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Clixon FAQ

What is Clixon?

Clixon is a YANG-based configuration manager, with interactive CLI, NETCONF and RESTCONF interfaces, an embedded database and transaction support.

Why should I use Clixon?

If you want an easy-to-use configuration toolkit based on yang with an open-source license. Typically for embedded devices requiring a config interface such as routers and switches.

What license is available?

Clixon is dual license. Either Apache License, Version 2.0 or GNU General Public License Version 2.

Is Clixon extendible?

Yes. All application semantics is defined in plugins with well-defined APIs. There are currently plugins for: CLI, Netconf, Restconf, the datastore and the backend.

Which programming language is used?

Clixon is written in C. The plugins are written in C. The CLI specification uses cligen (

How to best understand Clixon?

Run the Clixon example, in the example directory.

How do you build and install Clixon (and the example)?


	sudo make install; 
	sudo make install-include

The example:

	 cd example; 
	 sudo make install

How do I run Clixon example commands?

  • Start a backend server: clixon_backend -F -s init -f /usr/local/etc/example.xml
  • Start a cli session: clixon_cli -f /usr/local/etc/example.xml
  • Start a netconf session: clixon_netconf -f /usr/local/etc/example.xml
  • Start a restconf daemon: sudo su -c "/www-data/clixon_restconf -f /usr/local/etc/example.xml " -s /bin/sh www-data
  • Send a restconf command: curl -G

More info in the example directory.

Do I need to setup anything? (IMPORTANT)

The config demon requires a valid group to create a server UNIX domain socket. Define a valid CLICON_SOCK_GROUP in the config file or via the -g option or create the group and add the user to it. The default group is 'clicon'. Add yourself and www-data, if you intend to use restconf.

Using groupadd and usermod:

  sudo groupadd clicon # 
  sudo usermod -a -G clicon <user>
  sudo usermod -a -G clicon www-data

Using addgroup and adduser (eg on busybox):

  sudo addgroup clicon
  sudo adduser <user> clicon

(you may have to restart shell)


grep clicon /etc/group

How do I use the CLI?

The easiest way to use Clixon is via the CLI. Once the backend is started Example:

clixon_cli -f /usr/local/etc/example.xml 
cli> set interfaces interface eth9 ?
 description               enabled                   ipv4                     
 ipv6                      link-up-down-trap-enable  type                     
cli> set interfaces interface eth9 type ex:eth
cli> validate 
cli> commit 
cli> show configuration xml 
<interfaces xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces">
cli> delete interfaces interface eth9

How do I use netconf?

As an alternative to cli configuration, you can use netconf. Easiest is to just pipe netconf commands to the clixon_netconf application. Example:

clixon_netconf -qf /usr/local/etc/example.xml
<rpc-reply><data><interfaces xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces"><interface><name>eth9</name><type>ex:eth</type><enabled>true</enabled></interface></interfaces></data></rpc-reply>]]>]]>

However, more useful is to run clixon_netconf as an SSH subsystem. Register the subsystem in /etc/sshd_config:

	Subsystem netconf /usr/local/bin/clixon_netconf -f /usr/local/etc/example.xml

and then invoke it from a client using

	ssh -s <host> netconf

How do I use restconf?

You can access clixon via REST API using restconf, such as using curl. GET, PUT, POST are supported.

You need a web-server, such as nginx, and start a restconf fcgi daemon, clixon_restconf.

For example, using nginx, install, and edit config file: /etc/nginx/sites-available/default:

server {
  location /restconf {
    fastcgi_pass unix:/www-data/fastcgi_restconf.sock;
    include fastcgi_params;

Start nginx daemon

sudo /etc/init.d/nginx start

Start the clixon restconf daemon

sudo su -c "/www-data/clixon_restconf -f /usr/local/etc/example.xml " -s /bin/sh www-data

Then acess:

   curl -G
       "ietf-interfaces:type": "ex:eth" 

Read more in the restconf docs.

What about reference documentation?

Clixon uses Doxygen for reference documentation. You need to install doxygen and graphviz on your system. Build it in the doc directory and point the browser to .../clixon/doc/html/index.html as follows:

> cd doc
> make doc
> make graphs # detailed callgraphs

How is configuration data stored?

Configuration data is stored in an XML datastore. In the example the datastore are regular files found in /usr/local/var/example/.

What is validate and commit?

Clixon follows netconf in its validate and commit semantics. In short, you edit a 'candidate' configuration, which is first 'validated' for consistency and then 'committed' to the 'running' configuration.

A clixon developer writes commit functions to incrementaly upgrade a system state based on configuration changes. Writing commit callbacks is the core functionality of a clixon system.

What is a Clixon configuration file?

Clixon options are stored in an XML configuration file. The default configuration file is /usr/local/etc/clixon.xml. The example configuration file is installed at /usr/local/etc/example.xml. The YANG specification for the configuration file is clixon-config.yang.

How are Clixon configuration files found?

