linux-imx/tools/testing/selftests/net/forwarding

Motivation

One of the nice things about network namespaces is that they allow one to easily create and test complex environments.

Unfortunately, these namespaces can not be used with actual switching ASICs, as their ports can not be migrated to other network namespaces (NETIF_F_NETNS_LOCAL) and most of them probably do not support the L1-separation provided by namespaces.

However, a similar kind of flexibility can be achieved by using VRFs and by looping the switch ports together. For example:

                         br0
                          +
           vrf-h1         |           vrf-h2
             +        +---+----+        +
             |        |        |        |
192.0.2.1/24 +        +        +        + 192.0.2.2/24
           swp1     swp2     swp3     swp4
             +        +        +        +
             |        |        |        |
             +--------+        +--------+

The VRFs act as lightweight namespaces representing hosts connected to the switch.

This approach for testing switch ASICs has several advantages over the traditional method that requires multiple physical machines, to name a few:

1. Only the device under test (DUT) is being tested without noise from other system.

2. Ability to easily provision complex topologies. Testing bridging between 4-ports LAGs or 8-way ECMP requires many physical links that are not always available. With the VRF-based approach one merely needs to loopback more ports.

These tests are written with switch ASICs in mind, but they can be run on any Linux box using veth pairs to emulate physical loopbacks.

Guidelines for Writing Tests

o Where possible, reuse an existing topology for different tests instead of recreating the same topology. o Tests that use anything but the most trivial topologies should include an ASCII art showing the topology. o Where possible, IPv6 and IPv4 addresses shall conform to RFC 3849 and RFC 5737, respectively. o Where possible, tests shall be written so that they can be reused by multiple topologies and added to lib.sh. o Checks shall be added to lib.sh for any external dependencies. o Code shall be checked using ShellCheck [1] prior to submission.

1. https://www.shellcheck.net/

Customization

The forwarding selftests framework uses a number of variables that influence its behavior and tools it invokes, and how it invokes them, in various ways. A number of these variables can be overridden. The way these overridable variables are specified is typically one of the following two syntaxes:

: "${VARIABLE:=default_value}"
VARIABLE=${VARIABLE:=default_value}

Any of these variables can be overridden. Notably net/forwarding/lib.sh and net/lib.sh contain a number of overridable variables.

One way of overriding these variables is through the environment:

PAUSE_ON_FAIL=yes ./some_test.sh

The variable NETIFS is special. Since it is an array variable, there is no way to pass it through the environment. Its value can instead be given as consecutive arguments to the selftest:

./some_test.sh swp{1..8}

A way to customize variables in a persistent fashion is to create a file named forwarding.config in this directory. lib.sh sources the file if present, so it can contain any shell code. Typically it will contain assignments of variables whose value should be overridden.

forwarding.config.sample is available in the directory as an example of how forwarding.config might look.