YANG and XML is the heart of Clixon. Yang modules are used as a specification for handling XML configuration data. The YANG spec is used to generate an interactive CLI, netconf and restconf clients. It also specifies the format of the XML datastore.

The YANG standards that Clixon follows include:

However, the following YANG syntax modules are not implemented (reference to RFC7950 in parenthesis):

  • deviation (7.20.3)
  • action (7.15)
  • augment in a uses sub-clause (7.17) (module-level augment is implemented)
  • require-instance
  • instance-identifier type (9.13)
  • status (7.21.2)
  • YIN (13)
  • Yang extended XPath functions: re-match(), deref)(), derived-from(), derived-from-or-self(), enum-value(), bit-is-set() (10.2-10.6)
  • Default values on leaf-lists (7.7.2)
  • Lists without keys (non-config lists may lack keys)
  • if-feature-expr is restricted to single layer expressions with and/or: “x and y” and “x or y” is allowed but “x or (not y and z)” is not. (7.20.2)

Regular expressions

Clixon supports two regular expressions engines:

Posix is the default method, The regexps:s are translated to posix before matching with the standard Linux regex engine. This translation is not complete but can be considered “good-enough” for most yang use-cases. For reference, all standard Yang models have been tested.
Libxml2 uses the XSD regex engine. This is a complete XSD engine but you need to compile and link with libxml2 which may add overhead.

To use libxml2 in clixon you need enable libxml2 in both cligen and clixon:

./configure --with-libxml2 # both cligen and clixon

You then need to set the following configure option:


XML and XPath

Clixon has its own implementation of XML and XPath. See more in the detailed API reference.

The XML-related standards include:

The following XPath axes are supported:

  • child,
  • descendant,
  • descendant_or_self,
  • self
  • parent

The following xpath axes are not supported: preceeding, preceeding_sibling, namespace, following_sibling, following, ancestor,ancestor_or_self, and attribute


Unicode is not supported in YANG and XML


Clixon implements the following NETCONF RFC:s:

The following RFC6241 capabilities/features are hardcoded in Clixon:

  • :candidate (RFC6241 8.3)
  • :validate (RFC6241 8.6)
  • :xpath (RFC6241 8.9)
  • :notification (RFC5277)

The following features are optional and can be enabled by setting CLICON_FEATURE:

  • :startup (RFC6241 8.7)

Clixon does not support the following NETCONF features:

  • :url capability
  • copy-config source config
  • edit-config testopts
  • edit-config erropts
  • edit-config config-text
  • edit-config operation

Further, in get-config filter expressions, the RFC6241 XPath Capability is preferred over default subtrees. This has two reasons:

  1. XPath has better performance since the underlying system uses xpath, and subtree filtering is done after the complete tree is retreived.
  2. Subtree filtering does not support namespaces yet.

Default values

Clixon only stores explicit set default values in datastores, while unset values are populated in memory on retreival. This means that get-config will report all default values, not only those explicitly set.

RFC 6243: With-defaults Capability for NETCONF is not implemented. Among the modes descriibed in the RFC, Clixon implements “report-all” with-respect to GET and GET-CONFIG operations, but “explicit” with reespect to how configurations are saved in datastores.


Clixon Restconf is a daemon based on FastCGI C-API. Instructions are available to run with NGINX. The implementatation is based on RFC 8040: RESTCONF Protocol.

The following features of RFC8040 are supported:

  • stream notifications (Sec 6)
  • query parameters: “insert”, “point”, “content”, “depth”, “start-time” and “stop-time”.
  • Monitoring (Sec 9)

The following features are not implemented:

  • ETag/Last-Modified
  • Query parameters: “fields”, “filter”, “with-defaults”