dnsdist: Add support for early DoH HTTP responses

[rgacogne]

Short description

The HTTPPathRule and HTTPPathRegexRule, and the Lua bindings to access the HTTP informations are only invoked for valid DNS over HTTP queries. This PR adds support for responding to HTTP queries before the DNS payload has been parsed, thus allowing to respond to all HTTP queries.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[chbruyand]

Code LGTM, I'm just wondering if matching shouldn't be also done on HTTP method ?
I'm also wondering if we may want to send the possibly set custom headers in the response ?

[rgacogne]

I'm just wondering if matching shouldn't be also done on HTTP method ?

It might be a request at some point, yes, but for now I'm trying to keep it simple.

[rgacogne]

I'm also wondering if we may want to send the possibly set custom headers in the response ?

I'd really like dnsdist not to become a full-fledged HTTP server :-)

[chbruyand]

I'm also wondering if we may want to send the possibly set custom headers in the response ?

I'd really like dnsdist not to become a full-fledged HTTP server :-)

I know, this is just that you may expect to get the custom headers you set via customResponseHeaders in the response.

[rgacogne]

I know, this is just that you may expect to get the custom headers you set via customResponseHeaders in the response.

Oh, you are right! I'm not 100% sure it's a good idea to apply these to "early" responses, but I guess it might be something that users will expect..

[chbruyand]

I know, this is just that you may expect to get the custom headers you set via customResponseHeaders in the response.

Oh, you are right! I'm not 100% sure it's a good idea to apply these to "early" responses, but I guess it might be something that users will expect..

Yes, really depends on your custom headers usage. But I think you would expect it in use cases like #7900

[rgacogne]

I'm worried about the case where one does not want the customResponseHeaders applied here, so I guess we indeed need to accept an optional list of custom headers in newDOHResponseMapEntry(), where no list means to apply customResponseHeaders and any other value overrides it (so an empty list means no headers).

[rgacogne]

Fixed the default application/dns-message content-type header added to early 200 responses. Added support for custom HTTP headers in early responses as well.

[rgacogne]

Rebased to fix a conflict.