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+started by Ingo Molnar <mingo@redhat.com>, 2001.09.17
+2.6 port and netpoll api by Matt Mackall <mpm@selenic.com>, Sep 9 2003
+IPv6 support by Cong Wang <xiyou.wangcong@gmail.com>, Jan 1 2013
+Please send bug reports to Matt Mackall <mpm@selenic.com>
+Satyam Sharma <satyam.sharma@gmail.com>, and Cong Wang <xiyou.wangcong@gmail.com>
+This module logs kernel printk messages over UDP allowing debugging of
+problem 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 up
+the specified interface as soon as possible. While this doesn't allow
+capture of early kernel panics, it does capture most of the boot
+Sender and receiver configuration:
+It takes a string configuration parameter "netconsole" in the
+following format:
+ netconsole=[src-port]@[src-ip]/[<dev>],[tgt-port]@<tgt-ip>/[tgt-macaddr]
+ where
+ 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)
+ linux netconsole=4444@,9353@
+ or
+ insmod netconsole netconsole=@/,@
+ or using IPv6
+ insmod netconsole netconsole=@/,@fd00:1:2:3::1/
+It also supports logging to multiple remote agents by specifying
+parameters for the multiple agents separated by semicolons and the
+complete string enclosed in "quotes", thusly:
+ modprobe netconsole netconsole="@/,@;@/eth1,6892@"
+Built-in netconsole starts immediately after the TCP stack is
+initialized and attempts to bring up the supplied dev at the supplied
+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 without
+ the -p switch:
+ 'nc -u -l -p <port>' / 'nc -u -l <port>' or
+ 'netcat -u -l -p <port>' / 'netcat -u -l <port>'
+3) socat
+ 'socat udp-recv:<port> -'
+Dynamic reconfiguration:
+Dynamic reconfigurability is a useful addition to netconsole that enables
+remote logging targets to be dynamically added, removed, or have their
+parameters reconfigured at runtime from a configfs-based userspace interface.
+[ Note that the parameters of netconsole targets that were specified/created
+from the boot/module option are not exposed via this interface, and hence
+cannot be modified dynamically. ]
+To include this feature, select CONFIG_NETCONSOLE_DYNAMIC when building the
+netconsole module (or kernel, if netconsole is built-in).
+Some examples follow (where configfs is mounted at the /sys/kernel/config
+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 mentioned
+above) 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)
+ 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 of
+a target can be updated or not -- you can modify the parameters of only
+disabled targets (i.e. if "enabled" is 0).
+To update a target's parameters:
+ cat enabled # check if enabled is 1
+ echo 0 > enabled # disable the target (if required)
+ echo eth2 > dev_name # set local interface
+ echo > remote_ip # update some parameter
+ echo cb:a9:87:65:43:21 > remote_mac # update more parameters
+ echo 1 > enabled # enable target again
+You can also update the local interface dynamically. This is especially
+useful if you want to use interfaces that have newly come up (and may not
+have existed when netconsole was loaded / initialized).
+Miscellaneous notes:
+WARNING: the default target ethernet setting uses the broadcast
+ethernet address to send packets, which can cause increased load on
+other systems on the same ethernet segment.
+TIP: some LAN switches may be configured to suppress ethernet broadcasts
+so it is advised to explicitly specify the remote agents' MAC addresses
+from the config parameters passed to netconsole.
+TIP: to find out the MAC address of, say,, you may try using:
+ ping -c 1 ; /sbin/arp -n | grep
+TIP: in case the remote logging agent is on a separate LAN subnet than
+the sender, it is suggested to try specifying the MAC address of the
+default gateway (you may use /sbin/route -n to find it out) as the
+remote MAC address instead.
+NOTE: the network device (eth1 in the above case) can run any kind
+of other network traffic, netconsole is not intrusive. Netconsole
+might cause slight delays in other traffic if the volume of kernel
+messages is high, but should have no other impact.
+NOTE: if you find that the remote logging agent is not receiving or
+printing all messages from the sender, it is likely that you have set
+the "console_loglevel" parameter (on the sender) to only send high
+priority messages to the console. You can change this at runtime using:
+ dmesg -n 8
+or by specifying "debug" on the kernel command line at boot, to send
+all kernel messages to the console. A specific value for this parameter
+can also be set using the "loglevel" kernel boot option. See the
+dmesg(8) man page and Documentation/kernel-parameters.txt for details.
+Netconsole was designed to be as instantaneous as possible, to
+enable the logging of even the most critical kernel bugs. It works
+from IRQ contexts as well, and does not enable interrupts while
+sending packets. Due to these unique needs, configuration cannot
+be more automatic, and some fundamental limitations will remain:
+only IP networks, UDP packets and ethernet devices are supported.