4 Standards

4.1 YANG

YANG and XML are central to Clixon. Yang modules are used as a specification for encoding XML or JSON configuration and state data. The YANG spec is also used to generate an interactive CLI, NETCONF and RESTCONF clients, as well as the format of the XML datastore.

The YANG standards that Clixon follows include (see also netconf):

Clixon deviates from the YANG standard as follows (reference to RFC7950 sections in parenthesis):

Not implemented:

  • augment in a uses sub-clause (7.17) (module-level augment is implemented)

  • instance-identifier type (9.13)

  • status (7.21.2)

  • YIN (13)

  • Default values on leaf-lists (7.7.2)

  • error-message is not implemented as sub-statement of “range”, “length” and “pattern”


Clixon supports the following extended XPath functions (10):

  • current()

  • deref()

  • derived-from(),

  • derived-from-or-self()

  • bit-is-set()

  • The following extended XPath functions are not supported (10):

    • re-match()

    • enum-value()

See also support of standard XPath functions XML and XPath

4.1.1 Regular expressions

Clixon supports two regular expression engines:


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:


4.1.2 Metadata

Clixon implements Defining and Using Metadata with YANG RFC 7952 for XML and JSON.

This means that Yang-derived meta-data defined with:

md:annotation <name>

is defined for attributes so that they can be mapped from XML to JSON, for example.

Assigned meta-data are hardcoded. The following attributes are defined:

  • ietf-netconf-with-defaults:default from RFC 6243 / RFC 8040

4.1.3 Schema mount

Yang schema mount is supported as defined in: RFC 8528: YANG Schema Mount .

Enable by enable the CLICON_YANG_SCHEMA_MOUNT configuration option.


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)

  • :with-defaults (RFC6243)

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

  • :confirmed-commit:1.1 (RFC6241 8.4)

  • :startup (RFC6241 8.7)

  • :writable-running (RFC6241 8.2) - just write to running, no commit semantics

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 retrieved.

  2. Subtree filtering does not support namespaces yet.

Clixon supports netconf locks in default settings but not if CLICON_DATASTORE_CACHE is nocache mode.

4.2.1 RFC 6022

Clixon extends the RFC 6022 session parameter transport with “cli”, “restconf”, “netconf” and “snmp”. In particular, the clixon_netconf application uses stdio to get input and print output and is used in a “piping” fashion, for example directly in a terminal shell or as a part of a SSH sub-system, and therefore has no direct knowledge of whether the NETCONF transport is over SSH or not.

The source-host parameter is set only in certain circumstances when the source host is in fact known. This includes native RESTCONF for example.

Further, hello counters are backend based, ie the internal protocol, which means hellos from RESTCONF, SNMP and CLI clients are included and that eventual dropped hello messages from external NETCONF sessions are not.

4.2.2 Default handling

Clixon treats default data according to what is defined as explicit basic mode in RFC 6243: With-defaults Capability for NETCONF, i.e. the server consider any data node that is not explicitly set data to be default data.

One effect is that if you view the contents of datastores (or import/export them), they should be in explicit basic mode.

The :with-defaults capability indicates that clixon default behaviour is explicit and also indicates that additional retrieval modes supported by the server are:.

  • explicit

  • trim

  • report-all

  • report-all-tagged

Internally in memory, however, report-all is used.


Clixon supports the two RESTCONF compile-time variants: FCGI and Native. Both implements RFC 8040: RESTCONF Protocol.

The following features of RFC8040 are supported:


  • Stream notifications (Sec 6)

  • Query parameters: insert, point, content, depth, start-time, stop-time and with-defaults.

  • Monitoring (Sec 9)

The following features are not implemented:

  • ETag/Last-Modified

  • Query parameters: fields and filter

RESTCONF event notification as described in RFC7950 section 6 is supported as follows:

  • is not supported by native

  • is supported by FCGI

NMDA is partly supported according to RFC 8324 and RFC 8527. With-defaults and with-origin are not implemented.

RFC 8072: YANG Patch Media Type is not implemented.

In the native mode, Clixon also supports:

  • HTTP/1.1 as transport using a native implementation (RFC 7230),

  • HTTP/2 as transport implemented by libnghttp2 (RFC7540),

  • Transport Layer Security (TLS) implemented by libopenssl, versions 1.1.1 and 3.0

  • ALPN as defined in RFC 7301 for http/1, http/2 protocol selection by libopenssl

4.4 SNMP

The Clixon-SNMP frontend implements the MIB-YANG mapping as defined in RFC 6643.

4.5 XML and XPath

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

The XML-related standards include:

Clixon XML supports version and UTF-8 only.

The following XPath axes are supported:

  • child,

  • descendant,

  • descendant-or-self,

  • self

  • parent

The following xpath axes are not supported: preceding, preceding_sibling, namespace, following_sibling, following, ancestor,ancestor_or_self, and attribute

The following XPath functions as defined in Section 2.3 / 4 of the XPath 1.0 standard are supported:

  • contains()

  • count()

  • false()

  • name()

  • node()

  • boolean()

  • not()

  • position()

  • text()

  • true()

The following standard XPath functions are not supported:

  • ceiling

  • comment

  • concat

  • floor

  • id

  • lang

  • last

  • local-name

  • namespace-uri

  • normalize-space

  • number

  • processing-instructions

  • round

  • starts-with

  • string

  • substring

  • substring-after

  • substring-before

  • sum

  • translate

4.6 Pagination

The pagination solution is based on the following drafts:

See Pagination section for more info.

4.7 Unicode

Unicode is not supported in YANG and XML.

4.8 JSON

Clixon implements JSON according to: