Reject non-useable routes. Fixes nmap#2206

Author: nnposter

CHANGELOG

@@ -1,5 +1,9 @@
 #Nmap Changelog ($Id$); -*-text-*-
 
+o [GH#2206] Nmap no longer produces cryptic message "Failed to convert
+  source address to presentation format" when unable to find useable route
+  to the target. [nnposter]
+
 o [Ncat][GH#2202] Use safety-checked versions of FD_* macros to abort early if
   number of connections exceeds FD_SETSIZE. [Pavel Zhukov]
 

libnetutil/netutil.cc

@@ -3163,7 +3163,8 @@ static int route_dst_netlink(const struct sockaddr_storage *dst,
   len -= NLMSG_LENGTH(sizeof(*nlmsg));
 
   /* See rtnetlink(7). Anything matching this route is actually unroutable. */
-  if (rtmsg->rtm_type == RTN_UNREACHABLE)
+  if (rtmsg->rtm_type == RTN_UNREACHABLE || rtmsg->rtm_type == RTN_UNSPEC
+    || rtmsg->rtm_type == RTN_BLACKHOLE || rtmsg->rtm_type == RTN_PROHIBIT)
     return 0;
 
   /* Default values to be possibly overridden. */