caddytls: Support external certificate Managers (like Tailscale) (#4541)

Huge thank-you to Tailscale (https://tailscale.com) for making this change possible!
This is a great feature for Caddy and Tailscale is a great fit for a standard implementation.

* caddytls: GetCertificate modules; Tailscale

* Caddyfile support for get_certificate

Also fix AP provisioning in case of empty subject list (persist loaded
module on struct, much like Issuers, to surive reprovisioning).

And implement start of HTTP cert getter, still WIP.

* Update modules/caddytls/automation.go

Co-authored-by: Francis Lavoie <lavofr@gmail.com>

* Use tsclient package, check status for name

* Implement HTTP cert getter

And use reuse CertMagic's PEM functions for private keys.

* Remove cache option from Tailscale getter

Tailscale does its own caching and we don't need the added complexity...
for now, at least.

* Several updates

- Option to disable cert automation in auto HTTPS
- Support multiple cert managers
- Remove cache feature from cert manager modules
- Minor improvements to auto HTTPS logging

* Run go mod tidy

* Try to get certificates from Tailscale implicitly

Only for domains ending in .ts.net.

I think this is really cool!

Co-authored-by: Francis Lavoie <lavofr@gmail.com>

7 files changed, 339 insertions, 138 deletions

diff --git a/modules/caddyhttp/autohttps.go b/modules/caddyhttp/autohttps.go
index 6c37d70..3e38d1b 100644
--- a/modules/caddyhttp/autohttps.go
+++ b/modules/caddyhttp/autohttps.go
@@ -31,13 +31,20 @@ import (
 // HTTPS is enabled automatically and by default when
 // qualifying hostnames are available from the config.
 type AutoHTTPSConfig struct {
-	// If true, automatic HTTPS will be entirely disabled.
+	// If true, automatic HTTPS will be entirely disabled,
+	// including certificate management and redirects.
 	Disabled bool `json:"disable,omitempty"`
 
 	// If true, only automatic HTTP->HTTPS redirects will
-	// be disabled.
+	// be disabled, but other auto-HTTPS features will
+	// remain enabled.
 	DisableRedir bool `json:"disable_redirects,omitempty"`
 
+	// If true, automatic certificate management will be
+	// disabled, but other auto-HTTPS features will
+	// remain enabled.
+	DisableCerts bool `json:"disable_certificates,omitempty"`
+
 	// Hosts/domain names listed here will not be included
 	// in automatic HTTPS (they will not have certificates
 	// loaded nor redirects applied).
@@ -104,12 +111,13 @@ func (app *App) automaticHTTPSPhase1(ctx caddy.Context, repl *caddy.Replacer) er
 			srv.AutoHTTPS = new(AutoHTTPSConfig)
 		}
 		if srv.AutoHTTPS.Disabled {
+			app.logger.Warn("automatic HTTPS is completely disabled for server", zap.String("server_name", srvName))
 			continue
 		}
 
 		// skip if all listeners use the HTTP port
 		if !srv.listenersUseAnyPortOtherThan(app.httpPort()) {
-			app.logger.Info("server is listening only on the HTTP port, so no automatic HTTPS will be applied to this server",
+			app.logger.Warn("server is listening only on the HTTP port, so no automatic HTTPS will be applied to this server",
 				zap.String("server_name", srvName),
 				zap.Int("http_port", app.httpPort()),
 			)
@@ -166,30 +174,35 @@ func (app *App) automaticHTTPSPhase1(ctx caddy.Context, repl *caddy.Replacer) er
 
 		// for all the hostnames we found, filter them so we have
 		// a deduplicated list of names for which to obtain certs
-		for d := range serverDomainSet {
-			if certmagic.SubjectQualifiesForCert(d) &&
-				!srv.AutoHTTPS.Skipped(d, srv.AutoHTTPS.SkipCerts) {
-				// if a certificate for this name is already loaded,
-				// don't obtain another one for it, unless we are
-				// supposed to ignore loaded certificates
-				if !srv.AutoHTTPS.IgnoreLoadedCerts &&
-					len(app.tlsApp.AllMatchingCertificates(d)) > 0 {
-					app.logger.Info("skipping automatic certificate management because one or more matching certificates are already loaded",
-						zap.String("domain", d),
-						zap.String("server_name", srvName),
-					)
-					continue
-				}
+		// (only if cert management not disabled for this server)
+		if srv.AutoHTTPS.DisableCerts {
+			app.logger.Warn("skipping automated certificate management for server because it is disabled", zap.String("server_name", srvName))
+		} else {
+			for d := range serverDomainSet {
+				if certmagic.SubjectQualifiesForCert(d) &&
+					!srv.AutoHTTPS.Skipped(d, srv.AutoHTTPS.SkipCerts) {
+					// if a certificate for this name is already loaded,
+					// don't obtain another one for it, unless we are
+					// supposed to ignore loaded certificates
+					if !srv.AutoHTTPS.IgnoreLoadedCerts &&
+						len(app.tlsApp.AllMatchingCertificates(d)) > 0 {
+						app.logger.Info("skipping automatic certificate management because one or more matching certificates are already loaded",
+							zap.String("domain", d),
+							zap.String("server_name", srvName),
+						)
+						continue
+					}
 
