linux/Documentation/networking/netconsole.rst

.. 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

Release prepend support by Breno Leitao <leitao@debian.org>, Jul 7 2023

Userdata append support by Matthew Wood <thepacketgeek@gmail.com>, Jan 22 2024

Sysdata append support by Breno Leitao <leitao@debian.org>, Jan 15 2025

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=[+][r][src-port]@[src-ip]/[<dev>],[tgt-port]@<tgt-ip>/[tgt-macaddr]

   where
	+             if present, enable extended console support
	r             if present, prepend kernel version (release) to the message
	src-port      source for UDP packets (defaults to 6665)
	src-ip        source IP to use (interface address)
	dev           network interface name (eth0) or MAC address
	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/

or using a MAC address to select the egress interface::

   linux netconsole=4444@10.0.0.1/22:33:44:55:66:77,9353@10.0.0.2/12:34:56:78:9a:bc

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.

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)
	release		Prepend kernel release to message	(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)
	transmit_errors	Number of packet send errors		(read-only)
	=============== =================================       ============

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).

Netconsole targets defined at boot time (or module load time) with the
`netconsole=` param are assigned the name `cmdline<index>`.  For example, the
first target in the parameter is named `cmdline0`.  You can control and modify
these targets by creating configfs directories with the matching name.

Let's suppose you have two netconsole targets defined at boot time::

 netconsole=4444@10.0.0.1/eth1,9353@10.0.0.2/12:34:56:78:9a:bc;4444@10.0.0.1/eth1,9353@10.0.0.3/12:34:56:78:9a:bc

You can modify these targets in runtime by creating the following targets::

 mkdir cmdline0
 cat cmdline0/remote_ip
 10.0.0.2

 mkdir cmdline1
 cat cmdline1/remote_ip
 10.0.0.3

Append User Data
----------------

Custom user data can be appended to the end of messages with netconsole
dynamic configuration enabled. User data entries can be modified without
changing the "enabled" attribute of a target.

Directories (keys) under `userdata` are limited to 53 character length, and
data in `userdata/<key>/value` are limited to 200 bytes::

 cd /sys/kernel/config/netconsole && mkdir cmdline0
 cd cmdline0
 mkdir userdata/foo
 echo bar > userdata/foo/value
 mkdir userdata/qux
 echo baz > userdata/qux/value

Messages will now include this additional user data::

 echo "This is a message" > /dev/kmsg

Sends::

 12,607,22085407756,-;This is a message
  foo=bar
  qux=baz

Preview the userdata that will be appended with::

 cd /sys/kernel/config/netconsole/cmdline0/userdata
 for f in `ls userdata`; do echo $f=$(cat userdata/$f/value); done

If a `userdata` entry is created but no data is written to the `value` file,
the entry will be omitted from netconsole messages::

 cd /sys/kernel/config/netconsole && mkdir cmdline0
 cd cmdline0
 mkdir userdata/foo
 echo bar > userdata/foo/value
 mkdir userdata/qux

The `qux` key is omitted since it has no value::

 echo "This is a message" > /dev/kmsg
 12,607,22085407756,-;This is a message
  foo=bar

Delete `userdata` entries with `rmdir`::

 rmdir /sys/kernel/config/netconsole/cmdline0/userdata/qux

.. warning::
   When writing strings to user data values, input is broken up per line in
   configfs store calls and this can cause confusing behavior::

     mkdir userdata/testing
     printf "val1\nval2" > userdata/testing/value
     # userdata store value is called twice, first with "val1\n" then "val2"
     # so "val2" is stored, being the last value stored
     cat userdata/testing/value
     val2

   It is recommended to not write user data values with newlines.

Task name auto population in userdata
-------------------------------------

Inside the netconsole configfs hierarchy, there is a file called
`taskname_enabled` under the `userdata` directory. This file is used to enable
or disable the automatic task name population feature. This feature
automatically populates the current task name that is scheduled in the CPU
sneding the message.

