Enable CORS configuration

docs/content/middlewares/headers.md

@@ -14,11 +14,22 @@ The Headers middleware can manage the requests/responses headers.
 Add the `X-Script-Name` header to the proxied request and the `X-Custom-Response-Header` to the response
 
 ```yaml tab="Docker"
-a-container:
-image: a-container-image 
 labels:
-- "traefik.http.middlewares.testHeader.Headers.CustomRequestHeaders.X-Script-Name=test",
-- "traefik.http.middlewares.testHeader.Headers.CustomResponseHeaders.X-Custom-Response-Header=True",
+- "traefik.http.middlewares.testHeader.Headers.CustomRequestHeaders.X-Script-Name=test"
+- "traefik.http.middlewares.testHeader.Headers.CustomResponseHeaders.X-Custom-Response-Header=True"
+```
+
+```yaml tab="Kubernetes"
+apiVersion: traefik.containo.us/v1alpha1
+kind: Middleware
+metadata:
+  name: testHeader
+spec:
+  headers:
+    CustomRequestHeaders:
+      X-Script-Name: "test"
+    CustomResponseHeaders:
+      X-Custom-Response-Header: "True"
 ```
 
 ```toml tab="File"
@@ -34,51 +45,101 @@ labels:
 
 `X-Script-Name` header added to the proxied request, the `X-Custom-Request-Header` header removed from the request, and the `X-Custom-Response-Header` header removed from the response.
 
-??? example "File"
+```yaml tab="Docker"
+labels:
+  - "traefik.http.middlewares.testHeader.Headers.CustomRequestHeaders.X-Script-Name=test"
+  - "traefik.http.middlewares.testHeader.Headers.CustomResponseHeaders.X-Custom-Response-Header=True"
+```
 
-    ```toml    
-    [http.middlewares]
-      [http.middlewares.testHeader.headers]
-        [http.middlewares.testHeader.headers.CustomRequestHeaders]
-            X-Script-Name = "test"
-        [http.middlewares.testHeader.headers.CustomResponseHeaders]
-            X-Custom-Response-Header = "True"
-    ```
+```yaml tab="Kubernetes"
+apiVersion: traefik.containo.us/v1alpha1
+kind: Middleware
+metadata:
+  name: testHeader
+spec:
+  headers:
+    CustomRequestHeaders:
+      X-Script-Name: "test"
+    CustomResponseHeaders:
+      X-Custom-Response-Header: "True"
+```
 
-??? example "Docker"
-
-    ```yml
-    a-container:
-          image: a-container-image 
-            labels:
-              - "traefik.http.middlewares.testHeader.Headers.CustomRequestHeaders.X-Script-Name=test",
-              - "traefik.http.middlewares.testHeader.Headers.CustomResponseHeaders.X-Custom-Response-Header=True",
-    ```
+```toml tab="File"    
+[http.middlewares]
+  [http.middlewares.testHeader.headers]
+    [http.middlewares.testHeader.headers.CustomRequestHeaders]
+        X-Script-Name = "test"
+    [http.middlewares.testHeader.headers.CustomResponseHeaders]
+        X-Custom-Response-Header = "True"
+```
 
 ### Using Security Headers
 
 Security related headers (HSTS headers, SSL redirection, Browser XSS filter, etc) can be added and configured per frontend in a similar manner to the custom headers above.
 This functionality allows for some easy security features to quickly be set.
 
-??? example "File"
+```yaml tab="Docker"
+labels:
+  - "traefik.http.middlewares.testHeader.Headers.FrameDeny=true"
+  - "traefik.http.middlewares.testHeader.Headers.SSLRedirect=true"
+```
 
-    ```toml    
-    [http.middlewares]
-      [http.middlewares.testHeader.headers]
-        FrameDeny = true
-        SSLRedirect = true
-    ```
+```yaml tab="Kubernetes"
+apiVersion: traefik.containo.us/v1alpha1
+kind: Middleware
+metadata:
+  name: testHeader
+spec:
+  headers:
+    FrameDeny: "true"
+    SSLRedirect: "true"
+```
 
-??? example "Docker"
+```toml tab="File"    
+[http.middlewares]
+  [http.middlewares.testHeader.headers]
+    FrameDeny = true
+    SSLRedirect = true
+```
+
+### CORS Headers
+
+CORS (Cross-Origin Resource Sharing) headers can be added and configured per frontend in a similar manner to the custom headers above.
+This functionality allows for more advanced security features to quickly be set.
+
+```yaml tab="Docker"
+labels:
+  - "traefik.http.middlewares.testHeader.Headers.AccessControlAllowMethods=GET,OPTIONS,PUT"
+  - "traefik.http.middlewares.testHeader.Headers.AccessControlAllowOrigin=origin-list-or-null"
+  - "traefik.http.middlewares.testHeader.Headers.AccessControlMaxAge=100"
+  - "traefik.http.middlewares.testHeader.Headers.AddVaryHeader=true"
+```
+
+```yaml tab="Kubernetes"
+apiVersion: traefik.containo.us/v1alpha1
+kind: Middleware
+metadata:
+  name: testHeader
+spec:
+  headers:
+    AccessControlAllowMethods:
+      - "GET"
+      - "OPTIONS"
+      - "PUT"
+    AccessControlAllowOrigin: "origin-list-or-null"
+    AccessControlMaxAge: 100
+    AddVaryHeader: "true"
+```
+
+```toml tab="File"    
+[http.middlewares]
+  [http.middlewares.testHeader.headers]
+    AccessControlAllowMethods= ["GET", "OPTIONS", "PUT"]
+    AccessControlAllowOrigin = "origin-list-or-null"
+    AccessControlMaxAge = 100
+    AddVaryHeader = true
+```
 
-    ```yml
-    a-container:
-          image: a-container-image 
-            labels:
-              - "traefik.http.middlewares.testHeader.Headers.FrameDeny=true",
-              - "traefik.http.middlewares.testHeader.Headers.SSLRedirect=true",
-    ```
-       
 ## Configuration Options
 
 ### General
@@ -93,6 +154,42 @@ This functionality allows for some easy security features to quickly be set.