-				// most clients don't accept wildcards like *.tld... we
-				// can handle that, but as a courtesy, warn the user
-				if strings.Contains(d, "*") &&
-					strings.Count(strings.Trim(d, "."), ".") == 1 {
-					app.logger.Warn("most clients do not trust second-level wildcard certificates (*.tld)",
-						zap.String("domain", d))
-				}
+					// most clients don't accept wildcards like *.tld... we
+					// can handle that, but as a courtesy, warn the user
+					if strings.Contains(d, "*") &&
+						strings.Count(strings.Trim(d, "."), ".") == 1 {
+						app.logger.Warn("most clients do not trust second-level wildcard certificates (*.tld)",
+							zap.String("domain", d))
+					}
 
-				uniqueDomainsForCerts[d] = struct{}{}
+					uniqueDomainsForCerts[d] = struct{}{}
+				}
 			}
 		}
 
@@ -200,12 +213,11 @@ func (app *App) automaticHTTPSPhase1(ctx caddy.Context, repl *caddy.Replacer) er
 
 		// nothing left to do if auto redirects are disabled
 		if srv.AutoHTTPS.DisableRedir {
+			app.logger.Warn("automatic HTTP->HTTPS redirects are disabled", zap.String("server_name", srvName))
 			continue
 		}
 
-		app.logger.Info("enabling automatic HTTP->HTTPS redirects",
-			zap.String("server_name", srvName),
-		)
+		app.logger.Info("enabling automatic HTTP->HTTPS redirects", zap.String("server_name", srvName))
 
 		// create HTTP->HTTPS redirects
 		for _, addr := range srv.Listen {
@@ -459,6 +471,16 @@ func (app *App) createAutomationPolicies(ctx caddy.Context, internalNames []stri
 			}
 		}
 
+		// if no external managers were configured, enable
+		// implicit Tailscale support for convenience
+		if ap.Managers == nil {
+			ts, err := implicitTailscale(ctx)
+			if err != nil {
+				return err
+			}
+			ap.Managers = []certmagic.CertificateManager{ts}
+		}
+
 		// while we're here, is this the catch-all/base policy?
 		if !foundBasePolicy && len(ap.Subjects) == 0 {
 			basePolicy = ap
@@ -467,8 +489,14 @@ func (app *App) createAutomationPolicies(ctx caddy.Context, internalNames []stri
 	}
 
 	if basePolicy == nil {
-		// no base policy found, we will make one!
-		basePolicy = new(caddytls.AutomationPolicy)
+		// no base policy found, we will make one! (with implicit Tailscale integration)
+		ts, err := implicitTailscale(ctx)
+		if err != nil {
+			return err
+		}
+		basePolicy = &caddytls.AutomationPolicy{
+			Managers: []certmagic.CertificateManager{ts},
+		}
 	}
 
 	// if the basePolicy has an existing ACMEIssuer (particularly to
@@ -482,8 +510,8 @@ func (app *App) createAutomationPolicies(ctx caddy.Context, internalNames []stri
 		}
 	}
 	if baseACMEIssuer == nil {
-		// note that this happens if basePolicy.Issuer is nil
-		// OR if it is not nil but is not an ACMEIssuer
+		// note that this happens if basePolicy.Issuers is empty
+		// OR if it is not empty but does not have not an ACMEIssuer
 		baseACMEIssuer = new(caddytls.ACMEIssuer)
 	}
 
@@ -653,4 +681,11 @@ func (app *App) automaticHTTPSPhase2() error {
 	return nil
 }
 
