mirror of
https://mirrors.bfsu.edu.cn/git/linux.git
synced 2024-11-25 13:14:07 +08:00
7c9abe12b3
No direct transition from prerouting to forward hook, routing lookup
needs to happen first.
Fixes: 19b351f16f ("netfilter: add flowtable documentation")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
113 lines
5.7 KiB
Plaintext
113 lines
5.7 KiB
Plaintext
Netfilter's flowtable infrastructure
|
|
====================================
|
|
|
|
This documentation describes the software flowtable infrastructure available in
|
|
Netfilter since Linux kernel 4.16.
|
|
|
|
Overview
|
|
--------
|
|
|
|
Initial packets follow the classic forwarding path, once the flow enters the
|
|
established state according to the conntrack semantics (ie. we have seen traffic
|
|
in both directions), then you can decide to offload the flow to the flowtable
|
|
from the forward chain via the 'flow offload' action available in nftables.
|
|
|
|
Packets that find an entry in the flowtable (ie. flowtable hit) are sent to the
|
|
output netdevice via neigh_xmit(), hence, they bypass the classic forwarding
|
|
path (the visible effect is that you do not see these packets from any of the
|
|
netfilter hooks coming after the ingress). In case of flowtable miss, the packet
|
|
follows the classic forward path.
|
|
|
|
The flowtable uses a resizable hashtable, lookups are based on the following
|
|
7-tuple selectors: source, destination, layer 3 and layer 4 protocols, source
|
|
and destination ports and the input interface (useful in case there are several
|
|
conntrack zones in place).
|
|
|
|
Flowtables are populated via the 'flow offload' nftables action, so the user can
|
|
selectively specify what flows are placed into the flow table. Hence, packets
|
|
follow the classic forwarding path unless the user explicitly instruct packets
|
|
to use this new alternative forwarding path via nftables policy.
|
|
|
|
This is represented in Fig.1, which describes the classic 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 matches the initial packets that went
|
|
through the classic forwarding path. The TTL is decremented before calling
|
|
neigh_xmit(). Fragmented traffic is passed up to follow the classic forwarding
|
|
path given that the transport selectors are missing, therefore flowtable lookup
|
|
is not possible.
|
|
|
|
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 offload @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.
|
|
|
|
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
|
|
mailined [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
|