plugin/forward using pkg/up (#1493)

* plugin/forward: on demand healtchecking Only start doing health checks when we encouner an error (any error). This uses the new pluing/pkg/up package to abstract away the actual checking. This reduces the LOC quite a bit; does need more testing, unit testing and tcpdumping a bit. * fix tests * Fix readme * Use pkg/up for healthchecks * remove unused channel * more cleanups * update readme * * Again do go generate and go build; still referencing the wrong forward repo? Anyway fixed. * Use pkg/up for doing the healtchecks to cut back on unwanted queries * Change up.Func to return an error instead of a boolean. * Drop the string target argument as it doesn't make sense. * Add healthcheck test on failing to get an upstream answer. TODO(miek): double check Forward and Lookup and how they interact with HC, and if we correctly call close() on those * actual test * Tests here * more tests * try getting rid of host * Get rid of the host indirection * Finish removing hosts * moar testing * import fmt * field is not used * docs * move some stuff * bring back health_check * maxfails=0 test * git and merging, bah * review
2025-10-31 10:13:14 -04:00 · 2018-02-15 10:21:57 +01:00
parent 8b035fa938
commit 16504234e5
15 changed files with 306 additions and 221 deletions
--- a/plugin/forward/README.md
+++ b/plugin/forward/README.md
@@ -6,10 +6,17 @@

 ## Description

-The *forward* plugin is generally faster (~30+%) than *proxy* as it re-uses already opened sockets
-to the upstreams. It supports UDP, TCP and DNS-over-TLS and uses inband health checking that is
-enabled by default.
-When *all* upstreams are down it assumes healtchecking as a mechanism has failed and will try to
+The *forward* plugin re-uses already opened sockets to the upstreams. It supports UDP, TCP and
+DNS-over-TLS and uses in band health checking.
+
+When it detects an error a health check is performed. This checks runs in a loop, every *0.5s*, for
+as long as the upstream reports unhealthy. Once healthy we stop health checking (until the next
+error). The health checks use a recursive DNS query (`. IN NS`) to get upstream health. Any response
+that is not a network error (REFUSED, NOTIMPL, SERVFAIL, etc) is taken as a healthy upstream. The
+health check uses the same protocol as specified in **TO**. If `max_fails` is set to 0, no checking
+is performed and upstreams will always be considered healthy.
+
+When *all* upstreams are down it assumes health checking as a mechanism has failed and will try to
 connect to a random upstream (which may or may not work).

 ## Syntax
@@ -22,16 +29,11 @@ forward FROM TO...

 * **FROM** is the base domain to match for the request to be forwarded.
 * **TO...** are the destination endpoints to forward to. The **TO** syntax allows you to specify
-  a protocol, `tls://9.9.9.9` or `dns://` for plain DNS. The number of upstreams is limited to 15.
+  a protocol, `tls://9.9.9.9` or `dns://` (or no protocol) for plain DNS. The number of upstreams is
+  limited to 15.

-The health checks are done every *0.5s*. After *two* failed checks the upstream is considered
-unhealthy. The health checks use a recursive DNS query (`. IN NS`) to get upstream health. Any
-response that is not an error (REFUSED, NOTIMPL, SERVFAIL, etc) is taken as a healthy upstream. The
-health check uses the same protocol as specific in the **TO**. On startup each upstream is marked
-unhealthy until it passes a health check. A 0 duration will disable any health checks.
-
-Multiple upstreams are randomized (default policy) on first use. When a healthy proxy returns an
-error during the exchange the next upstream in the list is tried.
+Multiple upstreams are randomized (see `policy`) on first use. When a healthy proxy returns an error
+during the exchange the next upstream in the list is tried.

 Extra knobs are available with an expanded syntax:

@@ -39,12 +41,12 @@ Extra knobs are available with an expanded syntax:
 forward FROM TO... {
    except IGNORED_NAMES...
    force_tcp
-    health_check DURATION
    expire DURATION
    max_fails INTEGER
    tls CERT KEY CA
    tls_servername NAME
    policy random|round_robin
+    health_checks DURATION
 }
 ~~~

@@ -52,21 +54,16 @@ forward FROM TO... {
 * **IGNORED_NAMES** in `except` is a space-separated list of domains to exclude from forwarding.