 
 The `customRequestHeaders` option lists the Header names and values to apply to the request.
 
+### customResponseHeaders
+
+The `customResponseHeaders` option lists the Header names and values to apply to the response.
+
+### accessControlAllowCredentials
+
+The `accessControlAllowCredentials` indicates whether the request can include user credentials.
+
+### accessControlAllowHeaders
+
+The `accessControlAllowHeaders` indicates which header field names can be used as part of the request.
+
+### accessControlAllowMethods
+
+The  `accessControlAllowMethods` indicates which methods can be used during requests.
+
+### accessControlAllowOrigin
+
+The `accessControlAllowOrigin` indicates whether a resource can be shared by returning different values. The three options for this value are:
+
+- `origin-list-or-null`
+- `*`
+- `null`
+
+### accessControlExposeHeaders
+
+The `accessControlExposeHeaders` indicates which headers are safe to expose to the api of a CORS API specification.
+
+### accessControlMaxAge
+
+The `accessControlMaxAge` indicates how long a preflight request can be cached.
+
+### addVaryHeader
+
+The `addVaryHeader` is used in conjunction with `accessControlAllowOrigin` to determine whether the vary header should be added or modified to demonstrate that server responses can differ beased on the value of the origin header.
+
 ### allowedHosts 
 
 The `allowedHosts` option lists fully qualified domain names that are allowed.

docs/content/reference/providers/file.md

@@ -70,6 +70,13 @@
               excludedIPs = ["127.0.0.1/16", "192.168.1.7"]
       
           [http.middlewares.my-headers.Headers]
+            accessControlAllowCredentials = true
+            accessControlAllowHeaders = ["X-foobar", "X-fiibar"]
+            accessControlAllowMethods = ["GET", "PUT"]
+            accessControlAllowOrigin = "*"
+            accessControlExposeHeaders = ["X-foobar", "X-fiibar"]
+            accessControlMaxAge = 200
+            addVaryHeader = true
             allowedHosts = ["foobar", "foobar"]
             hostsProxyHeaders = ["foobar", "foobar"]
             sslRedirect = true

integration/fixtures/headers/basic.toml

@@ -0,0 +1,21 @@
+
+[entrypoints]
+  [entrypoints.web]
+    address = ":8000"
+
+[log]
+logLevel = "DEBUG"
+
+[providers]
+   [providers.file]
+
+[http.routers]
+  [http.routers.router1]
+    rule = "Host(`test.localhost`)"
+    service = "service1"
+
+[http.services]
+  [http.services.service1.loadbalancer]
+    [[http.services.service1.loadbalancer.servers]]
+      url = "http://172.17.0.2:80"
+      weight = 1

integration/fixtures/headers/cors.toml

@@ -0,0 +1,28 @@
+
+[entrypoints]
+  [entrypoints.web]
+    address = ":8000"
+
+[log]
+logLevel = "DEBUG"
+
+[providers]
+   [providers.file]
+
+[http.routers]
+  [http.routers.router1]
+    rule = "Host(`test.localhost`)"
+    service = "service1"
+
+[http.middlewares]
+  [http.middlewares.cors.Headers]
+    AccessControlAllowMethods= ["GET", "OPTIONS", "PUT"]
+    AccessControlAllowOrigin = "origin-list-or-null"
+    AccessControlMaxAge = 100
+    AddVaryHeader = true
+
+[http.services]
+  [http.services.service1.loadbalancer]
+    [[http.services.service1.loadbalancer.servers]]
+      url = "http://172.17.0.2:80"
+      weight = 1

integration/headers_test.go

@@ -0,0 +1,106 @@
+package integration
+
+import (
+	"net/http"
+	"time"
+
+	"github.com/containous/traefik/integration/try"
+	"github.com/go-check/check"
+	checker "github.com/vdemeester/shakers"
+)
+
+// Headers test suites
+type HeadersSuite struct{ BaseSuite }
+
+func (s *HeadersSuite) SetUpSuite(c *check.C) {
+	s.createComposeProject(c, "headers")
+
+	s.composeProject.Start(c)
+}
+
+func (s *HeadersSuite) TestSimpleConfiguration(c *check.C) {
+	cmd, display := s.traefikCmd(withConfigFile("fixtures/headers/basic.toml"))
+	defer display(c)
+	err := cmd.Start()
+	c.Assert(err, checker.IsNil)
+	defer cmd.Process.Kill()
+
+	// Expected a 404 as we did not configure anything
+	err = try.GetRequest("http://127.0.0.1:8000/", 1000*time.Millisecond, try.StatusCodeIs(http.StatusNotFound))
+	c.Assert(err, checker.IsNil)
+}
+
+func (s *HeadersSuite) TestCorsResponses(c *check.C) {
+	cmd, display := s.traefikCmd(withConfigFile("fixtures/headers/cors.toml"))
+	defer display(c)
+
+	err := cmd.Start()
+	c.Assert(err, checker.IsNil)
+	defer cmd.Process.Kill()
+
+	testCase := []struct {
+		desc           string
+		requestHeaders http.Header
+		expected       http.Header
+	}{
+		{
+			desc: "simple access control allow origin",
+			requestHeaders: http.Header{
+				"Origin": {"https://foo.bar.org"},
+			},
+			expected: http.Header{
+				"Access-Control-Allow-Origin": {"https://foo.bar.org"},
+				"Vary":                        {"Origin"},
+			},
+		},
+	}
+
+	for _, test := range testCase {
+		req, err := http.NewRequest(http.MethodGet, "http://127.0.0.1:8000/", nil)
+		c.Assert(err, checker.IsNil)
+		req.Host = "test.localhost"
+		req.Header = test.requestHeaders
+
+		err = try.Request(req, 500*time.Millisecond, try.HasBody(), try.HasHeaderStruct(test.expected))
+		c.Assert(err, checker.IsNil)
+	}
+}
+
+func (s *HeadersSuite) TestCorsPreflightResponses(c *check.C) {
