Merge pull request #40 from ChannelMeter/proxy-middleware

Proxy Middleware: Add support for multiple backends, load balancing & healthchecks

middleware/proxy/policy.go

@@ -0,0 +1,91 @@
+package proxy
+
+import (
+	"math/rand"
+	"sync/atomic"
+)
+
+type HostPool []*UpstreamHost
+
+// Policy decides how a host will be selected from a pool.
+type Policy interface {
+	Select(pool HostPool) *UpstreamHost
+}
+
+// The random policy randomly selected an up host from the pool.
+type Random struct{}
+
+func (r *Random) Select(pool HostPool) *UpstreamHost {
+	// instead of just generating a random index
+	// this is done to prevent selecting a down host
+	var randHost *UpstreamHost
+	count := 0
+	for _, host := range pool {
+		if host.Down() {
+			continue
+		}
+		count++
+		if count == 1 {
+			randHost = host
+		} else {
+			r := rand.Int() % count
+			if r == (count - 1) {
+				randHost = host
+			}
+		}
+	}
+	return randHost
+}
+
+// The least_conn policy selects a host with the least connections.
+// If multiple hosts have the least amount of connections, one is randomly
+// chosen.
+type LeastConn struct{}
+
+func (r *LeastConn) Select(pool HostPool) *UpstreamHost {
+	var bestHost *UpstreamHost
+	count := 0
+	leastConn := int64(1<<63 - 1)
+	for _, host := range pool {
+		if host.Down() {
+			continue
+		}
+		hostConns := host.Conns
+		if hostConns < leastConn {
+			bestHost = host
+			leastConn = hostConns
+			count = 1
+		} else if hostConns == leastConn {
+			// randomly select host among hosts with least connections
+			count++
+			if count == 1 {
+				bestHost = host
+			} else {
+				r := rand.Int() % count
+				if r == (count - 1) {
+					bestHost = host
+				}
+			}
+		}
+	}
+	return bestHost
+}
+
+// The round_robin policy selects a host based on round robin ordering.
+type RoundRobin struct {
+	Robin uint32
+}
+
+func (r *RoundRobin) Select(pool HostPool) *UpstreamHost {
+	poolLen := uint32(len(pool))
+	selection := atomic.AddUint32(&r.Robin, 1) % poolLen
+	host := pool[selection]
+	// if the currently selected host is down, just ffwd to up host
+	for i := uint32(1); host.Down() && i < poolLen; i++ {
+		host = pool[(selection+i)%poolLen]
+	}
+	if host.Down() {
+		return nil
+	}
+	return host
+}

middleware/proxy/policy_test.go

@@ -0,0 +1,57 @@
+package proxy
+
+import (
+	"testing"
+)
+
+func testPool() HostPool {
+	pool := []*UpstreamHost{
+		&UpstreamHost{
+			Name: "http://google.com", // this should resolve (healthcheck test)
+		},
+		&UpstreamHost{
+			Name: "http://shouldnot.resolve", // this shouldn't
+		},
+		&UpstreamHost{
+			Name: "http://C",
+		},
+	}
+	return HostPool(pool)
+}
+
+func TestRoundRobinPolicy(t *testing.T) {
+	pool := testPool()
+	rrPolicy := &RoundRobin{}
+	h := rrPolicy.Select(pool)
+	// First selected host is 1, because counter starts at 0
+	// and increments before host is selected
+	if h != pool[1] {
+		t.Error("Expected first round robin host to be second host in the pool.")
+	}
+	h = rrPolicy.Select(pool)
+	if h != pool[2] {
+		t.Error("Expected second round robin host to be third host in the pool.")
+	}
+	// mark host as down
+	pool[0].Unhealthy = true
+	h = rrPolicy.Select(pool)
+	if h != pool[1] {
+		t.Error("Expected third round robin host to be first host in the pool.")
+	}
+}
+
+func TestLeastConnPolicy(t *testing.T) {
+	pool := testPool()
+	lcPolicy := &LeastConn{}
+	pool[0].Conns = 10
+	pool[1].Conns = 10
+	h := lcPolicy.Select(pool)
+	if h != pool[2] {
+		t.Error("Expected least connection host to be third host.")
+	}
+	pool[2].Conns = 100
+	h = lcPolicy.Select(pool)
+	if h != pool[0] && h != pool[1] {
+		t.Error("Expected least connection host to be first or second host.")
+	}
+}

middleware/proxy/proxy.go

@@ -2,51 +2,118 @@
 package proxy
 
 import (
-	"net/http"
-	"net/http/httputil"
-	"net/url"
-	"strings"
-
+	"errors"
 	"github.com/mholt/caddy/middleware"
+	"net/http"
+	"net/url"
+	"sync/atomic"
+	"time"
 )
 
