diff --git a/CHANGELOG b/CHANGELOG
index c7ae48c07..617c27b1d 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,5 +1,10 @@
 #Nmap Changelog ($Id$); -*-text-*-
 
+o Nmap no longer considers an ICMP Host Unreachable as confirmation that a
+  target is down, in accordance with RFC 1122 which says these errors may be
+  transient. Instead, the probe will be destroyed and other probes used to
+  determine aliveness. [Daniel Miller]
+
 o [NSE][GH#711] New script openflow-info gathers preferred and supported
   protocol versions from OpenFlow devices [Jay Smith, Mak Kolybabi]
 
diff --git a/scan_engine.cc b/scan_engine.cc
index 33836aa10..cec3064cf 100644
--- a/scan_engine.cc
+++ b/scan_engine.cc
@@ -2014,8 +2014,8 @@ static bool ultrascan_host_pspec_update(UltraScanInfo *USI, HostScanStats *hss,
   int oldstate = hss->target->flags;
   /* If the host is already up, ignore any further updates. */
   if (hss->target->flags != HOST_UP) {
-    assert(newstate == HOST_UP || newstate == HOST_DOWN);
-    hss->target->flags = newstate;
+    // don't allow HOST_UNKNOWN to override a known state.
+    hss->target->flags = (newstate == HOST_UNKNOWN ? oldstate : newstate);
     /* For port scans (not -sn) where output may be delayed until more scan
      * phases are done, emit a hosthint element during host discovery when a
      * target is found to be up. */
@@ -2066,12 +2066,12 @@ void ultrascan_host_probe_update(UltraScanInfo *USI, HostScanStats *hss,
   bool adjust_timing = adjust_timing_hint;
   bool adjust_ping = adjust_timing_hint;
 
-  /* If we got a response that meant "down", then it was an ICMP error. These
+  /* If we got a response that meant "down" or "unknown", then it was an ICMP error. These
      are often rate-limited (RFC 1812) or generated by a different host. We only
      allow such responses to increase, not decrease, scanning speed by
      disallowing drops (probe->tryno > 0), and we don't allow changing the ping
      probe to something that's likely to get dropped. */
-  if (rcvdtime != NULL && newstate == HOST_DOWN) {
+  if (rcvdtime != NULL && newstate != HOST_UP) {
     if (probe->tryno > 0) {
       if (adjust_timing && o.debugging > 1)
         log_write(LOG_PLAIN, "Response for %s means new state is down; not adjusting timing.\n", hss->target->targetipstr());
diff --git a/scan_engine_raw.cc b/scan_engine_raw.cc
index 8eb3d093a..b31a4e667 100644
--- a/scan_engine_raw.cc
+++ b/scan_engine_raw.cc
@@ -666,23 +666,34 @@ int get_ping_pcap_result(UltraScanInfo *USI, struct timeval *stime) {
         if (probenum >= listsz)
           continue;
 
+        /* Destination unreachable. */
         if ((hdr.proto == IPPROTO_ICMP && ping->type == 3)
             || (hdr.proto == IPPROTO_ICMPV6 && ping->type == 1)) {
-          /* Destination unreachable. */
-          if (sockaddr_storage_cmp(&target_dst, &hdr.src) == 0) {
+          // If it's Port or Proto unreachable and the address matches, it's up.
+          if (((hdr.proto == IPPROTO_ICMP && (ping->code == 2 || ping->code == 3))
+                || (hdr.proto == IPPROTO_ICMPV6 && ping->code == 4))
+                && sockaddr_storage_cmp(&target_dst, &hdr.src) == 0) {
             /* The ICMP or ICMPv6 error came directly from the target, so it's up. */
             goodone = true;
             newstate = HOST_UP;
-          } else {
-            goodone = true;
-            newstate = HOST_DOWN;
+            if (o.debugging) {
+              log_write(LOG_STDOUT, "Got port/proto unreachable for %s\n", hss->target->targetipstr());
+            }
           }
-          if (o.debugging) {
-            if ((hdr.proto == IPPROTO_ICMP && ping->code == 3)
-                || (hdr.proto == IPPROTO_ICMPV6 && ping->code == 4))
-              log_write(LOG_STDOUT, "Got port unreachable for %s\n", hss->target->targetipstr());
-            else
+          else {
+            /* For other codes, see RFC 1122:
+             * A Destination Unreachable message that is received with code
+             * 0 (Net), 1 (Host), or 5 (Bad Source Route) may result from a
+             * routing transient and MUST therefore be interpreted as only
+             * a hint, not proof, that the specified destination is unreachable
+             */
+            // Still, it's a response so we should destroy the matching probe.
+            goodone = true;
+            newstate = HOST_UNKNOWN;
+            adjust_timing = false;
+            if (o.debugging) {
               log_write(LOG_STDOUT, "Got destination unreachable for %s\n", hss->target->targetipstr());
+            }
           }
         } else if ((hdr.proto == IPPROTO_ICMP && ping->type == 11)
                    || (hdr.proto == IPPROTO_ICMPV6 && ping->type == 3)) {
@@ -692,22 +703,26 @@ int get_ping_pcap_result(UltraScanInfo *USI, struct timeval *stime) {
           newstate = HOST_DOWN;
           /* I don't want anything to do with timing this. */
           adjust_timing = false;
-        } else if (hdr.proto == IPPROTO_ICMP && ping->type == 4) {
-          if (o.debugging)
-            log_write(LOG_STDOUT, "Got ICMP source quench\n");
-          usleep(50000);
-        } else if (hdr.proto == IPPROTO_ICMPV6 && ping->type == 4) {
-          if (o.debugging)
-            log_write(LOG_STDOUT, "Got ICMPv6 Parameter Problem\n");
         } else if (hdr.proto == IPPROTO_ICMP) {
-          if (o.debugging) {
-            log_write(LOG_STDOUT, "Got ICMP message type %d code %d\n",
-                      ping->type, ping->code);
+          if (ping->type == 4) {
+            if (o.debugging)
+              log_write(LOG_STDOUT, "Got ICMP source quench\n");
+            usleep(50000);
+          } else {
+            if (o.debugging) {
+              log_write(LOG_STDOUT, "Got ICMP message type %d code %d\n",
+                  ping->type, ping->code);
+            }
           }
         } else if (hdr.proto == IPPROTO_ICMPV6) {
-          if (o.debugging)
-            log_write(LOG_STDOUT, "Got ICMPv6 message type %d code %d\n",
-                      ping->type, ping->code);
+          if (ping->type == 4) {
+            if (o.debugging)
+              log_write(LOG_STDOUT, "Got ICMPv6 Parameter Problem\n");
+          } else {
+            if (o.debugging)
+              log_write(LOG_STDOUT, "Got ICMPv6 message type %d code %d\n",
+                  ping->type, ping->code);
+          }
         }
       }
     } else if (hdr.proto == IPPROTO_TCP && USI->ptech.rawtcpscan) {