+	cmd, display := s.traefikCmd(withConfigFile("fixtures/headers/cors.toml"))
+	defer display(c)
+
+	err := cmd.Start()
+	c.Assert(err, checker.IsNil)
+	defer cmd.Process.Kill()
+
+	testCase := []struct {
+		desc           string
+		requestHeaders http.Header
+		expected       http.Header
+	}{
+		{
+			desc: "simple preflight request",
+			requestHeaders: http.Header{
+				"Access-Control-Request-Headers": {"origin"},
+				"Access-Control-Request-Method":  {"GET", "OPTIONS"},
+				"Origin":                         {"https://foo.bar.org"},
+			},
+			expected: http.Header{
+				"Access-Control-Allow-Origin":  {"https://foo.bar.org"},
+				"Access-Control-Max-Age":       {"100"},
+				"Access-Control-Allow-Methods": {"GET,OPTIONS,PUT"},
+			},
+		},
+	}
+
+	for _, test := range testCase {
+		req, err := http.NewRequest(http.MethodOptions, "http://127.0.0.1:8000/", nil)
+		c.Assert(err, checker.IsNil)
+		req.Host = "test.localhost"
+		req.Header = test.requestHeaders
+
+		err = try.Request(req, 500*time.Millisecond, try.HasBody(), try.HasHeaderStruct(test.expected))
+		c.Assert(err, checker.IsNil)
+	}
+}

integration/integration_test.go

@@ -45,6 +45,7 @@ func init() {
 		check.Suite(&FileSuite{})
 		check.Suite(&GRPCSuite{})
 		check.Suite(&HealthCheckSuite{})
+		check.Suite(&HeadersSuite{})
 		check.Suite(&HostResolverSuite{})
 		check.Suite(&HTTPSSuite{})
 		check.Suite(&LogRotationSuite{})

integration/resources/compose/headers.yml

@@ -0,0 +1,4 @@
+whoami1:
+  image: containous/whoami
+  ports:
+    - "8881:80"

integration/try/condition.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io/ioutil"
 	"net/http"
+	"reflect"
 	"strings"
 
 	"github.com/abronan/valkeyrie/store"
@@ -125,6 +126,62 @@ func StatusCodeIs(status int) ResponseCondition {
 	}
 }
 
+// HasHeader returns a retry condition function.
+// The condition returns an error if the response does not have a header set.
+func HasHeader(header string) ResponseCondition {
+	return func(res *http.Response) error {
+		if _, ok := res.Header[header]; !ok {
+			return errors.New("response doesn't contain header: " + header)
+		}
+		return nil
+	}
+}
+
+// HasHeaderValue returns a retry condition function.
+// The condition returns an error if the response does not have a header set, and a value for that header.
+// Has an option to test for an exact header match only, not just contains.
+func HasHeaderValue(header, value string, exactMatch bool) ResponseCondition {
+	return func(res *http.Response) error {
+		if _, ok := res.Header[header]; !ok {
+			return errors.New("response doesn't contain header: " + header)
+		}
+
+		matchFound := false
+		for _, hdr := range res.Header[header] {
+			if value != hdr && exactMatch {
+				return fmt.Errorf("got header %s with value %s, wanted %s", header, hdr, value)
+			}
+			if value == hdr {
+				matchFound = true
+			}
+		}
+
+		if !matchFound {
+			return fmt.Errorf("response doesn't contain header %s with value %s", header, value)
+		}
+		return nil
+	}
+}
+
+// HasHeaderStruct returns a retry condition function.
+// The condition returns an error if the response does contain the headers set, and matching contents.
+func HasHeaderStruct(header http.Header) ResponseCondition {
+	return func(res *http.Response) error {
+		for key := range header {
+			if _, ok := res.Header[key]; ok {
+				// Header exists in the response, test it.
+				eq := reflect.DeepEqual(header[key], res.Header[key])
+				if !eq {
+					return fmt.Errorf("for header %s got values %v, wanted %v", key, res.Header[key], header[key])
+				}
+
+			}
+		}
+		return nil
+	}
+
+}
+
 // DoCondition is a retry condition function.
 // It returns an error
 type DoCondition func() error

pkg/config/middlewares.go

@@ -126,6 +126,21 @@ type Headers struct {
 	CustomRequestHeaders  map[string]string `json:"customRequestHeaders,omitempty"`
 	CustomResponseHeaders map[string]string `json:"customResponseHeaders,omitempty"`
 
+	// AccessControlAllowCredentials is only valid if true. false is ignored.
+	AccessControlAllowCredentials bool `json:"AccessControlAllowCredentials,omitempty"`
+	// AccessControlAllowHeaders must be used in response to a preflight request with Access-Control-Request-Headers set.
+	AccessControlAllowHeaders []string `json:"AccessControlAllowHeaders,omitempty"`
+	// AccessControlAllowMethods must be used in response to a preflight request with Access-Control-Request-Method set.
+	AccessControlAllowMethods []string `json:"AccessControlAllowMethods,omitempty"`
+	// AccessControlAllowOrigin Can be "origin-list-or-null" or "*". From (https://www.w3.org/TR/cors/#access-control-allow-origin-response-header)
+	AccessControlAllowOrigin string `json:"AccessControlAllowOrigin,omitempty"`
+	// AccessControlExposeHeaders sets valid headers for the response.
+	AccessControlExposeHeaders []string `json:"AccessControlExposeHeaders,omitempty"`
+	// AccessControlMaxAge sets the time that a preflight request may be cached.
+	AccessControlMaxAge int64 `json:"AccessControlMaxAge,omitempty"`
+	// AddVaryHeader controls if the Vary header is automatically added/updated when the AccessControlAllowOrigin is set.
