a1f60a2 Jun 20, 2016
@eugeneia @lukego
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Snabb Getting Started Guide


Welcome to Snabb, a software switch for the NFV world! The purpose of this guide is to introduce end users and developers to how to use Snabb. We'll delve into several aspects of Snabb Switch at a high level prior to implementing an example network function using the Snabb Switch concept of an App.


Running the code in this guide requires a recent distribution of Linux. The examples were written on a fresh install of a 64-bit Ubuntu 14.04 distribution on an IaaS virtual machine. There is no requirement for a hardware NIC supported by Snabb for these examples.

Downloading, Compiling, and Installing Snabb

The following commands clone the Snabb repository, compile the software, and install the snabb executable. Note that the output of the commands is omitted.

sudo apt-get update
sudo apt-get -y install build-essential git
git clone
cd snabb
make -j

Your First Snabb Program

Snabb provides you with many reusable network components out of the box. In Snabb terminology we call these components Apps, because they perform a specific function and can be combined with each other in arbitrary ways. In this guide we will use two of the apps bundled with Snabb to build a small but useful tool.

  1. PcapReader - This app reads packets from a PCAP capture file.
  2. RawSocket - This app receives and transmits packets over Linux network interfaces.

Each app can receive packets from an arbitrary number of input links and transmit packets to an arbitrary number of output links. Snabb Switch provides you with a little language to link together apps in a directed graph that governs the packet flow. We call this graph an App network. In our first example the output of PcapReader will be sent to the input of RawSocket.

Our example will implement a packet replay program that reads a PCAP file and then plays back the packets to an arbitrary Ethernet interface.

You can find the example_replay.lua program in the src/program/example_replay directory. Let us go over it and explain it step by step:

module(..., package.seeall)

local pcap = require("apps.pcap.pcap")
local raw = require("apps.socket.raw")

The first line contains a call to module, its a Lua specific function that creates a loadable module for the code defined in this file. Then we use require to load other modules we want to use: apps.pcap.pcap and apps.socket.raw.

function run (parameters)
   if not (#parameters == 2) then
      print("Usage: example_replay <pcap-file> <interface>")
   local pcap_file = parameters[1]
   local interface = parameters[2]

Snabb treats modules under src/program specially: if a module exposes a top-level run function it can be invoked from the snabb executable. E.g. to execute run you would invoke snabb like so:

src/snabb example_replay <args...>

Since this is a command line program we need to verify and parse the arguments we want to accept. The first argument to run will be an array containing the command line arguments. In this program we require two arguments, namely the PCAP file and interface to use.

   local c =, "capture", pcap.PcapReader, pcap_file), "playback", raw.RawSocket, interface), "capture.output -> playback.rx")

Now we get to the meat of this program: building the app network. First we get ourselves an an empty configuration c by calling We then add two app instances to our nework by calling on our configuration:

  • capture - an instance of the PcapReader app which will read pcap_file
  • playback - an instance of the RawSocket app that will receive from and transmit to interface

Then we use to define a connection between our apps: capture will transmit packets from its output port to playback's rx port. Since capture is a PcapReader it will transmit packets from a PCAP capture file to its output port. playback is a RawSocket and thus will transfer packets received on the rx port to the interface. In case you are curious, we could receive incoming packets from playback's underlying network interface by connecting its tx port to another app, e.g. a PcapWriter app.

   engine.main({duration=1, report = {showlinks=true}})

Finally we will load our configuration c into the engine by calling engine.configure. Now we can run our app network by calling engine.main. In this example run it for one second, naively assuming that will be more than enough time for the PCAP file to be processed.

Running The example_replay Program

We'll use a virtual interface as testing yields strange results when the sample program runs on "real" network interfaces in some IaaS environments. If you run the program in a controlled environment, you should be able to use eth0 or any other network interface without problems.

Create the veth pair using the following commands:

sudo ip link add veth0 type veth peer name veth1
sudo ip link set dev veth0 up
sudo ip link set dev veth1 up

An input.pcap file is included in the src/program/example_replay directory but you can just as well use any other PCAP file.

