Netconsole — The Linux Kernel documentation (2024)

Introduction:¶

This module logs kernel printk messages over UDP allowing debugging ofproblem 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 upthe specified interface as soon as possible. While this doesn’t allowcapture of early kernel panics, it does capture most of the bootprocess.

Sender and receiver configuration:¶

It takes a string configuration parameter “netconsole” in thefollowing 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 specifyingparameters for the multiple agents separated by semicolons and thecomplete 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 isinitialized and attempts to bring up the supplied dev at the suppliedaddress.

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 withoutthe -p switch:

    nc -u -l -p <port>' / 'nc -u -l <port>or:: netcat -u -l -p <port>' / 'netcat -u -l <port>

   See Also

   how to remove something from bin folder in ubuntu | Code Easesudo rmdir recursive | Code EaseInstall MyBB Forum with Nginx and Let’s Encrypt on CentOS and Rocky Linux – VITUXHow to Install Nextcloud on Ubuntu 24.04 LTS

3. socat

socat udp-recv:<port> -

Dynamic reconfiguration:¶

Dynamic reconfigurability is a useful addition to netconsole that enablesremote logging targets to be dynamically added, removed, or have theirparameters reconfigured at runtime from a configfs-based userspace interface.

To include this feature, select CONFIG_NETCONSOLE_DYNAMIC when building thenetconsole module (or kernel, if netconsole is built-in).

Some examples follow (where configfs is mounted at the /sys/kernel/configmountpoint).

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 mentionedabove) 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 ofa target can be updated or not -- you can modify the parameters of onlydisabled targets (i.e. if “enabled” is 0).

To update a target’s parameters:

cat enabled # check if enabled is 1echo 0 > enabled # disable the target (if required)echo eth2 > dev_name # set local interfaceecho 10.0.0.4 > remote_ip # update some parameterecho cb:a9:87:65:43:21 > remote_mac # update more parametersecho 1 > enabled # enable target again

You can also update the local interface dynamically. This is especiallyuseful if you want to use interfaces that have newly come up (and may nothave existed when netconsole was loaded / initialized).

Netconsole targets defined at boot time (or module load time) with thenetconsole= param are assigned the name cmdline<index>. For example, thefirst target in the parameter is named cmdline0. You can control and modifythese 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 cmdline0cat cmdline0/remote_ip10.0.0.2mkdir cmdline1cat cmdline1/remote_ip10.0.0.3

cd /sys/kernel/config/netconsole && mkdir cmdline0cd cmdline0mkdir userdata/fooecho bar > userdata/foo/valuemkdir userdata/quxecho baz > userdata/qux/value

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

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

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

cd /sys/kernel/config/netconsole && mkdir cmdline0cd cmdline0mkdir userdata/fooecho bar > userdata/foo/valuemkdir userdata/qux

echo "This is a message" > /dev/kmsg12,607,22085407756,-;This is a message foo=bar

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

mkdir userdata/testingprintf "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 storedcat userdata/testing/valueval2

Extended console:¶

If ‘+’ is prefixed to the configuration line or “extended” config fileis set to 1, extended console support is enabled. An example bootparam 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 thefollowing format which is the same as /dev/kmsg:

<level>,<sequnum>,<timestamp>,<contflag>;<message text>

If ‘r’ (release) feature is enabled, the kernel release version isprepended 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, verbatimnewline 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. Thesefragments are transmitted with “ncfrag” header field added:

ncfrag=<byte-offset>/<total-bytes>

For example, assuming a lot smaller chunk size, a message “the firstchunk, 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:¶

ping -c 1 10.0.0.2 ; /sbin/arp -n | grep 10.0.0.2

dmesg -n 8

Netconsole was designed to be as instantaneous as possible, toenable the logging of even the most critical kernel bugs. It worksfrom IRQ contexts as well, and does not enable interrupts whilesending packets. Due to these unique needs, configuration cannotbe more automatic, and some fundamental limitations will remain:only IP networks, UDP packets and ethernet devices are supported.