+	AddVaryHeader bool `json:"AddVaryHeader,omitempty"`
+
 	AllowedHosts            []string          `json:"allowedHosts,omitempty"`
 	HostsProxyHeaders       []string          `json:"hostsProxyHeaders,omitempty"`
 	SSLRedirect             bool              `json:"sslRedirect,omitempty"`
@@ -154,6 +169,17 @@ func (h *Headers) HasCustomHeadersDefined() bool {
 		len(h.CustomRequestHeaders) != 0)
 }
 
+// HasCorsHeadersDefined checks to see if any of the cors header elements have been set
+func (h *Headers) HasCorsHeadersDefined() bool {
+	return h != nil && (h.AccessControlAllowCredentials ||
+		len(h.AccessControlAllowHeaders) != 0 ||
+		len(h.AccessControlAllowMethods) != 0 ||
+		h.AccessControlAllowOrigin != "" ||
+		len(h.AccessControlExposeHeaders) != 0 ||
+		h.AccessControlMaxAge != 0 ||
+		h.AddVaryHeader)
+}
+
 // HasSecureHeadersDefined checks to see if any of the secure header elements have been set
 func (h *Headers) HasSecureHeadersDefined() bool {
 	return h != nil && (len(h.AllowedHosts) != 0 ||

pkg/middlewares/headers/headers.go

@@ -5,6 +5,8 @@ import (
 	"context"
 	"errors"
 	"net/http"
+	"strconv"
+	"strings"
 
 	"github.com/containous/traefik/pkg/config"
 	"github.com/containous/traefik/pkg/middlewares"
@@ -14,7 +16,8 @@ import (
 )
 
 const (
-	typeName = "Headers"
+	typeName        = "Headers"
+	originHeaderKey = "X-Request-Origin"
 )
 
 type headers struct {
@@ -28,22 +31,26 @@ func New(ctx context.Context, next http.Handler, config config.Headers, name str
 	logger := middlewares.GetLogger(ctx, name, typeName)
 	logger.Debug("Creating middleware")
 
-	if !config.HasSecureHeadersDefined() && !config.HasCustomHeadersDefined() {
+	hasSecureHeaders := config.HasSecureHeadersDefined()
+	hasCustomHeaders := config.HasCustomHeadersDefined()
+	hasCorsHeaders := config.HasCorsHeadersDefined()
+
+	if !hasSecureHeaders && !hasCustomHeaders && !hasCorsHeaders {
 		return nil, errors.New("headers configuration not valid")
 	}
 
 	var handler http.Handler
 	nextHandler := next
 
-	if config.HasSecureHeadersDefined() {
+	if hasSecureHeaders {
 		logger.Debug("Setting up secureHeaders from %v", config)
 		handler = newSecure(next, config)
 		nextHandler = handler
 	}
 
-	if config.HasCustomHeadersDefined() {
-		logger.Debug("Setting up customHeaders from %v", config)
-		handler = newHeader(nextHandler, config)
+	if hasCustomHeaders || hasCorsHeaders {
+		logger.Debug("Setting up customHeaders/Cors from %v", config)
+		handler = NewHeader(nextHandler, config)
 	}
 
 	return &headers{
@@ -102,29 +109,67 @@ func (s secureHeader) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 
 // Header is a middleware that helps setup a few basic security features. A single headerOptions struct can be
 // provided to configure which features should be enabled, and the ability to override a few of the default values.
-type header struct {
-	next http.Handler
-	// If Custom request headers are set, these will be added to the request
-	customRequestHeaders map[string]string
+type Header struct {
+	next    http.Handler
+	headers *config.Headers
 }
 
 // NewHeader constructs a new header instance from supplied frontend header struct.
-func newHeader(next http.Handler, headers config.Headers) *header {
-	return &header{
-		next:                 next,
-		customRequestHeaders: headers.CustomRequestHeaders,
+func NewHeader(next http.Handler, headers config.Headers) *Header {
+	return &Header{
+		next:    next,
+		headers: &headers,
 	}
 }
 
-func (s *header) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
+func (s *Header) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
+	reqAcMethod := req.Header.Get("Access-Control-Request-Method")
+	reqAcHeaders := req.Header.Get("Access-Control-Request-Headers")
+	originHeader := req.Header.Get("Origin")
+
+	if reqAcMethod != "" && reqAcHeaders != "" && originHeader != "" && req.Method == http.MethodOptions {
+		// If the request is an OPTIONS request with an Access-Control-Request-Method header, and  Access-Control-Request-Headers headers,
+		// and Origin headers, then it is a CORS preflight request, and we need to build a custom response: https://www.w3.org/TR/cors/#preflight-request
+		if s.headers.AccessControlAllowCredentials {
+			rw.Header().Set("Access-Control-Allow-Credentials", "true")
+		}
+
+		allowHeaders := strings.Join(s.headers.AccessControlAllowHeaders, ",")
+		if allowHeaders != "" {
+			rw.Header().Set("Access-Control-Allow-Headers", allowHeaders)
+		}
+
+		allowMethods := strings.Join(s.headers.AccessControlAllowMethods, ",")
+		if allowMethods != "" {
+			rw.Header().Set("Access-Control-Allow-Methods", allowMethods)
+		}
+
+		allowOrigin := s.getAllowOrigin(originHeader)
+
+		if allowOrigin != "" {
+			rw.Header().Set("Access-Control-Allow-Origin", allowOrigin)
+		}
+
+		rw.Header().Set("Access-Control-Max-Age", strconv.Itoa(int(s.headers.AccessControlMaxAge)))
+
+		return
+	}
+
+	if len(originHeader) > 0 {
+		rw.Header().Set(originHeaderKey, originHeader)
+	}
+
 	s.modifyRequestHeaders(req)
-	s.next.ServeHTTP(rw, req)
+	// If there is a next, call it.
+	if s.next != nil {
+		s.next.ServeHTTP(rw, req)
+	}
 }
 
 // modifyRequestHeaders set or delete request headers.