  Requests that match none of these names will be passed through.
 * `force_tcp`, use TCP even when the request comes in over UDP.
-* `health_checks`, use a different **DURATION** for health checking, the default duration is 0.5s.
-  A value of 0 disables the health checks completely.
 * `max_fails` is the number of subsequent failed health checks that are needed before considering
-  a backend to be down. If 0, the backend will never be marked as down. Default is 2.
+  an upstream to be down. If 0, the upstream will never be marked as down (nor health checked).
+  Default is 2.
 * `expire` **DURATION**, expire (cached) connections after this time, the default is 10s.
 * `tls` **CERT** **KEY** **CA** define the TLS properties for TLS; if you leave this out the
  system's configuration will be used.
 * `tls_servername` **NAME** allows you to set a server name in the TLS configuration; for instance 9.9.9.9
  needs this to be set to `dns.quad9.net`.
 * `policy` specifies the policy to use for selecting upstream servers. The default is `random`.
-
-The upstream selection is done via random (default policy) selection. If the socket for this client
-isn't known *forward* will randomly choose one. If this turns out to be unhealthy, the next one is
-tried. If *all* hosts are down, we assume health checking is broken and select a *random* upstream to
-try.
+* `health_checks`, use a different **DURATION** for health checking, the default duration is 0.5s.

 Also note the TLS config is "global" for the whole forwarding proxy if you need a different
 `tls-name` for different upstreams you're out of luck.
@@ -80,7 +77,7 @@ If monitoring is enabled (via the *prometheus* directive) then the following met
 * `coredns_forward_response_rcode_total{to, rcode}` - count of RCODEs per upstream.
 * `coredns_forward_healthcheck_failure_count_total{to}` - number of failed health checks per upstream.
 * `coredns_forward_healthcheck_broken_count_total{}` - counter of when all upstreams are unhealthy,
-  and we are randomly spraying to a target.
+  and we are randomly (this always uses the `random` policy) spraying to an upstream.
 * `coredns_forward_socket_count_total{to}` - number of cached sockets per upstream.

 Where `to` is one of the upstream servers (**TO** from the config), `proto` is the protocol used by
@@ -125,16 +122,10 @@ Proxy everything except `example.org` using the host's `resolv.conf`'s nameserve
 }
 ~~~

-Forward to a IPv6 host:
-
-~~~ corefile
-. {
-    forward . [::1]:1053
-}
-~~~
-
 Proxy all requests to 9.9.9.9 using the DNS-over-TLS protocol, and cache every answer for up to 30
-seconds.
+seconds. Note the `tls_servername` is mandatory if you want a working setup, as 9.9.9.9 can't be
+used in the TLS negotiation. Also set the health check duration to 5s to not completely swamp the
+service with health checks.

 ~~~ corefile
 . {
@@ -148,7 +139,7 @@ seconds.

 ## Bugs

-The TLS config is global for the whole forwarding proxy if you need a different `tls-name` for
+The TLS config is global for the whole forwarding proxy if you need a different `tls_serveraame` for
 different upstreams you're out of luck.

 ## Also See
--- a/plugin/forward/connect.go
+++ b/plugin/forward/connect.go
@@ -21,9 +21,6 @@ func (p *Proxy) connect(ctx context.Context, state request.Request, forceTCP, me
 	if forceTCP {
 		proto = "tcp"
 	}
-	if p.host.tlsConfig != nil {
-		proto = "tcp-tls"
-	}

 	conn, err := p.Dial(proto)
 	if err != nil {
@@ -57,9 +54,9 @@ func (p *Proxy) connect(ctx context.Context, state request.Request, forceTCP, me
 			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())
+		RequestCount.WithLabelValues(p.addr).Add(1)
+		RcodeCount.WithLabelValues(rc, p.addr).Add(1)
+		RequestDuration.WithLabelValues(p.addr).Observe(time.Since(start).Seconds())
 	}

 	return ret, nil
--- a/plugin/forward/forward.go
+++ b/plugin/forward/forward.go
@@ -20,8 +20,9 @@ import (
 // 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
+	proxies    []*Proxy
+	p          Policy
+	hcInterval time.Duration

 	from    string
 	ignored []string
@@ -31,22 +32,21 @@ type Forward struct {
 	maxfails      uint32
 	expire        time.Duration

-	forceTCP   bool          // also here for testing
-	hcInterval time.Duration // also here for testing
+	forceTCP bool // 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)}
+	f := &Forward{maxfails: 2, tlsConfig: new(tls.Config), expire: defaultExpire, p: new(random), from: ".", hcInterval: hcDuration}
 	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()
+	p.start(f.hcInterval)
 }

 // Len returns the number of configured proxies.
