Documentation/applications/system/conntrack: add conntrack man page

Documentation/applications/system/conntrack/index.rst

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+==============================
+``conntrack`` connection track
+==============================
+
+The ``conntrack`` command is used to display and monitor connection tracking
+entries in the NuttX kernel. It is similar to Linux's ``conntrack`` tool.
+
+Configuration
+=============
+
+- ``CONFIG_SYSTEM_CONNTRACK``
+- ``CONFIG_NETLINK_NETFILTER``
+
+The following additional options are available:
+
+- ``CONFIG_SYSTEM_CONNTRACK_PRIORITY`` - Task priority
+  (default: 100)
+- ``CONFIG_SYSTEM_CONNTRACK_STACKSIZE`` - Stack size
+  (default: ``DEFAULT_TASK_STACKSIZE``)
+
+Usage
+=====
+
+.. code-block:: text
+
+   conntrack -L [-f family]
+   conntrack -E
+
+Commands
+========
+
+``-L, --dump``
+   List all connection tracking entries.
+
+``-E, --event``
+   Display a real-time event log of connection tracking changes.
+   Press Ctrl+C to stop monitoring.
+
+Options
+=======
+
+``-f, --family PROTO``
+   Specify the L3 protocol family. Only valid with ``-L``.
+
+   Supported values:
+
+   - ``ipv4`` (default): Show IPv4 connection tracking entries.
+   - ``ipv6``: Show IPv6 connection tracking entries.
+
+Output Format
+=============
+
+Each connection tracking entry is displayed in the following format:
+
+.. code-block:: text
+
+   PROTO src=SRC_ADDR dst=DST_ADDR sport=SPORT dport=DPORT src=REPLY_SRC dst=REPLY_DST sport=REPLY_SPORT dport=REPLY_DPORT
+
+For ICMP/ICMPv6 entries, the format uses ``type``, ``code``, and ``id``
+instead of ``sport`` and ``dport``:
+
+.. code-block:: text
+
+   icmp src=SRC_ADDR dst=DST_ADDR type=TYPE code=CODE id=ID src=REPLY_SRC dst=REPLY_DST type=REPLY_TYPE code=REPLY_CODE id=REPLY_ID
+
+Event mode prefixes each entry with an event type:
+
+- ``[NEW]``: A new connection tracking entry was created.
+- ``[DESTROY]``: A connection tracking entry was removed.
+
+Examples
+========
+
+List all IPv4 connection tracking entries:
+
+.. code-block:: text
+
+   nsh> conntrack -L
+
+List all IPv6 connection tracking entries:
+
+.. code-block:: text
+
+   nsh> conntrack -L -f ipv6
+
+Monitor connection tracking events in real-time:
+
+.. code-block:: text
+
+   nsh> conntrack -E
+       [NEW] tcp src=10.0.0.1 dst=10.0.0.2 sport=12345 dport=80 src=10.0.0.2 dst=10.0.0.1 sport=80 dport=12345
+   [DESTROY] tcp src=10.0.0.1 dst=10.0.0.2 sport=12345 dport=80 src=10.0.0.2 dst=10.0.0.1 sport=80 dport=12345
+
+See Also
+========
+
+- :doc:`../iptables/index`
+- :doc:`../ip6tables/index`