docs: networking: convert netconsole.txt to ReST

- add SPDX header;
- add a document title;
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- add to networking/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

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+.. SPDX-License-Identifier: GPL-2.0
+
+==========
+Netconsole
+==========
+
+
+started by Ingo Molnar <mingo@redhat.com>, 2001.09.17
+
+2.6 port and netpoll api by Matt Mackall <mpm@selenic.com>, Sep 9 2003
+
+IPv6 support by Cong Wang <xiyou.wangcong@gmail.com>, Jan 1 2013
+
+Extended console support by Tejun Heo <tj@kernel.org>, May 1 2015
+
+Please send bug reports to Matt Mackall <mpm@selenic.com>
+Satyam Sharma <satyam.sharma@gmail.com>, and Cong Wang <xiyou.wangcong@gmail.com>
+
+Introduction:
+=============
+
+This module logs kernel printk messages over UDP allowing debugging of
+problem where disk logging fails and serial consoles are impractical.
+
+It can be used either built-in or as a module. As a built-in,
+netconsole initializes immediately after NIC cards and will bring up
+the specified interface as soon as possible. While this doesn't allow
+capture of early kernel panics, it does capture most of the boot
+process.
+
+Sender and receiver configuration:
+==================================
+
+It takes a string configuration parameter "netconsole" in the
+following format::
+
+ netconsole=[+][src-port]@[src-ip]/[<dev>],[tgt-port]@<tgt-ip>/[tgt-macaddr]
+
+   where
+	+             if present, enable extended console support
+	src-port      source for UDP packets (defaults to 6665)
+	src-ip        source IP to use (interface address)
+	dev           network interface (eth0)
+	tgt-port      port for logging agent (6666)
+	tgt-ip        IP address for logging agent
+	tgt-macaddr   ethernet MAC address for logging agent (broadcast)
+
+Examples::
+
+ linux netconsole=4444@10.0.0.1/eth1,9353@10.0.0.2/12:34:56:78:9a:bc
+
+or::
+
+ insmod netconsole netconsole=@/,@10.0.0.2/
+
+or using IPv6::
+
+ insmod netconsole netconsole=@/,@fd00:1:2:3::1/
+
+It also supports logging to multiple remote agents by specifying
+parameters for the multiple agents separated by semicolons and the
+complete string enclosed in "quotes", thusly::
+
+ modprobe netconsole netconsole="@/,@10.0.0.2/;@/eth1,6892@10.0.0.3/"
+
+Built-in netconsole starts immediately after the TCP stack is
+initialized and attempts to bring up the supplied dev at the supplied
+address.
+
+The remote host has several options to receive the kernel messages,
+for example:
+
+1) syslogd
+
+2) netcat
+
+   On distributions using a BSD-based netcat version (e.g. Fedora,
+   openSUSE and Ubuntu) the listening port must be specified without
+   the -p switch::
+
+	nc -u -l -p <port>' / 'nc -u -l <port>
+
+    or::
+
+	netcat -u -l -p <port>' / 'netcat -u -l <port>
+
+3) socat
+
+::
+
+   socat udp-recv:<port> -
+
+Dynamic reconfiguration:
+========================
+
+Dynamic reconfigurability is a useful addition to netconsole that enables
+remote logging targets to be dynamically added, removed, or have their
+parameters reconfigured at runtime from a configfs-based userspace interface.
+[ Note that the parameters of netconsole targets that were specified/created
+from the boot/module option are not exposed via this interface, and hence
+cannot be modified dynamically. ]
+
+To include this feature, select CONFIG_NETCONSOLE_DYNAMIC when building the
+netconsole module (or kernel, if netconsole is built-in).
+
+Some examples follow (where configfs is mounted at the /sys/kernel/config
+mountpoint).
+
+To add a remote logging target (target names can be arbitrary)::
+
+ cd /sys/kernel/config/netconsole/
+ mkdir target1
+
+Note that newly created targets have default parameter values (as mentioned
+above) and are disabled by default -- they must first be enabled by writing
+"1" to the "enabled" attribute (usually after setting parameters accordingly)
+as described below.