Clixon by default finds its configuration file at /usr/local/etc/clixon.xml. However, you can modify this location as follows:

  • Provide -f FILE option when starting a program (eg clixon_backend -f FILE)
  • Provide --with-configfile=FILE when configuring
  • Provide --with-sysconfig= when configuring. Then FILE is /clixon.xml
  • Provide --sysconfig= when configuring. Then FILE is /etc/clixon.xml
  • FILE is /usr/local/etc/clixon.xml

Can I modify clixon options at runtime?

Yes, when you start a clixon program, you can supply the -o option to modify the configuration specified in the configuration file. Options that are leafs are overriden, whereas options that are leaf-lists are added to.

Example, add the "usr/local/share/ietf" directory to the list of directories where yang files are searched for:

  clixon_cli -o CLICON_YANG_DIR=/usr/local/share/ietf

How are Yang files found?

Yang files contain the configuration specification. A Clixon application loads yang files and clixon itself loads system yang files. When Yang files are loaded modules are imported and submodules are included.

The following configuration file options control the loading of Yang files:

  • CLICON_YANG_DIR - A list of directories (yang dir path) where Clixon searches for module and submodules.
  • CLICON_YANG_MAIN_FILE - Load a specific Yang module given by a file.
  • CLICON_YANG_MODULE_MAIN - Specifies a single module to load. The module is searched for in the yang dir path.
  • CLICON_YANG_MODULE_REVISION : Specifies a revision to the main module.
  • CLICON_YANG_MAIN_DIR - Load all yang modules in this directory.

Note that the special CLIXON_DATADIR, by default /usr/local/share/clixon should be included in the yang dir path for Clixon system files to be found.

You can combine the options, however, if there are different variants of the same module, more specific options override less specific. The precedence of the options are as follows:


How do I enable Yang features?

Yang models have features, and parts of a specification can be conditional using the if-feature statement. In Clixon, features are enabled in the configuration file using <CLICON_FEATURE>.

The example below shows enabling a specific feature; enabling all features in module; and enabling all features in all modules, respectively:


Features can be probed by using RFC 7895 Yang module library which provides information on all modules and which features are enabled.

Can I run Clixon as a container?

Yes, Clixon has two examples on how to build docker containers. A base image and a complete example system.

The base image can only be used as a boilerplate for building clixon applications (it has no applications semantics); whereas the system is a complete example applications with CLI/Netconf/Restconf, and testing.

For example, the clixon-system container can be used as follows:

  • CLI: sudo docker exec -it clixon-system clixon_cli
  • Netconf: sudo docker exec -it clixon-system clixon_netconf
  • Restconf: curl -G http://localhost/restconf
  • Run tests: sudo docker exec -it clixon-system bash -c 'cd /clixon/clixon/test; ./'

See ../docker for more info.

Does Clixon support event streams?

Yes, Clixon supports event notification streams in the CLI, Netconf and Restconf API:s.

The example has a prebuilt notification stream called "EXAMPLE" that triggers every 5s. You enable the notification via the CLI:

cli> notify 
event-class fault;
reportingEntity {
    card Ethernet0;
severity major;

or via NETCONF:

clixon_netconf -qf /usr/local/etc/example.xml 
<rpc><create-subscription xmlns="urn:ietf:params:xml:ns:netmod:notification"><stream>EXAMPLE</stream></create-subscription></rpc>]]>]]>
<notification xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0"><eventTime>2018-09-30T12:44:59.657276</eventTime><event xmlns=""><event-class>fault</event-class><reportingEntity><card>Ethernet0</card></reportingEntity><severity>major</severity></event></notification>]]>]]>

or via restconf:

   curl -H "Accept: text/event-stream" -s -X GET http://localhost/streams/EXAMPLE

Consult clixon restconf on more information on how to setup a reverse proxy for restconf streams. It is also possible to configure a pub/sub system such as Nginx Nchan.

How should I start the backend daemon?

There are four different backend startup modes. There is differences in running state treatment, ie what state the machine is when you start the daemon and how loading the configuration affects it:

  • none - Do not touch running state. Typically after crash when running state and db are synched.
  • init - Initialize running state. Start with a completely clean running state.
  • running - Commit running db configuration into running state. Typically after reboot if a persistent running db exists.
  • startup - Commit startup configuration into running state. After reboot when no persistent running db exists.

You use the -s to select the mode:

clixon_backend ... -s running

You may also add a default method in the configuration file:


Can I use systemd with Clixon?

Yes. Systemd example files are provide for the backend and the restconf daemon as part of the example.

How can I add extra XML?

There are two ways to add extra XML to running database after start. Note that this XML is not "committed" into running.

The first way is via a file. Assume you want to add this xml (the config tag is a necessary top-level tag):

   <x xmlns="urn:example:clixon">extra</x>

You add this via the -c option:

clixon_backend ... -c extra.xml

The second way is by programming the plugin_reset() in the backend plugin. The example code contains an example on how to do this (see plugin_reset() in example_backend.c).