@@ -92,7 +92,27 @@ func (f *Forward) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 			child.Finish()
 		}

+		// If you query for instance ANY isc.org; you get a truncated query back which miekg/dns fails to unpack
+		// because the RRs are not finished. The returned message can be useful or useless. Return the original
+		// query with some header bits set that they should retry with TCP.
+		if err == dns.ErrTruncated {
+			// We may or may not have something sensible... if not reassemble something to send to the client.
+			if ret == nil {
+				ret = new(dns.Msg)
+				ret.SetReply(r)
+				ret.Truncated = true
+				ret.Authoritative = true
+				ret.Rcode = dns.RcodeSuccess
+			}
+			err = nil // and reset err to pass this back to the client.
+		}
+
 		if err != nil {
+			// Kick off health check to see if *our* upstream is broken.
+			if f.maxfails != 0 {
+				proxy.Healthcheck()
+			}
+
 			if fails < len(f.proxies) {
 				continue
 			}
@@ -140,8 +160,8 @@ func (f *Forward) isAllowedDomain(name string) bool {
 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")
+	errInvalidDomain = errors.New("invalid domain for forward")
+	errNoHealthy     = errors.New("no healthy proxies or upstream error")
 	errNoForward     = errors.New("no forwarder defined")
 )

--- a/plugin/forward/forward_test.go
+++ b/plugin/forward/forward_test.go
@@ -6,6 +6,7 @@ import (
 	"github.com/coredns/coredns/plugin/pkg/dnstest"
 	"github.com/coredns/coredns/plugin/test"
 	"github.com/coredns/coredns/request"
+
 	"github.com/miekg/dns"
 )

@@ -18,7 +19,7 @@ func TestForward(t *testing.T) {
 	})
 	defer s.Close()

-	p := NewProxy(s.Addr)
+	p := NewProxy(s.Addr, nil /* not TLS */)
 	f := New()
 	f.SetProxy(p)
 	defer f.Close()
--- a/plugin/forward/health.go
+++ b/plugin/forward/health.go
@@ -1,7 +1,6 @@
 package forward

 import (
-	"log"
 	"sync/atomic"

 	"github.com/miekg/dns"
@@ -10,41 +9,25 @@ import (
 // For HC we send to . IN NS +norec message to the upstream. Dial timeouts and empty
 // replies are considered fails, basically anything else constitutes a healthy upstream.

-func (h *host) Check() {
-	h.Lock()
-
-	if h.checking {
-		h.Unlock()
-		return
-	}
-
-	h.checking = true
-	h.Unlock()
-
-	err := h.send()
+// Check is used as the up.Func in the up.Probe.
+func (p *Proxy) Check() error {
+	err := p.send()
 	if err != nil {
-		log.Printf("[INFO] healtheck of %s failed with %s", h.addr, err)
-
-		HealthcheckFailureCount.WithLabelValues(h.addr).Add(1)
-
-		atomic.AddUint32(&h.fails, 1)
-	} else {
-		atomic.StoreUint32(&h.fails, 0)
+		HealthcheckFailureCount.WithLabelValues(p.addr).Add(1)
+		atomic.AddUint32(&p.fails, 1)
+		return err
 	}

-	h.Lock()
-	h.checking = false
-	h.Unlock()
-
-	return
+	atomic.StoreUint32(&p.fails, 0)
+	return nil
 }

-func (h *host) send() error {
+func (p *Proxy) send() error {
 	hcping := new(dns.Msg)
 	hcping.SetQuestion(".", dns.TypeNS)
 	hcping.RecursionDesired = false

-	m, _, err := h.client.Exchange(hcping, h.addr)
+	m, _, err := p.client.Exchange(hcping, p.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
@@ -55,13 +38,3 @@ func (h *host) send() error {

 	return err
 }
-
-// down returns true is this host has more than maxfails fails.