+var errUnreachable = errors.New("Unreachable backend")
+
 // Proxy represents a middleware instance that can proxy requests.
 type Proxy struct {
-	Next  middleware.Handler
-	Rules []Rule
+	Next      middleware.Handler
+	Upstreams []Upstream
+}
+
+// An upstream manages a pool of proxy upstream hosts. Select should return a
+// suitable upstream host, or nil if no such hosts are available.
+type Upstream interface {
+	// The path this upstream host should be routed on
+	From() string
+	// Selects an upstream host to be routed to.
+	Select() *UpstreamHost
+}
+
+type UpstreamHostDownFunc func(*UpstreamHost) bool
+
+// An UpstreamHost represents a single proxy upstream
+type UpstreamHost struct {
+	// The hostname of this upstream host
+	Name         string
+	ReverseProxy *ReverseProxy
+	Conns        int64
+	Fails        int32
+	FailTimeout  time.Duration
+	Unhealthy    bool
+	ExtraHeaders http.Header
+	CheckDown    UpstreamHostDownFunc
+}
+
+func (uh *UpstreamHost) Down() bool {
+	if uh.CheckDown == nil {
+		// Default settings
+		return uh.Unhealthy || uh.Fails > 0
+	}
+	return uh.CheckDown(uh)
 }
 
 // ServeHTTP satisfies the middleware.Handler interface.
 func (p Proxy) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, error) {
 
-	for _, rule := range p.Rules {
-		if middleware.Path(r.URL.Path).Matches(rule.From) {
-			var base string
+	for _, upstream := range p.Upstreams {
+		if middleware.Path(r.URL.Path).Matches(upstream.From()) {
+			var replacer middleware.Replacer
+			start := time.Now()
+			requestHost := r.Host
 
-			if strings.HasPrefix(rule.To, "http") { // includes https
-				// destination includes a scheme! no need to guess
-				base = rule.To
-			} else {
-				// no scheme specified; assume same as request
-				var scheme string
-				if r.TLS == nil {
-					scheme = "http"
-				} else {
-					scheme = "https"
+			// Since Select() should give us "up" hosts, keep retrying
+			// hosts until timeout (or until we get a nil host).
+			for time.Now().Sub(start) < (60 * time.Second) {
+				host := upstream.Select()
+				if host == nil {
+					return http.StatusBadGateway, errUnreachable
 				}
-				base = scheme + "://" + rule.To
-			}
+				proxy := host.ReverseProxy
+				r.Host = host.Name
 
-			baseUrl, err := url.Parse(base)
-			if err != nil {
-				return http.StatusInternalServerError, err
-			}
-			r.Host = baseUrl.Host
+				if baseUrl, err := url.Parse(host.Name); err == nil {
+					r.Host = baseUrl.Host
+					if proxy == nil {
+						proxy = NewSingleHostReverseProxy(baseUrl)
+					}
+				} else if proxy == nil {
+					return http.StatusInternalServerError, err
+				}
+				var extraHeaders http.Header
+				if host.ExtraHeaders != nil {
+					extraHeaders = make(http.Header)
+					if replacer == nil {
+						rHost := r.Host
+						r.Host = requestHost
+						replacer = middleware.NewReplacer(r, nil)
+						r.Host = rHost
+					}
+					for header, values := range host.ExtraHeaders {
+						for _, value := range values {
+							extraHeaders.Add(header,
+								replacer.Replace(value))
+							if header == "Host" {
+								r.Host = replacer.Replace(value)
+							}
+						}
+					}
+				}
 
