main: Protect /info from external access

Only accesses from localhost (considering reverse-proxied requests) are
allowed.

main.go

@@ -23,6 +23,12 @@ func indexHandler(w http.ResponseWriter, req *http.Request) {
 
 func newInfoHandler(server *server.Server) func(w http.ResponseWriter, req *http.Request) {
 	return func(w http.ResponseWriter, req *http.Request) {
+		if !isReqFromLocalhost(req) {
+			w.WriteHeader(http.StatusForbidden)
+			log.Warnf("Forbidden request for info from %s", requestAddr(req))
+			return
+		}
+
 		info, err := server.FetchManagedChannels(req.Context())
 		if err != nil {
 			w.WriteHeader(http.StatusInternalServerError)

util.go

@@ -1,11 +1,59 @@
 package main
 
 import (
+	"net"
+	"net/http"
 	"strings"
 
 	"github.com/gorilla/mux"
 )
 
+func requestAddr(req *http.Request) string {
+	res := req.RemoteAddr
+	for _, header := range []string{"X-Forwarded-For", "X-Real-IP"} {
+		if values, ok := req.Header[header]; ok {
+			for _, value := range values {
+				res += strings.TrimSpace(value)
+			}
+		}
+	}
+
+	return res
+}
+
+// isReqFromLocalhost checks if the request came from localhost. It checks the
+// X-Forwarded-For and X-Real-IP headers first and then falls back to the
+// RemoteAddr if necessary.
+func isReqFromLocalhost(req *http.Request) bool {
+	// Function to check if an IP from a given string is a loopback address.
+	// It splits the host and port if necessary and checks the IP.
+	isLoopback := func(addr string) bool {
+		ip := net.ParseIP(addr)
+		return ip != nil && ip.IsLoopback()
+	}
+
+	// Check X-Forwarded-For and X-Real-IP headers for the original client IP
+	for _, header := range []string{"X-Forwarded-For", "X-Real-IP"} {
+		if values, ok := req.Header[header]; ok {
+			for _, value := range values {
+				for _, ipStr := range strings.Split(value, ",") {
+					ipStr = strings.TrimSpace(ipStr)
+					if isLoopback(ipStr) {
+						return true
+					}
+				}
+			}
+		}
+	}
+
+	// If the headers are not present, fall back to the RemoteAddr.
+	host, _, err := net.SplitHostPort(req.RemoteAddr)
+	if err != nil {
+		host = req.RemoteAddr
+	}
+	return isLoopback(host)
+}
+
 func logRouterConfig(r *mux.Router) {
 	err := r.Walk(func(route *mux.Route, router *mux.Router, ancestors []*mux.Route) error {
 		pathTemplate, err := route.GetPathTemplate()