Add TCP Health Check using SYN, SYN-ACK, and RST packets

[eos175]

Add TCP Health Check using SYN, SYN-ACK, and RST packets

[nmengin]

Hey @eos175.

Thanks for your suggestion.

We are interested in this issue, but we’re unsure about the use case and the traction it will receive. We are going to leave the status as kind/proposal to give the community time to check your implementation and let us know if they would like this idea.

We will reevaluate as people respond.

Conversation is time-boxed to 6 months.

[eos175]

Some related issues are #5598 and #1657.

To use it, the procedure is as follows:

services:
  example_tcp_service:
    loadBalancer:
      servers:
        - address: 10.42.64.4:7445
        - address: 10.42.0.1:7445
        - address: 127.0.0.1:8448
      healthCheck:
        path: /health
        interval: "10s"
        timeout: "3s"
        mode: tcp

Note that the path is ignored due to the nature of the TCP protocol.

Additionally, it would be interesting to consider a mode: passive, which enables passive checks based on observing communication with back-end nodes. However, this is not as straightforward.