+// implicitTailscale returns a new and provisioned Tailscale module configured to be optional.
+func implicitTailscale(ctx caddy.Context) (caddytls.Tailscale, error) {
+	ts := caddytls.Tailscale{Optional: true}
+	err := ts.Provision(ctx)
+	return ts, err
+}
+
 type acmeCapable interface{ GetACMEIssuer() *caddytls.ACMEIssuer }
diff --git a/modules/caddypki/ca.go b/modules/caddypki/ca.go
index e3102fb..7fefee6 100644
--- a/modules/caddypki/ca.go
+++ b/modules/caddypki/ca.go
@@ -239,7 +239,7 @@ func (ca CA) loadOrGenRoot() (rootCert *x509.Certificate, rootKey interface{}, e
 		if err != nil {
 			return nil, nil, fmt.Errorf("loading root key: %v", err)
 		}
-		rootKey, err = pemDecodePrivateKey(rootKeyPEM)
+		rootKey, err = certmagic.PEMDecodePrivateKey(rootKeyPEM)
 		if err != nil {
 			return nil, nil, fmt.Errorf("decoding root key: %v", err)
 		}
@@ -263,7 +263,7 @@ func (ca CA) genRoot() (rootCert *x509.Certificate, rootKey interface{}, err err
 	if err != nil {
 		return nil, nil, fmt.Errorf("saving root certificate: %v", err)
 	}
-	rootKeyPEM, err := pemEncodePrivateKey(rootKey)
+	rootKeyPEM, err := certmagic.PEMEncodePrivateKey(rootKey)
 	if err != nil {
 		return nil, nil, fmt.Errorf("encoding root key: %v", err)
 	}
@@ -275,7 +275,7 @@ func (ca CA) genRoot() (rootCert *x509.Certificate, rootKey interface{}, err err
 	return rootCert, rootKey, nil
 }
 
-func (ca CA) loadOrGenIntermediate(rootCert *x509.Certificate, rootKey interface{}) (interCert *x509.Certificate, interKey interface{}, err error) {
+func (ca CA) loadOrGenIntermediate(rootCert *x509.Certificate, rootKey crypto.PrivateKey) (interCert *x509.Certificate, interKey crypto.PrivateKey, err error) {
 	interCertPEM, err := ca.storage.Load(ca.storageKeyIntermediateCert())
 	if err != nil {
 		if _, ok := err.(certmagic.ErrNotExist); !ok {
@@ -301,7 +301,7 @@ func (ca CA) loadOrGenIntermediate(rootCert *x509.Certificate, rootKey interface
 		if err != nil {
 			return nil, nil, fmt.Errorf("loading intermediate key: %v", err)
 		}
-		interKey, err = pemDecodePrivateKey(interKeyPEM)
+		interKey, err = certmagic.PEMDecodePrivateKey(interKeyPEM)
 		if err != nil {
 			return nil, nil, fmt.Errorf("decoding intermediate key: %v", err)
 		}
@@ -310,7 +310,7 @@ func (ca CA) loadOrGenIntermediate(rootCert *x509.Certificate, rootKey interface
 	return interCert, interKey, nil
 }
 
-func (ca CA) genIntermediate(rootCert *x509.Certificate, rootKey interface{}) (interCert *x509.Certificate, interKey interface{}, err error) {
+func (ca CA) genIntermediate(rootCert *x509.Certificate, rootKey crypto.PrivateKey) (interCert *x509.Certificate, interKey crypto.PrivateKey, err error) {
 	repl := ca.newReplacer()
 
 	interCert, interKey, err = generateIntermediate(repl.ReplaceAll(ca.IntermediateCommonName, ""), rootCert, rootKey)
@@ -325,7 +325,7 @@ func (ca CA) genIntermediate(rootCert *x509.Certificate, rootKey interface{}) (i
 	if err != nil {
 		return nil, nil, fmt.Errorf("saving intermediate certificate: %v", err)
 	}
-	interKeyPEM, err := pemEncodePrivateKey(interKey)
+	interKeyPEM, err := certmagic.PEMEncodePrivateKey(interKey)
 	if err != nil {
 		return nil, nil, fmt.Errorf("encoding intermediate key: %v", err)
 	}
diff --git a/modules/caddypki/certificates.go b/modules/caddypki/certificates.go
index a55c165..bd260da 100644
--- a/modules/caddypki/certificates.go
+++ b/modules/caddypki/certificates.go
@@ -15,6 +15,7 @@
 package caddypki
 
 import (
+	"crypto"
 	"crypto/x509"
 	"time"
 
