cmd/{k8s-operator,k8s-proxy}: add kube-apiserver ProxyGroup type (#16266)

Adds a new k8s-proxy command to convert operator's in-process proxy to
a separately deployable type of ProxyGroup: kube-apiserver. k8s-proxy
reads in a new config file written by the operator, modelled on tailscaled's
conffile but with some modifications to ensure multiple versions of the
config can co-exist within a file. This should make it much easier to
support reading that config file from a Kube Secret with a stable file name.

To avoid needing to give the operator ClusterRole{,Binding} permissions,
the helm chart now optionally deploys a new static ServiceAccount for
the API Server proxy to use if in auth mode.

Proxies deployed by kube-apiserver ProxyGroups currently work the same as
the operator's in-process proxy. They do not yet leverage Tailscale Services
for presenting a single HA DNS name.

Updates #13358

Change-Id: Ib6ead69b2173c5e1929f3c13fb48a9a5362195d8
Signed-off-by: Tom Proctor <tomhjp@users.noreply.github.com>

cmd/k8s-operator/proxy.go

@@ -0,0 +1,61 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !plan9
+
+package main
+
+import (
+	"fmt"
+	"log"
+	"os"
+)
+
+type apiServerProxyMode int
+
+func (a apiServerProxyMode) String() string {
+	switch a {
+	case apiServerProxyModeDisabled:
+		return "disabled"
+	case apiServerProxyModeEnabled:
+		return "auth"
+	case apiServerProxyModeNoAuth:
+		return "noauth"
+	default:
+		return "unknown"
+	}
+}
+
+const (
+	apiServerProxyModeDisabled apiServerProxyMode = iota
+	apiServerProxyModeEnabled
+	apiServerProxyModeNoAuth
+)
+
+func parseAPIProxyMode() apiServerProxyMode {
+	haveAuthProxyEnv := os.Getenv("AUTH_PROXY") != ""
+	haveAPIProxyEnv := os.Getenv("APISERVER_PROXY") != ""
+	switch {
+	case haveAPIProxyEnv && haveAuthProxyEnv:
+		log.Fatal("AUTH_PROXY (deprecated) and APISERVER_PROXY are mutually exclusive, please unset AUTH_PROXY")
+	case haveAuthProxyEnv:
+		var authProxyEnv = defaultBool("AUTH_PROXY", false) // deprecated
+		if authProxyEnv {
+			return apiServerProxyModeEnabled
+		}
+		return apiServerProxyModeDisabled
+	case haveAPIProxyEnv:
+		var apiProxyEnv = defaultEnv("APISERVER_PROXY", "") // true, false or "noauth"
+		switch apiProxyEnv {
+		case "true":
+			return apiServerProxyModeEnabled
+		case "false", "":
+			return apiServerProxyModeDisabled
+		case "noauth":
+			return apiServerProxyModeNoAuth
+		default:
+			panic(fmt.Sprintf("unknown APISERVER_PROXY value %q", apiProxyEnv))
+		}
+	}
+	return apiServerProxyModeDisabled
+}