connect(context.Background(), req, Options{})
+	if err == nil {
+		t.Fatal("Expected *not* to receive reply, but got one")
+	}
+}
+
+func TestProtocolSelection(t *testing.T) {
+	p := NewProxy("bad_address", transport.DNS)
+	p.readTimeout = 10 * time.Millisecond
+
+	stateUDP := request.Request{W: &test.ResponseWriter{}, Req: new(dns.Msg)}
+	stateTCP := request.Request{W: &test.ResponseWriter{TCP: true}, Req: new(dns.Msg)}
+	ctx := context.TODO()
+
+	go func() {
+		p.Connect(ctx, stateUDP, Options{})
+		p.Connect(ctx, stateUDP, Options{ForceTCP: true})
+		p.Connect(ctx, stateUDP, Options{PreferUDP: true})
+		p.Connect(ctx, stateUDP, Options{PreferUDP: true, ForceTCP: true})
+		p.Connect(ctx, stateTCP, Options{})
+		p.Connect(ctx, stateTCP, Options{ForceTCP: true})
+		p.Connect(ctx, stateTCP, Options{PreferUDP: true})
+		p.Connect(ctx, stateTCP, Options{PreferUDP: true, ForceTCP: true})
+	}()
+
+	for i, exp := range []string{"udp", "tcp", "udp", "tcp", "tcp", "tcp", "udp", "tcp"} {
+		proto := <-p.transport.dial
+		p.transport.ret <- nil
+		if proto != exp {
+			t.Errorf("Unexpected protocol in case %d, expected %q, actual %q", i, exp, proto)
+		}
+	}
+}

plugin/pkg/proxy/type.go

@@ -0,0 +1,39 @@
+package proxy
+
+import (
+	"net"
+)
+
+type transportType int
+
+const (
+	typeUDP transportType = iota
+	typeTCP
+	typeTLS
+	typeTotalCount // keep this last
+)
+
+func stringToTransportType(s string) transportType {
+	switch s {
+	case "udp":
+		return typeUDP
+	case "tcp":
+		return typeTCP
+	case "tcp-tls":
+		return typeTLS
+	}
+
+	return typeUDP
+}
+
+func (t *Transport) transportTypeFromConn(pc *persistConn) transportType {
+	if _, ok := pc.c.Conn.(*net.UDPConn); ok {
+		return typeUDP
+	}
+
+	if t.tlsConfig == nil {
+		return typeTCP
+	}
+
+	return typeTLS
+}