@@ -30,7 +31,7 @@ func generateRoot(commonName string) (rootCrt *x509.Certificate, privateKey inte
 	return newCert(rootProfile)
 }
 
-func generateIntermediate(commonName string, rootCrt *x509.Certificate, rootKey interface{}) (cert *x509.Certificate, privateKey interface{}, err error) {
+func generateIntermediate(commonName string, rootCrt *x509.Certificate, rootKey crypto.PrivateKey) (cert *x509.Certificate, privateKey crypto.PrivateKey, err error) {
 	interProfile, err := x509util.NewIntermediateProfile(commonName, rootCrt, rootKey)
 	if err != nil {
 		return
@@ -39,7 +40,7 @@ func generateIntermediate(commonName string, rootCrt *x509.Certificate, rootKey
 	return newCert(interProfile)
 }
 
-func newCert(profile x509util.Profile) (cert *x509.Certificate, privateKey interface{}, err error) {
+func newCert(profile x509util.Profile) (cert *x509.Certificate, privateKey crypto.PrivateKey, err error) {
 	certBytes, err := profile.CreateCertificate()
 	if err != nil {
 		return
diff --git a/modules/caddypki/crypto.go b/modules/caddypki/crypto.go
index dbc6f38..386ce62 100644
--- a/modules/caddypki/crypto.go
+++ b/modules/caddypki/crypto.go
@@ -17,14 +17,12 @@ package caddypki
 import (
 	"bytes"
 	"crypto"
-	"crypto/ecdsa"
-	"crypto/ed25519"
-	"crypto/rsa"
 	"crypto/x509"
 	"encoding/pem"
 	"fmt"
 	"os"
-	"strings"
+
+	"github.com/caddyserver/certmagic"
 )
 
 func pemDecodeSingleCert(pemDER []byte) (*x509.Certificate, error) {
@@ -45,70 +43,6 @@ func pemEncodeCert(der []byte) ([]byte, error) {
 	return pemEncode("CERTIFICATE", der)
 }
 
-// pemEncodePrivateKey marshals a EC or RSA private key into a PEM-encoded array of bytes.
-// TODO: this is the same thing as in certmagic. Should we reuse that code somehow? It's unexported.
-func pemEncodePrivateKey(key crypto.PrivateKey) ([]byte, error) {
-	var pemType string
-	var keyBytes []byte
-	switch key := key.(type) {
-	case *ecdsa.PrivateKey:
-		var err error
-		pemType = "EC"
-		keyBytes, err = x509.MarshalECPrivateKey(key)
-		if err != nil {
-			return nil, err
-		}
-	case *rsa.PrivateKey:
-		pemType = "RSA"
-		keyBytes = x509.MarshalPKCS1PrivateKey(key)
-	case *ed25519.PrivateKey:
-		var err error
-		pemType = "ED25519"
-		keyBytes, err = x509.MarshalPKCS8PrivateKey(key)
-		if err != nil {
-			return nil, err
-		}
-	default:
-		return nil, fmt.Errorf("unsupported key type: %T", key)
-	}
-	return pemEncode(pemType+" PRIVATE KEY", keyBytes)
-}
-
-// pemDecodePrivateKey loads a PEM-encoded ECC/RSA private key from an array of bytes.
-// Borrowed from Go standard library, to handle various private key and PEM block types.
-// https://github.com/golang/go/blob/693748e9fa385f1e2c3b91ca9acbb6c0ad2d133d/src/crypto/tls/tls.go#L291-L308
-// https://github.com/golang/go/blob/693748e9fa385f1e2c3b91ca9acbb6c0ad2d133d/src/crypto/tls/tls.go#L238)
-// TODO: this is the same thing as in certmagic. Should we reuse that code somehow? It's unexported.
-func pemDecodePrivateKey(keyPEMBytes []byte) (crypto.PrivateKey, error) {
-	keyBlockDER, _ := pem.Decode(keyPEMBytes)
-	if keyBlockDER == nil {
-		return nil, fmt.Errorf("no PEM data found")
-	}
-
-	if keyBlockDER.Type != "PRIVATE KEY" && !strings.HasSuffix(keyBlockDER.Type, " PRIVATE KEY") {
-		return nil, fmt.Errorf("unknown PEM header %q", keyBlockDER.Type)
-	}
-
-	if key, err := x509.ParsePKCS1PrivateKey(keyBlockDER.Bytes); err == nil {
-		return key, nil
-	}
-
-	if key, err := x509.ParsePKCS8PrivateKey(keyBlockDER.Bytes); err == nil {
-		switch key := key.(type) {
-		case *rsa.PrivateKey, *ecdsa.PrivateKey, ed25519.PrivateKey:
-			return key, nil
-		default:
-			return nil, fmt.Errorf("found unknown private key type in PKCS#8 wrapping: %T", key)
-		}
-	}
-
-	if key, err := x509.ParseECPrivateKey(keyBlockDER.Bytes); err == nil {
-		return key, nil
-	}
-
-	return nil, fmt.Errorf("unknown private key type")
-}
-
 func pemEncode(blockType string, b []byte) ([]byte, error) {
 	var buf bytes.Buffer
 	err := pem.Encode(&buf, &pem.Block{Type: blockType, Bytes: b})
@@ -137,7 +71,7 @@ type KeyPair struct {
 }
 
