Quickstart tutorial 2 – A simple counter

Congrats, you’ve made it to the second tutorial in the series. In the first instalment , we got our feet wet by writing a very simple hello world script. It didnt no much beyond printing a few log messages, but it taught us how to setup scripting. In this tutorial 2, we will write a simple script that meters packet lengths.

Packet Length Histogram counter

Once again as in Tutorial-1, lets run the script first and then explore details.

GOALS Develop useful Packet Length Histogram counter. How many packets > 1400,1000-1400,500-1000,etc. How to run in production mode, how to write a simplecounter script to plug into each packet, how to view results.
  • Get pktlen.lua

    • Download pktlen.lua into the lua scripts directory /usr/local/lib/trisul-probe/plugins/lua A sample run is shown below

      cd /usr/local/lib/trisul-probe/plugins/lua
      wget https://raw.githubusercontent.com/trisulnsm/trisul-scripts/master/lua/tutorial/tutorial2/pktlen.lua
  • Run Trisul over a PCAP file

    • Login as root or as the user specified in the trisulProbeConfig.xml file (default : trisul)

    • Run Trisul over a PCAP file, you can use your own or download tutorial2.pcap . This time will not be using the development tool testbench because we want to save the results into the database. Import the PCAP into a new context called “tut2”. The context=tut2 flag allows you to import the PCAP into its own context without erasing the previous data.

      trisulctl_probe importpcap /tmp/tutorial2.pcap context=tut2 

      You need to enter YES 2 times to confirm that you want to create a new context and import the PCAP.

    There will be no output. You need to login to Web Trisul to find out what just happened.

  • Login to Webtrisul context “tut2”

    • Login to Webtrisul, you will see a new context called “tut2” at the login screen. Select that and login. Then Go to Retro > Counters
    • Select “Packet Length” and voila ! You have detailed stats about distribution of packet lengths

    Figure: Topper sketches for packet length

    Figure: Meters added by LUA showing up in Trisul

    Pretty impressive for such a tiny script ? Lets see whats going on.

Exploring pktlen.lua

The pktlen.lua script plugs into Trisul to get a look at each packet. then counts the packet length againt a bucket. We will also demonstrate a way to count the bandwidth while we are at it.

Lets meter

  1. Traffic bandwidth – bytes/ bps per length bucket
  2. Packets – per length bucket

Meters and Keys

The two meters we maintain are

Bandwidth “Bytes” RATE_COUNTER Bandwidth per length-bucket. Out of 10Mbps bandwidth , 8Mbps is due to packets between 1000-1514 bytes in length, 100Kbps is due to small packets < 60 bytes etc
Packets “Packets” COUNTER Absolute count of number of packets in each length-bucket

Each bucket is identified by a unique string called a ‘key’. This script will use a string like “500-1000” to present packet lengths in range of 500 to 1000 bytes.

Structure of pktlen.lua

Lets look a bit deeper into th script countergroup and simplecounter sections and these are where the action happens.

Figure: Structure of pktlen.lua

The Trisul LUA API consists of about 16 different “script types”. In this tutorial , pktlen.lua uses the countergroup and simplecounter script types. You can even place the countergroup and simplecounter tables in separate files if you want.

  1. The id block just identifies the plugin to Trisul, we’ve already see this in Tutorial 1
  2. The countergroup block defines a new counter group
  3. The simplecounter block is where you count packets

The countergroup block

Reference : countergroup

Since we are creating a new counter group that meters Packet Lengths, we use a countergroup block to create a new group called “Packet Length”.

countergroup = {
  -- control section
  --   Id of the counter group 
  control = {
    guid = "{6ecb4ebb-d53b-470c-aca6-2f326b4c6c91}",
    name = "Packet Length",
    description = "Packet length distribution",
    bucketsize = 30,

  -- meter section
  --  What we're trying to count 
  --  meter 0 -  bytes by packet len
  --  meter 1 -  number of packets of each length 
  meters = {
      0, T.K.vartype.RATE_COUNTER, 10, "Bytes",   "bytes" ,   "B" },
      1, T.K.vartype.COUNTER,      10, "Packets", "packets",  "Pkts" },


You will notice the counter group has two sub blocks called control and meters.

The control section

guid for the counter group
There are tools to generate online, like http://www.guidgen.com/ – note the curly braces
Packet length
we select 30 seconds as the resolution for this group

The meters section

We define two meters

Meter 0 RATE_COUNTER This will track Mbps, Kbps for each packet length key 10 topper in sketch
Meter 1 COUNTER This will track total packets in each interval by length 10 toppers in sketch

The simplecounter block

Reference : simplecounter

Finally, we’ve landed at the most critical part of the plugin. This is where your LUA code actually goes.

The onpacket function is called for each packet at the appropriate protocol layer.

We want to tap packets at the IP layer, so we define the protocol_guid as

at which layer do we want to listen to packets. The guid {0A2C724B-5B9F-4ba6-9C97-B05080558574} in the example stands for IP. See Well Known GUIDs

simplecounter = {

  protocol_guid = "{0A2C724B-5B9F-4ba6-9C97-B05080558574}",

  onpacket = function(engine,layer)
    local packet= layer:packet()
    local len  = packet:wire_length()
    local key = ""

    if len > 1400 then key = "1400+"
    elseif len > 1000 then key = "1000-1400"
    elseif len > 500 then key = "500-1000"
    elseif len > 200 then key = "200-500"
    elseif len > 100 then key = "100-200"
    elseif len > 60 then key = "60-100"
    else key = "0-60" 

    -- update_counter methods is the "magic"  glue
    -- between C and LUA in trisul

        TrisulPlugin.countergroup.control.guid, key , 0)

        TrisulPlugin.countergroup.control.guid, key, 1, 1)


Key things to remember

  1. Your onpacket function is called for every packet. Try not to do any I/O.
  2. The two parameters engine and layer are objects passed with every packet
  3. Layer – represents the layer you mentioned in the protocol_guid section of the countergroup block
  4. Packet – from the layer you can get access to the entire packet using layer:packet()
  5. The engine parameter is the critical object that provides the methods you need to call to end your script.

What we’re doing

  1. We get the “wire length” from layer:packet(), then group it into 7 keys based on length
  2. Call the update_counter methods in the Engine object

Thats it ! The rest is taken care of by Trisul. We have a brand new counter group called Packet Length.

Congrats ! You are now well on your way to writing your own scripts. Just browse the code samples on GitHub and check out the documentaton on the left navigation.