Undocument `spin-client-addr` header

[lann]

The spin-client-addr header gives the IP and source port of some downstream client. I'd like to remove this from the "Spin contract" (and specifically developer.fermyon.com) for two reasons:

• in many environments it won't point at the end-user client but rather some proxy; this is controlled by the host network infrastructure, not Spin
• source port number for an http client is only very rarely useful info at the application layer

We can continue populating the header on a best-effort basis for backward compat but users should look to e.g. x-forwarded-for for this info like in any other environment.

[itowlson]

The Spin CLI doesn't seem to support x-forwarded-for? (or did this get added recently and I've not rebased)

spin-client-addr = Some(HeaderValue { inner: String("127.0.0.1:42608") })
x-forwarded-for = None

[itowlson]

Looking at the X-Forwarded-For docs, the original client address is the first item in the header value, followed by the list of proxies. So I think the CLI behaviour should be:

• If the incoming request already has a X-Forwarded-For header, append the originating IP address (the current spin-client-addr) to that header value
• Otherwise, insert a X-Forwarded-For header whose value is the originating IP address.

Does that sound right?

[lann]

append the originating IP address

I'm not sure about this; you don't usually stick the last source IP in x-forwarded-for. 🤔 On the other hand, I don't think we would otherwise expose that information. 🤔 On the otherer hand, it isn't clear to me that exposing internal IP addresses without explicit configuration is a good idea... 🤔

[lann]

The Spin CLI doesn't seem to support x-forwarded-for? (or did this get added recently and I've not rebased)

Ah, I was unclear. My suggestion is that we should leave it to the operator of the Spin host to ensure standard(-ish) headers like x-forwarded-for if that is desired.

[lann]

spinframework/spin-docs#112 documents spin-client-addr is "optional" which is probably good enough.