 // Load loads the certificate and key.
-func (kp KeyPair) Load() (*x509.Certificate, interface{}, error) {
+func (kp KeyPair) Load() (*x509.Certificate, crypto.Signer, error) {
 	switch kp.Format {
 	case "", "pem_file":
 		certData, err := os.ReadFile(kp.Certificate)
@@ -153,7 +87,7 @@ func (kp KeyPair) Load() (*x509.Certificate, interface{}, error) {
 		if err != nil {
 			return nil, nil, err
 		}
-		key, err := pemDecodePrivateKey(keyData)
+		key, err := certmagic.PEMDecodePrivateKey(keyData)
 		if err != nil {
 			return nil, nil, err
 		}
diff --git a/modules/caddytls/automation.go b/modules/caddytls/automation.go
index 2a701bf..95b1772 100644
--- a/modules/caddytls/automation.go
+++ b/modules/caddytls/automation.go
@@ -89,6 +89,14 @@ type AutomationPolicy struct {
 	// zerossl.
 	IssuersRaw []json.RawMessage `json:"issuers,omitempty" caddy:"namespace=tls.issuance inline_key=module"`
 
+	// Modules that can get a custom certificate to use for any
+	// given TLS handshake at handshake-time. Custom certificates
+	// can be useful if another entity is managing certificates
+	// and Caddy need only get it and serve it.
+	//
+	// TODO: This is an EXPERIMENTAL feature. It is subject to change or removal.
+	ManagersRaw []json.RawMessage `json:"get_certificate,omitempty" caddy:"namespace=tls.get_certificate inline_key=via"`
+
 	// If true, certificates will be requested with MustStaple. Not all
 	// CAs support this, and there are potentially serious consequences
 	// of enabling this feature without proper threat modeling.
@@ -113,7 +121,7 @@ type AutomationPolicy struct {
 
 	// If true, certificates will be managed "on demand"; that is, during
 	// TLS handshakes or when needed, as opposed to at startup or config
-	// load.
+	// load. This enables On-Demand TLS for this policy.
 	OnDemand bool `json:"on_demand,omitempty"`
 
 	// Disables OCSP stapling. Disabling OCSP stapling puts clients at
@@ -129,10 +137,12 @@ type AutomationPolicy struct {
 	// EXPERIMENTAL. Subject to change.
 	OCSPOverrides map[string]string `json:"ocsp_overrides,omitempty"`
 
-	// Issuers stores the decoded issuer parameters. This is only
-	// used to populate an underlying certmagic.Config's Issuers
-	// field; it is not referenced thereafter.
-	Issuers []certmagic.Issuer `json:"-"`
+	// Issuers and Managers store the decoded issuer and manager modules;
+	// they are only used to populate an underlying certmagic.Config's
+	// fields during provisioning so that the modules can survive a
+	// re-provisioning.
+	Issuers  []certmagic.Issuer             `json:"-"`
+	Managers []certmagic.CertificateManager `json:"-"`
 
 	magic   *certmagic.Config
 	storage certmagic.Storage
@@ -153,6 +163,7 @@ func (ap *AutomationPolicy) Provision(tlsApp *TLS) error {
 		ap.storage = cmStorage
 	}
 
+	// on-demand TLS
 	var ond *certmagic.OnDemandConfig
 	if ap.OnDemand {
 		ond = &certmagic.OnDemandConfig{
@@ -176,6 +187,22 @@ func (ap *AutomationPolicy) Provision(tlsApp *TLS) error {
 		}
 	}
 