To enable task name auto-population::

  echo 1 > /sys/kernel/config/netconsole/target1/userdata/taskname_enabled

When this option is enabled, the netconsole messages will include an additional
line in the userdata field with the format `taskname=<task name>`. This allows
the receiver of the netconsole messages to easily find which application was
currently scheduled when that message was generated, providing extra context
for kernel messages and helping to categorize them.

Example::

  echo "This is a message" > /dev/kmsg
  12,607,22085407756,-;This is a message
   taskname=echo

In this example, the message was generated while "echo" was the current
scheduled process.

Kernel release auto population in userdata
------------------------------------------

Within the netconsole configfs hierarchy, there is a file named `release_enabled`
located in the `userdata` directory. This file controls the kernel release
(version) auto-population feature, which appends the kernel release information
to userdata dictionary in every message sent.

To enable the release auto-population::

  echo 1 > /sys/kernel/config/netconsole/target1/userdata/release_enabled

Example::

  echo "This is a message" > /dev/kmsg
  12,607,22085407756,-;This is a message
   release=6.14.0-rc6-01219-g3c027fbd941d

.. note::

   This feature provides the same data as the "release prepend" feature.
   However, in this case, the release information is appended to the userdata
   dictionary rather than being included in the message header.


CPU number auto population in userdata
--------------------------------------

Inside the netconsole configfs hierarchy, there is a file called
`cpu_nr` under the `userdata` directory. This file is used to enable or disable
the automatic CPU number population feature. This feature automatically
populates the CPU number that is sending the message.

To enable the CPU number auto-population::

  echo 1 > /sys/kernel/config/netconsole/target1/userdata/cpu_nr

When this option is enabled, the netconsole messages will include an additional
line in the userdata field with the format `cpu=<cpu_number>`. This allows the
receiver of the netconsole messages to easily differentiate and demultiplex
messages originating from different CPUs, which is particularly useful when
dealing with parallel log output.

Example::

  echo "This is a message" > /dev/kmsg
  12,607,22085407756,-;This is a message
   cpu=42

In this example, the message was sent by CPU 42.

.. note::

   If the user has set a conflicting `cpu` key in the userdata dictionary,
   both keys will be reported, with the kernel-populated entry appearing after
   the user one. For example::

     # User-defined CPU entry
     mkdir -p /sys/kernel/config/netconsole/target1/userdata/cpu
     echo "1" > /sys/kernel/config/netconsole/target1/userdata/cpu/value

   Output might look like::

     12,607,22085407756,-;This is a message
      cpu=1
      cpu=42    # kernel-populated value


Message ID auto population in userdata
--------------------------------------

Within the netconsole configfs hierarchy, there is a file named `msgid_enabled`
located in the `userdata` directory. This file controls the message ID
auto-population feature, which assigns a numeric id to each message sent to a
given target and appends the ID to userdata dictionary in every message sent.

The message ID is generated using a per-target 32 bit counter that is
incremented for every message sent to the target. Note that this counter will
eventually wrap around after reaching uint32_t max value, so the message ID is
not globally unique over time. However, it can still be used by the target to
detect if messages were dropped before reaching the target by identifying gaps
in the sequence of IDs.

It is important to distinguish message IDs from the message <sequnum> field.
Some kernel messages may never reach netconsole (for example, due to printk
rate limiting). Thus, a gap in <sequnum> cannot be solely relied upon to
indicate that a message was dropped during transmission, as it may never have
been sent via netconsole. The message ID, on the other hand, is only assigned
to messages that are actually transmitted via netconsole.

Example::

  echo "This is message #1" > /dev/kmsg
  echo "This is message #2" > /dev/kmsg
  13,434,54928466,-;This is message #1
   msgid=1
  13,435,54934019,-;This is message #2
   msgid=2


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>

If 'r' (release) feature is enabled, the kernel release version is
prepended to the start of the message. Example::

 6.4.0,6,444,501151268,-;netconsole: network logging started

Non printable characters in <message text> are escaped using "\xff"
notation. If the message contains optional dictionary, verbatim
newline is used as the delimiter.

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.