docs/HTTPSRR.md: initial HTTPS RR documentation

Closes #16052

docs/HTTPSRR.md

@@ -0,0 +1,93 @@
+<!--
+Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+
+SPDX-License-Identifier: curl
+-->
+
+# HTTPS RR
+
+[RFC 9460](https://www.rfc-editor.org/rfc/rfc9460.html) documents the HTTPS
+DNS Resource Record.
+
+curl features **experimental** support for HTTPS RR.
+
+- The ALPN list from the retrieved HTTPS record is parsed
+- The ECH field is stored (when DoH is used)
+- The port number from the HTTPS RR is not used
+- The target name is not used
+- The IP addresses from the HTTPS RR are not used
+- It only supports a single HTTPS RR per hostname
+- consider cases without A/AAAA records but *with* HTTPS RR
+- consider service profiles where the RR provides different addresses for TCP
+  vs QUIC etc
+
+`HTTPSRR` is listed as a feature in the `curl -V` output if curl contains
+HTTPS RR support. If c-ares is not included in the build, the HTTPS RR support
+is limited to DoH.
+
+`c-ares-rr` is listed as a feature in the `curl -V` output if c-ares is used
+for additional resolves in addition to a "normal" resolve done with the
+threaded resolver.
+
+The data extracted from the HTTPS RR is stored in the in-memory DNS cache to
+be reused on subsequent uses of the same hostnames.
+
+## build
+
+    ./configure --enable-httpsrr
+
+or
+
+    cmake -DUSE_HTTPSRR=ON
+
+## ALPN
+
+The list of ALPN IDs is parsed but may not be completely respected because of
+what the HTTP version preference is set to, which is a problem we are working
+on. Also, getting an `HTTP/1.1` ALPN in the HTTPS RR field for an HTTP://
+transfer should imply switching to HTTPS, HSTS style. Which curl currently
+does not.
+
+## DoH
+
+When HTTPS RR is enabled in the curl build, The DoH code asks for an HTTPS
+record in addition to the A and AAA records, and if an HTTPS RR answer is
+returned, curl parses it and stores the retrieved information.
+
+## Non-DoH
+
+If DoH is not used for name resolving in an HTTPS RR enabled build, we must
+provide the ability using the regular resolver backends. We use the c-ares DNS
+library for the HTTPS RR lookup. Version 1.28.0 or later.
+
+### c-ares
+
+If curl is built to use the c-ares library for name resolves, an HTTPS RR
+enabled build makes a request for the HTTPS RR in addition to the regular
+lookup.
+
+### Threaded resolver
+
+When built to use the threaded resolver, which is the default, an HTTPS RR
+build still needs a c-ares installation provided so that a separate request
+for the HTTPS record can be done in parallel to the regular getaddrinfo()
+call.
+
+This is done by specifying both c-ares and threaded resolver to configure:
+
+    ./configure --enble-ares=... --enable-threaded-resolver
+
+or to cmake:
+
+    cmake -DENABLE_ARES=ON -DENABLE_THREADED_RESOLVER=ON
+
+Because the HTTPS record is handled separately from the A/AAAA record
+retrieval, by a separate library, there is a small risk for discrepancies.
+
+When building curl using the threaded resolver with HTTPS RR support (using
+c-ares), the `curl -V` output looks exactly like a c-ares resolver build.
+
+## HTTPS RR Options
+
+Because curl is a low level transfer tool for which users sometimes want
+detailed control, we need to offer options to control HTTPS RR use.

docs/Makefile.am

@@ -87,6 +87,7 @@ EXTRA_DIST =                                    \
  HSTS.md                                        \
  HTTP-COOKIES.md                                \
  HTTP3.md                                       \
+ HTTPSRR.md                                     \
  INFRASTRUCTURE.md                              \
  INSTALL                                        \
  INSTALL-CMAKE.md                               \