+	// we don't store loaded modules directly in the certmagic config since
+	// policy provisioning may happen more than once (during auto-HTTPS) and
+	// loading a module clears its config bytes; thus, load the module and
+	// store them on the policy before putting it on the config
+
+	// load and provision any cert manager modules
+	if ap.ManagersRaw != nil {
+		vals, err := tlsApp.ctx.LoadModule(ap, "ManagersRaw")
+		if err != nil {
+			return fmt.Errorf("loading external certificate manager modules: %v", err)
+		}
+		for _, getCertVal := range vals.([]interface{}) {
+			ap.Managers = append(ap.Managers, getCertVal.(certmagic.CertificateManager))
+		}
+	}
+
 	// load and provision any explicitly-configured issuer modules
 	if ap.IssuersRaw != nil {
 		val, err := tlsApp.ctx.LoadModule(ap, "IssuersRaw")
@@ -225,9 +252,10 @@ func (ap *AutomationPolicy) Provision(tlsApp *TLS) error {
 			DisableStapling:    ap.DisableOCSPStapling,
 			ResponderOverrides: ap.OCSPOverrides,
 		},
-		Storage: storage,
-		Issuers: issuers,
-		Logger:  tlsApp.logger,
+		Storage:  storage,
+		Issuers:  issuers,
+		Managers: ap.Managers,
+		Logger:   tlsApp.logger,
 	}
 	ap.magic = certmagic.New(tlsApp.certCache, template)
 