-			// TODO: Construct this before; not during every request, if possible
-			proxy := httputil.NewSingleHostReverseProxy(baseUrl)
-			proxy.ServeHTTP(w, r)
-			return 0, nil
+				atomic.AddInt64(&host.Conns, 1)
+				backendErr := proxy.ServeHTTP(w, r, extraHeaders)
+				atomic.AddInt64(&host.Conns, -1)
+				if backendErr == nil {
+					return 0, nil
+				}
+				timeout := host.FailTimeout
+				if timeout == 0 {
+					timeout = 10 * time.Second
+				}
+				atomic.AddInt32(&host.Fails, 1)
+				go func(host *UpstreamHost, timeout time.Duration) {
+					time.Sleep(timeout)
+					atomic.AddInt32(&host.Fails, -1)
+				}(host, timeout)
+			}
+			return http.StatusBadGateway, errUnreachable
 		}
 	}
 
@@ -55,30 +122,11 @@ func (p Proxy) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, error) {
 
 // New creates a new instance of proxy middleware.
 func New(c middleware.Controller) (middleware.Middleware, error) {
-	rules, err := parse(c)
-	if err != nil {
+	if upstreams, err := newStaticUpstreams(c); err == nil {
+		return func(next middleware.Handler) middleware.Handler {
+			return Proxy{Next: next, Upstreams: upstreams}
+		}, nil
+	} else {
 		return nil, err
 	}
-
-	return func(next middleware.Handler) middleware.Handler {
-		return Proxy{Next: next, Rules: rules}
-	}, nil
-}
-
-func parse(c middleware.Controller) ([]Rule, error) {
-	var rules []Rule
-
-	for c.Next() {
-		var rule Rule
-		if !c.Args(&rule.From, &rule.To) {
-			return rules, c.ArgErr()
-		}
-		rules = append(rules, rule)
-	}
-
-	return rules, nil
-}
-
-type Rule struct {
-	From, To string
 }

