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In case you are experiencing a kernel crash ([[oops]]) and have already [[Hardware testing|tested your hardware]], you should report what kernel says to the console (i.e. an [[oops]] text) to [https://bugs.openvz.org/ bug tracker]. Sometimes kernel crashes so badly that <tt>syslogd</tt> is not working and what kernel says is never written to a file. If this is the case, you have to catch what kernel says. There are several ways possible.

With RHEL6-based servers, kdump is pre-configured. See http://kb.odin.com/en/10044 to check the configuration. Dumps can be found under <code>/var/crash/</code> directory.

If kernel backtrace is not long enough there are chances that it can fit into a single screen. In that case, you can just make a photo of the kernel crash screen and attach it to the bug report. If you do not have a camera, you still can carefully write down (using a piece of paper and a pen, that is) what you see on the screen, and later type it into the bug report.

Here is a description of a common routine that is necessary to set up a serial console.

First of all you should make sure that your node has a [[w:Serial port|serial port]]. If there is no such port then

unfortunately this way is not for you.

Then you need to find a second machine with a serial port on it.

This machine will be used to collect logs from your primary machine. Further you need to acquire

so-called [[w:Serial cable|null modem cable (a.k.a. serial cable)]] and it must be long enough to connect these two machines.

In your boot loader add the following kernel parameters:

<pre>

</pre>

{{Warning|make sure kernel command line does '''not''' contain the word '''<code>quiet</code>''', otherwise most of the kernel messages will not be printed to console.}}

For example, in GRUB boot loader configuration file <tt>/boot/grub/grub.conf</tt> it looks like this:

<pre>

</pre>

Kernel loaded with such parameters will send all kernel messages to /dev/ttyS0 (first serial port, a.k.a. COM1). If you have several ports, make sure that your null modem cable is connected to the appropriate port.

On the second node you should run any software that can log from /dev/ttyS0.

It can be usual

<pre>

or something more sophisticated: syslogd, watchtty etc.

One more important thing. 115200 in the example above is the rate of emitting port. Receiving port must also work at the same rate. For example, to tune ttyS0 rate use stty program like this:

<pre>

</pre>

Some other serial port parameters, like parity, number of stop bits etc. should also be the same on both sides.

If you use binary kernel from openvz.org, it already has netconsole module compiled in, so just skip to next section.

If you build the kernel yourself, you might need to check if netconsole is compiled. To that effect, change to your kernel source directory and grep your kernel <tt>.config</tt> for NETCONSOLE:

<pre>

</pre>

If you see nothing or "# CONFIG_NETCONSOLE is not set" you need to recompile the kernel.

Edit your kernel configuration file <tt>.config</tt> with a text editor (<tt>nano .config</tt> or <tt>vi .config</tt>). Set netconsole to Y or M (depending on whether you want it as a module or built into the kernel; I have compiled it as a module):

<pre>CONFIG_NETCONSOLE=m</pre>

Save the file, then recompile the kernel:

<pre>

# make bzImage && make modules && make modules_install

Update your bootloader for the new updated kernel. In my case I use LILO so I just type lilo at the prompt.

Reboot into new kernel.

Next you want your netconsole to send the request to somewhere. Load netconsole module, specifying the remote server parameters:

<pre># modprobe netconsole netconsole=4444@10.0.2.1/eth0,6666@10.0.2.2/00:05:5D:34:11:AF</pre>

This will load the module with your settings. Replace your local IP address with where <tt>10.0.2.1</tt> is, <tt>eth0</tt> with your network interface card device, <tt>6666</tt> with the remote netconsole port (UDP), and <tt>10.0.2.2</tt> with your remote netconsole server IP. Also add in the mac address of your remote netconsole server, which in my case was 00:05:5D:34:11:AF. You can get the MAC address using arp utility:

<pre>

# ping -c 1 10.0.2.2

</pre>

If the remote netconsole server is outside of local network area, use mac address of default gateway or router on local network area and IP address of remote netconsole server (loging via WAN). Mac address of default gateway or router you can get the same way (ping to gateway/router and see mac address via arp command).

Netconsole documentation is available from <tt>Documentation/networking/netconsole.txt</tt> file under your kernel source directory.

To durable logging boot process before root filesystem mounting network device driver and netconsole module must be loaded from initd.

RedHat 5/CentOS 5:

echo 'MODULES+="<network-driver-module> netconsole "' > /etc/sysconfig/mkinitrd/netconsole

echo 'netconsole netconsole=<sport>@<saddr>/<dev>,<dport>@<daddr>/<dmac>' >> /etc/initramfs-tools/modules

and rebuild initrd

/etc/rsyslog.d/netconsole.conf

$template NetconsoleFormat,"%rawmsg%"

$UDPServerRun 6666

Setup '''netcat''' ('''nc''' on some Linux distributions) on your console server to listen on port 6666 UDP:

<pre>netcat -u -l -p6666</pre>

When your kernel prints something on the console, the text will be also captured on this netconsole server.

For automatic care about capturing on console server you can use init respawn feature in this way:

<pre>

echo "n1:23:respawn:/bin/netcat -u -l -p 6666 >> /var/log/netconsole" >> /etc/inittab

</pre>

If you want the log to contain date/time of each line, you can use '''awk''' like this:

See man strftime for info about how to tailor strftime() argument to your needs.

Note that if you want to add this to /etc/inittab, it should be done like this:

strftime("%d %b %Y %H:%M:%S"), \$0; fflush(stdout);}' \

>> /var/log/netconsole" > /usr/local/sbin/netconsole

telinit q

For long term capturing you would like to do log rotating some way. With logrotate you can do it by creating config file /etc/logrotate.d/netconsole:

<pre>

/var/log/netconsole {

</pre>

For more details, see man logrotate.

First check log level of console messages on OpenVZ side by:

<pre>cat /proc/sys/kernel/printk</pre>

First number should be 7 for testing. You can arrange it by:

<pre>sysctl -w kernel.printk="7 4 1 7"</pre>

After testing you can restore previous setting the same way.

Load '''netconsole''' module (see above) and on the console server run netcat (nc) command. On OpenVZ side provoke any console message, for example connect any USB hardware or try command:

If you see any console message on OpenVZ side, you should see message on console server too. If not, something is wrong. When debugging a problem, do not use tcpdump on OpenVZ side — it is not able to show netconsole packets. Instead, use tcpdump on console server. Quite a common source of problems with netconsole are firewalls.

* [[SysRq debugger]]

* [[Kernel debug options]]

* [http://kb.odin.com/en/10044 How to configure kdump (kernel crash dump)]

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