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Tap app (apps.tap.tap)

The Tap app is used to interact with a Linux tap device. Packets transmitted on the input port will be sent over the tap device, and packets that arrive on the tap device can be received on the output port.

DIAGRAM: Tap
          +-------+
          |       |
input --->*  Tap  *----> output
          |       |
          +-------+

Configuration

This app accepts either a single string or a table as its configuration argument. A single string is equivalent to the default configuration with the name attribute set to the string.

— Key name

Required. The name of the tap device.

If the device does not exist yet, which is inferred from the absence of the directory /sys/class/net/name, it will be created by the app and removed when the process terminates. Such a device is called ephemeral and its operational state is set to up after creation.

If the device already exists, it is called persistent. The app can attach to a persistent tap device and detaches from it when it terminates. The operational state is not changed. By default, the MTU is also not changed by the app, see the mtu_set option below.

One manner in which a persistent tap device can be created is by using the ip tool

ip tuntap add Tap345 mode tap
ip link set up dev Tap345
ip link set address 02:01:02:03:04:08 dev Tap0

— Key mtu

Optional. The L2 MTU of the device. The default is 1514.

By definition, the L2 MTU includes the size of the L2 header, e.g. 14 bytes in case of Ethernet without VLANs. However, the Linux ioctl methods only expose the L3 (IP) MTU, which does not include the L2 header. The following configuration options are used to correct this discrepancy.

— Key mtu_fixup

Optional. A boolean that indicates whether the mtu option should be corrected for the difference between the L2 and L3 MTU. The default is true.

— Key mtu_offset

Optional. The value by which the mtu is reduced when mtu_fixup is set to true. The default is 14.

The resulting MTU is called the effective MTU.

— Key mtu_set

Optional. Either nil or a boolean that indicates whether the MTU of the tap device should be set or checked. If mtu_set is true, the MTU of the tap device is set to the effective MTU. If mtu_set is false, the effective MTU is compared with the current value of the MTU of the tap device and an error is raised in case of a mismatch.

If mtu_set is nil, the MTU is set or checked if the tap device is ephemeral or persistent, respectively. The rationale is that if the device is persistent, the entity that created the device is responsible for the configuration and might not expect or react well to a change of the MTU.