linux/Documentation/networking/netconsole.txt
<<
>>
Prefs
   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 has several options to receive the kernel messages,
  55for example:
  56
  571) syslogd
  58
  592) netcat
  60
  61   On distributions using a BSD-based netcat version (e.g. Fedora,
  62   openSUSE and Ubuntu) the listening port must be specified without
  63   the -p switch:
  64
  65   'nc -u -l -p <port>' / 'nc -u -l <port>' or
  66   'netcat -u -l -p <port>' / 'netcat -u -l <port>'
  67
  683) socat
  69
  70   'socat udp-recv:<port> -'
  71
  72Dynamic reconfiguration:
  73========================
  74
  75Dynamic reconfigurability is a useful addition to netconsole that enables
  76remote logging targets to be dynamically added, removed, or have their
  77parameters reconfigured at runtime from a configfs-based userspace interface.
  78[ Note that the parameters of netconsole targets that were specified/created
  79from the boot/module option are not exposed via this interface, and hence
  80cannot be modified dynamically. ]
  81
  82To include this feature, select CONFIG_NETCONSOLE_DYNAMIC when building the
  83netconsole module (or kernel, if netconsole is built-in).
  84
  85Some examples follow (where configfs is mounted at the /sys/kernel/config
  86mountpoint).
  87
  88To add a remote logging target (target names can be arbitrary):
  89
  90 cd /sys/kernel/config/netconsole/
  91 mkdir target1
  92
  93Note that newly created targets have default parameter values (as mentioned
  94above) and are disabled by default -- they must first be enabled by writing
  95"1" to the "enabled" attribute (usually after setting parameters accordingly)
  96as described below.
  97
  98To remove a target:
  99
 100 rmdir /sys/kernel/config/netconsole/othertarget/
 101
 102The interface exposes these parameters of a netconsole target to userspace:
 103
 104        enabled         Is this target currently enabled?       (read-write)
 105        dev_name        Local network interface name            (read-write)
 106        local_port      Source UDP port to use                  (read-write)
 107        remote_port     Remote agent's UDP port                 (read-write)
 108        local_ip        Source IP address to use                (read-write)
 109        remote_ip       Remote agent's IP address               (read-write)
 110        local_mac       Local interface's MAC address           (read-only)
 111        remote_mac      Remote agent's MAC address              (read-write)
 112
 113The "enabled" attribute is also used to control whether the parameters of
 114a target can be updated or not -- you can modify the parameters of only
 115disabled targets (i.e. if "enabled" is 0).
 116
 117To update a target's parameters:
 118
 119 cat enabled                            # check if enabled is 1
 120 echo 0 > enabled                       # disable the target (if required)
 121 echo eth2 > dev_name                   # set local interface
 122 echo 10.0.0.4 > remote_ip              # update some parameter
 123 echo cb:a9:87:65:43:21 > remote_mac    # update more parameters
 124 echo 1 > enabled                       # enable target again
 125
 126You can also update the local interface dynamically. This is especially
 127useful if you want to use interfaces that have newly come up (and may not
 128have existed when netconsole was loaded / initialized).
 129
 130Miscellaneous notes:
 131====================
 132
 133WARNING: the default target ethernet setting uses the broadcast
 134ethernet address to send packets, which can cause increased load on
 135other systems on the same ethernet segment.
 136
 137TIP: some LAN switches may be configured to suppress ethernet broadcasts
 138so it is advised to explicitly specify the remote agents' MAC addresses
 139from the config parameters passed to netconsole.
 140
 141TIP: to find out the MAC address of, say, 10.0.0.2, you may try using:
 142
 143 ping -c 1 10.0.0.2 ; /sbin/arp -n | grep 10.0.0.2
 144
 145TIP: in case the remote logging agent is on a separate LAN subnet than
 146the sender, it is suggested to try specifying the MAC address of the
 147default gateway (you may use /sbin/route -n to find it out) as the
 148remote MAC address instead.
 149
 150NOTE: the network device (eth1 in the above case) can run any kind
 151of other network traffic, netconsole is not intrusive. Netconsole
 152might cause slight delays in other traffic if the volume of kernel
 153messages is high, but should have no other impact.
 154
 155NOTE: if you find that the remote logging agent is not receiving or
 156printing all messages from the sender, it is likely that you have set
 157the "console_loglevel" parameter (on the sender) to only send high
 158priority messages to the console. You can change this at runtime using:
 159
 160 dmesg -n 8
 161
 162or by specifying "debug" on the kernel command line at boot, to send
 163all kernel messages to the console. A specific value for this parameter
 164can also be set using the "loglevel" kernel boot option. See the
 165dmesg(8) man page and Documentation/kernel-parameters.txt for details.
 166
 167Netconsole was designed to be as instantaneous as possible, to
 168enable the logging of even the most critical kernel bugs. It works
 169from IRQ contexts as well, and does not enable interrupts while
 170sending packets. Due to these unique needs, configuration cannot
 171be more automatic, and some fundamental limitations will remain:
 172only IP networks, UDP packets and ethernet devices are supported.
 173
lxr.linux.no kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.