1.. SPDX-License-Identifier: GPL-2.0 2 3========== 4Netconsole 5========== 6 7 8started by Ingo Molnar <mingo@redhat.com>, 2001.09.17 9 102.6 port and netpoll api by Matt Mackall <mpm@selenic.com>, Sep 9 2003 11 12IPv6 support by Cong Wang <xiyou.wangcong@gmail.com>, Jan 1 2013 13 14Extended console support by Tejun Heo <tj@kernel.org>, May 1 2015 15 16Release prepend support by Breno Leitao <leitao@debian.org>, Jul 7 2023 17 18Userdata append support by Matthew Wood <thepacketgeek@gmail.com>, Jan 22 2024 19 20Please send bug reports to Matt Mackall <mpm@selenic.com> 21Satyam Sharma <satyam.sharma@gmail.com>, and Cong Wang <xiyou.wangcong@gmail.com> 22 23Introduction: 24============= 25 26This module logs kernel printk messages over UDP allowing debugging of 27problem where disk logging fails and serial consoles are impractical. 28 29It can be used either built-in or as a module. As a built-in, 30netconsole initializes immediately after NIC cards and will bring up 31the specified interface as soon as possible. While this doesn't allow 32capture of early kernel panics, it does capture most of the boot 33process. 34 35Sender and receiver configuration: 36================================== 37 38It takes a string configuration parameter "netconsole" in the 39following format:: 40 41 netconsole=[+][r][src-port]@[src-ip]/[<dev>],[tgt-port]@<tgt-ip>/[tgt-macaddr] 42 43 where 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) 52 53Examples:: 54 55 linux netconsole=4444@10.0.0.1/eth1,9353@10.0.0.2/12:34:56:78:9a:bc 56 57or:: 58 59 insmod netconsole netconsole=@/,@10.0.0.2/ 60 61or using IPv6:: 62 63 insmod netconsole netconsole=@/,@fd00:1:2:3::1/ 64 65It also supports logging to multiple remote agents by specifying 66parameters for the multiple agents separated by semicolons and the 67complete string enclosed in "quotes", thusly:: 68 69 modprobe netconsole netconsole="@/,@10.0.0.2/;@/eth1,6892@10.0.0.3/" 70 71Built-in netconsole starts immediately after the TCP stack is 72initialized and attempts to bring up the supplied dev at the supplied 73address. 74 75The remote host has several options to receive the kernel messages, 76for example: 77 781) syslogd 79 802) netcat 81 82 On distributions using a BSD-based netcat version (e.g. Fedora, 83 openSUSE and Ubuntu) the listening port must be specified without 84 the -p switch:: 85 86 nc -u -l -p <port>' / 'nc -u -l <port> 87 88 or:: 89 90 netcat -u -l -p <port>' / 'netcat -u -l <port> 91 923) socat 93 94:: 95 96 socat udp-recv:<port> - 97 98Dynamic reconfiguration: 99======================== 100 101Dynamic reconfigurability is a useful addition to netconsole that enables 102remote logging targets to be dynamically added, removed, or have their 103parameters reconfigured at runtime from a configfs-based userspace interface. 104 105To include this feature, select CONFIG_NETCONSOLE_DYNAMIC when building the 106netconsole module (or kernel, if netconsole is built-in). 107 108Some examples follow (where configfs is mounted at the /sys/kernel/config 109mountpoint). 110 111To add a remote logging target (target names can be arbitrary):: 112 113 cd /sys/kernel/config/netconsole/ 114 mkdir target1 115 116Note that newly created targets have default parameter values (as mentioned 117above) and are disabled by default -- they must first be enabled by writing 118"1" to the "enabled" attribute (usually after setting parameters accordingly) 119as described below. 120 121To remove a target:: 122 123 rmdir /sys/kernel/config/netconsole/othertarget/ 124 125The interface exposes these parameters of a netconsole target to userspace: 126 127 ============== ================================= ============ 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 ============== ================================= ============ 139 140The "enabled" attribute is also used to control whether the parameters of 141a target can be updated or not -- you can modify the parameters of only 142disabled targets (i.e. if "enabled" is 0). 143 144To update a target's parameters:: 145 146 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 152 153You can also update the local interface dynamically. This is especially 154useful if you want to use interfaces that have newly come up (and may not 155have existed when netconsole was loaded / initialized). 