CODEWITHSUNDEEP
network-programmingkubernetesiptables
LEQADA
LEQADA

Reputation: 1982

What iptables knows about pods?

Let's say we have 2 Nodes in a cluster.

Node A has 1 replica of a pod, Node B has 2 replicas. According to this talk (YouTube video with a time tag) from Google Cloud engineers, a request which was routed to Node A might be rerouted to the Node B by iptables which is inside the Node A. I have several questions regarding this behavior:

Upvotes: 0

Views: 187

Answers (2)

Vikram Hosakote
Vikram Hosakote

Reputation: 3684

Packets can move between nodes, services and pods before reaching the final destination.

All the intra-cluster routing (node-to-node, pod-to-pod, service-to-service, pod-to-service, service-to-pod, pod-to-node, node-to-pod, etc) in kubernetes is done by:

  • CNI
  • load-balancing algorithm
  • kube-proxy
  • iptables.

Packet route in k8s also depends on many things like load in the cluster, per-node load, affinity/anti-affinity rules, nodeSelectors, taints/tolerations, autoscaling, number of pod replicas, etc.

Intra-cluster routing is transparent to the router and ideally the user need not know about it unless there are networking issues to debug.

Doing sudo iptables -L -n -v on any k8s node shows the low-level iptables rules and chains used for packet-forwarding.

Upvotes: 0

coderanger
coderanger

Reputation: 54249

I think you might be mixing up two subsystems, service proxies and CNI. CNI is first, it’s a plug-in based system that sets up the routing rules across all your nodes so that the network appears flat. A pod IP will work like normal from any node. Exactly how that happens varies by plugin, Calico uses BGP between the nodes. Then there’s the service proxies, usually implemented using iptables though also somewhat pluggable. Those define the service IP -> endpoint IP (read: pod IP) load balancing. But the actual routing is handled by whatever your CNI plugin set up. There’s a lot of special modes and cases but that’s the basic overview.

Upvotes: 2

Related Questions