createServer crash on malformed packet

[ghost]

It is possible to crash a DNS server created with .createServer by sending it a bad packet. An exception is thrown by the DNS message parser which cannot be caught using the .createServer API.

The crash can be reproduced by running the following server code:

var dnsd = require('dnsd');

dnsd.createServer(function(req, res) {
  res.end('1.2.3.4');
}).listen(5300);

Once the server is started, send it a non-DNS packet:

netcat -u localhost 5300 <<< test

The server crashes with the following message:

buffer.js:582
    throw new RangeError('Trying to access beyond buffer length');
          ^
RangeError: Trying to access beyond buffer length
    at checkOffset (buffer.js:582:11)
    at Buffer.readUInt16BE (buffer.js:602:5)
    at record_count (/home/tim/test/node_modules/dnsd/parse.js:76:16)
    at Object.sections (/home/tim/test/node_modules/dnsd/parse.js:144:30)
    at Request.DNSMessage.parse (/home/tim/test/node_modules/dnsd/message.js:99:30)
    at Request.DNSMessage (/home/tim/test/node_modules/dnsd/message.js:55:10)
    at new Request (/home/tim/test/node_modules/dnsd/server.js:177:11)
    at Server.on_udp (/home/tim/test/node_modules/dnsd/server.js:167:13)
    at Socket.<anonymous> (/home/tim/test/node_modules/dnsd/server.js:45:54)
    at Socket.emit (events.js:98:17)

[ghost]

Here is the patch I used for silently ignoring malformed DNS packets:

commit 17cd4d84b0d35c4a34473b3bb37017cc7788e108
Author: Tim Cooper <[email protected]>
Date:   Thu Sep 4 11:38:32 2014 -0300

    silently ignore any malformed packets when using the .createServer API

diff --git a/server.js b/server.js
index 25e095d..9eec51f 100644
--- a/server.js
+++ b/server.js
@@ -142,11 +142,13 @@ Server.prototype.on_tcp_connection = function(connection) {

     if(length !== null && bytes_received == 2 + length) {
       // All of the data (plus the 2-byte length prefix) is received.
-      var data = Buffer.concat(bufs)
-        , req = new Request(data, connection)
-        , res = new Response(data, connection)
+      try {
+        var data = Buffer.concat(bufs)
+          , req = new Request(data, connection)
+          , res = new Response(data, connection)

-      self.emit('request', req, res)
+        self.emit('request', req, res)
+      } catch (ex) { }
     }
   })
 }
@@ -164,10 +166,12 @@ Server.prototype.on_udp = function(data, rinfo) {
                    , 'end'          : function() {}
                    }

-  var req = new Request(data, connection)
-    , res = new Response(data, connection)
+  try {
+    var req = new Request(data, connection)
+      , res = new Response(data, connection)

-  self.emit('request', req, res)
+    self.emit('request', req, res)
+  } catch (ex) { }
 }

[jhs]

Great catch! Thank you!