certs: update tscert dialer to connect to tsnet server

This provides an alternate implementation for tscert.TailscaledDialer
that tries to find a tsnet server for the requested certificate.
This allows caddy-tailscale to be used with caddy's auto_https support.

Fixes #19

Signed-off-by: Will Norris <will@tailscale.com>

module.go

@@ -14,13 +14,18 @@ import (
 	"strings"
 
 	"github.com/caddyserver/caddy/v2"
+	"github.com/caddyserver/caddy/v2/modules/caddytls"
+	"github.com/caddyserver/certmagic"
+	"github.com/tailscale/tscert"
 	"go.uber.org/zap"
 	"tailscale.com/tsnet"
 )
 
 func init() {
 	caddy.RegisterNetwork("tailscale", getPlainListener)
 	caddy.RegisterNetwork("tailscale+tls", getTLSListener)
+
+	tscert.TailscaledDialer = localAPIDialer
 }
 
 func getPlainListener(c context.Context, _ string, addr string, _ net.ListenConfig) (any, error) {
@@ -262,3 +267,44 @@ func (t *tsnetServerListener) Close() error {
 	_, err := nodes.Delete(t.name)
 	return err
 }
+
+// localAPIDialer finds the node that matches the requested certificate in ctx
+// and dials that node's local API.
+// If not match node is found, the default dailer is used,
+// which tries to connect to a local tailscaled on the machine.
+func localAPIDialer(ctx context.Context, network, addr string) (net.Conn, error) {
+	if addr != "local-tailscaled.sock:80" {
+		return nil, fmt.Errorf("unexpected URL address %q", addr)
+	}
+
+	serverName, ok := ctx.Value(caddytls.SNIServerNameCtxKey).(string)
+	if !ok {
+		return tscert.DefaultDialer(ctx, network, addr)
+	}
+
+	var tn *tailscaleNode
+	nodes.Range(func(key, value any) bool {
+		n, ok := value.(*tailscaleNode)
+		if !ok || n == nil {
+			return true
+		}
+		for _, d := range n.CertDomains() {
+			// MatchWildcard is not really necessary since Tailscale doesn't do wildcard certs,
+			// but caddy uses it for the built-in Tailscale CertGetter, so we do so here as well.
+			if certmagic.MatchWildcard(serverName, d) {
+				tn = n
+				return false
+			}
+		}
+
+		return true
+	})
+
+	if tn != nil {
+		if lc, err := tn.LocalClient(); err == nil {
+			return lc.Dial(ctx, network, addr)
+		}
+	}
+
+	return tscert.DefaultDialer(ctx, network, addr)
+}