Documentation/networking/netconsole.rst - linux - Git at Google

 .. 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

 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 (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.

 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)
 	==============  =================================       ============

 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.

 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.
	.. 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

	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 (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.

	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)
	============== ================================= ============

	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.

	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.