1 .. SPDX-License-Identifier: GPL-2.0
8 started by Ingo Molnar <mingo@redhat.com>, 2001.09.17
10 2.6 port and netpoll api by Matt Mackall <mpm@selenic.com>, Sep 9 2003
12 IPv6 support by Cong Wang <xiyou.wangcong@gmail.com>, Jan 1 2013
14 Extended console support by Tejun Heo <tj@kernel.org>, May 1 2015
16 Release prepend support by Breno Leitao <leitao@debian.org>, Jul 7 2023
18 Userdata append support by Matthew Wood <thepacketgeek@gmail.com>, Jan 22 2024
20 Please send bug reports to Matt Mackall <mpm@selenic.com>
21 Satyam Sharma <satyam.sharma@gmail.com>, and Cong Wang <xiyou.wangcong@gmail.com>
26 This module logs kernel printk messages over UDP allowing debugging of
27 problem where disk logging fails and serial consoles are impractical.
29 It can be used either built-in or as a module. As a built-in,
30 netconsole initializes immediately after NIC cards and will bring up
31 the specified interface as soon as possible. While this doesn't allow
32 capture of early kernel panics, it does capture most of the boot
35 Sender and receiver configuration:
36 ==================================
38 It takes a string configuration parameter "netconsole" in the
41 netconsole=[+][r][src-port]@[src-ip]/[<dev>],[tgt-port]@<tgt-ip>/[tgt-macaddr]
44 + if present, enable extended console support
45 r if present, prepend kernel version (release) to the message
46 src-port source for UDP packets (defaults to 6665)
47 src-ip source IP to use (interface address)
48 dev network interface (eth0)
49 tgt-port port for logging agent (6666)
50 tgt-ip IP address for logging agent
51 tgt-macaddr ethernet MAC address for logging agent (broadcast)
55 linux netconsole=4444@10.0.0.1/eth1,9353@10.0.0.2/12:34:56:78:9a:bc
59 insmod netconsole netconsole=@/,@10.0.0.2/
63 insmod netconsole netconsole=@/,@fd00:1:2:3::1/
65 It also supports logging to multiple remote agents by specifying
66 parameters for the multiple agents separated by semicolons and the
67 complete string enclosed in "quotes", thusly::
69 modprobe netconsole netconsole="@/,@10.0.0.2/;@/eth1,6892@10.0.0.3/"
71 Built-in netconsole starts immediately after the TCP stack is
72 initialized and attempts to bring up the supplied dev at the supplied
75 The remote host has several options to receive the kernel messages,
82 On distributions using a BSD-based netcat version (e.g. Fedora,
83 openSUSE and Ubuntu) the listening port must be specified without
86 nc -u -l -p <port>' / 'nc -u -l <port>
90 netcat -u -l -p <port>' / 'netcat -u -l <port>
96 socat udp-recv:<port> -
98 Dynamic reconfiguration:
99 ========================
101 Dynamic reconfigurability is a useful addition to netconsole that enables
102 remote logging targets to be dynamically added, removed, or have their
103 parameters reconfigured at runtime from a configfs-based userspace interface.
105 To include this feature, select CONFIG_NETCONSOLE_DYNAMIC when building the
106 netconsole module (or kernel, if netconsole is built-in).
108 Some examples follow (where configfs is mounted at the /sys/kernel/config
111 To add a remote logging target (target names can be arbitrary)::
113 cd /sys/kernel/config/netconsole/
116 Note that newly created targets have default parameter values (as mentioned
117 above) and are disabled by default -- they must first be enabled by writing
118 "1" to the "enabled" attribute (usually after setting parameters accordingly)
123 rmdir /sys/kernel/config/netconsole/othertarget/
125 The interface exposes these parameters of a netconsole target to userspace:
127 ============== ================================= ============
128 enabled Is this target currently enabled? (read-write)
129 extended Extended mode enabled (read-write)
130 release Prepend kernel release to message (read-write)
131 dev_name Local network interface name (read-write)
132 local_port Source UDP port to use (read-write)
133 remote_port Remote agent's UDP port (read-write)
134 local_ip Source IP address to use (read-write)
135 remote_ip Remote agent's IP address (read-write)
136 local_mac Local interface's MAC address (read-only)
137 remote_mac Remote agent's MAC address (read-write)
138 ============== ================================= ============
140 The "enabled" attribute is also used to control whether the parameters of
141 a target can be updated or not -- you can modify the parameters of only
142 disabled targets (i.e. if "enabled" is 0).
144 To update a target's parameters::
146 cat enabled # check if enabled is 1
147 echo 0 > enabled # disable the target (if required)
148 echo eth2 > dev_name # set local interface
149 echo 10.0.0.4 > remote_ip # update some parameter
150 echo cb:a9:87:65:43:21 > remote_mac # update more parameters
151 echo 1 > enabled # enable target again
153 You can also update the local interface dynamically. This is especially
154 useful if you want to use interfaces that have newly come up (and may not
155 have existed when netconsole was loaded / initialized).
