The backend daemon is the central Clixon component. It consists of a main module and a number of dynamically loaded plugins. The backend has four APIs:

An XML file read at startup, possibly amended with -o options.
Internal interface
A NETCONF socket to frontend clients. This is by default a UNIX domain socket but can also be an IPv4 or IPv6 TCP socket.
XML files storing configuration. The three main datastores are candidate, running and startup. A user edits the candidate datastore, commits the changes to running which triggers callbacks in the plugins.
Backend plugins configure the underlying system with application-specific APIs. These API:s depend on how the underlying system is configured, examples include configuration files or a socket, for example.

Note that a user typically does not access the datastores directly, it is possible to read, but write operations should not be done, since the backend daemon uses a datastore cache.

Command-line options

The backend have the following command-line options:
-h Help
-D <level> Debug level
-f <file> CLICON config file
-E <dir> Extra configuration directory
-l <option> Log on (s)yslog, std(e)rr, std(o)ut or (f)ile. Syslog is default. If foreground, then syslog and stderr is default. Filename is given after -f as follows: -lf<file>.
-d <dir> Specify backend plugin directory (default: none)
-p <dir> Yang directory path (see CLICON_YANG_DIR)
-b <dir> Specify XMLDB database directory
-F Run in foreground, do not run as daemon
-z Kill other config daemon and exit
-a <family> Internal backend socket family: UNIX|IPv4|IPv6
-u <path|addr> Internal socket domain path or IP addr (see -a)(default: /usr/var/hello.sock)
-P <file> PID filename (default: /usr/local/var/hello.pidfile)
-1 Run once and then quit (do not wait for events)
-s <mode> Specify backend startup mode: none|startup|running|init)
-c <file> Load extra xml configuration, but don’t commit.
-g <group> Client membership required to this group (default: clicon)
-U <user> Run backend daemon as this user AND drop privileges permanently
-y <file> Load yang spec file (override yang main module)
-o <option=value>
 Give configuration option overriding config file (see clixon-config.yang)

Logging and debugging

At debug, the backend can be run in the foreground and with debug flags:

clixon_backend -FD 1

Logging is done on syslog. Alternatively, logging can be made on a file using the -l option:

clixon_backend -lf<file>

When run in foreground, logging is by default done on both syslog and stderr.

In a debugging mode, it can be useful to run in once-only mode, where the backend quits directly after starting up, instead of waiting for events:

clixon_backend -F1D 1


There are four different backend startup modes selected by the -s option. The difference is in how the running state is handled, ie what state the machine is in when you start the daemon and how loading the configuration affects it:

Do not touch running state. Typically after crash when running state and db are synched.
Initialize running state. Start with a completely clean running state.
Commit running db configuration into running state. Typically after reboot if a persistent running db exists.
Commit startup configuration into running state. After reboot when no persistent running db exists.

You use the -s to select startup mode:

clixon_backend -s running

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

<clixon-config xmlns="">

When loading the startup/tmp configuration, the following actions are performed by the system:

  • Check syntax errors,
  • Upgrade callbacks.
  • Validation of the XML against the current Yang models
  • If errors are detected, enter failsafe mode.

The following config option is related to startup:

Enable process-control of restconf daemon, ie start/stop restconf daemon internally using fork/exec. Disable if you start the restconf daemon by other means.


Frontends:   socket  +----------+
CLI,           |     | backend  |
netconf      <--->   | daemon   |
restconf             |          |

The Clixon backend creates a socket that the frontends can connect to. Communication is made over this socket using internal Netconf. The following config options are related to the internal socket:

Address family for communicating with clixon_backend. One of: UNIX, IPv4, or IPv6. Can also be set with -a command-line option. Default is UNIX which denotes a UNIX socket.
If family above is AF_UNIX: Unix socket for communicating with clixon_backend. If family is AF_INET: IPv4 address”;
Inet socket port for communicating with clixon_backend (only IPv4|IPv6). Default is port 4535.
Group membership to access clixon_backend unix socket. Default is clicon.

Backend files

A couple of config options control files related to the backend, as follows:

Location of backend .so plugins. Load all .so plugins in this dir as backend plugins
Regexp of matching backend plugins in CLICON_BACKEND_DIR. default: *.so
Process-id file of backend daemon

Backend plugins

Backend plugins are the “glue” that binds the Clixon system to the underlying system. The backend invokes callbacks in the plugins when events.

Plugins are added by the application developer and adds “semantics” to a Clixon system. The CLI, Restconf and Netconf applications also use callbacks.

Plugins are written in C as dynamically loaded modules (.so files). At startup, the backend daemon looks in the directory pointed to by the config option CLICON_BACKEND_DIR, and loads all files with .so suffixes from that dir in alphabetical order.

For example, to load all backend plugins from: /usr/local/lib/example/backend:


You can filter which plugins to load by specifying a regular expression. For example, the following will only load backend plugins starting with “example”:

Callbacks are registered in two ways:
  • clixon_plugin_init : returning an API struct containing a fixed set of callbacks
  • register functions : Some specific clixon features use registering functions instead. This allows for multiple callbacks.


