src/kernel/linux/v4.19/Documentation/networking/netconsole.txt

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.