linux/Documentation/networking/netconsole.txt
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   1
   2started by Ingo Molnar <mingo@redhat.com>, 2001.09.17
   32.6 port and netpoll api by Matt Mackall <mpm@selenic.com>, Sep 9 2003
   4
   5Please send bug reports to Matt Mackall <mpm@selenic.com>
   6and Satyam Sharma <satyam.sharma@gmail.com>
   7
   8Introduction:
   9=============
  10
  11This module logs kernel printk messages over UDP allowing debugging of
  12problem where disk logging fails and serial consoles are impractical.
  13
  14It can be used either built-in or as a module. As a built-in,
  15netconsole initializes immediately after NIC cards and will bring up
  16the specified interface as soon as possible. While this doesn't allow
  17capture of early kernel panics, it does capture most of the boot
  18process.
  19
  20Sender and receiver configuration:
  21==================================
  22
  23It takes a string configuration parameter "netconsole" in the
  24following format:
  25
  26 netconsole=[src-port]@[src-ip]/[<dev>],[tgt-port]@<tgt-ip>/[tgt-macaddr]
  27
  28   where
  29        src-port      source for UDP packets (defaults to 6665)
  30        src-ip        source IP to use (interface address)
  31        dev           network interface (eth0)
  32        tgt-port      port for logging agent (6666)
  33        tgt-ip        IP address for logging agent
  34        tgt-macaddr   ethernet MAC address for logging agent (broadcast)
  35
  36Examples:
  37
  38 linux netconsole=4444@10.0.0.1/eth1,9353@10.0.0.2/12:34:56:78:9a:bc
  39
  40  or
  41
  42 insmod netconsole netconsole=@/,@10.0.0.2/
  43
  44It also supports logging to multiple remote agents by specifying
  45parameters for the multiple agents separated by semicolons and the
  46complete string enclosed in "quotes", thusly:
  47
  48 modprobe netconsole netconsole="@/,@10.0.0.2/;@/eth1,6892@10.0.0.3/"
  49
  50Built-in netconsole starts immediately after the TCP stack is
  51initialized and attempts to bring up the supplied dev at the supplied
  52address.
  53
  54The remote host can run either 'netcat -u -l -p <port>',
  55'nc -l -u <port>' or syslogd.
  56
  57Dynamic reconfiguration:
  58========================
  59
  60Dynamic reconfigurability is a useful addition to netconsole that enables
  61remote logging targets to be dynamically added, removed, or have their
  62parameters reconfigured at runtime from a configfs-based userspace interface.
  63[ Note that the parameters of netconsole targets that were specified/created
  64from the boot/module option are not exposed via this interface, and hence
  65cannot be modified dynamically. ]
  66
  67To include this feature, select CONFIG_NETCONSOLE_DYNAMIC when building the
  68netconsole module (or kernel, if netconsole is built-in).
  69
  70Some examples follow (where configfs is mounted at the /sys/kernel/config
  71mountpoint).
  72
  73To add a remote logging target (target names can be arbitrary):
  74
  75 cd /sys/kernel/config/netconsole/
  76 mkdir target1
  77
  78Note that newly created targets have default parameter values (as mentioned
  79above) and are disabled by default -- they must first be enabled by writing
  80"1" to the "enabled" attribute (usually after setting parameters accordingly)
  81as described below.
  82
  83To remove a target:
  84
  85 rmdir /sys/kernel/config/netconsole/othertarget/
  86
  87The interface exposes these parameters of a netconsole target to userspace:
  88
  89        enabled         Is this target currently enabled?       (read-write)
  90        dev_name        Local network interface name            (read-write)
  91        local_port      Source UDP port to use                  (read-write)
  92        remote_port     Remote agent's UDP port                 (read-write)
  93        local_ip        Source IP address to use                (read-write)
  94        remote_ip       Remote agent's IP address               (read-write)
  95        local_mac       Local interface's MAC address           (read-only)
  96        remote_mac      Remote agent's MAC address              (read-write)
  97
  98The "enabled" attribute is also used to control whether the parameters of
  99a target can be updated or not -- you can modify the parameters of only
 100disabled targets (i.e. if "enabled" is 0).
 101
 102To update a target's parameters:
 103
 104 cat enabled                            # check if enabled is 1
 105 echo 0 > enabled                       # disable the target (if required)
 106 echo eth2 > dev_name                   # set local interface
 107 echo 10.0.0.4 > remote_ip              # update some parameter
 108 echo cb:a9:87:65:43:21 > remote_mac    # update more parameters
 109 echo 1 > enabled                       # enable target again
 110
 111You can also update the local interface dynamically. This is especially
 112useful if you want to use interfaces that have newly come up (and may not
 113have existed when netconsole was loaded / initialized).
 114
 115Miscellaneous notes:
 116====================
 117
 118WARNING: the default target ethernet setting uses the broadcast
 119ethernet address to send packets, which can cause increased load on
 120other systems on the same ethernet segment.
 121
 122TIP: some LAN switches may be configured to suppress ethernet broadcasts
 123so it is advised to explicitly specify the remote agents' MAC addresses
 124from the config parameters passed to netconsole.
 125
 126TIP: to find out the MAC address of, say, 10.0.0.2, you may try using:
 127
 128 ping -c 1 10.0.0.2 ; /sbin/arp -n | grep 10.0.0.2
 129
 130TIP: in case the remote logging agent is on a separate LAN subnet than
 131the sender, it is suggested to try specifying the MAC address of the
 132default gateway (you may use /sbin/route -n to find it out) as the
 133remote MAC address instead.
 134
 135NOTE: the network device (eth1 in the above case) can run any kind
 136of other network traffic, netconsole is not intrusive. Netconsole
 137might cause slight delays in other traffic if the volume of kernel
 138messages is high, but should have no other impact.
 139
 140NOTE: if you find that the remote logging agent is not receiving or
 141printing all messages from the sender, it is likely that you have set
 142the "console_loglevel" parameter (on the sender) to only send high
 143priority messages to the console. You can change this at runtime using:
 144
 145 dmesg -n 8
 146
 147or by specifying "debug" on the kernel command line at boot, to send
 148all kernel messages to the console. A specific value for this parameter
 149can also be set using the "loglevel" kernel boot option. See the
 150dmesg(8) man page and Documentation/kernel-parameters.txt for details.
 151
 152Netconsole was designed to be as instantaneous as possible, to
 153enable the logging of even the most critical kernel bugs. It works
 154from IRQ contexts as well, and does not enable interrupts while
 155sending packets. Due to these unique needs, configuration cannot
 156be more automatic, and some fundamental limitations will remain:
 157only IP networks, UDP packets and ethernet devices are supported.
 158
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