diff --git a/src/libmain/shared.cc b/src/libmain/shared.cc
index 36b4ebdc9c..5a8f600e8d 100644
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@@ -217,7 +217,7 @@ static void initAndRun(int argc, char * * argv)
 
     /* Automatically clean up the temporary roots file when we
        exit. */
-    RemoveTempRoots removeTempRoots; /* unused variable - don't remove */
+    RemoveTempRoots removeTempRoots __attribute__((unused));
 
     /* Make sure that the database gets closed properly, even if
        terminate() is called (which happens sometimes due to bugs in
diff --git a/src/libstore/misc.cc b/src/libstore/misc.cc
index acf9346d4b..1759f521cc 100644
--- a/src/libstore/misc.cc
+++ b/src/libstore/misc.cc
@@ -46,7 +46,7 @@ Path findOutput(const Derivation & drv, string id)
 
 
 void queryMissing(const PathSet & targets,
-    PathSet & willBuild, PathSet & willSubstitute)
+    PathSet & willBuild, PathSet & willSubstitute, PathSet & unknown)
 {
     PathSet todo(targets.begin(), targets.end()), done;
 
@@ -57,7 +57,10 @@ void queryMissing(const PathSet & targets,
         done.insert(p);
 
         if (isDerivation(p)) {
-            if (!store->isValidPath(p)) continue;
+            if (!store->isValidPath(p)) {
+                unknown.insert(p);
+                continue;
+            }
             Derivation drv = derivationFromPath(p);
 
             bool mustBuild = false;
@@ -81,12 +84,11 @@ void queryMissing(const PathSet & targets,
         else {
             if (store->isValidPath(p)) continue;
             SubstitutablePathInfo info;
-            if (dynamic_cast<LocalStore *>(store.get())->querySubstitutablePathInfo(p, info)) {
+            if (store->querySubstitutablePathInfo(p, info)) {
                 willSubstitute.insert(p);
                 todo.insert(info.references.begin(), info.references.end());
-            }
-            /* Not substitutable and not buildable; should we flag
-               this? */
+            } else
+                unknown.insert(p);
         }
     }
 }
diff --git a/src/libstore/misc.hh b/src/libstore/misc.hh
index ad1ad80eb1..33d81ed4a0 100644
--- a/src/libstore/misc.hh
+++ b/src/libstore/misc.hh
@@ -29,7 +29,7 @@ Path findOutput(const Derivation & drv, string id);
    derivations that will be built, and the set of output paths that
    will be substituted. */
 void queryMissing(const PathSet & targets,
-    PathSet & willBuild, PathSet & willSubstitute);
+    PathSet & willBuild, PathSet & willSubstitute, PathSet & unknown);
 
 
 }
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index b7e60043d6..bc04c8adb9 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -216,7 +216,16 @@ bool RemoteStore::hasSubstitutes(const Path & path)
 bool RemoteStore::querySubstitutablePathInfo(const Path & path,
     SubstitutablePathInfo & info)
 {
-    throw Error("not implemented");
+    writeInt(wopQuerySubstitutablePathInfo, to);
+    writeString(path, to);
+    processStderr();
+    unsigned int reply = readInt(from);
+    if (reply == 0) return false;
+    info.deriver = readString(from);
+    if (info.deriver != "") assertStorePath(info.deriver);
+    info.references = readStorePaths(from);
+    info.downloadSize = readLongLong(from);
+    return true;
 }
 
 
diff --git a/src/libstore/worker-protocol.hh b/src/libstore/worker-protocol.hh
index d887ee59bd..1146f95770 100644
--- a/src/libstore/worker-protocol.hh
+++ b/src/libstore/worker-protocol.hh
@@ -33,6 +33,7 @@ typedef enum {
     wopQueryDeriver = 18,
     wopSetOptions = 19,
     wopCollectGarbage = 20,
+    wopQuerySubstitutablePathInfo = 21,
 } WorkerOp;
 
 
diff --git a/src/nix-env/nix-env.cc b/src/nix-env/nix-env.cc
index f561dc2e51..ee870f6906 100644
--- a/src/nix-env/nix-env.cc
+++ b/src/nix-env/nix-env.cc
@@ -531,7 +531,7 @@ static void queryInstSources(EvalState & state,
 
 static void printMissing(EvalState & state, const DrvInfos & elems)
 {
-    PathSet targets, willBuild, willSubstitute;
+    PathSet targets, willBuild, willSubstitute, unknown;
     for (DrvInfos::const_iterator i = elems.begin(); i != elems.end(); ++i) {
         Path drvPath = i->queryDrvPath(state);
         if (drvPath != "")
@@ -540,17 +540,24 @@ static void printMissing(EvalState & state, const DrvInfos & elems)
             targets.insert(i->queryOutPath(state));
     }
 
-    queryMissing(targets, willBuild, willSubstitute);
+    queryMissing(targets, willBuild, willSubstitute, unknown);
 
     if (!willBuild.empty()) {
         printMsg(lvlInfo, format("the following derivations will be built:"));
-        for (PathSet::iterator i = willBuild.begin(); i != willBuild.end(); ++i)
+        foreach (PathSet::iterator, i, willBuild)
             printMsg(lvlInfo, format("  %1%") % *i);
     }
 
     if (!willSubstitute.empty()) {
-        printMsg(lvlInfo, format("the following paths will be substituted:"));
-        for (PathSet::iterator i = willSubstitute.begin(); i != willSubstitute.end(); ++i)
+        printMsg(lvlInfo, format("the following paths will be downloaded/copied:"));
+        foreach (PathSet::iterator, i, willSubstitute)
+            printMsg(lvlInfo, format("  %1%") % *i);
+    }
+
+    if (!unknown.empty()) {
+        printMsg(lvlInfo, format("don't know how to build the following paths%1%:")
+            % (readOnlyMode ? " (may be caused by read-only store access)" : ""));
+        foreach (PathSet::iterator, i, unknown)
             printMsg(lvlInfo, format("  %1%") % *i);
     }
 }
diff --git a/src/nix-worker/nix-worker.cc b/src/nix-worker/nix-worker.cc
index d8d86434ea..377f2be73a 100644
--- a/src/nix-worker/nix-worker.cc
+++ b/src/nix-worker/nix-worker.cc
@@ -430,7 +430,21 @@ static void performOp(unsigned int clientVersion,
         stopWork();
         break;
     }
-            
+
+    case wopQuerySubstitutablePathInfo: {
+        Path path = absPath(readString(from));
+        startWork();
+        SubstitutablePathInfo info;
+        bool res = store->querySubstitutablePathInfo(path, info);
+        stopWork();
+        writeInt(res ? 1 : 0, to);
+        if (res) {
+            writeString(info.deriver, to);
+            writeStringSet(info.references, to);
+            writeLongLong(info.downloadSize, to);
+        }
+        break;
+    }
             
     default:
         throw Error(format("invalid operation %1%") % op);
@@ -440,7 +454,7 @@ static void performOp(unsigned int clientVersion,
 
 static void processConnection()
 {
-    RemoveTempRoots removeTempRoots; /* unused variable - don't remove */
+    RemoveTempRoots removeTempRoots __attribute__((unused));
 
     canSendStderr = false;
     myPid = getpid();