+
+To remove a target::
+
+ rmdir /sys/kernel/config/netconsole/othertarget/
+
+The interface exposes these parameters of a netconsole target to userspace:
+
+	==============  =================================       ============
+	enabled		Is this target currently enabled?	(read-write)
+	extended	Extended mode enabled			(read-write)
+	dev_name	Local network interface name		(read-write)
+	local_port	Source UDP port to use			(read-write)
+	remote_port	Remote agent's UDP port			(read-write)
+	local_ip	Source IP address to use		(read-write)
+	remote_ip	Remote agent's IP address		(read-write)
+	local_mac	Local interface's MAC address		(read-only)
+	remote_mac	Remote agent's MAC address		(read-write)
+	==============  =================================       ============
+
+The "enabled" attribute is also used to control whether the parameters of
+a target can be updated or not -- you can modify the parameters of only
+disabled targets (i.e. if "enabled" is 0).
+
+To update a target's parameters::
+
+ cat enabled				# check if enabled is 1
+ echo 0 > enabled			# disable the target (if required)
+ echo eth2 > dev_name			# set local interface
+ echo 10.0.0.4 > remote_ip		# update some parameter
+ echo cb:a9:87:65:43:21 > remote_mac	# update more parameters
+ echo 1 > enabled			# enable target again
+
+You can also update the local interface dynamically. This is especially
+useful if you want to use interfaces that have newly come up (and may not
+have existed when netconsole was loaded / initialized).
+
+Extended console:
+=================
+
+If '+' is prefixed to the configuration line or "extended" config file
+is set to 1, extended console support is enabled. An example boot
+param follows::
+
+ linux netconsole=+4444@10.0.0.1/eth1,9353@10.0.0.2/12:34:56:78:9a:bc
+
+Log messages are transmitted with extended metadata header in the
+following format which is the same as /dev/kmsg::
+
+ <level>,<sequnum>,<timestamp>,<contflag>;<message text>
+
+Non printable characters in <message text> are escaped using "\xff"
+notation. If the message contains optional dictionary, verbatim
+newline is used as the delimeter.
+
+If a message doesn't fit in certain number of bytes (currently 1000),
+the message is split into multiple fragments by netconsole. These
+fragments are transmitted with "ncfrag" header field added::
+
+ ncfrag=<byte-offset>/<total-bytes>
+
+For example, assuming a lot smaller chunk size, a message "the first
+chunk, the 2nd chunk." may be split as follows::
+
+ 6,416,1758426,-,ncfrag=0/31;the first chunk,
+ 6,416,1758426,-,ncfrag=16/31; the 2nd chunk.
+
+Miscellaneous notes:
+====================
+
+.. Warning::
+
+   the default target ethernet setting uses the broadcast
+   ethernet address to send packets, which can cause increased load on
+   other systems on the same ethernet segment.
+
+.. Tip::
+
+   some LAN switches may be configured to suppress ethernet broadcasts
+   so it is advised to explicitly specify the remote agents' MAC addresses
+   from the config parameters passed to netconsole.
+
+.. Tip::
+
+   to find out the MAC address of, say, 10.0.0.2, you may try using::
+
+	ping -c 1 10.0.0.2 ; /sbin/arp -n | grep 10.0.0.2
+
+.. Tip::
+
+   in case the remote logging agent is on a separate LAN subnet than
+   the sender, it is suggested to try specifying the MAC address of the
+   default gateway (you may use /sbin/route -n to find it out) as the
+   remote MAC address instead.
+
+.. note::
+
+   the network device (eth1 in the above case) can run any kind
+   of other network traffic, netconsole is not intrusive. Netconsole
+   might cause slight delays in other traffic if the volume of kernel
+   messages is high, but should have no other impact.
+
+.. note::
+
+   if you find that the remote logging agent is not receiving or
+   printing all messages from the sender, it is likely that you have set
+   the "console_loglevel" parameter (on the sender) to only send high
+   priority messages to the console. You can change this at runtime using::
+
+	dmesg -n 8
+
+   or by specifying "debug" on the kernel command line at boot, to send
+   all kernel messages to the console. A specific value for this parameter
+   can also be set using the "loglevel" kernel boot option. See the
+   dmesg(8) man page and Documentation/admin-guide/kernel-parameters.rst
+   for details.
+
+Netconsole was designed to be as instantaneous as possible, to
+enable the logging of even the most critical kernel bugs. It works
+from IRQ contexts as well, and does not enable interrupts while
+sending packets. Due to these unique needs, configuration cannot
+be more automatic, and some fundamental limitations will remain:
+only IP networks, UDP packets and ethernet devices are supported.