implement the On-Demand TLS feature

README.md

@@ -89,6 +89,20 @@ applications. To enable this, add the `--tls` flag when deploying an instance:
     kamal-proxy deploy service1 --target web-1:3000 --host app1.example.com --tls
 
 
+### On-demand TLS
+
+In addition of the automatic TLS functionality, Kamal Proxy can also dynamically obtain a TLS certificate 
+from any host allowed by an external API endpoint of your choice.
+This avoids hard-coding hosts in the configuration, especially when you don't know the hosts at the startup.
+
+    kamal-proxy deploy service1 --target web-1:3000 --host "" --tls --tls-on-demand-url="http://localhost:4567/check"
+
+The On-demand URL endpoint will have to answer a 200 HTTP status code. 
+Kamal Proxy will call the on-demand URL with a query string of `?host=` containing the host received by Kamal Proxy.
+
+It also must respond as fast as possible, a couple of milliseconds top.
+
+
 ### Custom TLS certificate
 
 When you obtained your TLS certificate manually, manage your own certificate authority,

internal/cmd/deploy.go

@@ -30,6 +30,7 @@ func newDeployCommand() *deployCommand {
 	deployCommand.cmd.Flags().StringSliceVar(&deployCommand.args.Hosts, "host", []string{}, "Host(s) to serve this target on (empty for wildcard)")
 
 	deployCommand.cmd.Flags().BoolVar(&deployCommand.args.ServiceOptions.TLSEnabled, "tls", false, "Configure TLS for this target (requires a non-empty host)")
+	deployCommand.cmd.Flags().StringVar(&deployCommand.args.ServiceOptions.TLSOnDemandUrl, "tls-on-demand-url", "", "Will make an HTTP request to the given URL, asking whether a host is allowed to have a certificate issued.")
 	deployCommand.cmd.Flags().BoolVar(&deployCommand.tlsStaging, "tls-staging", false, "Use Let's Encrypt staging environment for certificate provisioning")
 	deployCommand.cmd.Flags().StringVar(&deployCommand.args.ServiceOptions.TLSCertificatePath, "tls-certificate-path", "", "Configure custom TLS certificate path (PEM format)")
 	deployCommand.cmd.Flags().StringVar(&deployCommand.args.ServiceOptions.TLSPrivateKeyPath, "tls-private-key-path", "", "Configure custom TLS private key path (PEM format)")

internal/server/service.go

@@ -1,13 +1,16 @@
 package server
 
 import (
+	"context"
 	"crypto/sha256"
 	"encoding/hex"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"log/slog"
 	"net"
 	"net/http"
+	"net/url"
 	"os"
 	"path"
 	"strings"
@@ -66,6 +69,7 @@ type HealthCheckConfig struct {
 
 type ServiceOptions struct {
 	TLSEnabled         bool   `json:"tls_enabled"`
+	TLSOnDemandUrl     string `json:"tls_on_demand_url"`
 	TLSCertificatePath string `json:"tls_certificate_path"`
 	TLSPrivateKeyPath  string `json:"tls_private_key_path"`
 	ACMEDirectory      string `json:"acme_directory"`
@@ -318,14 +322,49 @@ func (s *Service) createCertManager(hosts []string, options ServiceOptions) (Cer
 		}
 	}
 
+	hostPolicy, err := s.createAutoCertHostPolicy(hosts, options)
+	if err != nil {
+		return nil, err
+	}
+
 	return &autocert.Manager{
 		Prompt:     autocert.AcceptTOS,
 		Cache:      autocert.DirCache(options.ScopedCachePath()),
-		HostPolicy: autocert.HostWhitelist(hosts...),
+		HostPolicy: hostPolicy,
 		Client:     &acme.Client{DirectoryURL: options.ACMEDirectory},
 	}, nil
 }
 
+func (s *Service) createAutoCertHostPolicy(hosts []string, options ServiceOptions) (autocert.HostPolicy, error) {
+	if options.TLSOnDemandUrl == "" {
+		return autocert.HostWhitelist(hosts...), nil
+	}
+
+	_, err := url.ParseRequestURI(options.TLSOnDemandUrl)
+	if err != nil {
+		slog.Error("Unable to parse the tls_on_demand_url URL")
+		return nil, err
+	}
+
+	slog.Info("Will use the tls_on_demand_url URL", "url", options.TLSOnDemandUrl)
+
+	return func(ctx context.Context, host string) error {
+		slog.Info("Get a certificate", "host", host)
+
+		resp, err := http.Get(fmt.Sprintf("%s?host=%s", options.TLSOnDemandUrl, url.QueryEscape(host)))
+		if err != nil {
+			slog.Error("Unable to reach the TLS on demand URL", host, err)
+			return err
+		}
+
+		if resp.StatusCode != http.StatusOK {
+			return fmt.Errorf("%s is not allowed to get a certificate", host)
+		}
+
+		return nil
+	}, nil
+}
+
 func (s *Service) createMiddleware(options ServiceOptions, certManager CertManager) (http.Handler, error) {
 	var err error
 	var handler http.Handler = http.HandlerFunc(s.serviceRequestWithTarget)