-func (h *host) down(maxfails uint32) bool {
-	if maxfails == 0 {
-		return false
-	}
-
-	fails := atomic.LoadUint32(&h.fails)
-	return fails > maxfails
-}
--- a/plugin/forward/health_test.go
+++ b/plugin/forward/health_test.go
@@ -0,0 +1,136 @@
+package forward
+
+import (
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/coredns/coredns/plugin/pkg/dnstest"
+	"github.com/coredns/coredns/plugin/test"
+
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+func TestHealth(t *testing.T) {
+	const expected = 0
+	i := uint32(0)
+	s := dnstest.NewServer(func(w dns.ResponseWriter, r *dns.Msg) {
+		if r.Question[0].Name == "." {
+			atomic.AddUint32(&i, 1)
+		}
+		ret := new(dns.Msg)
+		ret.SetReply(r)
+		w.WriteMsg(ret)
+	})
+	defer s.Close()
+
+	p := NewProxy(s.Addr, nil /* no TLS */)
+	f := New()
+	f.SetProxy(p)
+	defer f.Close()
+
+	req := new(dns.Msg)
+	req.SetQuestion("example.org.", dns.TypeA)
+
+	f.ServeDNS(context.TODO(), &test.ResponseWriter{}, req)
+
+	time.Sleep(1 * time.Second)
+	i1 := atomic.LoadUint32(&i)
+	if i1 != expected {
+		t.Errorf("Expected number of health checks to be %d, got %d", expected, i1)
+	}
+}
+
+func TestHealthTimeout(t *testing.T) {
+	const expected = 1
+	i := uint32(0)
+	s := dnstest.NewServer(func(w dns.ResponseWriter, r *dns.Msg) {
+		if r.Question[0].Name == "." {
+			// health check, answer
+			atomic.AddUint32(&i, 1)
+			ret := new(dns.Msg)
+			ret.SetReply(r)
+			w.WriteMsg(ret)
+		}
+		// not a health check, do a timeout
+	})
+	defer s.Close()
+
+	p := NewProxy(s.Addr, nil /* no TLS */)
+	f := New()
+	f.SetProxy(p)
+	defer f.Close()
+
+	req := new(dns.Msg)
+	req.SetQuestion("example.org.", dns.TypeA)
+
+	f.ServeDNS(context.TODO(), &test.ResponseWriter{}, req)
+
+	time.Sleep(1 * time.Second)
+	i1 := atomic.LoadUint32(&i)
+	if i1 != expected {
+		t.Errorf("Expected number of health checks to be %d, got %d", expected, i1)
+	}
+}
+
+func TestHealthFailTwice(t *testing.T) {
+	const expected = 2
+	i := uint32(0)
+	s := dnstest.NewServer(func(w dns.ResponseWriter, r *dns.Msg) {
+		if r.Question[0].Name == "." {
+			atomic.AddUint32(&i, 1)
+			i1 := atomic.LoadUint32(&i)
+			// Timeout health until we get the second one
+			if i1 < 2 {
+				return
+			}
+			ret := new(dns.Msg)
+			ret.SetReply(r)
+			w.WriteMsg(ret)
+		}
+	})
+	defer s.Close()
+
+	p := NewProxy(s.Addr, nil /* no TLS */)
+	f := New()
+	f.SetProxy(p)
+	defer f.Close()
+
+	req := new(dns.Msg)
+	req.SetQuestion("example.org.", dns.TypeA)
+
+	f.ServeDNS(context.TODO(), &test.ResponseWriter{}, req)
+
+	time.Sleep(3 * time.Second)
+	i1 := atomic.LoadUint32(&i)
+	if i1 != expected {
+		t.Errorf("Expected number of health checks to be %d, got %d", expected, i1)
+	}
+}
+
+func TestHealthMaxFails(t *testing.T) {
+	const expected = 0
+	i := uint32(0)
+	s := dnstest.NewServer(func(w dns.ResponseWriter, r *dns.Msg) {
+		// timeout
+	})
+	defer s.Close()
+
+	p := NewProxy(s.Addr, nil /* no TLS */)
+	f := New()
+	f.maxfails = 0