-func (s *header) modifyRequestHeaders(req *http.Request) {
+func (s *Header) modifyRequestHeaders(req *http.Request) {
 	// Loop through Custom request headers
-	for header, value := range s.customRequestHeaders {
+	for header, value := range s.headers.CustomRequestHeaders {
 		if value == "" {
 			req.Header.Del(header)
 		} else {
@@ -132,3 +177,57 @@ func (s *header) modifyRequestHeaders(req *http.Request) {
 		}
 	}
 }
+
+// ModifyResponseHeaders set or delete response headers
+func (s *Header) ModifyResponseHeaders(res *http.Response) error {
+	// Loop through Custom response headers
+	for header, value := range s.headers.CustomResponseHeaders {
+		if value == "" {
+			res.Header.Del(header)
+		} else {
+			res.Header.Set(header, value)
+		}
+	}
+	originHeader := res.Header.Get(originHeaderKey)
+	allowOrigin := s.getAllowOrigin(originHeader)
+	//	Delete the origin header key, since it is only used to pass data from the request for response handling
+	res.Header.Del(originHeaderKey)
+	if allowOrigin != "" {
+		res.Header.Set("Access-Control-Allow-Origin", allowOrigin)
+
+		if s.headers.AddVaryHeader {
+			varyHeader := res.Header.Get("Vary")
+			if varyHeader != "" {
+				varyHeader += ","
+			}
+			varyHeader += "Origin"
+
+			res.Header.Set("Vary", varyHeader)
+		}
+	}
+
+	if s.headers.AccessControlAllowCredentials {
+		res.Header.Set("Access-Control-Allow-Credentials", "true")
+	}
+
+	exposeHeaders := strings.Join(s.headers.AccessControlExposeHeaders, ",")
+	if exposeHeaders != "" {
+		res.Header.Set("Access-Control-Expose-Headers", exposeHeaders)
+	}
+
+	return nil
+}
+
+func (s *Header) getAllowOrigin(header string) string {
+	switch s.headers.AccessControlAllowOrigin {
+	case "origin-list-or-null":
+		if len(header) == 0 {
+			return "null"
+		}
+		return header
+	case "*":
+		return "*"
+	default:
+		return ""
+	}
+}

pkg/middlewares/headers/headers_test.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/containous/traefik/pkg/config"
 	"github.com/containous/traefik/pkg/testhelpers"
+	"github.com/containous/traefik/pkg/tracing"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -17,7 +18,7 @@ import (
 func TestCustomRequestHeader(t *testing.T) {
 	emptyHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {})
 
-	header := newHeader(emptyHandler, config.Headers{
+	header := NewHeader(emptyHandler, config.Headers{
 		CustomRequestHeaders: map[string]string{
 			"X-Custom-Request-Header": "test_request",
 		},
@@ -35,7 +36,7 @@ func TestCustomRequestHeader(t *testing.T) {
 func TestCustomRequestHeaderEmptyValue(t *testing.T) {
 	emptyHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {})
 
-	header := newHeader(emptyHandler, config.Headers{
+	header := NewHeader(emptyHandler, config.Headers{
 		CustomRequestHeaders: map[string]string{
 			"X-Custom-Request-Header": "test_request",
 		},
@@ -49,7 +50,7 @@ func TestCustomRequestHeaderEmptyValue(t *testing.T) {
 	assert.Equal(t, http.StatusOK, res.Code)
 	assert.Equal(t, "test_request", req.Header.Get("X-Custom-Request-Header"))
 
-	header = newHeader(emptyHandler, config.Headers{
+	header = NewHeader(emptyHandler, config.Headers{
 		CustomRequestHeaders: map[string]string{
 			"X-Custom-Request-Header": "",
 		},
@@ -188,3 +189,312 @@ func TestSSLForceHost(t *testing.T) {
 		})
 	}
 }
+
+func TestCORSPreflights(t *testing.T) {
+	emptyHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {})
+
+	testCases := []struct {
+		desc           string
+		header         *Header
+		requestHeaders http.Header
+		expected       http.Header
+	}{
+		{
+			desc: "Test Simple Preflight",
+			header: NewHeader(emptyHandler, config.Headers{
+				AccessControlAllowMethods: []string{"GET", "OPTIONS", "PUT"},
+				AccessControlAllowOrigin:  "origin-list-or-null",
+				AccessControlMaxAge:       600,
+			}),
+			requestHeaders: map[string][]string{
+				"Access-Control-Request-Headers": {"origin"},
+				"Access-Control-Request-Method":  {"GET", "OPTIONS"},
+				"Origin":                         {"https://foo.bar.org"},
+			},
+			expected: map[string][]string{
+				"Access-Control-Allow-Origin":  {"https://foo.bar.org"},
+				"Access-Control-Max-Age":       {"600"},
+				"Access-Control-Allow-Methods": {"GET,OPTIONS,PUT"},
+			},
+		},
+		{
+			desc: "Wildcard origin Preflight",
+			header: NewHeader(emptyHandler, config.Headers{
+				AccessControlAllowMethods: []string{"GET", "OPTIONS", "PUT"},
+				AccessControlAllowOrigin:  "*",
+				AccessControlMaxAge:       600,
+			}),
+			requestHeaders: map[string][]string{
+				"Access-Control-Request-Headers": {"origin"},
+				"Access-Control-Request-Method":  {"GET", "OPTIONS"},
+				"Origin":                         {"https://foo.bar.org"},
+			},
+			expected: map[string][]string{
+				"Access-Control-Allow-Origin":  {"*"},
+				"Access-Control-Max-Age":       {"600"},
+				"Access-Control-Allow-Methods": {"GET,OPTIONS,PUT"},
+			},
+		},
+		{
+			desc: "Allow Credentials Preflight",
+			header: NewHeader(emptyHandler, config.Headers{
