https plugin (#1)

This pull request adds https plugin that performs DNS-over-HTTPS proxying.

Author: v-byte-cpu

README.md

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+# https
+
+**https** is a [CoreDNS](https://github.com/coredns/coredns) plugin that proxies DNS messages to upstream resolvers using DNS-over-HTTPS protocol. See [RFC 8484](https://tools.ietf.org/html/rfc8484).
+
+## Installation
+
+External CoreDNS plugins can be enabled in one of two ways:
+  1. [Build with compile-time configuration file](https://coredns.io/2017/07/25/compile-time-enabling-or-disabling-plugins/#build-with-compile-time-configuration-file)
+  2. [Build with external golang source code](https://coredns.io/2017/07/25/compile-time-enabling-or-disabling-plugins/#build-with-external-golang-source-code)
+
+## Syntax
+
+In its most basic form:
+
+~~~
+https FROM TO...
+~~~
+
+* **FROM** is the base domain to match for the request to be proxied.
+* **TO...** are the destination endpoints to proxy to. The number of upstreams is
+  limited to 15.
+
+Multiple upstreams are randomized (see `policy`) on first use. When a proxy returns an error
+the next upstream in the list is tried.
+
+Extra knobs are available with an expanded syntax:
+
+~~~
+https FROM TO... {
+    except IGNORED_NAMES...
+    tls CERT KEY CA
+    tls_servername NAME
+    policy random|round_robin|sequential
+}
+~~~
+
+* **FROM** and **TO...** as above.
+* **IGNORED_NAMES** in `except` is a space-separated list of domains to exclude from proxying.
+  Requests that match none of these names will be passed through.
+* `tls` **CERT** **KEY** **CA** define the TLS properties for TLS connection. From 0 to 3 arguments can be
+  provided with the meaning as described below
+
+  * `tls` - no client authentication is used, and the system CAs are used to verify the server certificate (by default)
+  * `tls` **CA** - no client authentication is used, and the file CA is used to verify the server certificate
+  * `tls` **CERT** **KEY** - client authentication is used with the specified cert/key pair.
+    The server certificate is verified with the system CAs
+  * `tls` **CERT** **KEY**  **CA** - client authentication is used with the specified cert/key pair.
+    The server certificate is verified using the specified CA file
+
+* `policy` specifies the policy to use for selecting upstream servers. The default is `random`.
+
+
+## Metrics
+
+If monitoring is enabled (via the *prometheus* plugin) then the following metric are exported:
+
+* `coredns_https_request_duration_seconds{to}` - duration per upstream interaction.
+* `coredns_https_requests_total{to}` - query count per upstream.
+* `coredns_https_responses_total{to, rcode}` - count of RCODEs per upstream.
+  and we are randomly (this always uses the `random` policy) spraying to an upstream.
+
+## Examples
+
+Proxy all requests within `example.org.` to a DoH nameserver:
+
+~~~ corefile
+example.org {
+    https . cloudflare-dns.com/dns-query
+}
+~~~
+
+Forward everything except requests to `example.org`
+
+~~~ corefile
+. {
+    https . dns.quad9.net/dns-query {
+        except example.org
+    }
+}
+~~~
+
+Load balance all requests between multiple upstreams
+
+~~~ corefile
+. {
+    https . dns.quad9.net/dns-query cloudflare-dns.com:443/dns-query dns.google/dns-query
+}
+~~~
+
+Internal DoH server:
+
+~~~ corefile
+. {
+    https . 10.0.0.10:853/dns-query {
+      tls ca.crt
+      tls_servername internal.domain
+    }
+}
+~~~

go.mod

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+module github.com/v-byte-cpu/coredns-https
+
+go 1.19
+
+require (
+	github.com/coredns/caddy v1.1.1
+	github.com/coredns/coredns v1.9.3
+	github.com/miekg/dns v1.1.50
+	github.com/prometheus/client_golang v1.13.0
+	github.com/stretchr/testify v1.8.0
+)
+
+require (
+	github.com/apparentlymart/go-cidr v1.1.0 // indirect
+	github.com/beorn7/perks v1.0.1 // indirect
+	github.com/cespare/xxhash/v2 v2.1.2 // indirect
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/flynn/go-shlex v0.0.0-20150515145356-3f9db97f8568 // indirect
+	github.com/golang/protobuf v1.5.2 // indirect
+	github.com/grpc-ecosystem/grpc-opentracing v0.0.0-20180507213350-8e809c8a8645 // indirect
+	github.com/matttproud/golang_protobuf_extensions v1.0.1 // indirect
+	github.com/opentracing/opentracing-go v1.2.0 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/prometheus/client_model v0.2.0 // indirect
+	github.com/prometheus/common v0.37.0 // indirect
+	github.com/prometheus/procfs v0.8.0 // indirect
+	golang.org/x/mod v0.4.2 // indirect
+	golang.org/x/net v0.0.0-20220520000938-2e3eb7b945c2 // indirect
+	golang.org/x/sys v0.0.0-20220520151302-bc2c85ada10a // indirect
+	golang.org/x/text v0.3.7 // indirect
+	golang.org/x/tools v0.1.6-0.20210726203631-07bc1bf47fb2 // indirect
+	golang.org/x/xerrors v0.0.0-20220517211312-f3a8303e98df // indirect
+	google.golang.org/genproto v0.0.0-20220519153652-3a47de7e79bd // indirect
+	google.golang.org/grpc v1.46.2 // indirect
+	google.golang.org/protobuf v1.28.1 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)