A plugin must have a init function called clixon_plugin_init. If this function does not exist, the backend will fail.

The backend calls clixon_plugin_init and expects it to return an API struct (clixon_plugin_api) defining all callbacks. The init function may return NULL in which case the backend logs this and continues.

Once the plugin is loaded, it awaits callbacks from the backend.

The following callbacks are defined for backend plugins in clixon_plugin_api:

Clixon plugin init function, called immediately after plugin is loaded into the backend. The name of the function must be called clixon_plugin_init. It returns a struct with the name of the plugin, and all other callback names.
Called when application is started and initialization is complete, but before the application is placed in the background and drop privileges (see dropping privileges), if those operations are requested.
Called just before server daemonizes(forks). Not called if in foreground.
Called after the server has daemonized and before privileges are dropped.
Called just before plugin is unloaded
Called at parsing of yang modules containing an extension statement. A plugin may identify the extension by its name, and perform actions on the yang statement, such as transforming the yang in-memory. A callback is made for every statement, which means that several calls per extension can be made.
Provide state data XML from a plugin
Reset system status
General-purpose upgrade called once when loading the startup datastore
Transaction callbacks which are invoked for two reasons: validation requests or commits. These callbacks are further described in transactions section.

Registered callbacks

A second group of callbacks use register functions. This is a more detailed mechanism than the fixed callbacks described previously, but are only defined to a limited sets of functions:

  • rpc_callback_register() - for user-defined RPC callbacks.
  • upgrade_callback_register() - for upgrading, see Upgrade.
  • clixon_pagination_cb_register() - for pagination, as described in Pagination.

A user may register may register a callback for an incoming RPC, and that function will be called.

There may be several callbacks for the same RPC. The order the callbacks are registered are as follows:

  1. plugin_init
  2. backend_rpc_init (where system callbacks are registered)
  3. plugin_start

Which means if you register a copy-config callback in (1), it will be called before the system copy-config callback registered from (2) backend_rpc_init. If you register a copy-config in (3) plugin-start it will be called after the system copy-config.

Second, if there are more than one reply (eg <rpc-reply/><rpc-reply/>) only the first reply will be parsed and used by the cli/netconf/restconf clients.

If you want to take the original and modify it, you should therefore register the callback in plugin_start (3) so that your callback will be called after the system RPC. Then you should modify the original reply (not add a new reply).

Example: RPC callback

This example shows how to define a new RPC in YANG, register a callback function in C, read and write a parameter. It is revised slightly from the main example.


module clixon-example {
  namespace "urn:example:clixon";
  rpc example {
      input {
          leaf x {
            type string;
      output {
          leaf y {
            type string;

Register RPC in clixon_plugin_init():

clixon_plugin_api *clixon_plugin_init(clicon_handle h)
   rpc_callback_register(h, example_rpc, NULL, "urn:example:clixon", "example");

Callback function reading value input x, modifying value and writing it as output value y:

static int
example_rpc(clicon_handle h,            /* Clicon handle */
            cxobj        *xe,           /* Request: <rpc><xn></rpc> */
            cbuf         *cbret,        /* Reply eg <rpc-reply>... */
            void         *arg,          /* client_entry */
            void         *regarg)       /* Argument given at register */
    char *val;
    val = xml_find_body(xe, "x");       /* Read x value of incoming rpc */
    cprintf(cbret, "<rpc-reply xmlns=\"%s\">", NETCONF_BASE_NAMESPACE);
    val[0]++;                           /* Increment first char */
    /* Construct reply */
    cprintf(cbret, "<y xmlns=\"urn:example:clixon\">%s</y>", val);
    cprintf(cbret, "</rpc-reply>");

Result netconf session:

<rpc xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="42">
   <example xmlns="urn:example:clixon">
<rpc-reply xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="42">
   <y xmlns="urn:example:clixon">52</y>

Plugin callback guidelines


This information is important to understand for the stability of clixon

The Clixon programs run as non-blocking single-threaded applications. It calls functions from within dynamically loaded modules. The callback code must be written with this programming model in mind. The behavior of the callback directly impacts the behavior of the caller and the whole system.

The most serious effect is when crash within a callback happens. This will cause the whole program to crash.

A more subtle problem is the environment of the program. Clixon will configure the environment, and it expects that the callback will return with the exact same environment intact. If you change a signal handler, a terminal configuration, etc. you must restore the state as it was on entry. Failure to do this can cause problems that are difficult to isolate and fix.

A list of things to watch out for (but not complete!):

  • a crash in the plugin
  • change of signal behaviour, such as blocking or assigning signal handlers
  • change of process privileges
  • asynchronous calls
  • If you fork or create threads, ensure the main program continues uninterrupted


Clixon follows NETCONF in its validate and commit semantics. Using the CLI or another frontend, you edit the candidate configuration, which is first validated for consistency and then committed to the running configuration.

