Added new custom header structure

+ Moved custom header rewrite to dpcore
+ Restructure dpcore header rewrite sequence
+ Added advance custom header settings (zoraxy to upstream and zoraxy to downstream mode)
+ Added header remove feature
+ Removed password requirement for SMTP #80
+ Completed stream proxy module (TCP and UDP)
+ Optimized UX for reminding user to click Apply after port change
+ Added version number to footer #160

src/mod/dynamicproxy/dpcore/header.go

@@ -0,0 +1,121 @@
+package dpcore
+
+import (
+	"net"
+	"net/http"
+	"strings"
+)
+
+/*
+	Header.go
+
+	This script handles headers rewrite and remove
+	in dpcore.
+
+	Added in Zoraxy v3.0.6 by tobychui
+*/
+
+// removeHeaders Remove hop-by-hop headers listed in the "Connection" header, Remove hop-by-hop headers.
+func removeHeaders(header http.Header, noCache bool) {
+	// Remove hop-by-hop headers listed in the "Connection" header.
+	if c := header.Get("Connection"); c != "" {
+		for _, f := range strings.Split(c, ",") {
+			if f = strings.TrimSpace(f); f != "" {
+				header.Del(f)
+			}
+		}
+	}
+
+	// Remove hop-by-hop headers
+	for _, h := range hopHeaders {
+		if header.Get(h) != "" {
+			header.Del(h)
+		}
+	}
+
+	//Restore the Upgrade header if any
+	if header.Get("Zr-Origin-Upgrade") != "" {
+		header.Set("Upgrade", header.Get("Zr-Origin-Upgrade"))
+		header.Del("Zr-Origin-Upgrade")
+	}
+
+	//Disable cache if nocache is set
+	if noCache {
+		header.Del("Cache-Control")
+		header.Set("Cache-Control", "no-store")
+	}
+
+}
+
+// rewriteUserAgent rewrite the user agent based on incoming request
+func rewriteUserAgent(header http.Header, UA string) {
+	//Hide Go-HTTP-Client UA if the client didnt sent us one
+	if header.Get("User-Agent") == "" {
+		// If the outbound request doesn't have a User-Agent header set,
+		// don't send the default Go HTTP client User-Agent
+		header.Del("User-Agent")
+		header.Set("User-Agent", UA)
+	}
+}
+
+// Add X-Forwarded-For Header and rewrite X-Real-Ip according to sniffing logics
+func addXForwardedForHeader(req *http.Request) {
+	if clientIP, _, err := net.SplitHostPort(req.RemoteAddr); err == nil {
+		// If we aren't the first proxy retain prior
+		// X-Forwarded-For information as a comma+space
+		// separated list and fold multiple headers into one.
+		if prior, ok := req.Header["X-Forwarded-For"]; ok {
+			clientIP = strings.Join(prior, ", ") + ", " + clientIP
+		}
+		req.Header.Set("X-Forwarded-For", clientIP)
+		if req.TLS != nil {
+			req.Header.Set("X-Forwarded-Proto", "https")
+		} else {
+			req.Header.Set("X-Forwarded-Proto", "http")
+		}
+
+		if req.Header.Get("X-Real-Ip") == "" {
+			//Check if CF-Connecting-IP header exists
+			CF_Connecting_IP := req.Header.Get("CF-Connecting-IP")
+			Fastly_Client_IP := req.Header.Get("Fastly-Client-IP")
+			if CF_Connecting_IP != "" {
+				//Use CF Connecting IP
+				req.Header.Set("X-Real-Ip", CF_Connecting_IP)
+			} else if Fastly_Client_IP != "" {
+				//Use Fastly Client IP
+				req.Header.Set("X-Real-Ip", Fastly_Client_IP)
+			} else {
+				// Not exists. Fill it in with first entry in X-Forwarded-For
+				ips := strings.Split(clientIP, ",")
+				if len(ips) > 0 {
+					req.Header.Set("X-Real-Ip", strings.TrimSpace(ips[0]))
+				}
+			}
+
+		}
+
+	}
+}
+
+// injectUserDefinedHeaders inject the user headers from slice
+// if a value is empty string, the key will be removed from header.
+// if a key is empty string, the function will return immediately
+func injectUserDefinedHeaders(header http.Header, userHeaders [][]string) {
+	for _, userHeader := range userHeaders {
+		if len(userHeader) == 0 {
+			//End of header slice
+			return
+		}
+		headerKey := userHeader[0]
+		headerValue := userHeader[1]
+		if headerValue == "" {
+			//Remove header from head
+			header.Del(headerKey)
+			continue
+		}
+
+		//Default: Set header value
+		header.Del(headerKey) //Remove header if it already exists
+		header.Set(headerKey, headerValue)
+	}
+}