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 18Please send bug reports to Matt Mackall <mpm@selenic.com> 19Satyam Sharma <satyam.sharma@gmail.com>, and Cong Wang <xiyou.wangcong@gmail.com> 20 21Introduction: 22============= 23 24This module logs kernel printk messages over UDP allowing debugging of 25problem where disk logging fails and serial consoles are impractical. 26 27It can be used either built-in or as a module. As a built-in, 28netconsole initializes immediately after NIC cards and will bring up 29the specified interface as soon as possible. While this doesn't allow 30capture of early kernel panics, it does capture most of the boot 31process. 32 33Sender and receiver configuration: 34================================== 35 36It takes a string configuration parameter "netconsole" in the 37following format:: 38 39 netconsole=[+][r][src-port]@[src-ip]/[<dev>],[tgt-port]@<tgt-ip>/[tgt-macaddr] 40 41 where 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) 50 51Examples:: 52 53 linux netconsole=4444@10.0.0.1/eth1,9353@10.0.0.2/12:34:56:78:9a:bc 54 55or:: 56 57 insmod netconsole netconsole=@/,@10.0.0.2/ 58 59or using IPv6:: 60 61 insmod netconsole netconsole=@/,@fd00:1:2:3::1/ 62 63It also supports logging to multiple remote agents by specifying 64parameters for the multiple agents separated by semicolons and the 65complete string enclosed in "quotes", thusly:: 66 67 modprobe netconsole netconsole="@/,@10.0.0.2/;@/eth1,6892@10.0.0.3/" 68 69Built-in netconsole starts immediately after the TCP stack is 70initialized and attempts to bring up the supplied dev at the supplied 71address. 72 73The remote host has several options to receive the kernel messages, 74for example: 75 761) syslogd 77 782) netcat 79 80 On distributions using a BSD-based netcat version (e.g. Fedora, 81 openSUSE and Ubuntu) the listening port must be specified without 82 the -p switch:: 83 84 nc -u -l -p <port>' / 'nc -u -l <port> 85 86 or:: 87 88 netcat -u -l -p <port>' / 'netcat -u -l <port> 89 903) socat 91 92:: 93 94 socat udp-recv:<port> - 95 96Dynamic reconfiguration: 97======================== 98 99Dynamic reconfigurability is a useful addition to netconsole that enables 100remote logging targets to be dynamically added, removed, or have their 101parameters reconfigured at runtime from a configfs-based userspace interface. 102 103To include this feature, select CONFIG_NETCONSOLE_DYNAMIC when building the 104netconsole module (or kernel, if netconsole is built-in). 105 106Some examples follow (where configfs is mounted at the /sys/kernel/config 107mountpoint). 108 109To add a remote logging target (target names can be arbitrary):: 110 111 cd /sys/kernel/config/netconsole/ 112 mkdir target1 113 114Note that newly created targets have default parameter values (as mentioned 115above) and are disabled by default -- they must first be enabled by writing 116"1" to the "enabled" attribute (usually after setting parameters accordingly) 117as described below. 118 119To remove a target:: 120 121 rmdir /sys/kernel/config/netconsole/othertarget/ 122 123The interface exposes these parameters of a netconsole target to userspace: 124 125 ============== ================================= ============ 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 ============== ================================= ============ 137 138The "enabled" attribute is also used to control whether the parameters of 139a target can be updated or not -- you can modify the parameters of only 140disabled targets (i.e. if "enabled" is 0). 141 142To update a target's parameters:: 143 144 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 150 151You can also update the local interface dynamically. This is especially 152useful if you want to use interfaces that have newly come up (and may not 153have existed when netconsole was loaded / initialized). 154 155Netconsole targets defined at boot time (or module load time) with the 156`netconsole=` param are assigned the name `cmdline<index>`. For example, the 157first target in the parameter is named `cmdline0`. You can control and modify 158these targets by creating configfs directories with the matching name. 159 160Let's suppose you have two netconsole targets defined at boot time:: 161 162 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 163 164You can modify these targets in runtime by creating the following targets:: 165 166 mkdir cmdline0 167 cat cmdline0/remote_ip 168 10.0.0.2 169 170 mkdir cmdline1 171 cat cmdline1/remote_ip 172 10.0.0.3 173 174Extended console: 175================= 176 177If '+' is prefixed to the configuration line or "extended" config file 178is set to 1, extended console support is enabled. An example boot 179param follows:: 180 181 linux netconsole=+4444@10.0.0.1/eth1,9353@10.0.0.2/12:34:56:78:9a:bc 182 183Log messages are transmitted with extended metadata header in the 184following format which is the same as /dev/kmsg:: 185 186 <level>,<sequnum>,<timestamp>,<contflag>;<message text> 187 188If 'r' (release) feature is enabled, the kernel release version is 189prepended to the start of the message. Example:: 190 191 6.4.0,6,444,501151268,-;netconsole: network logging started 192 193Non printable characters in <message text> are escaped using "\xff" 194notation. If the message contains optional dictionary, verbatim 195newline is used as the delimiter. 196 197If a message doesn't fit in certain number of bytes (currently 1000), 198the message is split into multiple fragments by netconsole. These 199fragments are transmitted with "ncfrag" header field added:: 200 201 ncfrag=<byte-offset>/<total-bytes> 202 203For example, assuming a lot smaller chunk size, a message "the first 204chunk, the 2nd chunk." may be split as follows:: 205 206 6,416,1758426,-,ncfrag=0/31;the first chunk, 207 6,416,1758426,-,ncfrag=16/31; the 2nd chunk. 208 209Miscellaneous notes: 210==================== 211 212.. Warning:: 213 214 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. 217 218.. Tip:: 219 220 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. 223 224.. Tip:: 225 226 to find out the MAC address of, say, 10.0.0.2, you may try using:: 227 228 ping -c 1 10.0.0.2 ; /sbin/arp -n | grep 10.0.0.2 229 230.. Tip:: 231 232 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. 236 237.. note:: 238 239 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. 243 244.. note:: 245 246 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:: 250 251 dmesg -n 8 252 253 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 257 for details. 258 259Netconsole was designed to be as instantaneous as possible, to 260enable the logging of even the most critical kernel bugs. It works 261from IRQ contexts as well, and does not enable interrupts while 262sending packets. Due to these unique needs, configuration cannot 263be more automatic, and some fundamental limitations will remain: 264only IP networks, UDP packets and ethernet devices are supported. 265