156 157Netconsole targets defined at boot time (or module load time) with the 158`netconsole=` param are assigned the name `cmdline<index>`. For example, the 159first target in the parameter is named `cmdline0`. You can control and modify 160these targets by creating configfs directories with the matching name. 161 162Let's suppose you have two netconsole targets defined at boot time:: 163 164 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 165 166You can modify these targets in runtime by creating the following targets:: 167 168 mkdir cmdline0 169 cat cmdline0/remote_ip 170 10.0.0.2 171 172 mkdir cmdline1 173 cat cmdline1/remote_ip 174 10.0.0.3 175 176Append User Data 177---------------- 178 179Custom user data can be appended to the end of messages with netconsole 180dynamic configuration enabled. User data entries can be modified without 181changing the "enabled" attribute of a target. 182 183Directories (keys) under `userdata` are limited to 53 character length, and 184data in `userdata/<key>/value` are limited to 200 bytes:: 185 186 cd /sys/kernel/config/netconsole && mkdir cmdline0 187 cd cmdline0 188 mkdir userdata/foo 189 echo bar > userdata/foo/value 190 mkdir userdata/qux 191 echo baz > userdata/qux/value 192 193Messages will now include this additional user data:: 194 195 echo "This is a message" > /dev/kmsg 196 197Sends:: 198 199 12,607,22085407756,-;This is a message 200 foo=bar 201 qux=baz 202 203Preview the userdata that will be appended with:: 204 205 cd /sys/kernel/config/netconsole/cmdline0/userdata 206 for f in `ls userdata`; do echo $f=$(cat userdata/$f/value); done 207 208If a `userdata` entry is created but no data is written to the `value` file, 209the entry will be omitted from netconsole messages:: 210 211 cd /sys/kernel/config/netconsole && mkdir cmdline0 212 cd cmdline0 213 mkdir userdata/foo 214 echo bar > userdata/foo/value 215 mkdir userdata/qux 216 217The `qux` key is omitted since it has no value:: 218 219 echo "This is a message" > /dev/kmsg 220 12,607,22085407756,-;This is a message 221 foo=bar 222 223Delete `userdata` entries with `rmdir`:: 224 225 rmdir /sys/kernel/config/netconsole/cmdline0/userdata/qux 226 227.. warning:: 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:: 230 231 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 236 val2 237 238 It is recommended to not write user data values with newlines. 239 240Extended console: 241================= 242 243If '+' is prefixed to the configuration line or "extended" config file 244is set to 1, extended console support is enabled. An example boot 245param follows:: 246 247 linux netconsole=+4444@10.0.0.1/eth1,9353@10.0.0.2/12:34:56:78:9a:bc 248 249Log messages are transmitted with extended metadata header in the 250following format which is the same as /dev/kmsg:: 251 252 <level>,<sequnum>,<timestamp>,<contflag>;<message text> 253 254If 'r' (release) feature is enabled, the kernel release version is 255prepended to the start of the message. Example:: 256 257 6.4.0,6,444,501151268,-;netconsole: network logging started 258 259Non printable characters in <message text> are escaped using "\xff" 260notation. If the message contains optional dictionary, verbatim 261newline is used as the delimiter. 262 263If a message doesn't fit in certain number of bytes (currently 1000), 264the message is split into multiple fragments by netconsole. These 265fragments are transmitted with "ncfrag" header field added:: 266 267 ncfrag=<byte-offset>/<total-bytes> 268 269For example, assuming a lot smaller chunk size, a message "the first 270chunk, the 2nd chunk." may be split as follows:: 271 272 6,416,1758426,-,ncfrag=0/31;the first chunk, 273 6,416,1758426,-,ncfrag=16/31; the 2nd chunk. 274 275Miscellaneous notes: 276==================== 277 278.. Warning:: 279 280 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. 283 284.. Tip:: 285 286 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. 289 290.. Tip:: 291 292 to find out the MAC address of, say, 10.0.0.2, you may try using:: 293 294 ping -c 1 10.0.0.2 ; /sbin/arp -n | grep 10.0.0.2 295 296.. Tip:: 297 298 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. 302 303.. note:: 304 305 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. 309 310.. note:: 311 312 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:: 316 317 dmesg -n 8 318 319 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 323 for details. 324 325Netconsole was designed to be as instantaneous as possible, to 326enable the logging of even the most critical kernel bugs. It works 327from IRQ contexts as well, and does not enable interrupts while 328sending packets. Due to these unique needs, configuration cannot 329be more automatic, and some fundamental limitations will remain: 330only IP networks, UDP packets and ethernet devices are supported. 331