+				AccessControlAllowMethods:     []string{"GET", "OPTIONS", "PUT"},
+				AccessControlAllowOrigin:      "*",
+				AccessControlAllowCredentials: true,
+				AccessControlMaxAge:           600,
+			}),
+			requestHeaders: map[string][]string{
+				"Access-Control-Request-Headers": {"origin"},
+				"Access-Control-Request-Method":  {"GET", "OPTIONS"},
+				"Origin":                         {"https://foo.bar.org"},
+			},
+			expected: map[string][]string{
+				"Access-Control-Allow-Origin":      {"*"},
+				"Access-Control-Max-Age":           {"600"},
+				"Access-Control-Allow-Methods":     {"GET,OPTIONS,PUT"},
+				"Access-Control-Allow-Credentials": {"true"},
+			},
+		},
+		{
+			desc: "Allow Headers Preflight",
+			header: NewHeader(emptyHandler, config.Headers{
+				AccessControlAllowMethods: []string{"GET", "OPTIONS", "PUT"},
+				AccessControlAllowOrigin:  "*",
+				AccessControlAllowHeaders: []string{"origin", "X-Forwarded-For"},
+				AccessControlMaxAge:       600,
+			}),
+			requestHeaders: map[string][]string{
+				"Access-Control-Request-Headers": {"origin"},
+				"Access-Control-Request-Method":  {"GET", "OPTIONS"},
+				"Origin":                         {"https://foo.bar.org"},
+			},
+			expected: map[string][]string{
+				"Access-Control-Allow-Origin":  {"*"},
+				"Access-Control-Max-Age":       {"600"},
+				"Access-Control-Allow-Methods": {"GET,OPTIONS,PUT"},
+				"Access-Control-Allow-Headers": {"origin,X-Forwarded-For"},
+			},
+		},
+	}
+
+	for _, test := range testCases {
+		t.Run(test.desc, func(t *testing.T) {
+			req := testhelpers.MustNewRequest(http.MethodOptions, "/foo", nil)
+			req.Header = test.requestHeaders
+
+			rw := httptest.NewRecorder()
+			test.header.ServeHTTP(rw, req)
+
+			assert.Equal(t, test.expected, rw.Result().Header)
+		})
+	}
+}
+
+func TestEmptyHeaderObject(t *testing.T) {
+	next := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {})
+
+	_, err := New(context.Background(), next, config.Headers{}, "testing")
+	require.Errorf(t, err, "headers configuration not valid")
+}
+
+func TestCustomHeaderHandler(t *testing.T) {
+	next := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {})
+
+	header, _ := New(context.Background(), next, config.Headers{
+		CustomRequestHeaders: map[string]string{
+			"X-Custom-Request-Header": "test_request",
+		},
+	}, "testing")
+
+	res := httptest.NewRecorder()
+	req := testhelpers.MustNewRequest(http.MethodGet, "/foo", nil)
+
+	header.ServeHTTP(res, req)
+
+	assert.Equal(t, http.StatusOK, res.Code)
+	assert.Equal(t, "test_request", req.Header.Get("X-Custom-Request-Header"))
+}
+
+func TestGetTracingInformation(t *testing.T) {
+	next := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {})
+
+	header := &headers{
+		handler: next,
+		name:    "testing",
+	}
+
+	name, trace := header.GetTracingInformation()
+
+	assert.Equal(t, "testing", name)
+	assert.Equal(t, tracing.SpanKindNoneEnum, trace)
+
+}
+
+func TestCORSResponses(t *testing.T) {
+	emptyHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {})
+	nonEmptyHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { w.Header().Set("Vary", "Testing") })
+
+	testCases := []struct {
+		desc           string
+		header         *Header
+		requestHeaders http.Header
+		expected       http.Header
+	}{
+		{
+			desc: "Test Simple Request",
+			header: NewHeader(emptyHandler, config.Headers{
+				AccessControlAllowOrigin: "origin-list-or-null",
+			}),
+			requestHeaders: map[string][]string{
+				"Origin": {"https://foo.bar.org"},
+			},
+			expected: map[string][]string{
+				"Access-Control-Allow-Origin": {"https://foo.bar.org"},
+			},
+		},
+		{
+			desc: "Wildcard origin Request",
+			header: NewHeader(emptyHandler, config.Headers{
+				AccessControlAllowOrigin: "*",
+			}),
+			requestHeaders: map[string][]string{
+				"Origin": {"https://foo.bar.org"},
+			},
+			expected: map[string][]string{
+				"Access-Control-Allow-Origin": {"*"},
+			},
+		},
+		{
+			desc: "Empty origin Request",
+			header: NewHeader(emptyHandler, config.Headers{
+				AccessControlAllowOrigin: "origin-list-or-null",
+			}),
+			requestHeaders: map[string][]string{},
+			expected: map[string][]string{
+				"Access-Control-Allow-Origin": {"null"},
+			},
+		},
+		{
+			desc:           "Not Defined origin Request",
+			header:         NewHeader(emptyHandler, config.Headers{}),
+			requestHeaders: map[string][]string{},
+			expected:       map[string][]string{},
+		},
+		{
+			desc: "Allow Credentials Request",
+			header: NewHeader(emptyHandler, config.Headers{
+				AccessControlAllowOrigin:      "*",
+				AccessControlAllowCredentials: true,
+			}),
+			requestHeaders: map[string][]string{
+				"Origin": {"https://foo.bar.org"},
+			},
+			expected: map[string][]string{
+				"Access-Control-Allow-Origin":      {"*"},
+				"Access-Control-Allow-Credentials": {"true"},
+			},
+		},
+		{
+			desc: "Expose Headers Request",
+			header: NewHeader(emptyHandler, config.Headers{
+				AccessControlAllowOrigin:   "*",
