feat: add cns iptables reconciliation

[QxBytes]

Reason for Change:

CNS currently programs snat iptables rules only on NNC update/create. This PR makes it so that it also does so during cns bootup. Additionally, CNS will now attempt to "reconcile" the iptables on the node to what it expects. This allows us to "rollback" future updates to CNS iptables rules to match the expected state each time CNS reboots or restarts.

It is currently possible for CNI to add iptables rules to the node using chain "SWIFT". We want to jump to the cns-controlled chain "SWIFT-POSTROUTING" before we jump to the chain "SWIFT". The first piece of logic is to migrate to that:

Case 1: CNI already added a jump to SWIFT chain in nat POSTROUTING chain and it is at a higher priority than any SWIFT-POSTROUTING jump we may have added: We find the position of the SWIFT jump rule, and insert our SWIFT-POSTROUTING jump at that position, pushing the SWIFT jump rule to a lower priority

POSTROUTING
jump to KUBE POSTROUTING
...
jump to SWIFT
jump to SWIFT-POSTROUTING

becomes

POSTROUTING
jump to KUBE POSTROUTING
...
jump to SWIFT-POSTROUTING
jump to SWIFT

or

POSTROUTING
jump to KUBE POSTROUTING
...
jump to SWIFT

becomes

POSTROUTING
jump to KUBE POSTROUTING
...
jump to SWIFT-POSTROUTING
jump to SWIFT

Case 2: The SWIFT-POSTROUTING jump rule exists before the SWIFT jump rule, or exists without any SWIFT jump rule: We do not modify the nat POSTROUTING chain

POSTROUTING
jump to KUBE POSTROUTING
...
jump to SWIFT-POSTROUTING
jump to SWIFT

or

POSTROUTING
jump to KUBE POSTROUTING
...
jump to SWIFT-POSTROUTING

stays the same

Case 3: The nat POSTROUTING chain does not contain a SWIFT jump rule nor a SWIFT-POSTROUTING rule: We append the SWIFT-POSTROUTING jump rule to the nat POSTROUTING chain

POSTROUTING
jump to KUBE POSTROUTING
...

becomes

POSTROUTING
jump to KUBE POSTROUTING
...
jump to SWIFT-POSTROUTING

Now, packets will go to the nat SWIFT-POSTROUTING chain first. We then take a look at the SWIFT-POSTROUTING chain.
Case 1: The nat SWIFT-POSTROUTING chain does not contain every single rule we expect exactly, or the # of rules in the SWIFT-POSTROUTING chain does not match how many rules we expect: Flush the chain and reinstall the iptables rules we expect.

Case 2: The nat SWIFT-POSTROUTING chain contains every single rule we expect exactly, and the # of rules matches how many rules we expect: We do not modify or flush the SWIFT-POSTROUTING chain

Issue Fixed:

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• relevant PR labels added

Notes:

• Check iptables insert of bounds just in case
• When we list iptables rules, the first rule is always -N ... for custom chains or -P ... for built in chains (not shown in iptables -nvL -t nat). Feel free to check-- the library calls iptables -t <table> -S <chain> under the hood.
• When we list iptables rules, the rules use -A syntax when listed no matter how they were added (via insert, append, etc.)
• IPTables is 1-indexed, so insert at position 1 will be the first rule in the chain-- so when using iptables -nvL -t nat the first rule that shows up in a chain is at index 1.

[Copilot]

Pull Request Overview

This PR adds iptables reconciliation functionality to CNS, enabling it to restore iptables rules to the expected state during startup and handle migration from legacy SWIFT chains to SWIFT-POSTROUTING chains. The changes ensure CNS can recover from external iptables modifications and properly sequence rule execution.

• Adds iptables reconciliation during CNS bootup to restore expected SNAT rules
• Implements migration logic to transition from legacy SWIFT chain to SWIFT-POSTROUTING chain with proper priority ordering
• Enhances the iptables mock implementation to support new operations needed for testing

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
cns/restserver/util.go	Adds reconciliation logic during CNS startup to reprogram SNAT rules for all containers
cns/restserver/restserver.go	Extends iptables interface with List, ClearChain, and Delete methods
cns/restserver/internalapi_linux.go	Implements comprehensive iptables reconciliation and migration logic from SWIFT to SWIFT-POSTROUTING
cns/restserver/internalapi_linux_test.go	Adds extensive tests covering migration scenarios and rule reconciliation
cns/fakes/iptablesfake.go	Enhances mock implementation with proper iptables simulation and tracking

Comments suppressed due to low confidence (1)

cns/fakes/iptablesfake.go:192

• [nitpick] The method name 'ClearChainCallCount' is ambiguous - it could mean either getting the count or clearing the count. Consider renaming to 'GetClearChainCallCount' to clearly indicate it's a getter method.

func (c *IPTablesMock) ClearChainCallCount() int {

[santhoshmprabhu]

I think we could consider eliminating that blip from clearing the chain, but not a big deal.

[rbtr]

/azp run Azure Container Networking PR

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