REQUESTED_SERVER_NAME fallback to REQ(:AUTHORITY)

[zirain]

Currently, there're two substitution formatters in Envoy:

• REQUESTED_SERVER_NAME return sni for HTTP/TCP/THRIFT
• REQ(:AUTHORITY) for HTTP/HTTPS

it would be better if envoy can fallback to REQ(:AUTHORITY) when using REQUESTED_SERVER_NAME for HTTP, this would make #41697 #42303 more useful.

cc @wbpcode

[wbpcode]

SGTM.

[agrawroh]

Just a small clarification. Are we saying that we should fallback in case the SNI is null? Because there could be scenarios where the incoming SNI is different than the Authority. We have many such cases.

[zirain]

Are we saying that we should fallback in case the SNI is null?

that's my idea.

[agrawroh]

Two things we should think of:

1. Would this be a backward incompatible change that a lot of people are relying on? At least for us, we have use-cases where we use this to log requests that came with empty/null SNI.
2. If we do this then should we think of creating a dedicated formatter to extract just the SNI?

[zirain]

@agrawroh which is your prefer? a runtime flag or a new formatter?

[zirain]

@wbpcode @agrawroh
my proposal for this is that make %REQUESTED_SERVER_NAME(X:Y)%
The X parameter can be:

• FK FA(fallback): The output will retrive from REQ(:AUTHORITY)
• NF NFA(not fallback): The output will not retrive from REQ(:AUTHORITY)

The Y parameter can be:

• ORIG: Get the request host from the x-envoy-original-host header.

• HOST: Get the request host from the :authority header.

• ORIG_OR_HOST: Get the request host from the x-envoy-original-host header if it is present, otherwise get it from the :authority header. If the Y is not present, ORIG_OR_HOST will be used.

WDYT?

[wbpcode]

SGTM

[agrawroh]

It's looks a bit complicated but with documentation and example we can make it work. Thanks for the suggestion!

[zirain]

I feel like we need to change X to following options:

• SNI_ONLY: equals to current, return null if SNI is not set.
• SNI_FIRST: get from x-envoy-original-host/:authority when SNI is not set.
• HOST_FIRST: get from x-envoy-original-host/:authority first. this's useful when envoy terminate TLS and rewrite host, and we want put set the new host to UpstreamOperation.

@wbpcode @agrawroh WDYT?

[kyessenov]

What do you think about adding a coalescing operator like ??. Then you can compose and provide defaults:
%GET_OR_ELSE(REQUESTED_SERVER_NAME, REQUEST_HEADER(:authority))%. It is possible to implement a formatter that contains two formatters and defers if the first one evaluates to nil.