2 files changed, 107 insertions, 19 deletions

diff --git a/modules/caddypki/acmeserver/acmeserver.go b/modules/caddypki/acmeserver/acmeserver.go
index 6ecdfdc..0f739ec 100644
--- a/modules/caddypki/acmeserver/acmeserver.go
+++ b/modules/caddypki/acmeserver/acmeserver.go
@@ -15,7 +15,10 @@
 package acmeserver
 
 import (
+	"context"
 	"fmt"
+	weakrand "math/rand"
+	"net"
 	"net/http"
 	"os"
 	"path/filepath"
@@ -28,7 +31,7 @@ import (
 	"github.com/caddyserver/caddy/v2/modules/caddypki"
 	"github.com/go-chi/chi"
 	"github.com/smallstep/certificates/acme"
-	acmeAPI "github.com/smallstep/certificates/acme/api"
+	"github.com/smallstep/certificates/acme/api"
 	acmeNoSQL "github.com/smallstep/certificates/acme/db/nosql"
 	"github.com/smallstep/certificates/authority"
 	"github.com/smallstep/certificates/authority/provisioner"
@@ -76,8 +79,26 @@ type Handler struct {
 	// changed or removed in the future.
 	SignWithRoot bool `json:"sign_with_root,omitempty"`
 
+	// The addresses of DNS resolvers to use when looking up
+	// the TXT records for solving DNS challenges.
+	// It accepts [network addresses](/docs/conventions#network-addresses)
+	// with port range of only 1. If the host is an IP address,
+	// it will be dialed directly to resolve the upstream server.
+	// If the host is not an IP address, the addresses are resolved
+	// using the [name resolution convention](https://golang.org/pkg/net/#hdr-Name_Resolution)
+	// of the Go standard library. If the array contains more
+	// than 1 resolver address, one is chosen at random.
+	Resolvers []string `json:"resolvers,omitempty"`
+
+	logger    *zap.Logger
+	resolvers []caddy.NetworkAddress
+	ctx       caddy.Context
+
+	acmeDB        acme.DB
+	acmeAuth      *authority.Authority
+	acmeClient    acme.Client
+	acmeLinker    acme.Linker
 	acmeEndpoints http.Handler
-	logger        *zap.Logger
 }
 
 // CaddyModule returns the Caddy module information.
@@ -90,7 +111,9 @@ func (Handler) CaddyModule() caddy.ModuleInfo {
 
 // Provision sets up the ACME server handler.
 func (ash *Handler) Provision(ctx caddy.Context) error {
+	ash.ctx = ctx
 	ash.logger = ctx.Logger()
+
 	// set some defaults
 	if ash.CA == "" {
 		ash.CA = caddypki.DefaultCAID
@@ -142,31 +165,30 @@ func (ash *Handler) Provision(ctx caddy.Context) error {
 		DB: database,
 	}
 
-	auth, err := ca.NewAuthority(authorityConfig)
+	ash.acmeAuth, err = ca.NewAuthority(authorityConfig)
 	if err != nil {
 		return err
 	}
 
-	var acmeDB acme.DB
-	if authorityConfig.DB != nil {
-		acmeDB, err = acmeNoSQL.New(auth.GetDatabase().(nosql.DB))
-		if err != nil {
-			return fmt.Errorf("configuring ACME DB: %v", err)
-		}
+	ash.acmeDB, err = acmeNoSQL.New(ash.acmeAuth.GetDatabase().(nosql.DB))
+	if err != nil {
+		return fmt.Errorf("configuring ACME DB: %v", err)
 	}
 
-	// create the router for the ACME endpoints
-	acmeRouterHandler := acmeAPI.NewHandler(acmeAPI.HandlerOptions{
-		CA:     auth,
-		DB:     acmeDB,                            // stores all the server state
-		DNS:    ash.Host,                          // used for directory links
-		Prefix: strings.Trim(ash.PathPrefix, "/"), // used for directory links
-	})
+	ash.acmeClient, err = ash.makeClient()
+	if err != nil {
+		return err
+	}
+
+	ash.acmeLinker = acme.NewLinker(
+		ash.Host,
+		strings.Trim(ash.PathPrefix, "/"),
+	)
 