+	f.SetProxy(p)
+	defer f.Close()
+
+	req := new(dns.Msg)
+	req.SetQuestion("example.org.", dns.TypeA)
+
+	f.ServeDNS(context.TODO(), &test.ResponseWriter{}, req)
+
+	time.Sleep(1 * time.Second)
+	i1 := atomic.LoadUint32(&i)
+	if i1 != expected {
+		t.Errorf("Expected number of health checks to be %d, got %d", expected, i1)
+	}
+}
--- a/plugin/forward/host.go
+++ b/plugin/forward/host.go
@@ -1,44 +0,0 @@
-package forward
-
-import (
-	"crypto/tls"
-	"sync"
-	"time"
-
-	"github.com/miekg/dns"
-)
-
-type host struct {
-	addr   string
-	client *dns.Client
-
-	tlsConfig *tls.Config
-	expire    time.Duration
-
-	fails uint32
-	sync.RWMutex
-	checking bool
-}
-
-// newHost returns a new host, the fails are set to 1, i.e.
-// the first healthcheck must succeed before we use this host.
-func newHost(addr string) *host {
-	return &host{addr: addr, fails: 1, expire: defaultExpire}
-}
-
-// setClient sets and configures the dns.Client in host.
-func (h *host) SetClient() {
-	c := new(dns.Client)
-	c.Net = "udp"
-	c.ReadTimeout = 2 * time.Second
-	c.WriteTimeout = 2 * time.Second
-
-	if h.tlsConfig != nil {
-		c.Net = "tcp-tls"
-		c.TLSConfig = h.tlsConfig
-	}
-
-	h.client = c
-}
-
-const defaultExpire = 10 * time.Second
--- a/plugin/forward/lookup.go
+++ b/plugin/forward/lookup.go
@@ -5,10 +5,6 @@
 package forward

 import (
-	"crypto/tls"
-	"log"
-	"time"
-
 	"github.com/coredns/coredns/request"

 	"github.com/miekg/dns"
@@ -32,12 +28,10 @@ func (f *Forward) Forward(state request.Request) (*dns.Msg, error) {
 			// 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
 			}
@@ -68,10 +62,11 @@ func (f *Forward) Lookup(state request.Request, name string, typ uint16) (*dns.M
 }

 // NewLookup returns a Forward that can be used for plugin that need an upstream to resolve external names.
+// Note that the caller must run Close on the forward to stop the health checking goroutines.
 func NewLookup(addr []string) *Forward {
-	f := &Forward{maxfails: 2, tlsConfig: new(tls.Config), expire: defaultExpire, hcInterval: 2 * time.Second}
+	f := New()
 	for i := range addr {
-		p := NewProxy(addr[i])
+		p := NewProxy(addr[i], nil)
 		f.SetProxy(p)
 	}
 	return f
--- a/plugin/forward/lookup_test.go
+++ b/plugin/forward/lookup_test.go
@@ -6,6 +6,7 @@ import (
 	"github.com/coredns/coredns/plugin/pkg/dnstest"
 	"github.com/coredns/coredns/plugin/test"
 	"github.com/coredns/coredns/request"
+
 	"github.com/miekg/dns"
 )

@@ -18,7 +19,7 @@ func TestLookup(t *testing.T) {
 	})
 	defer s.Close()

-	p := NewProxy(s.Addr)
+	p := NewProxy(s.Addr, nil /* no TLS */)
 	f := New()
 	f.SetProxy(p)
 	defer f.Close()
--- a/plugin/forward/persistent.go
+++ b/plugin/forward/persistent.go
@@ -1,6 +1,7 @@
 package forward

 import (
+	"crypto/tls"
 	"net"
 	"time"

@@ -21,8 +22,10 @@ type connErr struct {

 // transport hold the persistent cache.
 type transport struct {
-	conns map[string][]*persistConn //  Buckets for udp, tcp and tcp-tls.
