28. Tun/Tap Poll Mode Driver

The rte_eth_tap.c PMD creates a device using TUN/TAP interfaces on the local host. The PMD allows for DPDK and the host to communicate using a raw device interface on the host and in the DPDK application.

The device created is a TAP device, which sends/receives packet in a raw format with a L2 header. The usage for a TAP PMD is for connectivity to the local host using a TAP interface. When the TAP PMD is initialized it will create a number of tap devices in the host accessed via ifconfig -a or ip command. The commands can be used to assign and query the virtual like device.

These TAP interfaces can be used with Wireshark or tcpdump or Pktgen-DPDK along with being able to be used as a network connection to the DPDK application. The method enable one or more interfaces is to use the --vdev=net_tap0 option on the DPDK application command line. Each --vdev=net_tap1 option given will create an interface named dtap0, dtap1, and so on.

The interface name can be changed by adding the iface=foo0, for example:

--vdev=net_tap0,iface=foo0 --vdev=net_tap1,iface=foo1, ...

Also the speed of the interface can be changed from 10G to whatever number needed, but the interface does not enforce that speed, for example:

--vdev=net_tap0,iface=foo0,speed=25000

Normally the PMD will generate a random MAC address, but when testing or with a static configuration the developer may need a fixed MAC address style. Using the option mac=fixed you can create a fixed known MAC address:

--vdev=net_tap0,mac=fixed

The MAC address will have a fixed value with the last octet incrementing by one for each interface string containing mac=fixed. The MAC address is formatted as 00:’d’:’t’:’a’:’p’:[00-FF]. Convert the characters to hex and you get the actual MAC address: 00:64:74:61:70:[00-FF].

It is possible to specify a remote netdevice to capture packets from by adding remote=foo1, for example:

--vdev=net_tap,iface=tap0,remote=foo1

If a remote is set, the tap MAC address will be set to match the remote one just after netdevice creation. Using TC rules, traffic from the remote netdevice will be redirected to the tap. If the tap is in promiscuous mode, then all packets will be redirected. In allmulti mode, all multicast packets will be redirected.

Using the remote feature is especially useful for capturing traffic from a netdevice that has no support in the DPDK. It is possible to add explicit rte_flow rules on the tap PMD to capture specific traffic (see next section for examples).

After the DPDK application is started you can send and receive packets on the interface using the standard rx_burst/tx_burst APIs in DPDK. From the host point of view you can use any host tool like tcpdump, Wireshark, ping, Pktgen and others to communicate with the DPDK application. The DPDK application may not understand network protocols like IPv4/6, UDP or TCP unless the application has been written to understand these protocols.

If you need the interface as a real network interface meaning running and has a valid IP address then you can do this with the following commands:

sudo ip link set dtap0 up; sudo ip addr add 192.168.0.250/24 dev dtap0
sudo ip link set dtap1 up; sudo ip addr add 192.168.1.250/24 dev dtap1

Please change the IP addresses as you see fit.

If routing is enabled on the host you can also communicate with the DPDK App over the internet via a standard socket layer application as long as you account for the protocol handling in the application.

If you have a Network Stack in your DPDK application or something like it you can utilize that stack to handle the network protocols. Plus you would be able to address the interface using an IP address assigned to the internal interface.

28.1. Flow API support

The tap PMD supports major flow API pattern items and actions, when running on linux kernels above 4.2 (“Flower” classifier required). The kernel support can be checked with this command:

zcat /proc/config.gz | ( grep 'CLS_FLOWER=' || echo 'not supported' ) |
tee -a /dev/stderr | grep -q '=m' &&
lsmod | ( grep cls_flower || echo 'try modprobe cls_flower' )

Supported items:

eth: src and dst (with variable masks), and eth_type (0xffff mask).
vlan: vid, pcp, tpid, but not eid. (requires kernel 4.9)
ipv4/6: src and dst (with variable masks), and ip_proto (0xffff mask).
udp/tcp: src and dst port (0xffff) mask.

Supported actions:

DROP
QUEUE
PASSTHRU

It is generally not possible to provide a “last” item. However, if the “last” item, once masked, is identical to the masked spec, then it is supported.

Only IPv4/6 and MAC addresses can use a variable mask. All other items need a full mask (exact match).

As rules are translated to TC, it is possible to show them with something like:

tc -s filter show dev tap1 parent 1:

28.1.1. Examples of testpmd flow rules

Drop packets for destination IP 192.0.2.1:

testpmd> flow create 0 priority 1 ingress pattern eth / ipv4 dst is 192.0.2.1 \
         / end actions drop / end

Ensure packets from a given MAC address are received on a queue 2:

testpmd> flow create 0 priority 2 ingress pattern eth src is 06:05:04:03:02:01 \
         / end actions queue index 2 / end

Drop UDP packets in vlan 3:

testpmd> flow create 0 priority 3 ingress pattern eth / vlan vid is 3 / \
         ipv4 proto is 17 / end actions drop / end

28.2. Example

The following is a simple example of using the TUN/TAP PMD with the Pktgen packet generator. It requires that the socat utility is installed on the test system.

Build DPDK, then pull down Pktgen and build pktgen using the DPDK SDK/Target used to build the dpdk you pulled down.

Run pktgen from the pktgen directory in a terminal with a commandline like the following:

sudo ./app/app/x86_64-native-linuxapp-gcc/app/pktgen -l 1-5 -n 4        \
 --proc-type auto --log-level 8 --socket-mem 512,512 --file-prefix pg   \
 --vdev=net_tap0 --vdev=net_tap1 -b 05:00.0 -b 05:00.1                  \
 -b 04:00.0 -b 04:00.1 -b 04:00.2 -b 04:00.3                            \
 -b 81:00.0 -b 81:00.1 -b 81:00.2 -b 81:00.3                            \
 -b 82:00.0 -b 83:00.0 -- -T -P -m [2:3].0 -m [4:5].1                   \
 -f themes/black-yellow.theme

Verify with ifconfig -a command in a different xterm window, should have a dtap0 and dtap1 interfaces created.

Next set the links for the two interfaces to up via the commands below:

sudo ip link set dtap0 up; sudo ip addr add 192.168.0.250/24 dev dtap0
sudo ip link set dtap1 up; sudo ip addr add 192.168.1.250/24 dev dtap1

Then use socat to create a loopback for the two interfaces:

sudo socat interface:dtap0 interface:dtap1

Then on the Pktgen command line interface you can start sending packets using the commands start 0 and start 1 or you can start both at the same time with start all. The command str is an alias for start all and stp is an alias for stop all.

While running you should see the 64 byte counters increasing to verify the traffic is being looped back. You can use set all size XXX to change the size of the packets after you stop the traffic. Use pktgen help command to see a list of all commands. You can also use the -f option to load commands at startup in command line or Lua script in pktgen.