forked from Minki/linux
794d9b2581
... cannot be used in block quote, it breaks compilation, remove it.
Fix warnings due to missing blank line such as:
net-next/Documentation/networking/nf_flowtable.rst:142: WARNING: Block quote ends without a blank line; unexpected unindent.
Fixes: 143490cde5 ("docs: nf_flowtable: update documentation with enhancements")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
236 lines
9.6 KiB
ReStructuredText
236 lines
9.6 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
====================================
|
|
Netfilter's flowtable infrastructure
|
|
====================================
|
|
|
|
This documentation describes the Netfilter flowtable infrastructure which allows
|
|
you to define a fastpath through the flowtable datapath. This infrastructure
|
|
also provides hardware offload support. The flowtable supports for the layer 3
|
|
IPv4 and IPv6 and the layer 4 TCP and UDP protocols.
|
|
|
|
Overview
|
|
--------
|
|
|
|
Once the first packet of the flow successfully goes through the IP forwarding
|
|
path, from the second packet on, you might decide to offload the flow to the
|
|
flowtable through your ruleset. The flowtable infrastructure provides a rule
|
|
action that allows you to specify when to add a flow to the flowtable.
|
|
|
|
A packet that finds a matching entry in the flowtable (ie. flowtable hit) is
|
|
transmitted to the output netdevice via neigh_xmit(), hence, packets bypass the
|
|
classic IP forwarding path (the visible effect is that you do not see these
|
|
packets from any of the Netfilter hooks coming after ingress). In case that
|
|
there is no matching entry in the flowtable (ie. flowtable miss), the packet
|
|
follows the classic IP forwarding path.
|
|
|
|
The flowtable uses a resizable hashtable. Lookups are based on the following
|
|
n-tuple selectors: layer 2 protocol encapsulation (VLAN and PPPoE), layer 3
|
|
source and destination, layer 4 source and destination ports and the input
|
|
interface (useful in case there are several conntrack zones in place).
|
|
|
|
The 'flow add' action allows you to populate the flowtable, the user selectively
|
|
specifies what flows are placed into the flowtable. Hence, packets follow the
|
|
classic IP forwarding path unless the user explicitly instruct flows to use this
|
|
new alternative forwarding path via policy.
|
|
|
|
The flowtable datapath is represented in Fig.1, which describes the classic IP
|
|
forwarding path including the Netfilter hooks and the flowtable fastpath bypass.
|
|
|
|
::
|
|
|
|
userspace process
|
|
^ |
|
|
| |
|
|
_____|____ ____\/___
|
|
/ \ / \
|
|
| input | | output |
|
|
\__________/ \_________/
|
|
^ |
|
|
| |
|
|
_________ __________ --------- _____\/_____
|
|
/ \ / \ |Routing | / \
|
|
--> ingress ---> prerouting ---> |decision| | postrouting |--> neigh_xmit
|
|
\_________/ \__________/ ---------- \____________/ ^
|
|
| ^ | ^ |
|
|
flowtable | ____\/___ | |
|
|
| | / \ | |
|
|
__\/___ | | forward |------------ |
|
|
|-----| | \_________/ |
|
|
|-----| | 'flow offload' rule |
|
|
|-----| | adds entry to |
|
|
|_____| | flowtable |
|
|
| | |
|
|
/ \ | |
|
|
/hit\_no_| |
|
|
\ ? / |
|
|
\ / |
|
|
|__yes_________________fastpath bypass ____________________________|
|
|
|
|
Fig.1 Netfilter hooks and flowtable interactions
|
|
|
|
The flowtable entry also stores the NAT configuration, so all packets are
|
|
mangled according to the NAT policy that is specified from the classic IP
|
|
forwarding path. The TTL is decremented before calling neigh_xmit(). Fragmented
|
|
traffic is passed up to follow the classic IP forwarding path given that the
|
|
transport header is missing, in this case, flowtable lookups are not possible.
|
|
TCP RST and FIN packets are also passed up to the classic IP forwarding path to
|
|
release the flow gracefully. Packets that exceed the MTU are also passed up to
|
|
the classic forwarding path to report packet-too-big ICMP errors to the sender.
|
|
|
|
Example configuration
|
|
---------------------
|
|
|
|
Enabling the flowtable bypass is relatively easy, you only need to create a
|
|
flowtable and add one rule to your forward chain::
|
|
|
|
table inet x {
|
|
flowtable f {
|
|
hook ingress priority 0; devices = { eth0, eth1 };
|
|
}
|
|
chain y {
|
|
type filter hook forward priority 0; policy accept;
|
|
ip protocol tcp flow add @f
|
|
counter packets 0 bytes 0
|
|
}
|
|
}
|
|
|
|
This example adds the flowtable 'f' to the ingress hook of the eth0 and eth1
|
|
netdevices. You can create as many flowtables as you want in case you need to
|
|
perform resource partitioning. The flowtable priority defines the order in which
|
|
hooks are run in the pipeline, this is convenient in case you already have a
|
|
nftables ingress chain (make sure the flowtable priority is smaller than the
|
|
nftables ingress chain hence the flowtable runs before in the pipeline).