-	host  *host
+	conns     map[string][]*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 connErr
@@ -35,10 +38,11 @@ type transport struct {
 	stop chan bool
 }

-func newTransport(h *host) *transport {
+func newTransport(addr string, tlsConfig *tls.Config) *transport {
 	t := &transport{
 		conns:   make(map[string][]*persistConn),
-		host:    h,
+		expire:  defaultExpire,
+		addr:    addr,
 		dial:    make(chan string),
 		yield:   make(chan connErr),
 		ret:     make(chan connErr),
@@ -51,7 +55,7 @@ func newTransport(h *host) *transport {
 }

 // len returns the number of connection, used for metrics. Can only be safely
-// used inside connManager() because of races.
+// used inside connManager() because of data races.
 func (t *transport) len() int {
 	l := 0
 	for _, conns := range t.conns {
@@ -79,7 +83,7 @@ Wait:
 			i := 0
 			for i = 0; i < len(t.conns[proto]); i++ {
 				pc := t.conns[proto][i]
-				if time.Since(pc.used) < t.host.expire {
+				if time.Since(pc.used) < t.expire {
 					// Found one, remove from pool and return this conn.
 					t.conns[proto] = t.conns[proto][i+1:]
 					t.ret <- connErr{pc.c, nil}
@@ -91,22 +95,22 @@ Wait:

 			// Not conns were found. Connect to the upstream to create one.
 			t.conns[proto] = t.conns[proto][i:]
-			SocketGauge.WithLabelValues(t.host.addr).Set(float64(t.len()))
+			SocketGauge.WithLabelValues(t.addr).Set(float64(t.len()))

 			go func() {
 				if proto != "tcp-tls" {
-					c, err := dns.DialTimeout(proto, t.host.addr, dialTimeout)
+					c, err := dns.DialTimeout(proto, t.addr, dialTimeout)
 					t.ret <- connErr{c, err}
 					return
 				}

-				c, err := dns.DialTimeoutWithTLS("tcp", t.host.addr, t.host.tlsConfig, dialTimeout)
+				c, err := dns.DialTimeoutWithTLS("tcp", t.addr, t.tlsConfig, dialTimeout)
 				t.ret <- connErr{c, err}
 			}()

 		case conn := <-t.yield:

-			SocketGauge.WithLabelValues(t.host.addr).Set(float64(t.len() + 1))
+			SocketGauge.WithLabelValues(t.addr).Set(float64(t.len() + 1))

 			// no proto here, infer from config and conn
 			if _, ok := conn.c.Conn.(*net.UDPConn); ok {
@@ -114,7 +118,7 @@ Wait:
 				continue Wait
 			}

-			if t.host.tlsConfig == nil {
+			if t.tlsConfig == nil {
 				t.conns["tcp"] = append(t.conns["tcp"], &persistConn{conn.c, time.Now()})
 				continue Wait
 			}
@@ -134,15 +138,30 @@ Wait:
 	}
 }

+// Dial dials the address configured in transport, potentially reusing a connection or creating a new one.
 func (t *transport) Dial(proto string) (*dns.Conn, error) {
+	// If tls has been configured; use it.
+	if t.tlsConfig != nil {
+		proto = "tcp-tls"
+	}
+
 	t.dial <- proto
 	c := <-t.ret
 	return c.c, c.err
 }

+// Yield return the connection to transport for reuse.
 func (t *transport) Yield(c *dns.Conn) {
 	t.yield <- connErr{c, nil}
 }

-// Stop stops the transports.
