For a long time, I have been confused about the iptables mode and IPVS mode of kube-proxy. I don’t understand the relationship and difference between IPVS and iptables. I spent a lot of time trying to figure it out. However, if someone asks me to explain it, I can’t still answer it clearly with confidence. Today, I decided to ask ChatGPT for help. Let’s see how ChatGPT helps me out.

I asked ChatGPT the below question:

Is IPVS built on iptables? If yes, how does it under the hood?

Here’s ChatGPT’s answer:

IPVS (IP Virtual Server) is not strictly "built on" iptables, but it integrates with it in specific scenarios for packet filtering and forwarding setup. Here's a detailed explanation of how this relationship works under the hood:

## How IPVS and iptables Work Together

### Initial Packet Filtering:

IPVS relies on iptables to perform the initial filtering of packets.
Specifically, the PREROUTING chain of the nat table in iptables is often used to intercept incoming packets and decide whether to pass them to IPVS for load balancing.

### Packet Forwarding to IPVS:

  When kube-proxy is in IPVS mode, it creates iptables rules to redirect service-related traffic to the IPVS virtual servers.
  This redirection is achieved using the -j IPVS target in iptables, which instructs iptables to hand off the packet to the IPVS subsystem for load balancing.

### IPVS's Role:
Once the packet reaches IPVS, the kernel-level IPVS module performs load balancing based on the rules it has set up (e.g., round-robin, least-connections, etc.).
IPVS maintains an efficient hash table for service and backend mappings, ensuring fast lookups and minimal overhead.

### Complementary Use:
IPVS does not completely replace iptables; instead, it complements it. For example:
IPVS handles the efficient distribution of packets to backend pods.
iptables manages generic packet filtering and other non-load-balancing rules (e.g., security).

## Why IPVS Uses iptables

### Simplicity of Integration:
Rather than re-implementing features like packet interception and pre-routing, IPVS leverages iptables for these functions.
### Modularity:
By relying on iptables for basic filtering, IPVS focuses solely on high-performance load balancing.
### Compatibility:
Using iptables ensures that IPVS can coexist with other iptables-based configurations, avoiding conflicts.

## Under the Hood Example: Kubernetes

In a Kubernetes cluster:

### kube-proxy in IPVS Mode:
kube-proxy sets up iptables rules in the nat table to intercept service traffic and forward it to IPVS.
IPVS manages the actual routing to backend pods based on its own rules.
    
### Example iptables Rule:

`iptables -t nat -A PREROUTING -p tcp --dport 80 -j IPVS`

This rule tells iptables to forward incoming TCP traffic on port 80 to IPVS.

### Example IPVS Configuration:

IPVS creates a virtual service (e.g., 192.168.1.100:80) and maps it to backend servers (e.g., 192.168.1.101:80, 192.168.1.102:80) with a load-balancing algorithm.

## Summary
While IPVS operates as a standalone kernel module for load balancing, it uses iptables for packet interception and pre-routing. This integration allows IPVS to focus on its strength (efficient load balancing) while reusing iptables for routing traffic into the IPVS subsystem. This synergy provides flexibility and performance, especially in Kubernetes environments where IPVS mode is used for scalable service management.

What do you think about this answer? I think it helps me to comprehend a lot. - If it’s correct:)