I want to program. How do I extend the example?

See ../apps/example

  • example.xml - Change the configuration file
  • The yang specifications - This is the central part. It changes the XML, database and the config cli.
  • example_cli.cli - Change the fixed part of the CLI commands
  • example_cli.c - Cli C-commands are placed here.
  • example_backend.c - Commit and validate functions.
  • example_backend_nacm.c - Secondary example plugin (for authorization)
  • example_netconf.c - Netconf plugin
  • example_restconf.c - Add restconf authentication, etc.

How is a plugin initiated?

Each plugin is initiated with an API struct followed by a plugin init function as follows:

   static clixon_plugin_api api = {
      "example",          /* name */
      ... /* more functions here */
   clixon_plugin_api *
   clixon_plugin_init(clicon_handle h)
      return &api; /* Return NULL on error */

For more info see [../example/]

How do I write a commit function?

In the example, you write a commit function in example_backend.c. Every time a commit is made, transaction_commit() is called in the backend. It has a 'transaction_data td' argument which is used to fetch information on added, deleted and changed entries. You access this information using access functions as defined in clixon_backend_transaction.h

How do I check what has changed on commit?

You use XPATHs on the XML trees in the transaction commit callback. Suppose you want to print all added interfaces:

	cxobj *target = transaction_target(td); # wanted XML tree
	vec = xpath_vec_flag(target, "//interface", &len, XML_FLAG_ADD); /* Get added i/fs */
	for (i=0; i<len; i++)             /* Loop over added i/fs */
	  clicon_xml2file(stdout, vec[i], 0, 1); /* Print the added interface */

You can look for added, deleted and changed entries in this way.

How do I access the XML tree?

Using XPATH, find and iteration functions defined in the XML library. Example library functions:


More are found in the doxygen reference.

How do I write a CLI callback function?

  1. You add an entry in example_cli.cli

example("This is a comment") var:int32("This is a variable"), mycallback("myarg");

  1. Then define a function in example_cli.c

mycallback(clicon_handle h, cvec *cvv, cvec *arv) where 'cvv' contains the value of the variable 'var' and 'argv' contains the string "myarg".

The 'cvv' datatype is a 'CLIgen variable vector'. They are documented in CLIgen tutorial

How do I write a validation function?

Similar to a commit function, but instead write the transaction_validate() function. Check for inconsistencies in the XML trees and if they fail, make an clicon_err() call.

    clicon_err(OE_PLUGIN, 0, "Route %s lacks ipv4 addr", name);
    return -1;

The validation or commit will then be aborted.

How do I write a state data callback function?

Netconf and restconf GET also returns state data, in contrast to config data. In YANG state data is specified with "config false;".

To return state data, you need to write a backend state data callback with the name "plugin_statedata()" where you return an XML tree.

Please look at the example for an example on how to write a state data callback.

How do I write an RPC function?

A YANG RPC is an application specific operation. Example:

   rpc example-rpc {
      input {
         leaf inarg { type string; }
      output {
         leaf outarg { type string; }

which defines the example-rpc operation present in the example (the arguments have been changed).

Clixon automatically relays the RPC to the clixon backend. To implement the RFC, you need to register an RPC callback in the backend plugin: Example:

clixon_plugin_init(clicon_handle h)
   rpc_callback_register(h, example_rpc, NULL, "example-rpc");

And then define the callback itself:

static int 
example_rpc(clicon_handle h,            /* Clicon handle */
	    cxobj        *xe,           /* Request: <rpc><xn></rpc> */
	    cbuf         *cbret,        /* Reply eg <rpc-reply>... */
	    void         *arg,          /* Client session */
	    void         *regarg)       /* Argument given at register */
    cprintf(cbret, "<rpc-reply><ok/></rpc-reply>");    
    return 0;

Here, the callback is over-simplified.

How do I write an authentication callback?

A restconf call may need to be authenticated. You can specify an authentication callback for restconf as follows:

plugin_credentials(clicon_handle h,     
		   void         *arg)
    FCGX_Request *r = (FCGX_Request *)arg;
    clicon_username_set(h, user);

To authenticate, the callback needs to return the value 1 and supply a username.

See [../apps/example/example_restconf.c] example_restconf_credentials() for an example of HTTP basic auth.

What about access control?

Clixon has experimental support of the Network Configuration Access Control Model defined in RFC8341

Incoming RPC and data node access points are supported with some limitations. See the (README)(../ for more information.

How do I write a CLI translator function?

The CLI can perform variable translation. This is useful if you want to process the input, such as hashing, encrypting or in other way translate the input.

Yang example:

list translate{
    leaf value{
        type string;

CLI specification:

translate value (<value:string translate:incstr()>),cli_set("/translate/value");

If you run this example using the incstr() function which increments the characters in the input, you get this result:

cli> translate value HAL
cli> show configuration
translate {
    value IBM;

You can perform translation on any type, not only strings.