Support custom ACME provider (#18340)

* Added ACMECAURL option to support custom ACME provider. Closes #18306
* Refactor setting.go https settings, renamed options and variables, and documented app.example.ini
* Refactored runLetsEncrypt to runACME
* Improved documentation

cmd/web_acme.go

@@ -0,0 +1,132 @@
+// Copyright 2020 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package cmd
+
+import (
+	"crypto/x509"
+	"encoding/pem"
+	"fmt"
+	"net/http"
+	"os"
+	"strconv"
+	"strings"
+
+	"code.gitea.io/gitea/modules/graceful"
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/setting"
+
+	"github.com/caddyserver/certmagic"
+)
+
+func getCARoot(path string) (*x509.CertPool, error) {
+	r, err := os.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+	block, _ := pem.Decode(r)
+	if block == nil {
+		return nil, fmt.Errorf("no PEM found in the file %s", path)
+	}
+	caRoot, err := x509.ParseCertificate(block.Bytes)
+	if err != nil {
+		return nil, err
+	}
+	certPool := x509.NewCertPool()
+	certPool.AddCert(caRoot)
+	return certPool, nil
+}
+
+func runACME(listenAddr string, m http.Handler) error {
+	// If HTTP Challenge enabled, needs to be serving on port 80. For TLSALPN needs 443.
+	// Due to docker port mapping this can't be checked programmatically
+	// TODO: these are placeholders until we add options for each in settings with appropriate warning
+	enableHTTPChallenge := true
+	enableTLSALPNChallenge := true
+	altHTTPPort := 0
+	altTLSALPNPort := 0
+
+	if p, err := strconv.Atoi(setting.PortToRedirect); err == nil {
+		altHTTPPort = p
+	}
+	if p, err := strconv.Atoi(setting.HTTPPort); err == nil {
+		altTLSALPNPort = p
+	}
+
+	magic := certmagic.NewDefault()
+	magic.Storage = &certmagic.FileStorage{Path: setting.AcmeLiveDirectory}
+	// Try to use private CA root if provided, otherwise defaults to system's trust
+	var certPool *x509.CertPool
+	if setting.AcmeCARoot != "" {
+		var err error
+		certPool, err = getCARoot(setting.AcmeCARoot)
+		if err != nil {
+			log.Warn("Failed to parse CA Root certificate, using default CA trust: %v", err)
+		}
+	}
+	myACME := certmagic.NewACMEManager(magic, certmagic.ACMEManager{
+		CA:                      setting.AcmeURL,
+		TrustedRoots:            certPool,
+		Email:                   setting.AcmeEmail,
+		Agreed:                  setting.AcmeTOS,
+		DisableHTTPChallenge:    !enableHTTPChallenge,
+		DisableTLSALPNChallenge: !enableTLSALPNChallenge,
+		ListenHost:              setting.HTTPAddr,
+		AltTLSALPNPort:          altTLSALPNPort,
+		AltHTTPPort:             altHTTPPort,
+	})
+
+	magic.Issuers = []certmagic.Issuer{myACME}
+
+	// this obtains certificates or renews them if necessary
+	err := magic.ManageSync(graceful.GetManager().HammerContext(), []string{setting.Domain})
+	if err != nil {
+		return err
+	}
+
+	tlsConfig := magic.TLSConfig()
+	tlsConfig.NextProtos = append(tlsConfig.NextProtos, "h2")
+
+	if version := toTLSVersion(setting.SSLMinimumVersion); version != 0 {
+		tlsConfig.MinVersion = version
+	}
+	if version := toTLSVersion(setting.SSLMaximumVersion); version != 0 {
+		tlsConfig.MaxVersion = version
+	}
+
+	// Set curve preferences
+	if curves := toCurvePreferences(setting.SSLCurvePreferences); len(curves) > 0 {
+		tlsConfig.CurvePreferences = curves
+	}
+
+	// Set cipher suites
+	if ciphers := toTLSCiphers(setting.SSLCipherSuites); len(ciphers) > 0 {
+		tlsConfig.CipherSuites = ciphers
+	}
+
+	if enableHTTPChallenge {
+		go func() {
+			log.Info("Running Let's Encrypt handler on %s", setting.HTTPAddr+":"+setting.PortToRedirect)
+			// all traffic coming into HTTP will be redirect to HTTPS automatically (LE HTTP-01 validation happens here)
+			err := runHTTP("tcp", setting.HTTPAddr+":"+setting.PortToRedirect, "Let's Encrypt HTTP Challenge", myACME.HTTPChallengeHandler(http.HandlerFunc(runLetsEncryptFallbackHandler)))
+			if err != nil {
+				log.Fatal("Failed to start the Let's Encrypt handler on port %s: %v", setting.PortToRedirect, err)
+			}
+		}()
+	}
+
+	return runHTTPSWithTLSConfig("tcp", listenAddr, "Web", tlsConfig, m)
+}
+
+func runLetsEncryptFallbackHandler(w http.ResponseWriter, r *http.Request) {
+	if r.Method != "GET" && r.Method != "HEAD" {
+		http.Error(w, "Use HTTPS", http.StatusBadRequest)
+		return
+	}
+	// Remove the trailing slash at the end of setting.AppURL, the request
+	// URI always contains a leading slash, which would result in a double
+	// slash
+	target := strings.TrimSuffix(setting.AppURL, "/") + r.URL.RequestURI()
+	http.Redirect(w, r, target, http.StatusFound)
+}