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Sometimes when some applications start logging to console, it can really bleed your eyes when something wrong happens. Not mentioning about standard output or standard errors which are rather easily controllable but it could be anything else invoked by kernel such as iptables logging when you  would like to log any rules to a log file or default syslog. But “hideously” such applications log to system console too and can surprise you when you plug monitor and see a messy mesh of error loggings all the way back and forth, specifically when a machine is virtual – of course in that case VM would be unresponsive

I was trying to look into syslog.conf to control but found out that this is not what a syslog daemon has dominance over; in fact, needs kernel-level  logging modified. And this can be done through either of these ways.

Instruct kernel to log only “errors” instead of informational notifications.

Controlled by ‘printk’ kernel parameter. Defaults are:

# sysctl -a | grep printk
kernel.printk_ratelimit_burst = 10
kernel.printk_ratelimit = 5
kernel.printk = 6    4    1    7
# find /proc -name '*printk*' -exec cat {} \;
6    4    1    7

Just change ’6 4 1 7′ to ’3 4 1 7′.

# echo "kernel.printk = 3 4 1 7" >> /etc/sysctl.conf
# sysctl -p

Documentation goes over here, if you’re intrested /usr/share/doc/kernel-doc-2.6.18/Documentation/sysctl/kernel.txt

printk:

The four values in printk denote:
 console_loglevel,
 default_message_loglevel,
 minimum_console_loglevel and
 default_console_loglevel respectively.

These values influence printk() behavior when printing or
 logging error messages. See 'man 2 syslog' for more info on
 the different loglevels.

- console_loglevel: messages with a higher priority than this will be printed to the console
 - default_message_level: messages without an explicit priority will be printed with this priority
 - minimum_console_loglevel: minimum (highest) value to which console_loglevel can be set
 - default_console_loglevel: default value for console_loglevel

Definitions of kernel logging numbers, from Syslog’s manual:

#define KERN_EMERG    "<0>"  /* system is unusable               */
 #define KERN_ALERT    "<1>"  /* action must be taken immediately */
 #define KERN_CRIT     "<2>"  /* critical conditions              */
 #define KERN_ERR      "<3>"  /* error conditions                 */
 #define KERN_WARNING  "<4>"  /* warning conditions               */
 #define KERN_NOTICE   "<5>"  /* normal but significant condition */
 #define KERN_INFO     "<6>"  /* informational                    */
 #define KERN_DEBUG    "<7>"  /* debug-level messages             */

Change kernel ring buffer logging level

Just keep in mind, using this approach only limits console logging and issuing dmesg would still print the over all logs.

# dmesg -n 3
or
# dmesg -n 4