+				AccessControlExposeHeaders: []string{"origin", "X-Forwarded-For"},
+			}),
+			requestHeaders: map[string][]string{
+				"Origin": {"https://foo.bar.org"},
+			},
+			expected: map[string][]string{
+				"Access-Control-Allow-Origin":   {"*"},
+				"Access-Control-Expose-Headers": {"origin,X-Forwarded-For"},
+			},
+		},
+		{
+			desc: "Test Simple Request with Vary Headers",
+			header: NewHeader(emptyHandler, config.Headers{
+				AccessControlAllowOrigin: "origin-list-or-null",
+				AddVaryHeader:            true,
+			}),
+			requestHeaders: map[string][]string{
+				"Origin": {"https://foo.bar.org"},
+			},
+			expected: map[string][]string{
+				"Access-Control-Allow-Origin": {"https://foo.bar.org"},
+				"Vary":                        {"Origin"},
+			},
+		},
+		{
+			desc: "Test Simple Request with Vary Headers and non-empty response",
+			header: NewHeader(nonEmptyHandler, config.Headers{
+				AccessControlAllowOrigin: "origin-list-or-null",
+				AddVaryHeader:            true,
+			}),
+			requestHeaders: map[string][]string{
+				"Origin": {"https://foo.bar.org"},
+			},
+			expected: map[string][]string{
+				"Access-Control-Allow-Origin": {"https://foo.bar.org"},
+				"Vary":                        {"Testing,Origin"},
+			},
+		},
+	}
+
+	for _, test := range testCases {
+		t.Run(test.desc, func(t *testing.T) {
+			req := testhelpers.MustNewRequest(http.MethodGet, "/foo", nil)
+			req.Header = test.requestHeaders
+
+			rw := httptest.NewRecorder()
+			test.header.ServeHTTP(rw, req)
+			err := test.header.ModifyResponseHeaders(rw.Result())
+			require.NoError(t, err)
+			assert.Equal(t, test.expected, rw.Result().Header)
+		})
+	}
+}
+
+func TestCustomResponseHeaders(t *testing.T) {
+	emptyHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {})
+
+	testCases := []struct {
+		desc     string
+		header   *Header
+		expected http.Header
+	}{
+		{
+			desc: "Test Simple Response",
+			header: NewHeader(emptyHandler, config.Headers{
+				CustomResponseHeaders: map[string]string{
+					"Testing":  "foo",
+					"Testing2": "bar",
+				},
+			}),
+			expected: map[string][]string{
+				"Testing":  {"foo"},
+				"Testing2": {"bar"},
+			},
+		},
+		{
+			desc: "Deleting Custom Header",
+			header: NewHeader(emptyHandler, config.Headers{
+				CustomResponseHeaders: map[string]string{
+					"Testing":  "foo",
+					"Testing2": "",
+				},
+			}),
+			expected: map[string][]string{
+				"Testing": {"foo"},
+			},
+		},
+	}
+
+	for _, test := range testCases {
+		t.Run(test.desc, func(t *testing.T) {
+			req := testhelpers.MustNewRequest(http.MethodGet, "/foo", nil)
+			rw := httptest.NewRecorder()
+			test.header.ServeHTTP(rw, req)
+			err := test.header.ModifyResponseHeaders(rw.Result())
+			require.NoError(t, err)
+			assert.Equal(t, test.expected, rw.Result().Header)
+		})
+	}
+}

pkg/provider/label/parser_test.go

@@ -42,7 +42,14 @@ func TestDecodeConfiguration(t *testing.T) {
 		"traefik.http.middlewares.Middleware7.forwardauth.tls.insecureskipverify":              "true",
 		"traefik.http.middlewares.Middleware7.forwardauth.tls.key":                             "foobar",
 		"traefik.http.middlewares.Middleware7.forwardauth.trustforwardheader":                  "true",
+		"traefik.http.middlewares.Middleware8.headers.accesscontrolallowcredentials":           "true",
 		"traefik.http.middlewares.Middleware8.headers.allowedhosts":                            "foobar, fiibar",
+		"traefik.http.middlewares.Middleware8.headers.accesscontrolallowheaders":               "X-foobar, X-fiibar",
+		"traefik.http.middlewares.Middleware8.headers.accesscontrolallowmethods":               "GET, PUT",
+		"traefik.http.middlewares.Middleware8.headers.accesscontrolalloworigin":                "foobar",
+		"traefik.http.middlewares.Middleware8.headers.accesscontrolexposeheaders":              "X-foobar, X-fiibar",
+		"traefik.http.middlewares.Middleware8.headers.accesscontrolmaxage":                     "200",
+		"traefik.http.middlewares.Middleware8.headers.addvaryheader":                           "true",
 		"traefik.http.middlewares.Middleware8.headers.browserxssfilter":                        "true",
 		"traefik.http.middlewares.Middleware8.headers.contentsecuritypolicy":                   "foobar",
 		"traefik.http.middlewares.Middleware8.headers.contenttypenosniff":                      "true",
@@ -377,6 +384,22 @@ func TestDecodeConfiguration(t *testing.T) {
 						"name0": "foobar",
 						"name1": "foobar",
 					},
+					AccessControlAllowCredentials: true,
+					AccessControlAllowHeaders: []string{
+						"X-foobar",
+						"X-fiibar",
+					},
+					AccessControlAllowMethods: []string{
+						"GET",
+						"PUT",
+					},
+					AccessControlAllowOrigin: "foobar",
+					AccessControlExposeHeaders: []string{
+						"X-foobar",
+						"X-fiibar",
+					},
+					AccessControlMaxAge: 200,
+					AddVaryHeader:       true,
 					AllowedHosts: []string{
 						"foobar",
 						"fiibar",
@@ -710,6 +733,22 @@ func TestEncodeConfiguration(t *testing.T) {
 							"name0": "foobar",