middleware/proxy/reverseproxy.go

@@ -0,0 +1,215 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// HTTP reverse proxy handler
+
+package proxy
+
+import (
+	"io"
+	"net"
+	"net/http"
+	"net/url"
+	"strings"
+	"sync"
+	"time"
+)
+
+// onExitFlushLoop is a callback set by tests to detect the state of the
+// flushLoop() goroutine.
+var onExitFlushLoop func()
+
+// ReverseProxy is an HTTP Handler that takes an incoming request and
+// sends it to another server, proxying the response back to the
+// client.
+type ReverseProxy struct {
+	// Director must be a function which modifies
+	// the request into a new request to be sent
+	// using Transport. Its response is then copied
+	// back to the original client unmodified.
+	Director func(*http.Request)
+
+	// The transport used to perform proxy requests.
+	// If nil, http.DefaultTransport is used.
+	Transport http.RoundTripper
+
+	// FlushInterval specifies the flush interval
+	// to flush to the client while copying the
+	// response body.
+	// If zero, no periodic flushing is done.
+	FlushInterval time.Duration
+}
+
+func singleJoiningSlash(a, b string) string {
+	aslash := strings.HasSuffix(a, "/")
+	bslash := strings.HasPrefix(b, "/")
+	switch {
+	case aslash && bslash:
+		return a + b[1:]
+	case !aslash && !bslash:
+		return a + "/" + b
+	}
+	return a + b
+}
+
+// NewSingleHostReverseProxy returns a new ReverseProxy that rewrites
+// URLs to the scheme, host, and base path provided in target. If the
+// target's path is "/base" and the incoming request was for "/dir",
+// the target request will be for /base/dir.
+func NewSingleHostReverseProxy(target *url.URL) *ReverseProxy {
+	targetQuery := target.RawQuery
+	director := func(req *http.Request) {
+		req.URL.Scheme = target.Scheme
+		req.URL.Host = target.Host
+		req.URL.Path = singleJoiningSlash(target.Path, req.URL.Path)
+		if targetQuery == "" || req.URL.RawQuery == "" {
+			req.URL.RawQuery = targetQuery + req.URL.RawQuery
+		} else {
+			req.URL.RawQuery = targetQuery + "&" + req.URL.RawQuery
+		}
+	}
+	return &ReverseProxy{Director: director}
+}
+
+func copyHeader(dst, src http.Header) {
+	for k, vv := range src {
+		for _, v := range vv {
+			dst.Add(k, v)
+		}
+	}
+}
+
+// Hop-by-hop headers. These are removed when sent to the backend.
+// http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html
+var hopHeaders = []string{
+	"Connection",
+	"Keep-Alive",
+	"Proxy-Authenticate",
+	"Proxy-Authorization",
+	"Te", // canonicalized version of "TE"
+	"Trailers",
+	"Transfer-Encoding",
+	"Upgrade",
+}
+
+func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request, extraHeaders http.Header) error {
+	transport := p.Transport
+	if transport == nil {
+		transport = http.DefaultTransport
+	}
+
+	outreq := new(http.Request)
+	*outreq = *req // includes shallow copies of maps, but okay
+
+	p.Director(outreq)
+	outreq.Proto = "HTTP/1.1"
+	outreq.ProtoMajor = 1
+	outreq.ProtoMinor = 1
+	outreq.Close = false
+
+	// Remove hop-by-hop headers to the backend.  Especially
+	// important is "Connection" because we want a persistent
+	// connection, regardless of what the client sent to us.  This
+	// is modifying the same underlying map from req (shallow
+	// copied above) so we only copy it if necessary.
+	copiedHeaders := false
+	for _, h := range hopHeaders {
+		if outreq.Header.Get(h) != "" {
+			if !copiedHeaders {
+				outreq.Header = make(http.Header)
+				copyHeader(outreq.Header, req.Header)
+				copiedHeaders = true
+			}
+			outreq.Header.Del(h)
+		}
+	}
+
+	if clientIP, _, err := net.SplitHostPort(req.RemoteAddr); err == nil {
+		// If we aren't the first proxy retain prior
+		// X-Forwarded-For information as a comma+space
+		// separated list and fold multiple headers into one.
+		if prior, ok := outreq.Header["X-Forwarded-For"]; ok {
+			clientIP = strings.Join(prior, ", ") + ", " + clientIP
+		}
+		outreq.Header.Set("X-Forwarded-For", clientIP)
+	}
+
+	if extraHeaders != nil {
+		for k, v := range extraHeaders {
+			outreq.Header[k] = v
+		}
+	}
+
+	res, err := transport.RoundTrip(outreq)
+	if err != nil {
+		return err
+	}
+	defer res.Body.Close()
+
+	for _, h := range hopHeaders {
+		res.Header.Del(h)
+	}
+
+	copyHeader(rw.Header(), res.Header)
+
+	rw.WriteHeader(res.StatusCode)
+	p.copyResponse(rw, res.Body)
+	return nil
+}
+
+func (p *ReverseProxy) copyResponse(dst io.Writer, src io.Reader) {
+	if p.FlushInterval != 0 {
+		if wf, ok := dst.(writeFlusher); ok {
+			mlw := &maxLatencyWriter{
+				dst:     wf,
+				latency: p.FlushInterval,
+				done:    make(chan bool),
+			}
+			go mlw.flushLoop()
+			defer mlw.stop()
+			dst = mlw
+		}
+	}
+
+	io.Copy(dst, src)
+}
+
+type writeFlusher interface {
+	io.Writer
+	http.Flusher
+}
+
+type maxLatencyWriter struct {
+	dst     writeFlusher
+	latency time.Duration
+
+	lk   sync.Mutex // protects Write + Flush
+	done chan bool
+}
+
+func (m *maxLatencyWriter) Write(p []byte) (int, error) {
+	m.lk.Lock()
+	defer m.lk.Unlock()
+	return m.dst.Write(p)
+}
+
+func (m *maxLatencyWriter) flushLoop() {
+	t := time.NewTicker(m.latency)
+	defer t.Stop()
+	for {
+		select {
+		case <-m.done:
+			if onExitFlushLoop != nil {
+				onExitFlushLoop()
+			}
+			return
+		case <-t.C:
+			m.lk.Lock()
+			m.dst.Flush()
+			m.lk.Unlock()
+		}
+	}
+}
+
+func (m *maxLatencyWriter) stop() { m.done <- true }

