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 Please send bug reports to Matt Mackall <mpm@selenic.com>
19 Satyam Sharma <satyam.sharma@gmail.com>, and Cong Wang <xiyou.wangcong@gmail.com>
24 This module logs kernel printk messages over UDP allowing debugging of
25 problem where disk logging fails and serial consoles are impractical.
27 It can be used either built-in or as a module. As a built-in,
28 netconsole initializes immediately after NIC cards and will bring up
29 the specified interface as soon as possible. While this doesn't allow
30 capture of early kernel panics, it does capture most of the boot
33 Sender and receiver configuration:
34 ==================================
36 It takes a string configuration parameter "netconsole" in the
39 netconsole=[+][r][src-port]@[src-ip]/[<dev>],[tgt-port]@<tgt-ip>/[tgt-macaddr]
42 + if present, enable extended console support
43 r if present, prepend kernel version (release) to the message
44 src-port source for UDP packets (defaults to 6665)
45 src-ip source IP to use (interface address)
46 dev network interface (eth0)
47 tgt-port port for logging agent (6666)
48 tgt-ip IP address for logging agent
49 tgt-macaddr ethernet MAC address for logging agent (broadcast)
53 linux netconsole=4444@10.0.0.1/eth1,9353@10.0.0.2/12:34:56:78:9a:bc
57 insmod netconsole netconsole=@/,@10.0.0.2/
61 insmod netconsole netconsole=@/,@fd00:1:2:3::1/
63 It also supports logging to multiple remote agents by specifying
64 parameters for the multiple agents separated by semicolons and the
65 complete string enclosed in "quotes", thusly::
67 modprobe netconsole netconsole="@/,@10.0.0.2/;@/eth1,6892@10.0.0.3/"
69 Built-in netconsole starts immediately after the TCP stack is
70 initialized and attempts to bring up the supplied dev at the supplied
73 The remote host has several options to receive the kernel messages,
80 On distributions using a BSD-based netcat version (e.g. Fedora,
81 openSUSE and Ubuntu) the listening port must be specified without
84 nc -u -l -p <port>' / 'nc -u -l <port>
88 netcat -u -l -p <port>' / 'netcat -u -l <port>
94 socat udp-recv:<port> -
96 Dynamic reconfiguration:
97 ========================
99 Dynamic reconfigurability is a useful addition to netconsole that enables
100 remote logging targets to be dynamically added, removed, or have their
101 parameters reconfigured at runtime from a configfs-based userspace interface.
103 To include this feature, select CONFIG_NETCONSOLE_DYNAMIC when building the
104 netconsole module (or kernel, if netconsole is built-in).
106 Some examples follow (where configfs is mounted at the /sys/kernel/config
109 To add a remote logging target (target names can be arbitrary)::
111 cd /sys/kernel/config/netconsole/
114 Note that newly created targets have default parameter values (as mentioned
115 above) and are disabled by default -- they must first be enabled by writing
116 "1" to the "enabled" attribute (usually after setting parameters accordingly)
121 rmdir /sys/kernel/config/netconsole/othertarget/
123 The interface exposes these parameters of a netconsole target to userspace:
125 ============== ================================= ============
126 enabled Is this target currently enabled? (read-write)
127 extended Extended mode enabled (read-write)
128 release Prepend kernel release to message (read-write)
129 dev_name Local network interface name (read-write)
130 local_port Source UDP port to use (read-write)
131 remote_port Remote agent's UDP port (read-write)
132 local_ip Source IP address to use (read-write)
133 remote_ip Remote agent's IP address (read-write)
134 local_mac Local interface's MAC address (read-only)
135 remote_mac Remote agent's MAC address (read-write)
136 ============== ================================= ============
138 The "enabled" attribute is also used to control whether the parameters of
139 a target can be updated or not -- you can modify the parameters of only
140 disabled targets (i.e. if "enabled" is 0).
