TCP-packet processing in outbound proxy in linkerd-proxy

[NikitaSkrynnik]

Hello. I'm trying to understand how linkerd-proxy works internally. I'm not very familiar with Rust. I wonder where in code TCP-packet processing in outbound proxy begins. Can somebody help me, please?

[kleimkuhler]

In order to start understanding the proxy, understanding tower, one of the core libraries it uses, will be very helpful. If you read through those docs, you'll start to get a sense about how different layers and stacks compose to make any of the various "services" that you'll see in the proxy code.

As for the outbound TCP-related code in the proxy, you'll want to start in linkerd/app/outbound/src/lib.rs, specifically in fn mk_proxy. You can see how we start with creating the logical and forward stacks, and then use those to create a single stack that handles things like discovery and instrumentation.

If you're not familiar with Rust, I'd recommend working in an environment with the Rust language server so that you can click through to function definitions and get an idea about what types are being worked with.

[NikitaSkrynnik]

Thank you for the answer. Could you, please, also tell me for what types of requests this branch in switch_logical happens? I can't understand how we can provide logical_addr in TCP packets coming to outbound proxy.

logical_addr consists of dns name and port, doesn't it?

[kleimkuhler]

logical_addr is part of the destination profile response from the proxy API. If we have a logical_addr, that means we're dealing with a destination that is named (i.e. a Service). This means that there are multiple endpoints that we can perform load balancing, traffic splitting, etc. on.

Above that switch you can see we do the same check on if it's a single endpoint (if let Some((addr, metadata)) = rx.endpoint()). In that case, we can't perform those things I mentioned above which is why we create a different "stack".