Wow, what a coincidence, someone named OVS using OVS. Who would have
thought!!1!
On Mon, Jun 01, 2015 at 09:54:55AM +0530, OVS wrote:
> I am new to OVS and was going through code to understand the flow. I
> was looking at userspace ovs handling where no kernel module specific
> to OVS is needed. I understand that Bridge functionality is handled
> using TAP interface in this scenario.
TAP interfaces are used to implement "internal" devices.
> 1. First question, will other interfaces like eth0, eth1 etc will have
> corresponding link through TAP devices?
No.
> Ideally i think it should be TAP as we want to process the packet in
> userspace.
I don't follow.
> 2. Second question, if everything is handled in user space, what would
> netlink used for?
It's only used internally within the OVS process. Also a lot of Linux
networking (not just OVS) works through Netlink in any case, so Netlink
is used to talk to the network stack in those cases.
You might find some enlightenment in this FAQ entry, especially in the
last few paragraphs.
### Q: I created a tap device tap0, configured an IP address on it, and
added it to a bridge, like this:
tunctl -t tap0
ifconfig tap0 192.168.0.123
ovs-vsctl add-br br0
ovs-vsctl add-port br0 tap0
I expected that I could then use this IP address to contact other
hosts on the network, but it doesn't work. Why not?
A: The short answer is that this is a misuse of a "tap" device. Use
an "internal" device implemented by Open vSwitch, which works
differently and is designed for this use. To solve this problem
with an internal device, instead run:
ovs-vsctl add-br br0
ovs-vsctl add-port br0 int0 -- set Interface int0 type=internal
ifconfig int0 192.168.0.123
Even more simply, you can take advantage of the internal port that
every bridge has under the name of the bridge:
ovs-vsctl add-br br0
ifconfig br0 192.168.0.123
In more detail, a "tap" device is an interface between the Linux
(or *BSD) network stack and a user program that opens it as a
socket. When the "tap" device transmits a packet, it appears in
the socket opened by the userspace program. Conversely, when the
userspace program writes to the "tap" socket, the kernel TCP/IP
stack processes the packet as if it had been received by the "tap"
device.
Consider the configuration above. Given this configuration, if you
"ping" an IP address in the 192.168.0.x subnet, the Linux kernel
routing stack will transmit an ARP on the tap0 device. Open
vSwitch userspace treats "tap" devices just like any other network
device; that is, it doesn't open them as "tap" sockets. That means
that the ARP packet will simply get dropped.
You might wonder why the Open vSwitch kernel module doesn't
intercept the ARP packet and bridge it. After all, Open vSwitch
intercepts packets on other devices. The answer is that Open
vSwitch only intercepts *received* packets, but this is a packet
being transmitted. The same thing happens for all other types of
network devices, except for Open vSwitch "internal" ports. If you,
for example, add a physical Ethernet port to an OVS bridge,
configure an IP address on a physical Ethernet port, and then issue
a "ping" to an address in that subnet, the same thing happens: an
ARP gets transmitted on the physical Ethernet port and Open vSwitch
never sees it. (You should not do that, as documented at the
beginning of this section.)
It can make sense to add a "tap" device to an Open vSwitch bridge,
if some userspace program (other than Open vSwitch) has opened the
tap socket. This is the case, for example, if the "tap" device was
created by KVM (or QEMU) to simulate a virtual NIC. In such a
case, when OVS bridges a packet to the "tap" device, the kernel
forwards that packet to KVM in userspace, which passes it along to
the VM, and in the other direction, when the VM sends a packet, KVM
writes it to the "tap" socket, which causes OVS to receive it and
bridge it to the other OVS ports. Please note that in such a case
no IP address is configured on the "tap" device (there is normally
an IP address configured in the virtual NIC inside the VM, but this
is not visible to the host Linux kernel or to Open vSwitch).
There is one special case in which Open vSwitch does directly read
and write "tap" sockets. This is an implementation detail of the
Open vSwitch userspace switch, which implements its "internal"
ports as Linux (or *BSD) "tap" sockets. In such a userspace
switch, OVS receives packets sent on the "tap" device used to
implement an "internal" port by reading the associated "tap"
socket, and bridges them to the rest of the switch. In the other
direction, OVS transmits packets bridged to the "internal" port by
writing them to the "tap" socket, causing them to be processed by
the kernel TCP/IP stack as if they had been received on the "tap"
device. Users should not need to be concerned with this
implementation detail.
Open vSwitch has a network device type called "tap". This is
intended only for implementing "internal" ports in the OVS
userspace switch and should not be used otherwise. In particular,
users should not configure KVM "tap" devices as type "tap" (use
type "system", the default, instead).
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