kubernetes/pkg/proxy/nftables

NFTables kube-proxy

This is an implementation of service proxying via the nftables API of the kernel netfilter subsystem.

General theory of netfilter

Packet flow through netfilter looks something like:

             +================+      +=====================+
             | hostNetwork IP |      | hostNetwork process |
             +================+      +=====================+
                         ^                |
  -  -  -  -  -  -  -  - | -  -  -  -  - [*] -  -  -  -  -  -  -  -  -
                         |                v
                     +-------+        +--------+
                     | input |        | output |
                     +-------+        +--------+
                         ^                |
      +------------+     |   +---------+  v      +-------------+
      | prerouting |-[*]-+-->| forward |--+-[*]->| postrouting |
      +------------+         +---------+         +-------------+
            ^                                           |
 -  -  -  - | -  -  -  -  -  -  -  -  -  -  -  -  -  -  |  -  -  -  -
            |                                           v
       +---------+                                  +--------+
   --->| ingress |                                  | egress |--->
       +---------+                                  +--------+

where the [*] represents a routing decision, and all of the boxes except in the top row represent netfilter hooks. More detailed versions of this diagram can be seen at https://en.wikipedia.org/wiki/Netfilter#/media/File:Netfilter-packet-flow.svg and https://wiki.nftables.org/wiki-nftables/index.php/Netfilter_hooks but note that in the the standard version of this diagram, the top two boxes are squished together into "local process" which (a) fails to make a few important distinctions, and (b) makes it look like a single packet can go input -> "local process" -> output, which it cannot. Note also that the ingress and egress hooks are special and mostly not available to us; kube-proxy lives in the middle section of diagram, with the five main netfilter hooks.

There are three paths through the diagram, called the "input", "forward", and "output" paths, depending on which of those hooks it passes through. Packets coming from host network namespace processes always take the output path, while packets coming in from outside the host network namespace (whether that's from an external host or from a pod network namespace) arrive via ingress and take the input or forward path, depending on the routing decision made after prerouting; packets destined for an IP which is assigned to a network interface in the host network namespace get routed along the input path; anything else (including, in particular, packets destined for a pod IP) gets routed along the forward path.

kube-proxy's use of nftables hooks

Kube-proxy uses nftables for four things:

• Using DNAT to rewrite traffic from service IPs (cluster IPs, external IPs, load balancer IP, and NodePorts on node IPs) to the corresponding endpoint IPs.

• Using SNAT to masquerade traffic as needed to ensure that replies to it will come back to this node/namespace (so that they can be un-DNAT-ed).

• Dropping packets that are filtered out by the LoadBalancerSourceRanges feature.

• Dropping packets for services with Local traffic policy but no local endpoints.

• Rejecting packets for services with no local or remote endpoints.

This is implemented as follows:

• We do the DNAT for inbound traffic in prerouting: this covers traffic coming from off-node to all types of service IPs, and traffic coming from pods to all types of service IPs. (We must do this in prerouting, because the choice of endpoint IP may affect whether the packet then gets routed along the input path or the forward path.)

• We do the DNAT for outbound traffic in output: this covers traffic coming from host-network processes to all types of service IPs. Regardless of the final destination, the traffic will take the "output path". (In the case where a host-network process connects to a service IP that DNATs it to a host-network endpoint IP, the traffic will still initially take the "output path", but then reappear on the "input path".)

• LoadBalancerSourceRanges firewalling has to happen before service DNAT, so we do that on prerouting and output as well, with a lower (i.e. more urgent) priority than the DNAT chains.

• The drop and reject rules for services with no endpoints don't need to happen explicitly before or after any other rules (since they match packets that wouldn't be matched by any other rules). But with kernels before 5.9, reject is not allowed in prerouting, so we can't just do them in the same place as the source ranges firewall. So we do these checks from input, forward, and output, to cover all three paths. (In fact, we only need to check @no-endpoint-nodeports on the input hook, but it's easier to just check them both in one place, and this code is likely to be rewritten later anyway. Note that the converse statement "we only need to check @no-endpoint-services on the forward and output hooks" is not true, because @no-endpoint-services may include externalIPs/LB IPs that are assigned to local interfaces.)

• Masquerading has to happen in the postrouting hook, because "masquerade" means "SNAT to the IP of the interface the packet is going out on", so it has to happen after the final routing decision. (We don't need to masquerade packets that are going to a host network IP, because masquerading is about ensuring that the packet eventually gets routed back to the host network namespace on this node, so if it's never getting routed away from there, there's nothing to do.)