proxy: Add, remove, or replace upstream and downstream headers (closes #666) (PR #788)

* Overwrite proxy headers based on directive

Headers of the request sent by the proxy upstream can now be modified in
the following way:

Prefix header with `+`: Header will be added if it doesn't exist
otherwise, the values will be merge
Prefix header with `-': Header will be removed
No prefix: Header will be replaced with given value

* Add missing formating directive reported by go vet

* Overwrite up/down stream proxy headers

Add Up/DownStreamHeaders to UpstreamHost

Split `proxy_header` option in `proxy` directive into `header_upstream`
and `header_downstream`. By splitting into two, it makes it clear in
what direction the given headers must be applied.

`proxy_header` can still be used (to maintain backward compatability)
but its assumed to be `header_upstream`

Response headers received by the reverse proxy from the upstream host
are updated according the `header_downstream` rules.
The update occurs through a func given to the reverse proxy, which is
applied once a response is received.

Headers (for upstream and downstream) can now be modified in
the following way:

Prefix header with `+`: Header will be added if it doesn't exist
otherwise, the values will be merge
Prefix header with `-': Header will be removed
No prefix: Header will be replaced with given value

Updated branch with changes from master

* minor refactor to make intent clearer

* Make Up/Down stream headers naming consistent

* Fix error descriptions to be more clear

* Fix lint issue

middleware/proxy/proxy.go

@@ -43,7 +43,8 @@ type UpstreamHost struct {
 	Fails             int32
 	FailTimeout       time.Duration
 	Unhealthy         bool
-	ExtraHeaders      http.Header
+	UpstreamHeaders   http.Header
+	DownstreamHeaders http.Header
 	CheckDown         UpstreamHostDownFunc
 	WithoutPathPrefix string
 	MaxConns          int64
@@ -99,26 +100,33 @@ func (p Proxy) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, error) {
 			}
 
 			outreq.Host = host.Name
-			if host.ExtraHeaders != nil {
-				extraHeaders := make(http.Header)
+			if host.UpstreamHeaders != nil {
 				if replacer == nil {
 					rHost := r.Host
 					replacer = middleware.NewReplacer(r, nil, "")
 					outreq.Host = rHost
 				}
-				for header, values := range host.ExtraHeaders {
-					for _, value := range values {
-						extraHeaders.Add(header, replacer.Replace(value))
-						if header == "Host" {
-							outreq.Host = replacer.Replace(value)
+				if v, ok := host.UpstreamHeaders["Host"]; ok {
+					r.Host = replacer.Replace(v[len(v)-1])
 				}
-					}
-				}
-				for k, v := range extraHeaders {
+				// Modify headers for request that will be sent to the upstream host
+				upHeaders := createHeadersByRules(host.UpstreamHeaders, r.Header, replacer)
+				for k, v := range upHeaders {
 					outreq.Header[k] = v
 				}
 			}
 
+			var downHeaderUpdateFn respUpdateFn
+			if host.DownstreamHeaders != nil {
+				if replacer == nil {
+					rHost := r.Host
+					replacer = middleware.NewReplacer(r, nil, "")
+					outreq.Host = rHost
+				}
+				//Creates a function that is used to update headers the response received by the reverse proxy
+				downHeaderUpdateFn = createRespHeaderUpdateFn(host.DownstreamHeaders, replacer)
+			}
+
 			proxy := host.ReverseProxy
 			if baseURL, err := url.Parse(host.Name); err == nil {
 				r.Host = baseURL.Host
@@ -130,7 +138,7 @@ func (p Proxy) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, error) {
 			}
 
 			atomic.AddInt64(&host.Conns, 1)
-			backendErr := proxy.ServeHTTP(w, outreq)
+			backendErr := proxy.ServeHTTP(w, outreq, downHeaderUpdateFn)
 			atomic.AddInt64(&host.Conns, -1)
 			if backendErr == nil {
 				return 0, nil
@@ -182,3 +190,48 @@ func createUpstreamRequest(r *http.Request) *http.Request {
 
 	return outreq
 }
+
+func createRespHeaderUpdateFn(rules http.Header, replacer middleware.Replacer) respUpdateFn {
+	return func(resp *http.Response) {
+		newHeaders := createHeadersByRules(rules, resp.Header, replacer)
+		for h, v := range newHeaders {
+			resp.Header[h] = v
+		}
+	}
+}
+
+func createHeadersByRules(rules http.Header, base http.Header, repl middleware.Replacer) http.Header {
+	newHeaders := make(http.Header)
+	for header, values := range rules {
+		if strings.HasPrefix(header, "+") {
+			header = strings.TrimLeft(header, "+")
+			add(newHeaders, header, base[header])
+			applyEach(values, repl.Replace)
+			add(newHeaders, header, values)
+		} else if strings.HasPrefix(header, "-") {
+			base.Del(strings.TrimLeft(header, "-"))
+		} else if _, ok := base[header]; ok {
+			applyEach(values, repl.Replace)
+			for _, v := range values {
+				newHeaders.Set(header, v)
+			}
+		} else {
+			applyEach(values, repl.Replace)
+			add(newHeaders, header, values)
+			add(newHeaders, header, base[header])
+		}
+	}
+	return newHeaders
+}
+
+func applyEach(values []string, mapFn func(string) string) {
+	for i, v := range values {
+		values[i] = mapFn(v)
+	}
+}
+
+func add(base http.Header, header string, values []string) {
+	for _, v := range values {
+		base.Add(header, v)
+	}
+}

