Add protection against UDP inetd looping attacks

This patch adds protection against a common inetd attack vector wherein
the reply port to UDP inetd services is forged to an internal inetd
service port.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

inetd.h

@@ -46,6 +46,7 @@ typedef struct {
 	int    proto;
 	int    port;
 	int    forking;
+	int    builtin;		/* Set by built-in inetd services only */
 	int    next_id;		/* Next child job's id */
 	char   name[10];
 	int  (*cmd)(int type);	/* internal inetd service, like 'time' */
@@ -53,6 +54,8 @@ typedef struct {
 	TAILQ_HEAD(, inetd_filter) filters;
 } inetd_t;
 
+int  inetd_check_loop  (struct sockaddr *sa, socklen_t len, char *name);
+
 int  inetd_start       (inetd_t *inetd);
 void inetd_stop        (inetd_t *inetd);