diff --git a/modules/caddytls/certmanagers.go b/modules/caddytls/certmanagers.go
new file mode 100644
index 0000000..653e9f5
--- /dev/null
+++ b/modules/caddytls/certmanagers.go
@@ -0,0 +1,208 @@
+package caddytls
+
+import (
+	"context"
+	"crypto/tls"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"strings"
+
+	"github.com/caddyserver/caddy/v2"
+	"github.com/caddyserver/caddy/v2/caddyconfig/caddyfile"
+	"github.com/caddyserver/certmagic"
+	"github.com/tailscale/tscert"
+	"go.uber.org/zap"
+)
+
+func init() {
+	caddy.RegisterModule(Tailscale{})
+	caddy.RegisterModule(HTTPCertGetter{})
+}
+
+// Tailscale is a module that can get certificates from the local Tailscale process.
+type Tailscale struct {
+	// If true, this module will operate in "best-effort" mode and
+	// ignore "soft" errors; i.e. try Tailscale, and if it doesn't connect
+	// or return a certificate, oh well. Failure to connect to Tailscale
+	// results in a no-op instead of an error. Intended for the use case
+	// where this module is added implicitly for convenience, even if
+	// Tailscale isn't necessarily running.
+	Optional bool `json:"optional,omitempty"`
+
+	logger *zap.Logger
+}
+
+// CaddyModule returns the Caddy module information.
+func (Tailscale) CaddyModule() caddy.ModuleInfo {
+	return caddy.ModuleInfo{
+		ID:  "tls.get_certificate.tailscale",
+		New: func() caddy.Module { return new(Tailscale) },
+	}
+}
+
+func (ts *Tailscale) Provision(ctx caddy.Context) error {
+	ts.logger = ctx.Logger(ts)
+	return nil
+}
+
+func (ts Tailscale) GetCertificate(ctx context.Context, hello *tls.ClientHelloInfo) (*tls.Certificate, error) {
+	canGetCert, err := ts.canHazCertificate(ctx, hello)
+	if err == nil && !canGetCert {
+		return nil, nil // pass-thru: Tailscale can't offer a cert for this name
+	}
+	if err != nil {
+		ts.logger.Warn("could not get status; will try to get certificate anyway", zap.Error(err))
+	}
+	return tscert.GetCertificate(hello)
+}
+
+// canHazCertificate returns true if Tailscale reports it can get a certificate for the given ClientHello.
+func (ts Tailscale) canHazCertificate(ctx context.Context, hello *tls.ClientHelloInfo) (bool, error) {
+	if ts.Optional && !strings.HasSuffix(strings.ToLower(hello.ServerName), tailscaleDomainAliasEnding) {
+		return false, nil
+	}
+	status, err := tscert.GetStatus(ctx)
+	if err != nil {
+		if ts.Optional {
+			return false, nil // ignore error if we don't expect/require it to work anyway
+		}
+		return false, err
+	}
+	for _, domain := range status.CertDomains {
+		if certmagic.MatchWildcard(hello.ServerName, domain) {
+			return true, nil
+		}
+	}
+	return false, nil
+}
+
+// UnmarshalCaddyfile deserializes Caddyfile tokens into ts.
+//
+//     ... tailscale
+//
+func (Tailscale) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
+	for d.Next() {
+		if d.NextArg() {
+			return d.ArgErr()
+		}
+	}
+	return nil
+}
+
+// tailscaleDomainAliasEnding is the ending for all Tailscale custom domains.
+const tailscaleDomainAliasEnding = ".ts.net"
+
+// HTTPCertGetter can get a certificate via HTTP(S) request.
+type HTTPCertGetter struct {
+	// The URL from which to download the certificate. Required.
+	//
+	// The URL will be augmented with query string parameters taken
+	// from the TLS handshake:
+	//
+	// - server_name: The SNI value
+	// - signature_schemes: Comma-separated list of hex IDs of signatures
+	// - cipher_suites: Comma-separated list of hex IDs of cipher suites
+	//
+	// To be valid, the response must be HTTP 200 with a PEM body
+	// consisting of blocks for the certificate chain and the private
+	// key.
+	URL string `json:"url,omitempty"`
+
+	ctx context.Context
+}
+
+// CaddyModule returns the Caddy module information.
+func (hcg HTTPCertGetter) CaddyModule() caddy.ModuleInfo {
+	return caddy.ModuleInfo{
+		ID:  "tls.get_certificate.http",
+		New: func() caddy.Module { return new(HTTPCertGetter) },
+	}
+}
+
+func (hcg *HTTPCertGetter) Provision(ctx caddy.Context) error {
+	hcg.ctx = ctx
+	if hcg.URL == "" {
+		return fmt.Errorf("URL is required")
+	}
+	return nil
+}
+
+func (hcg HTTPCertGetter) GetCertificate(ctx context.Context, hello *tls.ClientHelloInfo) (*tls.Certificate, error) {
+	sigs := make([]string, len(hello.SignatureSchemes))
+	for i, sig := range hello.SignatureSchemes {
+		sigs[i] = fmt.Sprintf("%x", uint16(sig)) // you won't believe what %x uses if the val is a Stringer
+	}
+	suites := make([]string, len(hello.CipherSuites))
+	for i, cs := range hello.CipherSuites {
+		suites[i] = fmt.Sprintf("%x", cs)
+	}
+
+	parsed, err := url.Parse(hcg.URL)
+	if err != nil {
+		return nil, err
+	}
+	qs := parsed.Query()
+	qs.Set("server_name", hello.ServerName)
+	qs.Set("signature_schemes", strings.Join(sigs, ","))
+	qs.Set("cipher_suites", strings.Join(suites, ","))
+	parsed.RawQuery = qs.Encode()
+
+	req, err := http.NewRequestWithContext(hcg.ctx, http.MethodGet, parsed.String(), nil)
+	if err != nil {
+		return nil, err
+	}
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("got HTTP %d", resp.StatusCode)
+	}
+
+	bodyBytes, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("error reading response body: %v", err)
+	}
+
+	cert, err := tlsCertFromCertAndKeyPEMBundle(bodyBytes)
+	if err != nil {
+		return nil, err
+	}
+
+	return &cert, nil
+}
+
+// UnmarshalCaddyfile deserializes Caddyfile tokens into ts.
+//
+//     ... http <url>
+//
+func (hcg *HTTPCertGetter) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
+	for d.Next() {
+		if !d.NextArg() {
+			return d.ArgErr()
+		}
+		hcg.URL = d.Val()
+		if d.NextArg() {
+			return d.ArgErr()
+		}
+		for nesting := d.Nesting(); d.NextBlock(nesting); {
+			return d.Err("block not allowed here")
+		}
+	}
+	return nil
+}
+
+// Interface guards
+var (
+	_ certmagic.CertificateManager = (*Tailscale)(nil)
+	_ caddy.Provisioner            = (*Tailscale)(nil)
+	_ caddyfile.Unmarshaler        = (*Tailscale)(nil)
+
+	_ certmagic.CertificateManager = (*HTTPCertGetter)(nil)
+	_ caddy.Provisioner            = (*HTTPCertGetter)(nil)
+	_ caddyfile.Unmarshaler        = (*HTTPCertGetter)(nil)
+)
diff --git a/modules/caddytls/folderloader.go b/modules/caddytls/folderloader.go
index 0ff0629..33b31a5 100644
--- a/modules/caddytls/folderloader.go
+++ b/modules/caddytls/folderloader.go
@@ -61,10 +61,14 @@ func (fl FolderLoader) LoadCertificates() ([]Certificate, error) {
 				return nil
 			}
 
