Added support for custom header variables

- Added support for using nginx-like variables in custom headers
- Supported variables includes: $host, $remote_addr, $request_uri, $request_method, $content_length, $content_type, $uri, $args, $scheme, $query_string, $http_user_agent and $http_referer
- Added test case for custom header variable rewriter

src/mod/dynamicproxy/rewrite/headervars.go

@@ -0,0 +1,63 @@
+package rewrite
+
+import (
+	"fmt"
+	"net/http"
+	"strings"
+)
+
+// GetHeaderVariableValuesFromRequest returns a map of header variables and their values
+// note that variables behavior is not exactly identical to nginx variables
+func GetHeaderVariableValuesFromRequest(r *http.Request) map[string]string {
+	vars := make(map[string]string)
+
+	// Request-specific variables
+	vars["$host"] = r.Host
+	vars["$remote_addr"] = r.RemoteAddr
+	vars["$request_uri"] = r.RequestURI
+	vars["$request_method"] = r.Method
+	vars["$content_length"] = fmt.Sprintf("%d", r.ContentLength)
+	vars["$content_type"] = r.Header.Get("Content-Type")
+
+	// Parsed URI elements
+	vars["$uri"] = r.URL.Path
+	vars["$args"] = r.URL.RawQuery
+	vars["$scheme"] = r.URL.Scheme
+	vars["$query_string"] = r.URL.RawQuery
+
+	// User agent and referer
+	vars["$http_user_agent"] = r.UserAgent()
+	vars["$http_referer"] = r.Referer()
+
+	return vars
+}
+
+// CustomHeadersIncludeDynamicVariables checks if the user-defined headers contain dynamic variables
+// use for early exit when processing the headers
+func CustomHeadersIncludeDynamicVariables(userDefinedHeaders []*UserDefinedHeader) bool {
+	for _, header := range userDefinedHeaders {
+		if strings.Contains(header.Value, "$") {
+			return true
+		}
+	}
+	return false
+}
+
+// PopulateRequestHeaderVariables populates the user-defined headers with the values from the request
+func PopulateRequestHeaderVariables(r *http.Request, userDefinedHeaders []*UserDefinedHeader) []*UserDefinedHeader {
+	if !CustomHeadersIncludeDynamicVariables(userDefinedHeaders) {
+		// Early exit if there are no dynamic variables
+		return userDefinedHeaders
+	}
+	vars := GetHeaderVariableValuesFromRequest(r)
+	populatedHeaders := []*UserDefinedHeader{}
+	// Populate the user-defined headers with the values from the request
+	for _, header := range userDefinedHeaders {
+		thisHeader := header.Copy()
+		for key, value := range vars {
+			thisHeader.Value = strings.ReplaceAll(thisHeader.Value, key, value)
+		}
+		populatedHeaders = append(populatedHeaders, thisHeader)
+	}
+	return populatedHeaders
+}