+// Stop stops the transport's connection manager.
 func (t *transport) Stop() { t.stop <- true }
+
+// 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
--- a/plugin/forward/persistent_test.go
+++ b/plugin/forward/persistent_test.go
@@ -16,8 +16,7 @@ func TestPersistent(t *testing.T) {
 	})
 	defer s.Close()

-	h := newHost(s.Addr)
-	tr := newTransport(h)
+	tr := newTransport(s.Addr, nil /* no TLS */)
 	defer tr.Stop()

 	c1, _ := tr.Dial("udp")
--- a/plugin/forward/proxy.go
+++ b/plugin/forward/proxy.go
@@ -2,47 +2,60 @@ package forward

 import (
 	"crypto/tls"
-	"sync"
+	"sync/atomic"
 	"time"

+	"github.com/coredns/coredns/plugin/pkg/up"
+
 	"github.com/miekg/dns"
 )

 // Proxy defines an upstream host.
 type Proxy struct {
-	host *host
+	addr   string
+	client *dns.Client

+	// Connection caching
+	expire    time.Duration
 	transport *transport

-	// copied from Forward.
-	hcInterval time.Duration
-	forceTCP   bool
-
-	stop chan bool
-
-	sync.RWMutex
+	// health checking
+	probe *up.Probe
+	fails uint32
 }

 // NewProxy returns a new proxy.
-func NewProxy(addr string) *Proxy {
-	host := newHost(addr)
-
+func NewProxy(addr string, tlsConfig *tls.Config) *Proxy {
 	p := &Proxy{
-		host:       host,
-		hcInterval: hcDuration,
-		stop:       make(chan bool),
-		transport:  newTransport(host),
+		addr:      addr,
+		fails:     0,
+		probe:     up.New(),
+		transport: newTransport(addr, tlsConfig),
 	}
+	p.client = dnsClient(tlsConfig)
 	return p
 }

-// SetTLSConfig sets the TLS config in the lower p.host.
-func (p *Proxy) SetTLSConfig(cfg *tls.Config) { p.host.tlsConfig = cfg }
+// dnsClient returns a client used for health checking.
+func dnsClient(tlsConfig *tls.Config) *dns.Client {
+	c := new(dns.Client)
+	c.Net = "udp"
+	// TODO(miek): this should be half of hcDuration?
+	c.ReadTimeout = 1 * time.Second
+	c.WriteTimeout = 1 * time.Second

-// SetExpire sets the expire duration in the lower p.host.
-func (p *Proxy) SetExpire(expire time.Duration) { p.host.expire = expire }
+	if tlsConfig != nil {
+		c.Net = "tcp-tls"
+		c.TLSConfig = tlsConfig
+	}
+	return c
+}

-func (p *Proxy) close() { p.stop <- true }
+// SetTLSConfig sets the TLS config in the lower p.transport.
+func (p *Proxy) SetTLSConfig(cfg *tls.Config) { p.transport.SetTLSConfig(cfg) }
+
+// SetExpire sets the expire duration in the lower p.transport.
+func (p *Proxy) SetExpire(expire time.Duration) { p.transport.SetExpire(expire) }

 // 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) }
@@ -50,26 +63,28 @@ func (p *Proxy) Dial(proto string) (*dns.Conn, error) { return p.transport.Dial(
 // 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) }
+// Healthcheck kicks of a round of health checks for this proxy.
+func (p *Proxy) Healthcheck() { p.probe.Do(p.Check) }

-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
-		}
+// 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
 }

+// close stops the health checking goroutine.
+func (p *Proxy) close() {
+	p.probe.Stop()
+	p.transport.Stop()
+}
+
+// start starts the proxy's healthchecking.
+func (p *Proxy) start(duration time.Duration) { p.probe.Start(duration) }
+
 const (
 	dialTimeout = 4 * time.Second
 	timeout     = 2 * time.Second
--- a/plugin/forward/setup.go
+++ b/plugin/forward/setup.go
@@ -62,25 +62,14 @@ func setup(c *caddy.Controller) error {

 // OnStartup starts a goroutines for all proxies.
 func (f *Forward) OnStartup() (err error) {
-	if f.hcInterval == 0 {
-		for _, p := range f.proxies {
-			p.host.fails = 0
-		}
-		return nil
-	}
-
 	for _, p := range f.proxies {
-		go p.healthCheck()
+		p.start(f.hcInterval)
 	}
 	return nil
 }

 // OnShutdown stops all configured proxies.
 func (f *Forward) OnShutdown() error {
-	if f.hcInterval == 0 {
-		return nil
-	}
-
 	for _, p := range f.proxies {
 		p.close()
 	}
@@ -88,9 +77,7 @@ func (f *Forward) OnShutdown() error {
 }

 // Close is a synonym for OnShutdown().