142 To update a target's parameters::
144 cat enabled # check if enabled is 1
145 echo 0 > enabled # disable the target (if required)
146 echo eth2 > dev_name # set local interface
147 echo 10.0.0.4 > remote_ip # update some parameter
148 echo cb:a9:87:65:43:21 > remote_mac # update more parameters
149 echo 1 > enabled # enable target again
151 You can also update the local interface dynamically. This is especially
152 useful if you want to use interfaces that have newly come up (and may not
153 have existed when netconsole was loaded / initialized).
155 Netconsole targets defined at boot time (or module load time) with the
156 `netconsole=` param are assigned the name `cmdline<index>`. For example, the
157 first target in the parameter is named `cmdline0`. You can control and modify
158 these targets by creating configfs directories with the matching name.
160 Let's suppose you have two netconsole targets defined at boot time::
162 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
164 You can modify these targets in runtime by creating the following targets::
167 cat cmdline0/remote_ip
171 cat cmdline1/remote_ip
177 If '+' is prefixed to the configuration line or "extended" config file
178 is set to 1, extended console support is enabled. An example boot
181 linux netconsole=+4444@10.0.0.1/eth1,9353@10.0.0.2/12:34:56:78:9a:bc
183 Log messages are transmitted with extended metadata header in the
184 following format which is the same as /dev/kmsg::
186 <level>,<sequnum>,<timestamp>,<contflag>;<message text>
188 If 'r' (release) feature is enabled, the kernel release version is
189 prepended to the start of the message. Example::
191 6.4.0,6,444,501151268,-;netconsole: network logging started
193 Non printable characters in <message text> are escaped using "\xff"
194 notation. If the message contains optional dictionary, verbatim
195 newline is used as the delimiter.
197 If a message doesn't fit in certain number of bytes (currently 1000),
198 the message is split into multiple fragments by netconsole. These
199 fragments are transmitted with "ncfrag" header field added::
201 ncfrag=<byte-offset>/<total-bytes>
203 For example, assuming a lot smaller chunk size, a message "the first
204 chunk, the 2nd chunk." may be split as follows::
206 6,416,1758426,-,ncfrag=0/31;the first chunk,
207 6,416,1758426,-,ncfrag=16/31; the 2nd chunk.
214 the default target ethernet setting uses the broadcast
215 ethernet address to send packets, which can cause increased load on
216 other systems on the same ethernet segment.
220 some LAN switches may be configured to suppress ethernet broadcasts
221 so it is advised to explicitly specify the remote agents' MAC addresses
222 from the config parameters passed to netconsole.
226 to find out the MAC address of, say, 10.0.0.2, you may try using::
228 ping -c 1 10.0.0.2 ; /sbin/arp -n | grep 10.0.0.2
232 in case the remote logging agent is on a separate LAN subnet than
233 the sender, it is suggested to try specifying the MAC address of the
234 default gateway (you may use /sbin/route -n to find it out) as the
235 remote MAC address instead.
239 the network device (eth1 in the above case) can run any kind
240 of other network traffic, netconsole is not intrusive. Netconsole
241 might cause slight delays in other traffic if the volume of kernel
242 messages is high, but should have no other impact.
246 if you find that the remote logging agent is not receiving or
247 printing all messages from the sender, it is likely that you have set
248 the "console_loglevel" parameter (on the sender) to only send high
249 priority messages to the console. You can change this at runtime using::
253 or by specifying "debug" on the kernel command line at boot, to send
254 all kernel messages to the console. A specific value for this parameter
255 can also be set using the "loglevel" kernel boot option. See the
256 dmesg(8) man page and Documentation/admin-guide/kernel-parameters.rst
259 Netconsole was designed to be as instantaneous as possible, to
260 enable the logging of even the most critical kernel bugs. It works
261 from IRQ contexts as well, and does not enable interrupts while
262 sending packets. Due to these unique needs, configuration cannot
263 be more automatic, and some fundamental limitations will remain:
264 only IP networks, UDP packets and ethernet devices are supported.