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package handlers
import (
"net/http"
"strconv"
"strings"
)
// CORSOption represents a functional option for configuring the CORS middleware.
type CORSOption func(*cors) error
type cors struct {
h http.Handler
allowedHeaders []string
allowedMethods []string
allowedOrigins []string
allowedOriginValidator OriginValidator
exposedHeaders []string
maxAge int
ignoreOptions bool
allowCredentials bool
optionStatusCode int
}
// OriginValidator takes an origin string and returns whether or not that origin is allowed.
type OriginValidator func(string) bool
var (
defaultCorsOptionStatusCode = 200
defaultCorsMethods = []string{"GET", "HEAD", "POST"}
defaultCorsHeaders = []string{"Accept", "Accept-Language", "Content-Language", "Origin"}
// (WebKit/Safari v9 sends the Origin header by default in AJAX requests)
)
const (
corsOptionMethod string = "OPTIONS"
corsAllowOriginHeader string = "Access-Control-Allow-Origin"
corsExposeHeadersHeader string = "Access-Control-Expose-Headers"
corsMaxAgeHeader string = "Access-Control-Max-Age"
corsAllowMethodsHeader string = "Access-Control-Allow-Methods"
corsAllowHeadersHeader string = "Access-Control-Allow-Headers"
corsAllowCredentialsHeader string = "Access-Control-Allow-Credentials"
corsRequestMethodHeader string = "Access-Control-Request-Method"
corsRequestHeadersHeader string = "Access-Control-Request-Headers"
corsOriginHeader string = "Origin"
corsVaryHeader string = "Vary"
corsOriginMatchAll string = "*"
)
func (ch *cors) ServeHTTP(w http.ResponseWriter, r *http.Request) {
origin := r.Header.Get(corsOriginHeader)
if !ch.isOriginAllowed(origin) {
if r.Method != corsOptionMethod || ch.ignoreOptions {
ch.h.ServeHTTP(w, r)
}
return
}
if r.Method == corsOptionMethod {
if ch.ignoreOptions {
ch.h.ServeHTTP(w, r)
return
}
if _, ok := r.Header[corsRequestMethodHeader]; !ok {
w.WriteHeader(http.StatusBadRequest)
return
}
method := r.Header.Get(corsRequestMethodHeader)
if !ch.isMatch(method, ch.allowedMethods) {
w.WriteHeader(http.StatusMethodNotAllowed)
return
}
requestHeaders := strings.Split(r.Header.Get(corsRequestHeadersHeader), ",")
allowedHeaders := []string{}
for _, v := range requestHeaders {
canonicalHeader := http.CanonicalHeaderKey(strings.TrimSpace(v))
if canonicalHeader == "" || ch.isMatch(canonicalHeader, defaultCorsHeaders) {
continue
}
if !ch.isMatch(canonicalHeader, ch.allowedHeaders) {
w.WriteHeader(http.StatusForbidden)
return
}
allowedHeaders = append(allowedHeaders, canonicalHeader)
}
if len(allowedHeaders) > 0 {
w.Header().Set(corsAllowHeadersHeader, strings.Join(allowedHeaders, ","))
}
if ch.maxAge > 0 {
w.Header().Set(corsMaxAgeHeader, strconv.Itoa(ch.maxAge))
}
if !ch.isMatch(method, defaultCorsMethods) {
w.Header().Set(corsAllowMethodsHeader, method)
}
} else {
if len(ch.exposedHeaders) > 0 {
w.Header().Set(corsExposeHeadersHeader, strings.Join(ch.exposedHeaders, ","))
}
}
if ch.allowCredentials {
w.Header().Set(corsAllowCredentialsHeader, "true")
}
if len(ch.allowedOrigins) > 1 {
w.Header().Set(corsVaryHeader, corsOriginHeader)
}
returnOrigin := origin
if ch.allowedOriginValidator == nil && len(ch.allowedOrigins) == 0 {
returnOrigin = "*"
} else {
for _, o := range ch.allowedOrigins {
// A configuration of * is different than explicitly setting an allowed
// origin. Returning arbitrary origin headers in an access control allow
// origin header is unsafe and is not required by any use case.
if o == corsOriginMatchAll {
returnOrigin = "*"
break
}
}
}
w.Header().Set(corsAllowOriginHeader, returnOrigin)
if r.Method == corsOptionMethod {
w.WriteHeader(ch.optionStatusCode)
return
}
ch.h.ServeHTTP(w, r)
}
// CORS provides Cross-Origin Resource Sharing middleware.
// Example:
//
// import (
// "net/http"
//
// "github.com/gorilla/handlers"
// "github.com/gorilla/mux"
// )
//
// func main() {
// r := mux.NewRouter()
// r.HandleFunc("/users", UserEndpoint)
// r.HandleFunc("/projects", ProjectEndpoint)
//
// // Apply the CORS middleware to our top-level router, with the defaults.
