Improve proxy address sourcing for VM auto-discovery (gravitational#3…

…0884)

This change adds a few more places for the Discovery service to get
the Teleport cluster's public proxy address for VM discovery.

lib/config/configuration.go

@@ -1277,16 +1277,23 @@ func applySSHConfig(fc *FileConfig, cfg *servicecfg.Config) (err error) {
 // getInstallerProxyAddr determines the address of the proxy for discovered
 // nodes to connect to.
 func getInstallerProxyAddr(installParams *InstallParams, fc *FileConfig) string {
+	// Explicit proxy address.
 	if installParams != nil && installParams.PublicProxyAddr != "" {
 		return installParams.PublicProxyAddr
 	}
+	// Proxy address from config.
 	if fc.ProxyServer != "" {
 		return fc.ProxyServer
 	}
 	if fc.Proxy.Enabled() && len(fc.Proxy.PublicAddr) > 0 {
 		return fc.Proxy.PublicAddr[0]
 	}
-	return ""
+	// Possible proxy address for v1/v2 config.
+	if len(fc.AuthServers) > 0 {
+		return fc.AuthServers[0]
+	}
+	// Probably not a proxy address, but we have nothing better.
+	return fc.AuthServer
 }
 
 func applyDiscoveryConfig(fc *FileConfig, cfg *servicecfg.Config) error {

lib/config/configuration_test.go

@@ -4117,3 +4117,79 @@ func TestApplyKubeConfig(t *testing.T) {
 		})
 	}
 }
+
+func TestGetInstallerProxyAddr(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name              string
+		installParams     *InstallParams
+		fc                *FileConfig
+		expectedProxyAddr string
+	}{
+		{
+			name:              "empty",
+			fc:                &FileConfig{},
+			expectedProxyAddr: "",
+		},
+		{
+			name: "explicit proxy addr",
+			installParams: &InstallParams{
+				PublicProxyAddr: "explicit.example.com",
+			},
+			fc: &FileConfig{
+				Global: Global{
+					ProxyServer: "proxy.example.com",
+				},
+			},
+			expectedProxyAddr: "explicit.example.com",
+		},
+		{
+			name: "proxy server",
+			fc: &FileConfig{
+				Global: Global{
+					ProxyServer: "proxy.example.com",
+				},
+			},
+			expectedProxyAddr: "proxy.example.com",
+		},
+		{
+			name: "local proxy service",
+			fc: &FileConfig{
+				Global: Global{
+					AuthServer: "auth.example.com",
+				},
+				Proxy: Proxy{
+					Service: Service{
+						EnabledFlag: "yes",
+					},
+					PublicAddr: apiutils.Strings{"proxy.example.com"},
+				},
+			},
+			expectedProxyAddr: "proxy.example.com",
+		},
+		{
+			name: "v1/v2 auth servers",
+			fc: &FileConfig{
+				Version: "v2",
+				Global: Global{
+					AuthServers: []string{"proxy.example.com"},
+				},
+			},
+			expectedProxyAddr: "proxy.example.com",
+		},
+		{
+			name: "auth server",
+			fc: &FileConfig{
+				Global: Global{
+					AuthServer: "auth.example.com",
+				},
+			},
+			expectedProxyAddr: "auth.example.com",
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			assert.Equal(t, tc.expectedProxyAddr, getInstallerProxyAddr(tc.installParams, tc.fc))
+		})
+	}
+}