|
|
|
|
The 'flow offload' action from the forward chain 'y' adds an entry to the
|
|
flowtable for the TCP syn-ack packet coming in the reply direction. Once the
|
|
flow is offloaded, you will observe that the counter rule in the example above
|
|
does not get updated for the packets that are being forwarded through the
|
|
forwarding bypass.
|
|
|
|
You can identify offloaded flows through the [OFFLOAD] tag when listing your
|
|
connection tracking table.
|
|
|
|
::
|
|
|
|
# conntrack -L
|
|
tcp 6 src=10.141.10.2 dst=192.168.10.2 sport=52728 dport=5201 src=192.168.10.2 dst=192.168.10.1 sport=5201 dport=52728 [OFFLOAD] mark=0 use=2
|
|
|
|
|
|
Layer 2 encapsulation
|
|
---------------------
|
|
|
|
Since Linux kernel 5.13, the flowtable infrastructure discovers the real
|
|
netdevice behind VLAN and PPPoE netdevices. The flowtable software datapath
|
|
parses the VLAN and PPPoE layer 2 headers to extract the ethertype and the
|
|
VLAN ID / PPPoE session ID which are used for the flowtable lookups. The
|
|
flowtable datapath also deals with layer 2 decapsulation.
|
|
|
|
You do not need to add the PPPoE and the VLAN devices to your flowtable,
|
|
instead the real device is sufficient for the flowtable to track your flows.
|
|
|
|
Bridge and IP forwarding
|
|
------------------------
|
|
|
|
Since Linux kernel 5.13, you can add bridge ports to the flowtable. The
|
|
flowtable infrastructure discovers the topology behind the bridge device. This
|
|
allows the flowtable to define a fastpath bypass between the bridge ports
|
|
(represented as eth1 and eth2 in the example figure below) and the gateway
|
|
device (represented as eth0) in your switch/router.
|
|
|
|
::
|
|
|
|
fastpath bypass
|
|
.-------------------------.
|
|
/ \
|
|
| IP forwarding |
|
|
| / \ \/
|
|
| br0 eth0 ..... eth0
|
|
. / \ *host B*
|
|
-> eth1 eth2
|
|
. *switch/router*
|
|
.
|
|
.
|
|
eth0
|
|
*host A*
|
|
|
|
The flowtable infrastructure also supports for bridge VLAN filtering actions
|
|
such as PVID and untagged. You can also stack a classic VLAN device on top of
|
|
your bridge port.
|
|
|
|
If you would like that your flowtable defines a fastpath between your bridge
|
|
ports and your IP forwarding path, you have to add your bridge ports (as
|
|
represented by the real netdevice) to your flowtable definition.
|
|
|
|
Counters
|
|
--------
|
|
|
|
The flowtable can synchronize packet and byte counters with the existing
|
|
connection tracking entry by specifying the counter statement in your flowtable
|
|
definition, e.g.
|
|
|
|
::
|
|
|
|
table inet x {
|
|
flowtable f {
|
|
hook ingress priority 0; devices = { eth0, eth1 };
|
|
counter
|
|
}
|
|
}
|
|
|
|
Counter support is available since Linux kernel 5.7.
|
|
|
|
Hardware offload
|
|
----------------
|
|
|
|
If your network device provides hardware offload support, you can turn it on by
|
|
means of the 'offload' flag in your flowtable definition, e.g.
|
|
|
|
::
|
|
|
|
table inet x {
|
|
flowtable f {
|
|
hook ingress priority 0; devices = { eth0, eth1 };
|
|
flags offload;
|
|
}
|
|
}
|
|
|
|
There is a workqueue that adds the flows to the hardware. Note that a few
|
|
packets might still run over the flowtable software path until the workqueue has
|
|
a chance to offload the flow to the network device.
|
|
|
|
You can identify hardware offloaded flows through the [HW_OFFLOAD] tag when
|
|
listing your connection tracking table. Please, note that the [OFFLOAD] tag
|
|
refers to the software offload mode, so there is a distinction between [OFFLOAD]
|
|
which refers to the software flowtable fastpath and [HW_OFFLOAD] which refers
|
|
to the hardware offload datapath being used by the flow.
|
|
|
|
The flowtable hardware offload infrastructure also supports for the DSA
|
|
(Distributed Switch Architecture).
|
|
|
|
Limitations
|
|
-----------
|
|
|
|
The flowtable behaves like a cache. The flowtable entries might get stale if
|
|
either the destination MAC address or the egress netdevice that is used for
|
|
transmission changes.
|
|
|
|
This might be a problem if:
|
|
|
|
- You run the flowtable in software mode and you combine bridge and IP
|
|
forwarding in your setup.
|
|
- Hardware offload is enabled.
|
|
|
|
More reading
|
|
------------
|
|
|
|
This documentation is based on the LWN.net articles [1]_\ [2]_. Rafal Milecki
|
|
also made a very complete and comprehensive summary called "A state of network
|
|
acceleration" that describes how things were before this infrastructure was
|
|
mainlined [3]_ and it also makes a rough summary of this work [4]_.
|
|
|
|
.. [1] https://lwn.net/Articles/738214/
|
|
.. [2] https://lwn.net/Articles/742164/
|
|
.. [3] http://lists.infradead.org/pipermail/lede-dev/2018-January/010830.html
|
|
.. [4] http://lists.infradead.org/pipermail/lede-dev/2018-January/010829.html
|