Moikka! Here's Adrián. I work for Igalia.

SnabbWall's Traffic Analyzer: L7Spy

Wow, two posts just a few days apart from each other! Today I am writing about L7Spy, the application from the SnabbWall suite used to analyze network traffic and detect which applications they belong to. SnabbWall is being developed at Igalia with sponsorship from the NLnet Foundation. (Thanks!)


A Digression

There are two things related to L7Spy I want to mention beforehand:

nDPI 1.8 is Now Supported

The changes from the recently released ljndpi v0.3.3 have been merged into the SnabbWall repository, which means that now it also supports using version 1.8 of the nDPI library.

Following Upstream

Snabb follows a monthly release schedule. Initially, I thought it would be better to do all the development for SnabbWall starting from a given Snabb release, and incorporate the changes from upstream later on, after the first one of SnabbWall is done. I noticed how it might end up being difficult to rebase SnabbWall on top of the changes accumulated from the upstream repository, and switched to the following scheme:

  • The branch containing the most up-to-date development code is always in the snabbwall branch.
  • Whenever a new Snabb version is released, changes are merged from the upstream repository using its corresponding release tag.
  • When a SnabbWall release is done, it tagged from current contents of the snabbwall branch, which means it is automatically based on the latest Snabb release.
  • If a released version needs fixing, a maintenance branch snabbwall-vX.Y for it is created from the release tag. Patches have to be backported or purposedfully made for that this new branch.

This approach reduces the burden of keeping up with changes in Snabb, while still allowing to make bugfix releases of any SnabbWall version when needed.


The L7Spy is a Snabb application, and as such it can be reused in your own application networks. It is made to be connected in a few different ways, which should allow for painless integration. The application has two bidirectional endpoints called south and north:

The L7Spy application.

Packets entering one of the ends are forwarded unmodified to the other end, in both directions, and they are scanned as they pass through the application.

The only mandatory connection is the receiving side of either south or north: this allows for using L7Spy as a “kitchen sink”, swallowing the scanned packets without sending them anywhere. As an example, this can be used to connect a RawSocket application to L7Spy, attach the RawSocket to an existing network interface, and do passive analysis of the traffic passing through that interface:

Passive analysis made easy

If you swap the RawSocket application with the one driving Intel NICs, and divert a copy of packets passing through your firewall towards the NIC, you would be doing essentially the same but at lightning speed and moving the load of packet analysis to a separate machine. Add a bit more of smarts on top, and this kind of setup would already look like a standalone Intrusion Detection System!

snabb wall

While implementing the L7Spy application, the snabb wall command has also come to life. Like all the other Snabb programs, the wall command is built into the snabb executable, and it has subcommands itself:

aperez@hikari ~/snabb/src % sudo ./snabb wall
[sudo] password for aperez:
  snabb wall <subcommand> <arguments...>
  snabb wall --help

Available subcommands:

  spy     Analyze traffic and report statistics

Use --help for per-command usage. Example:

  snabb wall spy --help

aperez@hikari ~/snabb/src %

For the moment being, only the spy subcommand is provided: it exercises the L7Spy application by feeding into it packets captured from one of the supported sources. It can work in batch mode —the default—, which is useful to analyze a static pcap file. The following example analyzes the full contents of the pcap file, reporting at the end all the traffic flows detected:

aperez@hikari ~/snabb/src % sudo ./snabb wall spy \
      pcap program/wall/tests/data/EmergeSync.cap
0xfffffffff19ba8be   18p -  RSYNC
0xffffffffc6f96d0f 4538p -  RSYNC
aperez@hikari ~/snabb/src %

Two flows are identified, which correspond to two different connections between the client with IPv4 address (from two different ports) to the server with address Both flows are correctly identified as belonging to the rsync application. Notice how the application is properly detected even though the rsync daemon is running in port 26883 which is a non-standard one for this kind of service.

It is also possible to ingest traffic directly from a network interface in live mode. Apart from the drivers for Intel cards included with Snabb (selectable using the intel10g and intel1g sources) there are two hardware-independent sources for TAP devices (tap) and RAW sockets (raw). Let’s use a RAW socket to sniff a copy of every packet which passes through the a kernel-managed network interface, while opening a well known search engine in a browser using their https:// URL:

aperez@hikari ~/snabb/src % sudo ./snabb wall spy --live eth0
0x77becdcb    2p    -  SERVICE_GOOGLE:DNS
0x6e111c73    4p   -  SERVICE_GOOGLE:SSL

Note how even the connection is encrypted, our nDPI-based analyzer has been able to correlate a DNS query immediately followed by a connection to the address from the DNS response at port 443 as a typical pattern used to browse web pages, and it has gone the extra mile to tell us that it belongs to a Google-operated service.

There is one more feature in snabb wall spy. Passing --stats will print a line at the end (in normal mode), or every two seconds (in live mode), with a summary of the amount of data processed:

aperez@hikari ~/snabb/src % sudo ./snabb wall spy --live --stats eth0
=== 2016-08-17T18:05:54+0300 === 71866 Bytes, 99 packets, 35933.272 B/s, 49.500 PPS
=== 2016-08-17T18:19:35+0300 === 3105473 Bytes, 2884 packets, 1.599 B/s, 0.001 PPS

One last note: even though it is not shown in the examples, scanning and identifying IPv6 traffic is already supported.

Get that IPv6 flowing!

What Next?

Now that traffic analysis and flow identification is working nicely, it has to be used for actual filtering. I am now in the process of implementing the firewall component of SnabbWall (L7fw) as a Snabb application. Also, there will be a snabb wall filter subcommand, providing an off-the-shelf L7 filtering solution. Expect a couple more of blog posts about them.

In the meanwhile, if you want to play with L7Spy, I will be updating its WIP documentation in the following days. Happy hacking!