? tcpdump
Index: print-udp.c
===================================================================
RCS file: /cvs/src/usr.sbin/tcpdump/print-udp.c,v
retrieving revision 1.40
diff -u -p -r1.40 print-udp.c
--- print-udp.c	22 Dec 2015 21:01:07 -0000	1.40
+++ print-udp.c	3 Jul 2018 02:48:13 -0000
@@ -374,6 +374,7 @@ static int udp6_cksum(const struct ip6_h
 #define GTP_U_PORT		2152
 #define GTP_PRIME_PORT		3386
 #define UDPENCAP_PORT		4500		/*XXX*/
+#define GRE_PORT		4754
 #define MULTICASTDNS_PORT	5353
 
 #ifdef INET6
@@ -640,6 +641,10 @@ udp_print(const u_char *bp, u_int length
 			iapp_print((const u_char *)(up + 1), length);
 		else if (ISPORT(VQP_PORT))
 			vqp_print((const u_char *)(up + 1), length);
+		else if (ISPORT(GRE_PORT)) {
+			printf(" ");
+			gre_print((const u_char *)(up + 1), length);
+		}
 #ifdef INET6
 		else if (ISPORT(RIPNG_PORT))
 			ripng_print((const u_char *)(up + 1), length);