Use caddy's certmagic library for extensible/robust ACME handling (#14177)

* use certmagic for more extensible/robust ACME cert handling

* accept TOS based on config option

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lauris BH <lauris@nix.lv>

vendor/github.com/caddyserver/certmagic/httphandler.go

@@ -0,0 +1,133 @@
+// Copyright 2015 Matthew Holt
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package certmagic
+
+import (
+	"encoding/json"
+	"net/http"
+	"strings"
+
+	"github.com/mholt/acmez/acme"
+	"go.uber.org/zap"
+)
+
+// HTTPChallengeHandler wraps h in a handler that can solve the ACME
+// HTTP challenge. cfg is required, and it must have a certificate
+// cache backed by a functional storage facility, since that is where
+// the challenge state is stored between initiation and solution.
+//
+// If a request is not an ACME HTTP challenge, h will be invoked.
+func (am *ACMEManager) HTTPChallengeHandler(h http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if am.HandleHTTPChallenge(w, r) {
+			return
+		}
+		h.ServeHTTP(w, r)
+	})
+}
+
+// HandleHTTPChallenge uses am to solve challenge requests from an ACME
+// server that were initiated by this instance or any other instance in
+// this cluster (being, any instances using the same storage am does).
+//
+// If the HTTP challenge is disabled, this function is a no-op.
+//
+// If am is nil or if am does not have a certificate cache backed by
+// usable storage, solving the HTTP challenge will fail.
+//
+// It returns true if it handled the request; if so, the response has
+// already been written. If false is returned, this call was a no-op and
+// the request has not been handled.
+func (am *ACMEManager) HandleHTTPChallenge(w http.ResponseWriter, r *http.Request) bool {
+	if am == nil {
+		return false
+	}
+	if am.DisableHTTPChallenge {
+		return false
+	}
+	if !LooksLikeHTTPChallenge(r) {
+		return false
+	}
+	return am.distributedHTTPChallengeSolver(w, r)
+}
+
+// distributedHTTPChallengeSolver checks to see if this challenge
+// request was initiated by this or another instance which uses the
+// same storage as am does, and attempts to complete the challenge for
+// it. It returns true if the request was handled; false otherwise.
+func (am *ACMEManager) distributedHTTPChallengeSolver(w http.ResponseWriter, r *http.Request) bool {
+	if am == nil {
+		return false
+	}
+
+	host := hostOnly(r.Host)
+
+	tokenKey := distributedSolver{acmeManager: am, caURL: am.CA}.challengeTokensKey(host)
+	chalInfoBytes, err := am.config.Storage.Load(tokenKey)
+	if err != nil {
+		if _, ok := err.(ErrNotExist); !ok {
+			if am.Logger != nil {
+				am.Logger.Error("opening distributed HTTP challenge token file",
+					zap.String("host", host),
+					zap.Error(err))
+			}
+		}
+		return false
+	}
+
+	var challenge acme.Challenge
+	err = json.Unmarshal(chalInfoBytes, &challenge)
+	if err != nil {
+		if am.Logger != nil {
+			am.Logger.Error("decoding HTTP challenge token file (corrupted?)",
+				zap.String("host", host),
+				zap.String("token_key", tokenKey),
+				zap.Error(err))
+		}
+		return false
+	}
+
+	return am.answerHTTPChallenge(w, r, challenge)
+}
+
+// answerHTTPChallenge solves the challenge with chalInfo.
+// Most of this code borrowed from xenolf's built-in HTTP-01
+// challenge solver in March 2018.
+func (am *ACMEManager) answerHTTPChallenge(w http.ResponseWriter, r *http.Request, challenge acme.Challenge) bool {
+	challengeReqPath := challenge.HTTP01ResourcePath()
+	if r.URL.Path == challengeReqPath &&
+		strings.EqualFold(hostOnly(r.Host), challenge.Identifier.Value) && // mitigate DNS rebinding attacks
+		r.Method == "GET" {
+		w.Header().Add("Content-Type", "text/plain")
+		w.Write([]byte(challenge.KeyAuthorization))
+		r.Close = true
+		if am.Logger != nil {
+			am.Logger.Info("served key authentication",
+				zap.String("identifier", challenge.Identifier.Value),
+				zap.String("challenge", "http-01"),
+				zap.String("remote", r.RemoteAddr))
+		}
+		return true
+	}
+	return false
+}
+
+// LooksLikeHTTPChallenge returns true if r looks like an ACME
+// HTTP challenge request from an ACME server.
+func LooksLikeHTTPChallenge(r *http.Request) bool {
+	return r.Method == "GET" && strings.HasPrefix(r.URL.Path, challengeBasePath)
+}
+
+const challengeBasePath = "/.well-known/acme-challenge"