feat(vpcnatgw): send gratuitous arp for nexthops at natgw initialization

[SkalaNetworks]

Pull Request

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What type of this PR

Examples of user facing changes:

• Features

This PR adds a one-liner to the script launched at the initialization of a VPC NAT gateway. The one-liner picks up every IPv4 on the external interface (the one that connects to the physical network) and sends gratuitous ARP for each of the interface.

When the NAT gateway is in pure ARP mode (which is the default mode), it doesn't serve any specific purpose.

But when the NAT gateway is in BGP mode, the external BGP router will see a nexthop address mounted on this interface (whereas in ARP mode, the nexthop is the EIP directly, which already has a gratuitous ARP feature).

The BGP router will forward traffic to this nexthop by resolving the MAC address through ARP and will cache it. If the NAT gateway restarts and is scheduled on another node, the MAC address changes, but no announcement of that change is propagated to the BGP router.

The BGP router will keep forwarding traffic to the wrong node until its cache expire. In the meantime, all the traffic is blackholed.

This fix will ensure the cache for the nexthop is updated on any neighbour of the NAT gateway when it boots up.

Which issue(s) this PR fixes

Fixes #(issue-number)

[gemini-code-assist]

Summary of Changes

Hello @SkalaNetworks, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the robustness of the VPC NAT gateway by ensuring that its MAC address is promptly updated on connected BGP routers upon initialization or restart. By sending gratuitous ARP for all IPv4 addresses on the external network interface, it prevents traffic blackholing that could occur when a NAT gateway is rescheduled to a different node, thereby improving network stability and availability, especially in BGP mode.

Highlights

• VPC NAT Gateway Initialization: Implemented a mechanism to send gratuitous ARP for all IPv4 addresses on the external network interface (net1) during the NAT gateway's initialization process.
• BGP Mode Stability: Addresses an issue in BGP mode where a NAT gateway restarting on a new node would lead to outdated MAC address caches on BGP routers, causing traffic blackholing. This change ensures immediate cache updates.
• Network Reliability: Improves the overall reliability and availability of the NAT gateway service by preventing traffic disruptions caused by stale ARP caches on upstream BGP routers.

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[gemini-code-assist]

Code Review

This pull request adds a command to send gratuitous ARP packets at NAT gateway initialization, which is a crucial fix to prevent traffic blackholing in BGP mode. The implementation is a good start, but the shell command pipeline can fail silently, potentially reintroducing the very issue it aims to solve. I've provided a suggestion to make the command more robust by adding error handling and also simplifying the pipeline.