diff --git a/usr.sbin/rtsold/rtsol.c b/usr.sbin/rtsold/rtsol.c
index 30027fc65ac..76756bfd839 100644
--- a/usr.sbin/rtsold/rtsol.c
+++ b/usr.sbin/rtsold/rtsol.c
@@ -363,13 +363,19 @@ rtsol_input(int sock)
 		case ND_OPT_RDNSS:
 			rdnss = (struct nd_opt_rdnss *)raoptp;
 
-			/* Optlen sanity check (Section 5.3.1 in RFC 6106) */
-			if (rdnss->nd_opt_rdnss_len < 3) {
+			/*
+			 * The option header is 8 bytes long and each address
+			 * occupies 16 bytes, so the option length must be
+			 * greater than or equal to 24 bytes and an odd multiple
+			 * of 8 bytes.  See section 5.1 in RFC 6106.
+			 */
+			if (rdnss->nd_opt_rdnss_len < 3 ||
+			    rdnss->nd_opt_rdnss_len % 2 == 0) {
 				warnmsg(LOG_INFO, __func__,
-		    			"too short RDNSS option"
-					"in RA from %s was ignored.",
-					inet_ntop(AF_INET6, &from.sin6_addr,
-					    ntopbuf, sizeof(ntopbuf)));
+				    "too short RDNSS option in RA from %s "
+				    "was ignored.",
+				inet_ntop(AF_INET6, &from.sin6_addr, ntopbuf,
+				    sizeof(ntopbuf)));
 				break;
 			}