Auth: add authentication via env variable

This commit provides a "cloud-native" way of authenticating clients
by comparing the provided password to the MOSQUITTO_PASSWORD environment
variable.

This allows to quickly and easily adding authentication by simply
providing the password to the environment variable of the docker
container / kubernetes pod.

Signed-off-by: Frank Villaro-Dixon <frank@vi-di.fr>

plugins/CMakeLists.txt

@@ -1,3 +1,4 @@
+add_subdirectory(auth-by-env)
 add_subdirectory(dynamic-security)
 add_subdirectory(persist-sqlite)
 add_subdirectory(examples)

plugins/auth-by-env/CMakeLists.txt

@@ -0,0 +1,11 @@
+include_directories(${mosquitto_SOURCE_DIR} ${mosquitto_SOURCE_DIR}/include
+			${OPENSSL_INCLUDE_DIR} ${STDBOOL_H_PATH} ${STDINT_H_PATH})
+
+add_library(mosquitto_auth_by_env MODULE mosquitto_auth_by_env.c)
+set_target_properties(mosquitto_auth_by_env PROPERTIES
+	POSITION_INDEPENDENT_CODE 1
+)
+set_target_properties(mosquitto_auth_by_env PROPERTIES PREFIX "")
+
+# Don't install, these are example plugins only. XXX
+#install(TARGETS mosquitto_auth_by_env RUNTIME DESTINATION "${CMAKE_INSTALL_BINDIR}" LIBRARY DESTINATION "${CMAKE_INSTALL_LIBDIR}")

plugins/auth-by-env/mosquitto_auth_by_env.c

@@ -0,0 +1,104 @@
+/*
+Copyright (c) 2021 Frank Villaro-Dixon <frank@villaro-dixon.eu>
+
+This plugin is under the WTFPL. Do what you want with it.
+
+SPDX-License-Identifier: WTFPL
+
+Contributors:
+   Frank Villaro-Dixon - initial implementation and documentation.
+*/
+
+/*
+ * This plugin allows users to authenticate with any username, as long as
+ * the provided password matches the MOSQUITTO_PASSWORD environment variable.
+ * If the MOSQUITTO_PASSWORD env variable is empty, then authentication is rejected.
+ *
+ * Compile with:
+ *   gcc -I<path to mosquitto-repo/include> -fPIC -shared mosquitto_auth_by_env.c -o mosquitto_auth_by_env.so
+ *
+ * Use in config with:
+ *
+ *   plugin /path/to/mosquitto_auth_by_env.so
+ *
+ * Note that this only works on Mosquitto 2.0 or later.
+ */
+#include "config.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "mosquitto_broker.h"
+#include "mosquitto_plugin.h"
+#include "mosquitto.h"
+#include "mqtt_protocol.h"
+
+#define ENV_MOSQUITTO_PASSWORD "MOSQUITTO_PASSWORD"
+
+static mosquitto_plugin_id_t *mosq_pid = NULL;
+static char *environment_password;
+
+static int basic_auth_callback(int event, void *event_data, void *userdata)
+{
+	struct mosquitto_evt_basic_auth *ed = event_data;
+
+	UNUSED(event);
+	UNUSED(userdata);
+
+	if(!environment_password || !ed->password){
+		return MOSQ_ERR_PLUGIN_DEFER;
+	}
+	if(!strcmp(ed->password, environment_password)){
+		/* Password matched MOSQUITTO_PASSWORD */
+		return MOSQ_ERR_SUCCESS;
+	}
+	else{
+		return MOSQ_ERR_PLUGIN_DEFER;
+	}
+}
+
+int mosquitto_plugin_version(int supported_version_count, const int *supported_versions)
+{
+	int i;
+
+	for(i=0; i<supported_version_count; i++){
+		if(supported_versions[i] == 5){
+			return 5;
+		}
+	}
+	return -1;
+}
+
+int mosquitto_plugin_init(mosquitto_plugin_id_t *identifier, void **user_data, struct mosquitto_opt *opts, int opt_count)
+{
+	UNUSED(user_data);
+	UNUSED(opts);
+	UNUSED(opt_count);
+
+	char *env_var_content;
+
+	mosq_pid = identifier;
+
+	env_var_content = getenv(ENV_MOSQUITTO_PASSWORD);
+	if(env_var_content){
+		if(strlen(env_var_content) > 0){
+			environment_password = strdup(env_var_content);
+			return mosquitto_callback_register(mosq_pid, MOSQ_EVT_BASIC_AUTH, basic_auth_callback, NULL, NULL);
+		}
+	}
+
+	log__printf(NULL, MOSQ_LOG_INFO, "Auth-by-env plugin called, but "ENV_MOSQUITTO_PASSWORD" env var is empty\n");
+	return 0;
+}
+
+int mosquitto_plugin_cleanup(void *user_data, struct mosquitto_opt *opts, int opt_count)
+{
+	UNUSED(user_data);
+	UNUSED(opts);
+	UNUSED(opt_count);
+
+	free(environment_password);
+
+	return mosquitto_callback_unregister(mosq_pid, MOSQ_EVT_BASIC_AUTH, basic_auth_callback, NULL);
+}