fix: handle HTTP header names case-insensitively (#1506)

Details: HTTP header names should be case-insensitive per RFC 7230. When the users
configure the headers like 'user-agent' (lowercase), gatus was adding its own 'User-Agent'
header because the case-sensitive map lookup failed to find the existing header.

Fixed headers:
- User-Agent
- Content-Type
- Host

Fixes #1491"

Co-authored-by: TwiN <twin@linux.com>

config/endpoint/endpoint.go

@@ -221,12 +221,12 @@ func (e *Endpoint) ValidateAndSetDefaults() error {
 		e.Headers = make(map[string]string)
 	}
 	// Automatically add user agent header if there isn't one specified in the endpoint configuration
-	if _, userAgentHeaderExists := e.Headers[UserAgentHeader]; !userAgentHeaderExists {
+	if !hasHeader(e.Headers, UserAgentHeader) {
 		e.Headers[UserAgentHeader] = GatusUserAgent
 	}
 	// Automatically add "Content-Type: application/json" header if there's no Content-Type set
 	// and endpoint.GraphQL is set to true
-	if _, contentTypeHeaderExists := e.Headers[ContentTypeHeader]; !contentTypeHeaderExists && e.GraphQL {
+	if !hasHeader(e.Headers, ContentTypeHeader) && e.GraphQL {
 		e.Headers[ContentTypeHeader] = "application/json"
 	}
 	if len(e.Conditions) == 0 {
@@ -493,8 +493,8 @@ func (e *Endpoint) call(result *Result) {
 				wsHeaders[k] = v
 			}
 		}
-		if _, exists := wsHeaders["User-Agent"]; !exists {
+		if !hasHeader(wsHeaders, UserAgentHeader) {
-			wsHeaders["User-Agent"] = GatusUserAgent
+			wsHeaders[UserAgentHeader] = GatusUserAgent
 		}
 		result.Connected, result.Body, err = client.QueryWebSocket(e.URL, e.getParsedBody(), wsHeaders, e.ClientConfig)
 		if err != nil {
@@ -582,7 +582,7 @@ func (e *Endpoint) buildHTTPRequest() *http.Request {
 	request, _ := http.NewRequest(e.Method, e.URL, bodyBuffer)
 	for k, v := range e.Headers {
 		request.Header.Set(k, v)
-		if k == HostHeader {
+		if strings.EqualFold(k, HostHeader) {
 			request.Host = v
 		}
 	}
@@ -626,3 +626,13 @@ func (e *Endpoint) needsToRetrieveIP() bool {
 	}
 	return false
 }
+
+// hasHeader checks if a header exists in the map using a case-insensitive lookup
+func hasHeader(headers map[string]string, name string) bool {
+	for k := range headers {
+		if strings.EqualFold(k, name) {
+			return true
+		}
+	}
+	return false
+}

config/endpoint/endpoint_test.go

@@ -21,6 +21,30 @@ import (
 	"github.com/TwiN/gatus/v5/test"
 )
 
+func TestHasHeader(t *testing.T) {
+	scenarios := []struct {
+		name     string
+		headers  map[string]string
+		lookup   string
+		expected bool
+	}{
+		{name: "exact-match", headers: map[string]string{"User-Agent": "test"}, lookup: "User-Agent", expected: true},
+		{name: "lowercase-lookup", headers: map[string]string{"User-Agent": "test"}, lookup: "user-agent", expected: true},
+		{name: "uppercase-lookup", headers: map[string]string{"user-agent": "test"}, lookup: "USER-AGENT", expected: true},
+		{name: "mixed-case", headers: map[string]string{"UsEr-AgEnT": "test"}, lookup: "uSeR-aGeNt", expected: true},
+		{name: "not-found", headers: map[string]string{"Content-Type": "test"}, lookup: "User-Agent", expected: false},
+		{name: "empty-headers", headers: map[string]string{}, lookup: "User-Agent", expected: false},
+		{name: "nil-headers", headers: nil, lookup: "User-Agent", expected: false},
+	}
+	for _, scenario := range scenarios {
+		t.Run(scenario.name, func(t *testing.T) {
+			if result := hasHeader(scenario.headers, scenario.lookup); result != scenario.expected {
+				t.Errorf("expected %v, got %v", scenario.expected, result)
+			}
+		})
+	}
+}
+
 func TestEndpoint(t *testing.T) {
 	defer client.InjectHTTPClient(nil)
 	scenarios := []struct {
@@ -722,6 +746,76 @@ func TestEndpoint_buildHTTPRequestWithHostHeader(t *testing.T) {
 	}
 }
 
+func TestEndpoint_buildHTTPRequestWithLowercaseUserAgent(t *testing.T) {
+	condition := Condition("[STATUS] == 200")
+	endpoint := Endpoint{
+		Name:       "website-health",
+		URL:        "https://twin.sh/health",
+		Conditions: []Condition{condition},
+		Headers: map[string]string{
+			"user-agent": "CustomAgent/1.0",
+		},
+	}
+	err := endpoint.ValidateAndSetDefaults()
+	if err != nil {
+		t.Fatal("did not expect an error, got", err)
+	}
+	if _, exists := endpoint.Headers[UserAgentHeader]; exists {
+		t.Error("User-Agent header should not have been added since user-agent was already specified")
+	}
+	request := endpoint.buildHTTPRequest()
+	if userAgent := request.Header.Get("User-Agent"); userAgent != "CustomAgent/1.0" {
+		t.Errorf("request.Header.Get(User-Agent) should've been CustomAgent/1.0, but was %s", userAgent)
+	}
+}
+
+func TestEndpoint_buildHTTPRequestWithLowercaseContentType(t *testing.T) {
+	condition := Condition("[STATUS] == 200")
+	endpoint := Endpoint{
+		Name:       "website-graphql",
+		URL:        "https://twin.sh/graphql",
+		Method:     "POST",
+		Conditions: []Condition{condition},
+		GraphQL:    true,
+		Headers: map[string]string{
+			"content-type": "application/graphql",
+		},
+		Body: `{ users { id } }`,
+	}
+	err := endpoint.ValidateAndSetDefaults()
+	if err != nil {
+		t.Fatal("did not expect an error, got", err)
+	}
+	if _, exists := endpoint.Headers[ContentTypeHeader]; exists {
+		t.Error("Content-Type header should not have been added since content-type was already specified")
+	}
+	request := endpoint.buildHTTPRequest()
+	if contentType := request.Header.Get("Content-Type"); contentType != "application/graphql" {
+		t.Errorf("request.Header.Get(Content-Type) should've been application/graphql, but was %s", contentType)
+	}
+}
+
+func TestEndpoint_buildHTTPRequestWithLowercaseHostHeader(t *testing.T) {
+	condition := Condition("[STATUS] == 200")
+	endpoint := Endpoint{
+		Name:       "website-health",
+		URL:        "https://twin.sh/health",
+		Method:     "POST",
+		Conditions: []Condition{condition},
+		Headers: map[string]string{
+			"host": "example.com",
+		},
+	}
+	err := endpoint.ValidateAndSetDefaults()
+	if err != nil {
+		t.Fatal("did not expect an error, got", err)
+	}
+	request := endpoint.buildHTTPRequest()
+	if request.Host != "example.com" {
+		t.Error("request.Host should've been example.com, but was", request.Host)
+	}
+}
+
 func TestEndpoint_buildHTTPRequestWithGraphQLEnabled(t *testing.T) {
 	condition := Condition("[STATUS] == 200")
 	endpoint := Endpoint{