diff --git a/CHANGES b/CHANGES
index 547e0b8..f6694a6 100644
--- a/CHANGES
+++ b/CHANGES
@@ -7,6 +7,10 @@ untagged yet (2018-09-05)
 	- fix ipv6 prefix-ranges. Reported by Jay Ford.
     - OpenBGPd change: -E now generates prefix-set instead of prefix-list.
     Based on submission by Denis Fondras
+    - new option -w, allowing to 'validate' AS numbers included in as-path
+    access-lists: only those AS having registered route-objects are allowed.
+    By default checks route[4] presence, to check route6 objects shall be
+    used together with -6.
 
 0.1.35-rc2 (2017-06-14)
     - OpenBSD need <sys/select.h>. Reported by Denis Fondras.
diff --git a/bgpq3.c b/bgpq3.c
index 5098b51..d43fea7 100644
--- a/bgpq3.c
+++ b/bgpq3.c
@@ -27,7 +27,7 @@ int
 usage(int ecode)
 {
 	printf("\nUsage: bgpq3 [-h host[:port]] [-S sources] [-P|E|G <num>|f <num>]"
-		" [-2346ABbDJjXd] [-R len] <OBJECTS>...\n");
+		" [-2346ABbDdJjwXz] [-R len] <OBJECTS>...\n");
 	printf(" -2        : allow routes belonging to as23456 (transition-as) "
 		"(default: false)\n");
 	printf(" -3        : assume that your device is asn32-safe\n");
@@ -69,6 +69,8 @@ usage(int ecode)
 	printf(" -U        : generate config for Huawei (Cisco IOS by default)\n");
 	printf(" -W len    : specify max-entries on as-path line (use 0 for "
 		"infinity)\n");
+	printf(" -w        : 'validate' AS numbers: accept only ones with "
+		"registered routes\n");
 	printf(" -X        : generate config for IOS XR (Cisco IOS by default)\n");
 	printf("\n" PACKAGE_NAME " version: " PACKAGE_VERSION "\n");
 	printf("Copyright(c) Alexandre Snarskii <snar@snar.spb.ru> 2007-2018\n\n");
@@ -140,7 +142,7 @@ main(int argc, char* argv[])
 	if (getenv("IRRD_SOURCES"))
 		expander.sources=getenv("IRRD_SOURCES");
 
-	while((c=getopt(argc,argv,"2346a:AbBdDEF:S:jJf:l:L:m:M:NW:Ppr:R:G:Th:UXsz"))
+	while((c=getopt(argc,argv,"2346a:AbBdDEF:S:jJf:l:L:m:M:NW:Ppr:R:G:Th:UwXsz"))
 		!=EOF) {
 	switch(c) {
 		case '2':
@@ -312,6 +314,8 @@ main(int argc, char* argv[])
 			};
 			widthSet=1;
 			break;
+		case 'w': expander.validate_asns=1;
+			break;
 		case 'X': if(expander.vendor) vendor_exclusive();
 			expander.vendor=V_CISCO_XR;
 			break;
@@ -530,11 +534,17 @@ main(int argc, char* argv[])
 	};
 
 	if((expander.generation==T_ASPATH || expander.generation==T_OASPATH) &&
-		af != AF_INET) {
+		af != AF_INET && !expander.validate_asns) {
 		sx_report(SX_FATAL, "Sorry, -6 makes no sense with as-path (-f/-G) "
 			"generation\n");
 	};
 