middleware/proxy/proxy_test.go

@@ -348,6 +348,141 @@ func TestUnixSocketProxyPaths(t *testing.T) {
 	}
 }
 
+func TestUpstreamHeadersUpdate(t *testing.T) {
+	log.SetOutput(ioutil.Discard)
+	defer log.SetOutput(os.Stderr)
+
+	var actualHeaders http.Header
+	backend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Write([]byte("Hello, client"))
+		actualHeaders = r.Header
+	}))
+	defer backend.Close()
+
+	upstream := newFakeUpstream(backend.URL, false)
+	upstream.host.UpstreamHeaders = http.Header{
+		"Connection": {"{>Connection}"},
+		"Upgrade":    {"{>Upgrade}"},
+		"+Merge-Me":  {"Merge-Value"},
+		"+Add-Me":    {"Add-Value"},
+		"-Remove-Me": {""},
+		"Replace-Me": {"{hostname}"},
+	}
+	// set up proxy
+	p := &Proxy{
+		Upstreams: []Upstream{upstream},
+	}
+
+	// create request and response recorder
+	r, err := http.NewRequest("GET", "/", nil)
+	if err != nil {
+		t.Fatalf("Failed to create request: %v", err)
+	}
+	w := httptest.NewRecorder()
+
+	//add initial headers
+	r.Header.Add("Merge-Me", "Initial")
+	r.Header.Add("Remove-Me", "Remove-Value")
+	r.Header.Add("Replace-Me", "Replace-Value")
+
+	p.ServeHTTP(w, r)
+
+	replacer := middleware.NewReplacer(r, nil, "")
+
+	headerKey := "Merge-Me"
+	values, ok := actualHeaders[headerKey]
+	if !ok {
+		t.Errorf("Request sent to upstream backend does not contain expected %v header. Expected header to be added", headerKey)
+	} else if len(values) < 2 && (values[0] != "Initial" || values[1] != replacer.Replace("{hostname}")) {
+		t.Errorf("Values for proxy header `+Merge-Me` should be merged. Got %v", values)
+	}
+
+	headerKey = "Add-Me"
+	if _, ok := actualHeaders[headerKey]; !ok {
+		t.Errorf("Request sent to upstream backend does not contain expected %v header", headerKey)
+	}
+
+	headerKey = "Remove-Me"
+	if _, ok := actualHeaders[headerKey]; ok {
+		t.Errorf("Request sent to upstream backend should not contain %v header", headerKey)
+	}
+
+	headerKey = "Replace-Me"
+	headerValue := replacer.Replace("{hostname}")
+	value, ok := actualHeaders[headerKey]
+	if !ok {
+		t.Errorf("Request sent to upstream backend should not remove %v header", headerKey)
+	} else if len(value) > 0 && headerValue != value[0] {
+		t.Errorf("Request sent to upstream backend should replace value of %v header with %v. Instead value was %v", headerKey, headerValue, value)
+	}
+
+}
+
+func TestDownstreamHeadersUpdate(t *testing.T) {
+	log.SetOutput(ioutil.Discard)
+	defer log.SetOutput(os.Stderr)
+
+	backend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Add("Merge-Me", "Initial")
+		w.Header().Add("Remove-Me", "Remove-Value")
+		w.Header().Add("Replace-Me", "Replace-Value")
+		w.Write([]byte("Hello, client"))
+	}))
+	defer backend.Close()
+
+	upstream := newFakeUpstream(backend.URL, false)
+	upstream.host.DownstreamHeaders = http.Header{
+		"+Merge-Me":  {"Merge-Value"},
+		"+Add-Me":    {"Add-Value"},
+		"-Remove-Me": {""},
+		"Replace-Me": {"{hostname}"},
+	}
+	// set up proxy
+	p := &Proxy{
+		Upstreams: []Upstream{upstream},
+	}
+
+	// create request and response recorder
+	r, err := http.NewRequest("GET", "/", nil)
+	if err != nil {
+		t.Fatalf("Failed to create request: %v", err)
+	}
+	w := httptest.NewRecorder()
+
+	p.ServeHTTP(w, r)
+
+	replacer := middleware.NewReplacer(r, nil, "")
+	actualHeaders := w.Header()
+
+	headerKey := "Merge-Me"
+	values, ok := actualHeaders[headerKey]
+	if !ok {
+		t.Errorf("Downstream response does not contain expected %v header. Expected header should be added", headerKey)
+	} else if len(values) < 2 && (values[0] != "Initial" || values[1] != replacer.Replace("{hostname}")) {
+		t.Errorf("Values for header `+Merge-Me` should be merged. Got %v", values)
+	}
+
+	headerKey = "Add-Me"
+	if _, ok := actualHeaders[headerKey]; !ok {
+		t.Errorf("Downstream response does not contain expected %v header", headerKey)
+	}
+
+	headerKey = "Remove-Me"
+	if _, ok := actualHeaders[headerKey]; ok {
+		t.Errorf("Downstream response should not contain %v header received from upstream", headerKey)
+	}
+
+	headerKey = "Replace-Me"
+	headerValue := replacer.Replace("{hostname}")
+	value, ok := actualHeaders[headerKey]
+	if !ok {
+		t.Errorf("Downstream response should contain %v header and not remove it", headerKey)
+	} else if len(value) > 0 && headerValue != value[0] {
+		t.Errorf("Downstream response should have header %v with value %v. Instead value was %v", headerKey, headerValue, value)
+	}
+
+}
+
 func newFakeUpstream(name string, insecure bool) *fakeUpstream {
 	uri, _ := url.Parse(name)
 	u := &fakeUpstream{
@@ -410,7 +545,7 @@ func (u *fakeWsUpstream) Select() *UpstreamHost {
 	return &UpstreamHost{
 		Name:         u.name,
 		ReverseProxy: NewSingleHostReverseProxy(uri, u.without),
-		ExtraHeaders: http.Header{
+		UpstreamHeaders: http.Header{
 			"Connection": {"{>Connection}"},
 			"Upgrade":    {"{>Upgrade}"}},
 	}