-			cert, err := x509CertFromCertAndKeyPEMFile(fpath)
+			bundle, err := os.ReadFile(fpath)
 			if err != nil {
 				return err
 			}
+			cert, err := tlsCertFromCertAndKeyPEMBundle(bundle)
+			if err != nil {
+				return fmt.Errorf("%s: %w", fpath, err)
+			}
 
 			certs = append(certs, Certificate{Certificate: cert})
 
@@ -77,12 +81,7 @@ func (fl FolderLoader) LoadCertificates() ([]Certificate, error) {
 	return certs, nil
 }
 
-func x509CertFromCertAndKeyPEMFile(fpath string) (tls.Certificate, error) {
-	bundle, err := os.ReadFile(fpath)
-	if err != nil {
-		return tls.Certificate{}, err
-	}
-
+func tlsCertFromCertAndKeyPEMBundle(bundle []byte) (tls.Certificate, error) {
 	certBuilder, keyBuilder := new(bytes.Buffer), new(bytes.Buffer)
 	var foundKey bool // use only the first key in the file
 
@@ -96,8 +95,7 @@ func x509CertFromCertAndKeyPEMFile(fpath string) (tls.Certificate, error) {
 
 		if derBlock.Type == "CERTIFICATE" {
 			// Re-encode certificate as PEM, appending to certificate chain
-			err = pem.Encode(certBuilder, derBlock)
-			if err != nil {
+			if err := pem.Encode(certBuilder, derBlock); err != nil {
 				return tls.Certificate{}, err
 			}
 		} else if derBlock.Type == "EC PARAMETERS" {
@@ -105,18 +103,16 @@ func x509CertFromCertAndKeyPEMFile(fpath string) (tls.Certificate, error) {
 			// parameters and key (parameter block should come first)
 			if !foundKey {
 				// Encode parameters
-				err = pem.Encode(keyBuilder, derBlock)
-				if err != nil {
+				if err := pem.Encode(keyBuilder, derBlock); err != nil {
 					return tls.Certificate{}, err
 				}
 
 				// Key must immediately follow
 				derBlock, bundle = pem.Decode(bundle)
 				if derBlock == nil || derBlock.Type != "EC PRIVATE KEY" {
-					return tls.Certificate{}, fmt.Errorf("%s: expected elliptic private key to immediately follow EC parameters", fpath)
+					return tls.Certificate{}, fmt.Errorf("expected elliptic private key to immediately follow EC parameters")
 				}
-				err = pem.Encode(keyBuilder, derBlock)
-				if err != nil {
+				if err := pem.Encode(keyBuilder, derBlock); err != nil {
 					return tls.Certificate{}, err
 				}
 				foundKey = true
@@ -124,28 +120,27 @@ func x509CertFromCertAndKeyPEMFile(fpath string) (tls.Certificate, error) {
 		} else if derBlock.Type == "PRIVATE KEY" || strings.HasSuffix(derBlock.Type, " PRIVATE KEY") {
 			// RSA key
 			if !foundKey {
-				err = pem.Encode(keyBuilder, derBlock)
-				if err != nil {
+				if err := pem.Encode(keyBuilder, derBlock); err != nil {
 					return tls.Certificate{}, err
 				}
 				foundKey = true
 			}
 		} else {
-			return tls.Certificate{}, fmt.Errorf("%s: unrecognized PEM block type: %s", fpath, derBlock.Type)
+			return tls.Certificate{}, fmt.Errorf("unrecognized PEM block type: %s", derBlock.Type)
 		}
 	}
 
 	certPEMBytes, keyPEMBytes := certBuilder.Bytes(), keyBuilder.Bytes()
 	if len(certPEMBytes) == 0 {
-		return tls.Certificate{}, fmt.Errorf("%s: failed to parse PEM data", fpath)
+		return tls.Certificate{}, fmt.Errorf("failed to parse PEM data")
 	}
 	if len(keyPEMBytes) == 0 {
-		return tls.Certificate{}, fmt.Errorf("%s: no private key block found", fpath)
+		return tls.Certificate{}, fmt.Errorf("no private key block found")
 	}
 
 	cert, err := tls.X509KeyPair(certPEMBytes, keyPEMBytes)
 	if err != nil {
-		return tls.Certificate{}, fmt.Errorf("%s: making X509 key pair: %v", fpath, err)
+		return tls.Certificate{}, fmt.Errorf("making X509 key pair: %v", err)
 	}
 
 	return cert, nil