-func (f *Forward) Close() {
-	f.OnShutdown()
-}
+func (f *Forward) Close() { f.OnShutdown() }

 func parseForward(c *caddy.Controller) (*Forward, error) {
 	f := New()
@@ -140,8 +127,8 @@ func parseForward(c *caddy.Controller) (*Forward, error) {
 			}

 			// We can't set tlsConfig here, because we haven't parsed it yet.
-			// We set it below at the end of parseBlock.
-			p := NewProxy(h)
+			// We set it below at the end of parseBlock, use nil now.
+			p := NewProxy(h, nil /* no TLS */)
 			f.proxies = append(f.proxies, p)
 		}

@@ -200,17 +187,11 @@ func parseBlock(c *caddy.Controller, f *Forward) error {
 			return fmt.Errorf("health_check can't be negative: %d", dur)
 		}
 		f.hcInterval = dur
-		for i := range f.proxies {
-			f.proxies[i].hcInterval = dur
-		}
 	case "force_tcp":
 		if c.NextArg() {
 			return c.ArgErr()
 		}
 		f.forceTCP = true
-		for i := range f.proxies {
-			f.proxies[i].forceTCP = true
-		}
 	case "tls":
 		args := c.RemainingArgs()
 		if len(args) != 3 {
--- a/plugin/pkg/up/up.go
+++ b/plugin/pkg/up/up.go
@@ -17,8 +17,8 @@ type Probe struct {
 	inprogress bool
 }

-// Func is used to determine if a target is alive. If so this function must return true.
-type Func func(target string) bool
+// Func is used to determine if a target is alive. If so this function must return nil.
+type Func func() error

 // New returns a pointer to an intialized Probe.
 func New() *Probe {
@@ -32,9 +32,9 @@ func (p *Probe) Do(f Func) { p.do <- f }
 func (p *Probe) Stop() { p.stop <- true }

 // Start will start the probe manager, after which probes can be initialized with Do.
-func (p *Probe) Start(target string, interval time.Duration) { go p.start(target, interval) }
+func (p *Probe) Start(interval time.Duration) { go p.start(interval) }

-func (p *Probe) start(target string, interval time.Duration) {
+func (p *Probe) start(interval time.Duration) {
 	for {
 		select {
 		case <-p.stop:
@@ -52,9 +52,10 @@ func (p *Probe) start(target string, interval time.Duration) {
 			// we return from the goroutine and we can accept another Func to run.
 			go func() {
 				for {
-					if ok := f(target); ok {
+					if err := f(); err == nil {
 						break
 					}
+					// TODO(miek): little bit of exponential backoff here?
 					time.Sleep(interval)
 				}
 				p.Lock()
--- a/plugin/pkg/up/up_test.go
+++ b/plugin/pkg/up/up_test.go
@@ -12,20 +12,20 @@ func TestUp(t *testing.T) {
 	wg := sync.WaitGroup{}
 	hits := int32(0)

-	upfunc := func(s string) bool {
+	upfunc := func() error {
 		atomic.AddInt32(&hits, 1)
 		// Sleep tiny amount so that our other pr.Do() calls hit the lock.
 		time.Sleep(3 * time.Millisecond)
 		wg.Done()
-		return true
+		return nil
 	}

-	pr.Start("nonexistent", 5*time.Millisecond)
+	pr.Start(5 * time.Millisecond)
 	defer pr.Stop()

 	// These functions AddInt32 to the same hits variable, but we only want to wait when
 	// upfunc finishes, as that only calls Done() on the waitgroup.
-	upfuncNoWg := func(s string) bool { atomic.AddInt32(&hits, 1); return true }
+	upfuncNoWg := func() error { atomic.AddInt32(&hits, 1); return nil }
 	wg.Add(1)
 	pr.Do(upfunc)
 	pr.Do(upfuncNoWg)