diff --git a/webui/src/main/java/com/muwire/webui/ConfigurationServlet.java b/webui/src/main/java/com/muwire/webui/ConfigurationServlet.java
index d466364f..4b6f74cc 100644
--- a/webui/src/main/java/com/muwire/webui/ConfigurationServlet.java
+++ b/webui/src/main/java/com/muwire/webui/ConfigurationServlet.java
@@ -51,24 +51,24 @@ public class ConfigurationServlet extends HttpServlet {
         core.getMuOptions().setBrowseFiles(false);
     }
     
-    private void update(String name, String value) {
+    private void update(String name, String value) throws Exception {
         switch(name) {
         case "allowUntrusted" : core.getMuOptions().setAllowUntrusted(false); break;
         case "searchExtraHop" : core.getMuOptions().setSearchExtraHop(true); break;
         case "allowTrustLists": core.getMuOptions().setAllowTrustLists(true); break;
-        case "trustListInterval" : core.getMuOptions().setTrustListInterval(Integer.parseInt(value)); break;
-        case "downloadRetryInterval" : core.getMuOptions().setDownloadRetryInterval(Integer.parseInt(value)); break;
+        case "trustListInterval" : core.getMuOptions().setTrustListInterval(getPositiveInteger(value)); break;
+        case "downloadRetryInterval" : core.getMuOptions().setDownloadRetryInterval(getPositiveInteger(value)); break;
         case "totalUploadSlots" : core.getMuOptions().setTotalUploadSlots(Integer.parseInt(value)); break;
         case "uploadSlotsPerUser" : core.getMuOptions().setUploadSlotsPerUser(Integer.parseInt(value)); break;
-        case "downloadLocation" : core.getMuOptions().setDownloadLocation(new File(value)); break;
-        case "incompleteLocation" : core.getMuOptions().setIncompleteLocation(new File(value)); break;
+        case "downloadLocation" : core.getMuOptions().setDownloadLocation(getDirectory(value)); break;
+        case "incompleteLocation" : core.getMuOptions().setIncompleteLocation(getDirectory(value)); break;
         case "shareDownloadedFiles" : core.getMuOptions().setShareDownloadedFiles(true); break;
         case "shareHiddenFiles" : core.getMuOptions().setShareHiddenFiles(true); break;
         case "searchComments" : core.getMuOptions().setSearchComments(true); break;
         case "browseFiles" : core.getMuOptions().setBrowseFiles(true); break;
-        case "speedSmoothSeconds" : core.getMuOptions().setSpeedSmoothSeconds(Integer.parseInt(value)); break;
-        case "inBw" : core.getMuOptions().setInBw(Integer.parseInt(value)); break;
-        case "outBw" : core.getMuOptions().setOutBw(Integer.parseInt(value)); break;
+        case "speedSmoothSeconds" : core.getMuOptions().setSpeedSmoothSeconds(getPositiveInteger(value)); break;
+        case "inBw" : core.getMuOptions().setInBw(getPositiveInteger(value)); break;
+        case "outBw" : core.getMuOptions().setOutBw(getPositiveInteger(value)); break;
         case "inbound.length" : core.getI2pOptions().setProperty(name, value); break;
         case "inbound.quantity" : core.getI2pOptions().setProperty(name, value); break;
         case "outbound.length" : core.getI2pOptions().setProperty(name, value); break;
@@ -76,6 +76,24 @@ public class ConfigurationServlet extends HttpServlet {
         // TODO: ui settings
         }
     }
+    
+    private static int getPositiveInteger(String s) throws Exception {
+        int rv = Integer.parseInt(s);
+        if (rv <= 0)
+            throw new Exception(s + " is negative");
+        return rv;
+    }
+    
+    private static File getDirectory(String s) throws Exception {
+        File f = new File(s);
+        if (!f.exists())
+            throw new Exception(s + " does not exist");
+        if (!f.isDirectory())
+            throw new Exception(s + " is not a directory");
+        if (!f.canWrite())
+            throw new Exception(s + " cannot be written to");
+        return f;
+    }
 
     @Override
     public void init(ServletConfig config) throws ServletException {