sFlow for tunnels: Tunnel extension & IPv6 encapsulation support

[simonschdev]

Hi Peter, we have another suggestion for tunneling support in sFlow, building on our previous pull request.

Let us know what you think! :)

Background

Motivated by pull request #559, sfcapd was recently extended with the capability to parse the nested headers used in GRE tunnels. Given the new command-line option -o gre, sfcapd will consider the innermost header stack (i.e., the tunneled flow) to be relevant for the stored flow information. In absence of that command-line option, the outermost header stack (i.e., the tunneling flow) is considered critical.

Problem

While being a valuable first step, previous sfcapd tunnel functionality would benefit from the following two extensions:

• Complete tunnel information: The current command-line option forces the user to choose between analyzing the tunneled flow and analyzing the tunneling flow. However, it is often interesting to know which tunnels encapsulated which flows, and thus to store the combination of the tunneling flow and tunneled flow.
• IPv6 encapsulation support: To tunnel IPv6 traffic in IPv4 traffic, using GRE for tunneling is actually unnecessary, as IPv6 encapsulation (IP protocol number 41, e.g. 6to4 and 6in4) allows to simply concatenate an IPv4 header and an IPv6 header (without intermediate GRE header). Since such encapsulation tunnels might thus appear frequently in practice, sfcapd support for such IPv6 encapsulation would be desirable.

Solution approach

We implement the extensions mentioned above as follows:

• Complete tunnel information: The information from the outermost IP header is stored in the already existing tunnel extension, which was already suggested in the discussion of pull request GRE tunnel introspection for sFlow #559.
• IPv6 encapsulation support: Upon discovery of an IP header with protocol number 41, the header parsing is restarted recursively after that header - analogously to the parsing behavior upon discovery of a GRE header. The user can now use the option 6in4 (i.e., with -o 6in4) to convey that the parsing should advance beyond encapsulation headers.

Result & Tests

For demonstations, we provide a PCAP file (extended with a 6in4 flow compared to pull request #559), containing sFlow records for 6 different flows that are encapsulated in an IP flow from 1.1.1.1 to 2.2.2.2. When recording these packets with sfcapd using -o gre,6in4 , nfdump now shows the following records:

Date first seen         Duration         Proto      Src IP Addr:Port          Dst IP Addr:Port   Packets    Bytes Flows
2024-11-01 16:24:19.768     00:00:00.000 GRE            1.1.1.1:53    ->          2.2.2.2:53           1       84     1
2024-11-01 16:24:19.768     00:00:00.000 UDP            3.3.3.3:53    ->          4.4.4.4:53           1       84     1
2024-11-01 16:24:19.768     00:00:00.000 GRE            1.1.1.1:53    ->          2.2.2.2:53           1       70     1
2024-11-01 16:24:19.768     00:00:00.000 UDP            5.5.5.5:53    ->          6.6.6.6:53           1       70     1
2024-11-01 16:24:19.768     00:00:00.000 GRE            1.1.1.1:53    ->          2.2.2.2:53           1       90     1
2024-11-01 16:24:19.768     00:00:00.000 UDP    7:7:7:7:7:7:7:7.53    ->  8:8:8:8:8:8:8:8.53           1       90     1
2024-11-01 16:24:19.768     00:00:00.000 GRE            1.1.1.1:53    ->          2.2.2.2:53           1      106     1
2024-11-01 16:24:19.768     00:00:00.000 UDP        11.11.11.11:53    ->      12.12.12.12:53           1      106     1
2024-11-01 16:24:19.768     00:00:00.000 GRE            1.1.1.1:0     ->          2.2.2.2:0            1       42     1
2024-11-01 16:24:19.768     00:00:00.000 IPv6           1.1.1.1:53    ->          2.2.2.2:53           1       86     1
2024-11-01 16:24:19.768     00:00:00.000 UDP   13:13:1..3:13:13.53    -> 14:14:1..4:14:14.53           1       86     1

(The third-to-last flow record comes from a pathological packet with an empty GRE payload).

Moreover, all tests run with make check still pass.

Contributors

This pull request is proposed by Simon Scherrer from NetFabric.ai.