middleware/proxy/upstream.go

@@ -0,0 +1,203 @@
+package proxy
+
+import (
+	"github.com/mholt/caddy/middleware"
+	"io"
+	"io/ioutil"
+	"net/http"
+	"net/url"
+	"strconv"
+	"strings"
+	"time"
+)
+
+type staticUpstream struct {
+	from   string
+	Hosts  HostPool
+	Policy Policy
+
+	FailTimeout time.Duration
+	MaxFails    int32
+	HealthCheck struct {
+		Path     string
+		Interval time.Duration
+	}
+}
+
+func newStaticUpstreams(c middleware.Controller) ([]Upstream, error) {
+	var upstreams []Upstream
+
+	for c.Next() {
+		upstream := &staticUpstream{
+			from:        "",
+			Hosts:       nil,
+			Policy:      &Random{},
+			FailTimeout: 10 * time.Second,
+			MaxFails:    1,
+		}
+		var proxyHeaders http.Header
+		if !c.Args(&upstream.from) {
+			return upstreams, c.ArgErr()
+		}
+		to := c.RemainingArgs()
+		if len(to) == 0 {
+			return upstreams, c.ArgErr()
+		}
+
+		for c.NextBlock() {
+			switch c.Val() {
+			case "policy":
+				if !c.NextArg() {
+					return upstreams, c.ArgErr()
+				}
+				switch c.Val() {
+				case "random":
+					upstream.Policy = &Random{}
+				case "round_robin":
+					upstream.Policy = &RoundRobin{}
+				case "least_conn":
+					upstream.Policy = &LeastConn{}
+				default:
+					return upstreams, c.ArgErr()
+				}
+			case "fail_timeout":
+				if !c.NextArg() {
+					return upstreams, c.ArgErr()
+				}
+				if dur, err := time.ParseDuration(c.Val()); err == nil {
+					upstream.FailTimeout = dur
+				} else {
+					return upstreams, err
+				}
+			case "max_fails":
+				if !c.NextArg() {
+					return upstreams, c.ArgErr()
+				}
+				if n, err := strconv.Atoi(c.Val()); err == nil {
+					upstream.MaxFails = int32(n)
+				} else {
+					return upstreams, err
+				}
+			case "health_check":
+				if !c.NextArg() {
+					return upstreams, c.ArgErr()
+				}
+				upstream.HealthCheck.Path = c.Val()
+				upstream.HealthCheck.Interval = 30 * time.Second
+				if c.NextArg() {
+					if dur, err := time.ParseDuration(c.Val()); err == nil {
+						upstream.HealthCheck.Interval = dur
+					} else {
+						return upstreams, err
+					}
+				}
+			case "proxy_header":
+				var header, value string
+				if !c.Args(&header, &value) {
+					return upstreams, c.ArgErr()
+				}
+				if proxyHeaders == nil {
+					proxyHeaders = make(map[string][]string)
+				}
+				proxyHeaders.Add(header, value)
+			}
+		}
+
+		upstream.Hosts = make([]*UpstreamHost, len(to))
+		for i, host := range to {
+			if !strings.HasPrefix(host, "http") {
+				host = "http://" + host
+			}
+			uh := &UpstreamHost{
+				Name:         host,
+				Conns:        0,
+				Fails:        0,
+				FailTimeout:  upstream.FailTimeout,
+				Unhealthy:    false,
+				ExtraHeaders: proxyHeaders,
+				CheckDown: func(upstream *staticUpstream) UpstreamHostDownFunc {
+					return func(uh *UpstreamHost) bool {
+						if uh.Unhealthy {
+							return true
+						}
+						if uh.Fails >= upstream.MaxFails &&
+							upstream.MaxFails != 0 {
+							return true
+						}
+						return false
+					}
+				}(upstream),
+			}
+			if baseUrl, err := url.Parse(uh.Name); err == nil {
+				uh.ReverseProxy = NewSingleHostReverseProxy(baseUrl)
+			} else {
+				return upstreams, err
+			}
+			upstream.Hosts[i] = uh
+		}
+
+		if upstream.HealthCheck.Path != "" {
+			go upstream.healthCheckWorker(nil)
+		}
+		upstreams = append(upstreams, upstream)
+	}
+	return upstreams, nil
+}
+
+func (u *staticUpstream) healthCheck() {
+	for _, host := range u.Hosts {
+		hostUrl := host.Name + u.HealthCheck.Path
+		if r, err := http.Get(hostUrl); err == nil {
+			io.Copy(ioutil.Discard, r.Body)
+			r.Body.Close()
+			host.Unhealthy = r.StatusCode < 200 || r.StatusCode >= 400
+		} else {
+			host.Unhealthy = true
+		}
+	}
+}
+
+func (u *staticUpstream) healthCheckWorker(stop chan struct{}) {
+	ticker := time.NewTicker(u.HealthCheck.Interval)
+	u.healthCheck()
+	for {
+		select {
+		case <-ticker.C:
+			u.healthCheck()
+		case <-stop:
+			// TODO: the library should provide a stop channel and global
+			// waitgroup to allow goroutines started by plugins a chance
+			// to clean themselves up.
+		}
+	}
+}
+
+func (u *staticUpstream) From() string {
+	return u.from
+}
+
+func (u *staticUpstream) Select() *UpstreamHost {
+	pool := u.Hosts
+	if len(pool) == 1 {
+		if pool[0].Down() {
+			return nil
+		}
+		return pool[0]
+	}
+	allDown := true
+	for _, host := range pool {
+		if !host.Down() {
+			allDown = false
+			break
+		}
+	}
+	if allDown {
+		return nil
+	}
+
+	if u.Policy == nil {
+		return (&Random{}).Select(pool)
+	} else {
+		return u.Policy.Select(pool)
+	}
+}