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

The netconf validation and commit operation is implemented in Clixon by a transaction mechanism, which ensures that user-written plugin callbacks are invoked atomically and revert on error. The transaction callbacks are the following:

Transaction start
Transaction validation
Transaction validation complete
Transaction commit
Transaction when commit done. After this, the source db is copied to target and default values removed.
Transaction completed.
Transaction revert
Transaction aborted

In a system with two plugins, for example, a transaction sequence looks like the following:

Backend   Plugin1    Plugin2
|          |          |
+--------->+--------->+ begin
|          |          |
+--------->+--------->+ validate
|          |          |
+--------->+--------->+ complete
|          |          |
+--------->+--------->+ commit
|          |          |
+--------->+--------->+ commit_done
|          |          |
+--------->+--------->+ end

If an error occurs in the commit call of plugin2, for example, the transaction is aborted and the commit reverted:

Backend   Plugin1    Plugin2
|          |          |
+--------->+--------->+ begin
|          |          |
+--------->+--------->+ validate
|          |          |
+--------->+---->X    + commit error
|          |          |
+--------->+          + revert
|          |          |
+--------->+--------->+ abort


There are two XML trees:

This is the original XML tree, such as “running”
This is the new XML tree, such as “candidate”

There are three vectors pointing into the XML trees:

The delete vector consists of XML nodes in “src” that are removed in “target”
The add vector consists of nodes that exists in “target” and do not exist in “src”
The change vector consists of nodes that exists in both “src” and “target” but are different

All transaction callbacks are called with a transaction-data argument (td). The transaction data describes a system transition from a src to target state. The struct contains source and target XML tree (e.g. candidate/running) in the form of XML tree vectors (dvec, avec, cvec) and associated lengths. These contain the difference between src and target XML, ie “what has changed”. It is up to the validate callbacks to ensure that these changes are OK. It is up to the commit callbacks to enforce these changes in the configuration of the system.

The “td” parameter can be accessed with the following functions:

uint64_t transaction_id(transaction_data td)
Get the unique transaction-id
void   *transaction_arg(transaction_data td)
Get plugin/application specific callback argument
int     transaction_arg_set(transaction_data td, void *arg)
Set callback argument
cxobj  *transaction_src(transaction_data td)
Get source database xml tree
cxobj  *transaction_target(transaction_data td)
Get target database xml tree
cxobj **transaction_dvec(transaction_data td)
Get vector of xml nodes that are deleted src->target
size_t  transaction_dlen(transaction_data td)
Get length of delete xml vector
cxobj **transaction_avec(transaction_data td)
Get vector of xml nodes that are added src->target
size_t  transaction_alen(transaction_data td)
Get length of add xml vector
cxobj **transaction_scvec(transaction_data td)
Get vector of xml nodes that changed
cxobj **transaction_tcvec(transaction_data td)
Get vector of xml nodes that changed
size_t  transaction_clen(transaction_data td)
Get length of changed xml vector

A programmer can also use XML flags that are set in “src” and “target” XML trees to identify what has changed. The following flags are used to makr the trees:

All deleted XML nodes in “src” and all its descendants
All added XML nodes in “target” and all its descendants
All changed XML nodes in both “src” and “target” and all its descendants. Also all ancestors of all added, deleted and changed nodes.

For example, assume the tree (A B) is replaced with (B C), then the two trees are marked with the following flags:

          src            target
           o CHANGE        o CHANGE
           |               |
           o CHANGE        o CHANGE
          /|               |\
DELETE   A B               B C ADD


The backend process itself does not really require any specific access, but it may be an important topic for an application using clixon when the plugins are designed. A plugin may need to access privileged system resources (such as configure files).

The backend itself is usually started as root: sudo clixon_backend -s init, which means that the plugins also run as root (being part of the same process).

The backend can also be started as a non-root user. However, you may need to set some config options to allow user write access, for example as follows(there may be others):


Dropping privileges

You may want to start the backend as root and then drop privileges to a non-root user which is a common technique to limit exposure of exploits.

This can be done either by command line-options: sudo clicon_backend -s init -U clicon or (more generally) using configure options:


This will initialize resources as root and then permanently drop uid:s to the unprivileged user (clicon in the example abobe). It will also change ownership of several files to the user, including datastores and the clicon socket (if the socket is unix domain).

Note that the unprivileged user must exist on the system, see Installation.

Drop privileges temporary

If you drop privileges permanently, you need to access all privileged resources initially before the drop. For a plugin designer, this means that you need to access privileges system resources in the plugin_init or plugin_start callbacks. The transaction callbacks, for example, will be run in unprivileged mode.

An alternative is to drop privileges temporary and then be able to raise privileges when needed:


In this mode, a plugin callback (eg commit), can temporarily raise the privileges when accessing system resources, and the lower them when done.

An example C-code for raising privileges in a plugin is as follows:

uid_t euid = geteuid();
... make high privilege stuff...