WIP: add headers support

README.md

@@ -207,6 +207,8 @@ Here are some examples of conditions you can use:
 | `[BODY].name == pat(john*)`  | String at JSONPath `$.name` matches pattern `john*`     | `{"name":"john.doe"}`      | `{"name":"bob"}` |
 | `[BODY].id == any(1, 2)`     | Value at JSONPath `$.id` is equal to `1` or `2`         | 1, 2                       | 3, 4, 5 |
 | `[CERTIFICATE_EXPIRATION] > 48h` | Certificate expiration is more than 48h away        | 49h, 50h, 123h             | 1h, 24h, ... |
+| `[HEADER].Content-Type == text/html` | HTTP response header `Content-Type` must be `text/html` | `text/html`        | `application/json`, ...  |
+| `[HEADER].Server == pat(nginx/1.*)` | HTTP response header `Server` matches parrern `nginx/1.*` | `nginx/1.21.1`, `nginx/1.19.7`, ... | `nginx/0.9.1`, ...  |
 
 
 #### Placeholders
@@ -219,6 +221,7 @@ Here are some examples of conditions you can use:
 | `[CONNECTED]`              | Resolves into whether a connection could be established         | `true`
 | `[CERTIFICATE_EXPIRATION]` | Resolves into the duration before certificate expiration        | `24h`, `48h`, 0 (if not using HTTPS)
 | `[DNS_RCODE]`              | Resolves into the DNS status of the response                    | NOERROR
+| `[HEADER].<header_name>`   | Resolves into the HTTP response specified header   | `text/html`
 
 
 #### Functions

core/condition.go

@@ -5,6 +5,7 @@ import (
 	"strconv"
 	"strings"
 	"time"
+	"net/http"
 
 	"github.com/TwinProduction/gatus/jsonpath"
 	"github.com/TwinProduction/gatus/pattern"
@@ -46,6 +47,11 @@ const (
 	// Values that could replace the placeholder: 4461677039 (~52 days)
 	CertificateExpirationPlaceholder = "[CERTIFICATE_EXPIRATION]"
 
+	// HeaderPlaceholder is a placeholder for a HTTP header
+	//
+	// Values that could replace the placeholder: []
+	HeaderPlaceholder = "[HEADER]"
+
 	// LengthFunctionPrefix is the prefix for the length function
 	//
 	// Usage: len([BODY].articles) == 10, len([BODY].name) > 5
@@ -220,40 +226,54 @@ func sanitizeAndResolve(elements []string, result *Result) ([]string, []string)
 		case CertificateExpirationPlaceholder:
 			element = strconv.FormatInt(result.CertificateExpiration.Milliseconds(), 10)
 		default:
-			// if contains the BodyPlaceholder, then evaluate json path
-			if strings.Contains(element, BodyPlaceholder) {
-				checkingForLength := false
-				checkingForExistence := false
-				if strings.HasPrefix(element, LengthFunctionPrefix) && strings.HasSuffix(element, FunctionSuffix) {
-					checkingForLength = true
-					element = strings.TrimSuffix(strings.TrimPrefix(element, LengthFunctionPrefix), FunctionSuffix)
-				}
-				if strings.HasPrefix(element, HasFunctionPrefix) && strings.HasSuffix(element, FunctionSuffix) {
-					checkingForExistence = true
-					element = strings.TrimSuffix(strings.TrimPrefix(element, HasFunctionPrefix), FunctionSuffix)
+			// if contains the HeaderPlaceholder
+			if strings.HasPrefix(strings.ToUpper(element), HeaderPlaceholder) {
+				headerName := http.CanonicalHeaderKey(strings.Split(element, ".")[1])
+				headerValues, headerExists := result.Headers[headerName]
+				if headerExists {
+					headerValuesFormatted := strings.Join(headerValues, ", ")
+					element = "any(" + headerValuesFormatted + ")"
+					fmt.Println(element)
+				} else {
+					// Specified header key isn't present in response headers
+					element = "any()"
 				}
-				resolvedElement, resolvedElementLength, err := jsonpath.Eval(strings.TrimPrefix(strings.TrimPrefix(element, BodyPlaceholder), "."), result.body)
-				if checkingForExistence {
-					if err != nil {
-						element = "false"
-					} else {
-						element = "true"
+			} else {
+				// if contains the BodyPlaceholder, then evaluate json path
+				if strings.Contains(element, BodyPlaceholder) {
+					checkingForLength := false
+					checkingForExistence := false
+					if strings.HasPrefix(element, LengthFunctionPrefix) && strings.HasSuffix(element, FunctionSuffix) {
+						checkingForLength = true
+						element = strings.TrimSuffix(strings.TrimPrefix(element, LengthFunctionPrefix), FunctionSuffix)
 					}
-				} else {
-					if err != nil {
-						if err.Error() != "unexpected end of JSON input" {
-							result.AddError(err.Error())
-						}
-						if checkingForLength {
-							element = LengthFunctionPrefix + element + FunctionSuffix + " " + InvalidConditionElementSuffix
+					if strings.HasPrefix(element, HasFunctionPrefix) && strings.HasSuffix(element, FunctionSuffix) {
+						checkingForExistence = true
+						element = strings.TrimSuffix(strings.TrimPrefix(element, HasFunctionPrefix), FunctionSuffix)
+					}
+					resolvedElement, resolvedElementLength, err := jsonpath.Eval(strings.TrimPrefix(strings.TrimPrefix(element, BodyPlaceholder), "."), result.body)
+					if checkingForExistence {
+						if err != nil {
+							element = "false"
 						} else {
-							element = element + " " + InvalidConditionElementSuffix
+							element = "true"
 						}
 					} else {
-						if checkingForLength {
-							element = strconv.Itoa(resolvedElementLength)
+						if err != nil {
+							if err.Error() != "unexpected end of JSON input" {
+								result.AddError(err.Error())
+							}
+							if checkingForLength {
+								element = LengthFunctionPrefix + element + FunctionSuffix + " " + InvalidConditionElementSuffix
+							} else {
+								element = element + " " + InvalidConditionElementSuffix
+							}
 						} else {
-							element = resolvedElement
+							if checkingForLength {
+								element = strconv.Itoa(resolvedElementLength)
+							} else {
+								element = resolvedElement
+							}
 						}
 					}
 				}

core/result.go

@@ -39,6 +39,9 @@ type Result struct {
 	// CertificateExpiration is the duration before the certificate expires
 	CertificateExpiration time.Duration `json:"-"`
 
+	// Headers headers of HTTP response
+	Headers map[string][]string `json:"headers"`
+
 	// body is the response body
 	//
 	// Note that this variable is only used during the evaluation of a service's health.

core/service.go

@@ -252,6 +252,7 @@ func (service *Service) call(result *Result) {
 		}
 		result.HTTPStatus = response.StatusCode
 		result.Connected = response.StatusCode > 0
+		result.Headers = response.Header
 		// Only read the body if there's a condition that uses the BodyPlaceholder
 		if service.needsToReadBody() {
 			result.body, err = ioutil.ReadAll(response.Body)