middleware/proxy/upstream_test.go

@@ -0,0 +1,43 @@
+package proxy
+
+import (
+	"testing"
+	"time"
+)
+
+func TestHealthCheck(t *testing.T) {
+	upstream := &staticUpstream{
+		from:        "",
+		Hosts:       testPool(),
+		Policy:      &Random{},
+		FailTimeout: 10 * time.Second,
+		MaxFails:    1,
+	}
+	upstream.healthCheck()
+	if upstream.Hosts[0].Down() {
+		t.Error("Expected first host in testpool to not fail healthcheck.")
+	}
+	if !upstream.Hosts[1].Down() {
+		t.Error("Expected second host in testpool to fail healthcheck.")
+	}
+}
+
+func TestSelect(t *testing.T) {
+	upstream := &staticUpstream{
+		from:        "",
+		Hosts:       testPool()[:3],
+		Policy:      &Random{},
+		FailTimeout: 10 * time.Second,
+		MaxFails:    1,
+	}
+	upstream.Hosts[0].Unhealthy = true
+	upstream.Hosts[1].Unhealthy = true
+	upstream.Hosts[2].Unhealthy = true
+	if h := upstream.Select(); h != nil {
+		t.Error("Expected select to return nil as all host are down")
+	}
+	upstream.Hosts[2].Unhealthy = false
+	if h := upstream.Select(); h == nil {
+		t.Error("Expected select to not return nil")
+	}
+}

middleware/replacer.go

@@ -8,17 +8,21 @@ import (
 	"time"
 )
 
-// replacer is a type which can replace placeholder
+// Replacer is a type which can replace placeholder
 // substrings in a string with actual values from a
 // http.Request and responseRecorder. Always use
 // NewReplacer to get one of these.
+type Replacer interface {
+	Replace(string) string
+}
+
 type replacer map[string]string
 
 // NewReplacer makes a new replacer based on r and rr.
 // Do not create a new replacer until r and rr have all
 // the needed values, because this function copies those
 // values into the replacer.
-func NewReplacer(r *http.Request, rr *responseRecorder) replacer {
+func NewReplacer(r *http.Request, rr *responseRecorder) Replacer {
 	rep := replacer{
 		"{method}": r.Method,
 		"{scheme}": func() string {
@@ -33,6 +37,9 @@ func NewReplacer(r *http.Request, rr *responseRecorder) replacer {
 		"{fragment}": r.URL.Fragment,
 		"{proto}":    r.Proto,
 		"{remote}": func() string {
+			if fwdFor := r.Header.Get("X-Forwarded-For"); fwdFor != "" {
+				return fwdFor
+			}
 			host, _, err := net.SplitHostPort(r.RemoteAddr)
 			if err != nil {
 				return r.RemoteAddr
@@ -50,9 +57,11 @@ func NewReplacer(r *http.Request, rr *responseRecorder) replacer {
 		"{when}": func() string {
 			return time.Now().Format(timeFormat)
 		}(),
-		"{status}":  strconv.Itoa(rr.status),
-		"{size}":    strconv.Itoa(rr.size),
-		"{latency}": time.Since(rr.start).String(),
+	}
+	if rr != nil {
+		rep["{status}"] = strconv.Itoa(rr.status)
+		rep["{size}"] = strconv.Itoa(rr.size)
+		rep["{latency}"] = time.Since(rr.start).String()
 	}
 
 	// Header placeholders