// http.ListenAndServe(":8000", handlers.CORS()(r))
// }
//
func CORS(opts ...CORSOption) func(http.Handler) http.Handler {
return func(h http.Handler) http.Handler {
ch := parseCORSOptions(opts...)
ch.h = h
return ch
}
}
func parseCORSOptions(opts ...CORSOption) *cors {
ch := &cors{
allowedMethods: defaultCorsMethods,
allowedHeaders: defaultCorsHeaders,
allowedOrigins: []string{},
optionStatusCode: defaultCorsOptionStatusCode,
}
for _, option := range opts {
option(ch)
}
return ch
}
//
// Functional options for configuring CORS.
//
// AllowedHeaders adds the provided headers to the list of allowed headers in a
// CORS request.
// This is an append operation so the headers Accept, Accept-Language,
// and Content-Language are always allowed.
// Content-Type must be explicitly declared if accepting Content-Types other than
// application/x-www-form-urlencoded, multipart/form-data, or text/plain.
func AllowedHeaders(headers []string) CORSOption {
return func(ch *cors) error {
for _, v := range headers {
normalizedHeader := http.CanonicalHeaderKey(strings.TrimSpace(v))
if normalizedHeader == "" {
continue
}
if !ch.isMatch(normalizedHeader, ch.allowedHeaders) {
ch.allowedHeaders = append(ch.allowedHeaders, normalizedHeader)
}
}
return nil
}
}
// AllowedMethods can be used to explicitly allow methods in the
// Access-Control-Allow-Methods header.
// This is a replacement operation so you must also
// pass GET, HEAD, and POST if you wish to support those methods.
func AllowedMethods(methods []string) CORSOption {
return func(ch *cors) error {
ch.allowedMethods = []string{}
for _, v := range methods {
normalizedMethod := strings.ToUpper(strings.TrimSpace(v))
if normalizedMethod == "" {
continue
}
if !ch.isMatch(normalizedMethod, ch.allowedMethods) {
ch.allowedMethods = append(ch.allowedMethods, normalizedMethod)
}
}
return nil
}
}
// AllowedOrigins sets the allowed origins for CORS requests, as used in the
// 'Allow-Access-Control-Origin' HTTP header.
// Note: Passing in a []string{"*"} will allow any domain.
func AllowedOrigins(origins []string) CORSOption {
return func(ch *cors) error {
for _, v := range origins {
if v == corsOriginMatchAll {
ch.allowedOrigins = []string{corsOriginMatchAll}
return nil
}
}
ch.allowedOrigins = origins
return nil
}
}
// AllowedOriginValidator sets a function for evaluating allowed origins in CORS requests, represented by the
// 'Allow-Access-Control-Origin' HTTP header.
func AllowedOriginValidator(fn OriginValidator) CORSOption {
return func(ch *cors) error {
ch.allowedOriginValidator = fn
return nil
}
}
// OptionStatusCode sets a custom status code on the OPTIONS requests.
// Default behaviour sets it to 200 to reflect best practices. This is option is not mandatory
// and can be used if you need a custom status code (i.e 204).
//
// More informations on the spec:
// https://fetch.spec.whatwg.org/#cors-preflight-fetch
func OptionStatusCode(code int) CORSOption {
return func(ch *cors) error {
ch.optionStatusCode = code
return nil
}
}
// ExposedHeaders can be used to specify headers that are available
// and will not be stripped out by the user-agent.
func ExposedHeaders(headers []string) CORSOption {
return func(ch *cors) error {
ch.exposedHeaders = []string{}
for _, v := range headers {
normalizedHeader := http.CanonicalHeaderKey(strings.TrimSpace(v))
if normalizedHeader == "" {
continue
}
if !ch.isMatch(normalizedHeader, ch.exposedHeaders) {
ch.exposedHeaders = append(ch.exposedHeaders, normalizedHeader)
}
}
return nil
}
}
// MaxAge determines the maximum age (in seconds) between preflight requests. A
// maximum of 10 minutes is allowed. An age above this value will default to 10
// minutes.
func MaxAge(age int) CORSOption {
return func(ch *cors) error {
// Maximum of 10 minutes.
if age > 600 {
age = 600
}
ch.maxAge = age
return nil
}
}
// IgnoreOptions causes the CORS middleware to ignore OPTIONS requests, instead
// passing them through to the next handler. This is useful when your application
// or framework has a pre-existing mechanism for responding to OPTIONS requests.
func IgnoreOptions() CORSOption {
return func(ch *cors) error {
ch.ignoreOptions = true
return nil
}
}
// AllowCredentials can be used to specify that the user agent may pass
// authentication details along with the request.
func AllowCredentials() CORSOption {
return func(ch *cors) error {
ch.allowCredentials = true
return nil
}
}
func (ch *cors) isOriginAllowed(origin string) bool {
if origin == "" {
return false
}
if ch.allowedOriginValidator != nil {
return ch.allowedOriginValidator(origin)
}
if len(ch.allowedOrigins) == 0 {
return true
}
for _, allowedOrigin := range ch.allowedOrigins {
if allowedOrigin == origin || allowedOrigin == corsOriginMatchAll {
return true
}
}
return false
}
func (ch *cors) isMatch(needle string, haystack []string) bool {
for _, v := range haystack {
if v == needle {
return true
}
}
return false
}