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 .. SPDX-License-Identifier: GPL-2.0
 
 ==========
 Netconsole
 ==========
 
 
 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
 
 Extended console support by Tejun Heo <tj@kernel.org>, May 1 2015
 
 Please send bug reports to Matt Mackall <mpm@selenic.com>
 Satyam Sharma <satyam.sharma@gmail.com>, and Cong Wang <xiyou.wangcong@gmail.com>
 
 Introduction:
 =============
 
 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
 process.
 
 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
         +             if present, enable extended console support
         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)
 
 Examples::
 
  linux netconsole=4444@10.0.0.1/eth1,9353@10.0.0.2/12:34:56:78:9a:bc
 
 or::
 
  insmod netconsole netconsole=@/,@10.0.0.2/
 
 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="@/,@10.0.0.2/;@/eth1,6892@10.0.0.3/"
 
 Built-in netconsole starts immediately after the TCP stack is
 initialized and attempts to bring up the supplied dev at the supplied
 address.
 
 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
 mountpoint).
 
 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)
         extended        Extended mode 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 10.0.0.4 > 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).
 
 Extended console:
 =================
 
 If '+' is prefixed to the configuration line or "extended" config file
 is set to 1, extended console support is enabled. An example boot
 param follows::
 
  linux netconsole=+4444@10.0.0.1/eth1,9353@10.0.0.2/12:34:56:78:9a:bc
 
 Log messages are transmitted with extended metadata header in the
 following format which is the same as /dev/kmsg::
 
  <level>,<sequnum>,<timestamp>,<contflag>;<message text>
 
 Non printable characters in <message text> are escaped using "\xff"
 notation. If the message contains optional dictionary, verbatim
 newline is used as the delimeter.
 
 If a message doesn't fit in certain number of bytes (currently 1000),
 the message is split into multiple fragments by netconsole. These
 fragments are transmitted with "ncfrag" header field added::
 
  ncfrag=<byte-offset>/<total-bytes>
 
 For example, assuming a lot smaller chunk size, a message "the first
 chunk, the 2nd chunk." may be split as follows::
 
  6,416,1758426,-,ncfrag=0/31;the first chunk,
  6,416,1758426,-,ncfrag=16/31; the 2nd chunk.
 
 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, 10.0.0.2, you may try using::
 
         ping -c 1 10.0.0.2 ; /sbin/arp -n | grep 10.0.0.2
 
 .. 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/admin-guide/kernel-parameters.rst
    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.

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