157 Netconsole targets defined at boot time (or module load time) with the
158 `netconsole=` param are assigned the name `cmdline<index>`. For example, the
159 first target in the parameter is named `cmdline0`. You can control and modify
160 these targets by creating configfs directories with the matching name.
162 Let's suppose you have two netconsole targets defined at boot time::
164 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
166 You can modify these targets in runtime by creating the following targets::
169 cat cmdline0/remote_ip
173 cat cmdline1/remote_ip
179 Custom user data can be appended to the end of messages with netconsole
180 dynamic configuration enabled. User data entries can be modified without
181 changing the "enabled" attribute of a target.
183 Directories (keys) under `userdata` are limited to 53 character length, and
184 data in `userdata/<key>/value` are limited to 200 bytes::
186 cd /sys/kernel/config/netconsole && mkdir cmdline0
189 echo bar > userdata/foo/value
191 echo baz > userdata/qux/value
193 Messages will now include this additional user data::
195 echo "This is a message" > /dev/kmsg
199 12,607,22085407756,-;This is a message
203 Preview the userdata that will be appended with::
205 cd /sys/kernel/config/netconsole/cmdline0/userdata
206 for f in `ls userdata`; do echo $f=$(cat userdata/$f/value); done
208 If a `userdata` entry is created but no data is written to the `value` file,
209 the entry will be omitted from netconsole messages::
211 cd /sys/kernel/config/netconsole && mkdir cmdline0
214 echo bar > userdata/foo/value
217 The `qux` key is omitted since it has no value::
219 echo "This is a message" > /dev/kmsg
220 12,607,22085407756,-;This is a message
223 Delete `userdata` entries with `rmdir`::
225 rmdir /sys/kernel/config/netconsole/cmdline0/userdata/qux
228 When writing strings to user data values, input is broken up per line in
229 configfs store calls and this can cause confusing behavior::
231 mkdir userdata/testing
232 printf "val1\nval2" > userdata/testing/value
233 # userdata store value is called twice, first with "val1\n" then "val2"
234 # so "val2" is stored, being the last value stored
235 cat userdata/testing/value
238 It is recommended to not write user data values with newlines.
243 If '+' is prefixed to the configuration line or "extended" config file
244 is set to 1, extended console support is enabled. An example boot
247 linux netconsole=+4444@10.0.0.1/eth1,9353@10.0.0.2/12:34:56:78:9a:bc
249 Log messages are transmitted with extended metadata header in the
250 following format which is the same as /dev/kmsg::
252 <level>,<sequnum>,<timestamp>,<contflag>;<message text>
254 If 'r' (release) feature is enabled, the kernel release version is
255 prepended to the start of the message. Example::
257 6.4.0,6,444,501151268,-;netconsole: network logging started
259 Non printable characters in <message text> are escaped using "\xff"
260 notation. If the message contains optional dictionary, verbatim
261 newline is used as the delimiter.
263 If a message doesn't fit in certain number of bytes (currently 1000),
264 the message is split into multiple fragments by netconsole. These
265 fragments are transmitted with "ncfrag" header field added::
267 ncfrag=<byte-offset>/<total-bytes>
269 For example, assuming a lot smaller chunk size, a message "the first
270 chunk, the 2nd chunk." may be split as follows::
272 6,416,1758426,-,ncfrag=0/31;the first chunk,
273 6,416,1758426,-,ncfrag=16/31; the 2nd chunk.
280 the default target ethernet setting uses the broadcast
281 ethernet address to send packets, which can cause increased load on
282 other systems on the same ethernet segment.
286 some LAN switches may be configured to suppress ethernet broadcasts
287 so it is advised to explicitly specify the remote agents' MAC addresses
288 from the config parameters passed to netconsole.
292 to find out the MAC address of, say, 10.0.0.2, you may try using::
294 ping -c 1 10.0.0.2 ; /sbin/arp -n | grep 10.0.0.2
298 in case the remote logging agent is on a separate LAN subnet than
299 the sender, it is suggested to try specifying the MAC address of the
300 default gateway (you may use /sbin/route -n to find it out) as the
301 remote MAC address instead.
305 the network device (eth1 in the above case) can run any kind
306 of other network traffic, netconsole is not intrusive. Netconsole
307 might cause slight delays in other traffic if the volume of kernel
308 messages is high, but should have no other impact.
312 if you find that the remote logging agent is not receiving or
313 printing all messages from the sender, it is likely that you have set
314 the "console_loglevel" parameter (on the sender) to only send high
315 priority messages to the console. You can change this at runtime using::
319 or by specifying "debug" on the kernel command line at boot, to send
320 all kernel messages to the console. A specific value for this parameter
321 can also be set using the "loglevel" kernel boot option. See the
322 dmesg(8) man page and Documentation/admin-guide/kernel-parameters.rst
325 Netconsole was designed to be as instantaneous as possible, to
326 enable the logging of even the most critical kernel bugs. It works
327 from IRQ contexts as well, and does not enable interrupts while
328 sending packets. Due to these unique needs, configuration cannot
329 be more automatic, and some fundamental limitations will remain:
330 only IP networks, UDP packets and ethernet devices are supported.