Open a second terminal window and run tcpdump on veth0:

sudo tcpdump -i veth0

From Snabb directory, run the following invocation of our example program:

sudo src/snabb example_replay src/program/example_replay/input.pcap veth0

You should see the following output:

link report:
                   5 sent on capture.output -> playback.rx (loss rate: 0%)

In your other window, tcpdump will capture the outbound packets on veth0:

tcpdump: WARNING: veth0: no IPv4 address assigned
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on veth0, link-type EN10MB (Ethernet), capture size 65535 bytes
17:45:34.097183 IP > ICMP echo request, id 2320, seq 1, length 64
17:45:34.097228 IP > ICMP echo request, id 2320, seq 2, length 64
17:45:34.097242 IP > ICMP echo request, id 2320, seq 3, length 64
17:45:34.097254 IP > ICMP echo request, id 2320, seq 4, length 64
17:45:34.097268 IP > ICMP echo request, id 2320, seq 5, length 64
5 packets captured
5 packets received by filter
0 packets dropped by kernel

Congratulations! You have successfully run your first Snabb program.

Your First Snabb App

The example program described above configures an app network using apps already included with Snabb. But writing custom Snabb Switch apps its easy. Let's inspect an example app to get you going!

Our forwarding logic for the app will be simple (and silly): the app will send every other packet on its output port. The odd numbered packets will be silently discarded. While this is not useful, the purpose is to show the anatomy of a Snabb app.

Snabb apps are required to implement one method: new. The new method returns an instance of your app. An optional method we'll use is push, which moves packets from the input to output ports. Let's examine the example app defined in src/program/example_spray/sprayer.lua.

module(..., package.seeall)

Sprayer = {}

Again we use module to declare a module. Then we create an empty Sprayer table to define some methods on.

function Sprayer:new ()
   local o = { packet_counter = 1 }
   return setmetatable(o, {__index = Sprayer})

This is the new method of our app. It returns an instance of Sprayer with a packet_counter field initialized to 1. We will use the counter to determine which packet to drop.

function Sprayer:push()
   local i = assert(self.input.input, "input port not found")
   local o = assert(self.output.output, "output port not found")

The push method of our app will be called by the engine to pump packets through the app network. First of all we get a hold of the ports we want to receive and transmit packets on, i and o. For each app instance self.input and self.output will be bound to tables mapping port names to the underlying link data structures. We assert that the links actually exists in order to raise an exception immediately if the app was not properly connected.

   while not link.empty(i) do
      self:process_packet(i, o)
      self.packet_counter = self.packet_counter + 1

Now we loop over the available packets on iand process each individually. This is a common Snabb idiom. The actual logic of our app is performed by a call to the process_packet method which is defined below. Note that we increment the packet_counter of our instance for every packet processed.

function Sprayer:process_packet(i, o)
   local p = link.receive(i)

   -- drop every other packet
   if self.packet_counter % 2 == 0 then
      link.transmit(o, p)

In the process_packet method we first receive a packet p from i using link.receive. We then decide whether p should be transmitted to o using link.transmit or dropped, depending on whether self.packet_counter is even or odd. Note that packets which are not transmitted to another link must be freed using

We'll use the example_spray program defined in src/program/example_spray/example_spray.lua to run our example app. We will not go over it in detail because it is very similar to the example_replay program explained before. Note though how we require the newly defined program.example_spray.sprayer module and use it when creating the app network.

module(..., package.seeall)

local pcap = require("apps.pcap.pcap")
local sprayer = require("program.example_spray.sprayer")

function run (parameters)
   if not (#parameters == 2) then
      print("Usage: example_spray <input> <output>")
   local input = parameters[1]
   local output = parameters[2]

   local c =, "capture", pcap.PcapReader, input), "spray_app", sprayer.Sprayer), "output_file", pcap.PcapWriter, output), "capture.output -> spray_app.input"), "spray_app.output -> output_file.input")

   engine.main({duration=1, report = {showlinks=true}})

Here is the expected output if you use the provided input.pcap file to run the example_spray program:

src/snabb example_spray src/program/example_replay/input.pcap /tmp/out.cap

link report:
                   5 sent on capture.output -> spray_app.input (loss rate: 0%)
                   2 sent on spray_app.output -> output_file.input (loss rate: 0%)

The app sent packets numbered 2 and 4. Packets numbered 1, 3, and 5 are discarded.

Next Steps

Here are some suggested steps to continue learning about Snabb.

  1. Read the source documentation. Start with the in the src directory.
  2. Read the code for the example apps in basic_apps.lua.
  3. Continue reading the source for other apps in the apps directory.
  4. Modify the sprayer.lua program to make decisions based on the contents of the packet's Layer 3 header. Hint: The snabb executable must be rebuilt (using make) any time you make changes to apps and/or programs in order to include your changes.

Don't hesitate to contact the Snabb community on the mailing list.