 							"name1": "foobar",
 						},
+						AccessControlAllowCredentials: true,
+						AccessControlAllowHeaders: []string{
+							"X-foobar",
+							"X-fiibar",
+						},
+						AccessControlAllowMethods: []string{
+							"GET",
+							"PUT",
+						},
+						AccessControlAllowOrigin: "foobar",
+						AccessControlExposeHeaders: []string{
+							"X-foobar",
+							"X-fiibar",
+						},
+						AccessControlMaxAge: 200,
+						AddVaryHeader:       true,
 						AllowedHosts: []string{
 							"foobar",
 							"fiibar",
@@ -854,6 +893,13 @@ func TestEncodeConfiguration(t *testing.T) {
 		"traefik.HTTP.Middlewares.Middleware7.ForwardAuth.TLS.InsecureSkipVerify":              "true",
 		"traefik.HTTP.Middlewares.Middleware7.ForwardAuth.TLS.Key":                             "foobar",
 		"traefik.HTTP.Middlewares.Middleware7.ForwardAuth.TrustForwardHeader":                  "true",
+		"traefik.HTTP.Middlewares.Middleware8.Headers.AccessControlAllowCredentials":           "true",
+		"traefik.HTTP.Middlewares.Middleware8.Headers.AccessControlAllowHeaders":               "X-foobar, X-fiibar",
+		"traefik.HTTP.Middlewares.Middleware8.Headers.AccessControlAllowMethods":               "GET, PUT",
+		"traefik.HTTP.Middlewares.Middleware8.Headers.AccessControlAllowOrigin":                "foobar",
+		"traefik.HTTP.Middlewares.Middleware8.Headers.AccessControlExposeHeaders":              "X-foobar, X-fiibar",
+		"traefik.HTTP.Middlewares.Middleware8.Headers.AccessControlMaxAge":                     "200",
+		"traefik.HTTP.Middlewares.Middleware8.Headers.AddVaryHeader":                           "true",
 		"traefik.HTTP.Middlewares.Middleware8.Headers.AllowedHosts":                            "foobar, fiibar",
 		"traefik.HTTP.Middlewares.Middleware8.Headers.BrowserXSSFilter":                        "true",
 		"traefik.HTTP.Middlewares.Middleware8.Headers.ContentSecurityPolicy":                   "foobar",

pkg/responsemodifiers/headers.go

@@ -4,46 +4,43 @@ import (
 	"net/http"
 
 	"github.com/containous/traefik/pkg/config"
+	"github.com/containous/traefik/pkg/middlewares/headers"
 	"github.com/unrolled/secure"
 )
 
-func buildHeaders(headers *config.Headers) func(*http.Response) error {
+func buildHeaders(hdrs *config.Headers) func(*http.Response) error {
 	opt := secure.Options{
-		BrowserXssFilter:        headers.BrowserXSSFilter,
-		ContentTypeNosniff:      headers.ContentTypeNosniff,
-		ForceSTSHeader:          headers.ForceSTSHeader,
-		FrameDeny:               headers.FrameDeny,
-		IsDevelopment:           headers.IsDevelopment,
-		SSLRedirect:             headers.SSLRedirect,
-		SSLForceHost:            headers.SSLForceHost,
-		SSLTemporaryRedirect:    headers.SSLTemporaryRedirect,
-		STSIncludeSubdomains:    headers.STSIncludeSubdomains,
-		STSPreload:              headers.STSPreload,
-		ContentSecurityPolicy:   headers.ContentSecurityPolicy,
-		CustomBrowserXssValue:   headers.CustomBrowserXSSValue,
-		CustomFrameOptionsValue: headers.CustomFrameOptionsValue,
-		PublicKey:               headers.PublicKey,
-		ReferrerPolicy:          headers.ReferrerPolicy,
-		SSLHost:                 headers.SSLHost,
-		AllowedHosts:            headers.AllowedHosts,
-		HostsProxyHeaders:       headers.HostsProxyHeaders,
-		SSLProxyHeaders:         headers.SSLProxyHeaders,
-		STSSeconds:              headers.STSSeconds,
+		BrowserXssFilter:        hdrs.BrowserXSSFilter,
+		ContentTypeNosniff:      hdrs.ContentTypeNosniff,
+		ForceSTSHeader:          hdrs.ForceSTSHeader,
+		FrameDeny:               hdrs.FrameDeny,
+		IsDevelopment:           hdrs.IsDevelopment,
+		SSLRedirect:             hdrs.SSLRedirect,
+		SSLForceHost:            hdrs.SSLForceHost,
+		SSLTemporaryRedirect:    hdrs.SSLTemporaryRedirect,
+		STSIncludeSubdomains:    hdrs.STSIncludeSubdomains,
+		STSPreload:              hdrs.STSPreload,
+		ContentSecurityPolicy:   hdrs.ContentSecurityPolicy,
+		CustomBrowserXssValue:   hdrs.CustomBrowserXSSValue,
+		CustomFrameOptionsValue: hdrs.CustomFrameOptionsValue,
+		PublicKey:               hdrs.PublicKey,
+		ReferrerPolicy:          hdrs.ReferrerPolicy,
+		SSLHost:                 hdrs.SSLHost,
+		AllowedHosts:            hdrs.AllowedHosts,
+		HostsProxyHeaders:       hdrs.HostsProxyHeaders,
+		SSLProxyHeaders:         hdrs.SSLProxyHeaders,
+		STSSeconds:              hdrs.STSSeconds,
 	}
 
 	return func(resp *http.Response) error {
-		if headers.HasCustomHeadersDefined() {
-			// Loop through Custom response headers
-			for header, value := range headers.CustomResponseHeaders {
-				if value == "" {
-					resp.Header.Del(header)
-				} else {
-					resp.Header.Set(header, value)
-				}
+		if hdrs.HasCustomHeadersDefined() || hdrs.HasCorsHeadersDefined() {
+			err := headers.NewHeader(nil, *hdrs).ModifyResponseHeaders(resp)
+			if err != nil {
+				return err
 			}
 		}
 
-		if headers.HasSecureHeadersDefined() {
+		if hdrs.HasSecureHeadersDefined() {
 			err := secure.New(opt).ModifyResponseHeaders(resp)
 			if err != nil {
 				return err