middleware/proxy/reverseproxy.go

@@ -154,7 +154,9 @@ var InsecureTransport http.RoundTripper = &http.Transport{
 	TLSClientConfig:     &tls.Config{InsecureSkipVerify: true},
 }
 
-func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, outreq *http.Request) error {
+type respUpdateFn func(resp *http.Response)
+
+func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, outreq *http.Request, respUpdateFn respUpdateFn) error {
 	transport := p.Transport
 	if transport == nil {
 		transport = http.DefaultTransport
@@ -169,6 +171,8 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, outreq *http.Request) e
 	res, err := transport.RoundTrip(outreq)
 	if err != nil {
 		return err
+	} else if respUpdateFn != nil {
+		respUpdateFn(res)
 	}
 
 	if res.StatusCode == http.StatusSwitchingProtocols && strings.ToLower(res.Header.Get("Upgrade")) == "websocket" {

middleware/proxy/upstream.go

@@ -20,7 +20,8 @@ var (
 
 type staticUpstream struct {
 	from               string
-	proxyHeaders       http.Header
+	upstreamHeaders    http.Header
+	downstreamHeaders  http.Header
 	Hosts              HostPool
 	Policy             Policy
 	insecureSkipVerify bool
@@ -43,7 +44,8 @@ func NewStaticUpstreams(c parse.Dispenser) ([]Upstream, error) {
 	for c.Next() {
 		upstream := &staticUpstream{
 			from:              "",
-			proxyHeaders: make(http.Header),
+			upstreamHeaders:   make(http.Header),
+			downstreamHeaders: make(http.Header),
 			Hosts:             nil,
 			Policy:            &Random{},
 			FailTimeout:       10 * time.Second,
@@ -102,7 +104,8 @@ func (u *staticUpstream) NewHost(host string) (*UpstreamHost, error) {
 		Fails:             0,
 		FailTimeout:       u.FailTimeout,
 		Unhealthy:         false,
-		ExtraHeaders: u.proxyHeaders,
+		UpstreamHeaders:   u.upstreamHeaders,
+		DownstreamHeaders: u.downstreamHeaders,
 		CheckDown: func(u *staticUpstream) UpstreamHostDownFunc {
 			return func(uh *UpstreamHost) bool {
 				if uh.Unhealthy {
@@ -182,15 +185,23 @@ func parseBlock(c *parse.Dispenser, u *staticUpstream) error {
 			}
 			u.HealthCheck.Interval = dur
 		}
+	case "header_upstream":
+		fallthrough
 	case "proxy_header":
 		var header, value string
 		if !c.Args(&header, &value) {
 			return c.ArgErr()
 		}
-		u.proxyHeaders.Add(header, value)
+		u.upstreamHeaders.Add(header, value)
+	case "header_downstream":
+		var header, value string
+		if !c.Args(&header, &value) {
+			return c.ArgErr()
+		}
+		u.downstreamHeaders.Add(header, value)
 	case "websocket":
-		u.proxyHeaders.Add("Connection", "{>Connection}")
-		u.proxyHeaders.Add("Upgrade", "{>Upgrade}")
+		u.upstreamHeaders.Add("Connection", "{>Connection}")
+		u.upstreamHeaders.Add("Upgrade", "{>Upgrade}")
 	case "without":
 		if !c.NextArg() {
 			return c.ArgErr()