+	if (expander.validate_asns && expander.generation != T_ASPATH &&
+		expander.generation != T_OASPATH) {
+		sx_report(SX_FATAL, "Sorry, -w makes sense only for as-path (-f/-G) "
+			"generation\n");
+	};
+
 	if(!argv[0])
 		usage(1);
 
diff --git a/bgpq3.h b/bgpq3.h
index 9becd99..870406d 100644
--- a/bgpq3.h
+++ b/bgpq3.h
@@ -56,6 +56,7 @@ struct bgpq_expander {
 	int identify;
 	int sequence;
 	int maxdepth;
+	int validate_asns;
 	unsigned char asn32;
 	unsigned char* asn32s[65536];
 	struct bgpq_prequest* firstpipe, *lastpipe;
diff --git a/bgpq_expander.c b/bgpq_expander.c
index cde6f08..a970f04 100644
--- a/bgpq_expander.c
+++ b/bgpq_expander.c
@@ -13,6 +13,7 @@
 #include <ctype.h>
 #include <errno.h>
 #include <netdb.h>
+#include <limits.h>
 #include <string.h>
 #include <strings.h>
 #include <stdarg.h>
@@ -380,6 +381,38 @@ bgpq_pipeline(struct bgpq_expander* b,
 	return bp;
 };
 
+static void
+bgpq_expander_invalidate_asn(struct bgpq_expander* b, const char* q)
+{
+	   if (!strncmp(q, "!gas", 4) || !strncmp(q, "!6as", 4)) {
+		   char* eptr;
+		   unsigned long asn = strtoul(q+4, &eptr, 10), asn0, asn1 = 0;
+		   if (!asn || asn == ULONG_MAX || asn >= 4294967295 ||
+			   (eptr && *eptr != '\n' && *eptr != '\r' && *eptr != '.')) {
+			   sx_report(SX_ERROR, "some problem invalidating asn %s\n", q);
+			   return;
+		   };
+		   if (eptr && (*eptr == '\r' || *eptr == '\n')) {
+			   asn1 = asn;
+			   asn0 = 0;
+		   } else if (eptr && *eptr == '.') {
+			   asn0 = asn;
+			   asn1 = strtoul(eptr+1, &eptr, 10);
+		   };
+		   if (asn >= 65536) {
+			   asn1 = asn % 65536;
+			   asn0 = asn / 65536;
+		   };
+		   if (!b->asn32s[asn0] ||
+			   !(b->asn32s[asn0][asn1/8] & (0x80 >> (asn1 % 8)))) {
+			   sx_report(SX_NOTICE, "strange, invalidating inactive asn %lu(%s)\n",
+				   asn, q);
+		   } else {
+			   b->asn32s[asn0][asn1/8] &= ~(0x80 >> (asn1 % 8));
+		   };
+	   };
+};
+
 static void
 bgpq_write(struct bgpq_expander* b)
 {
@@ -561,10 +594,12 @@ have3:
 		} else if(response[0]=='C') {
 			/* No data */
 			SX_DEBUG(debug_expander,"No data expanding %s\n", req->request);
+			if (b->validate_asns) bgpq_expander_invalidate_asn(b, req->request);
 		} else if(response[0]=='D') {
 			/* .... */
 			SX_DEBUG(debug_expander,"Key not found expanding %s\n",
 				req->request);
+			if (b->validate_asns) bgpq_expander_invalidate_asn(b, req->request);
 		} else if(response[0]=='E') {
 			sx_report(SX_ERROR, "Multiple keys expanding %s: %s\n",
 				req->request, response);
@@ -707,8 +742,10 @@ have3:
 		free(recvbuffer);
 	} else if(response[0]=='C') {
 		/* no data */
+		if (b->validate_asns) bgpq_expander_invalidate_asn(b, request);
 	} else if(response[0]=='D') {
 		/* ... */
+		if (b->validate_asns) bgpq_expander_invalidate_asn(b, request);
 	} else if(response[0]=='E') {
 		/* XXXXXX */
 	} else if(response[0]=='F') {
@@ -831,7 +868,7 @@ bgpq_expand(struct bgpq_expander* b)
 			bgpq_read(b);
 	};
 
-	if(b->generation>=T_PREFIXLIST) {
+	if(b->generation>=T_PREFIXLIST || b->validate_asns) {
 		unsigned i, j, k;
 		STAILQ_FOREACH(mc, &b->rsets, next) {
 			if(b->family==AF_INET) {