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diff --git a/Documentation/networking/nf_flowtable.txt b/Documentation/networking/nf_flowtable.txt new file mode 100644 index 000000000..b01c91893 --- /dev/null +++ b/Documentation/networking/nf_flowtable.txt @@ -0,0 +1,112 @@ +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 |