 	// extract its http.Handler so we can use it directly
 	r := chi.NewRouter()
 	r.Route(ash.PathPrefix, func(r chi.Router) {
-		acmeRouterHandler.Route(r)
+		api.Route(r)
 	})
 	ash.acmeEndpoints = r
 
@@ -175,6 +197,16 @@ func (ash *Handler) Provision(ctx caddy.Context) error {
 
 func (ash Handler) ServeHTTP(w http.ResponseWriter, r *http.Request, next caddyhttp.Handler) error {
 	if strings.HasPrefix(r.URL.Path, ash.PathPrefix) {
+		acmeCtx := acme.NewContext(
+			r.Context(),
+			ash.acmeDB,
+			ash.acmeClient,
+			ash.acmeLinker,
+			nil,
+		)
+		acmeCtx = authority.NewContext(acmeCtx, ash.acmeAuth)
+		r = r.WithContext(acmeCtx)
+
 		ash.acmeEndpoints.ServeHTTP(w, r)
 		return nil
 	}
@@ -227,6 +259,55 @@ func (ash Handler) openDatabase() (*db.AuthDB, error) {
 	return database.(databaseCloser).DB, err
 }
 
+// makeClient creates an ACME client which will use a custom
+// resolver instead of net.DefaultResolver.
+func (ash Handler) makeClient() (acme.Client, error) {
+	for _, v := range ash.Resolvers {
+		addr, err := caddy.ParseNetworkAddressWithDefaults(v, "udp", 53)
+		if err != nil {
+			return nil, err
+		}
+		if addr.PortRangeSize() != 1 {
+			return nil, fmt.Errorf("resolver address must have exactly one address; cannot call %v", addr)
+		}
+		ash.resolvers = append(ash.resolvers, addr)
+	}
+
+	var resolver *net.Resolver
+	if len(ash.resolvers) != 0 {
+		dialer := &net.Dialer{
+			Timeout: 2 * time.Second,
+		}
+		resolver = &net.Resolver{
+			PreferGo: true,
+			Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
+				//nolint:gosec
+				addr := ash.resolvers[weakrand.Intn(len(ash.resolvers))]
+				return dialer.DialContext(ctx, addr.Network, addr.JoinHostPort(0))
+			},
+		}
+	} else {
+		resolver = net.DefaultResolver
+	}
+
+	return resolverClient{
+		Client:   acme.NewClient(),
+		resolver: resolver,
+		ctx:      ash.ctx,
+	}, nil
+}
+
+type resolverClient struct {
+	acme.Client
+
+	resolver *net.Resolver
+	ctx      context.Context
+}
+
+func (c resolverClient) LookupTxt(name string) ([]string, error) {
+	return c.resolver.LookupTXT(c.ctx, name)
+}
+
 const defaultPathPrefix = "/acme/"
 
 var keyCleaner = regexp.MustCompile(`[^\w.-_]`)
diff --git a/modules/caddypki/acmeserver/caddyfile.go b/modules/caddypki/acmeserver/caddyfile.go
index ae2d8ef..3b52113 100644
--- a/modules/caddypki/acmeserver/caddyfile.go
+++ b/modules/caddypki/acmeserver/caddyfile.go
@@ -29,8 +29,9 @@ func init() {
 // parseACMEServer sets up an ACME server handler from Caddyfile tokens.
 //
 //	acme_server [<matcher>] {
-//	    ca <id>
-//	    lifetime <duration>
+//		ca        <id>
+//		lifetime  <duration>
+//		resolvers <addresses...>
 //	}
 func parseACMEServer(h httpcaddyfile.Helper) ([]httpcaddyfile.ConfigValue, error) {
 	if !h.Next() {
@@ -74,6 +75,12 @@ func parseACMEServer(h httpcaddyfile.Helper) ([]httpcaddyfile.ConfigValue, error
 				}
 
 				acmeServer.Lifetime = caddy.Duration(dur)
+
+			case "resolvers":
+				acmeServer.Resolvers = h.RemainingArgs()
+				if len(acmeServer.Resolvers) == 0 {
+					return nil, h.Errf("must specify at least one resolver address")
+				}
 			}
 		}
 	}