linux/Documentation/networking/netconsole.txt

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

Please send bug reports to Matt Mackall <mpm@selenic.com>

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.

It takes a string configuration parameter "netconsole" in the
following format:

 netconsole=[src-port]@[src-ip]/[<dev>],[tgt-port]@<tgt-ip>/[tgt-macaddr]

   where
        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/

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 can run either 'netcat -u -l -p <port>' or syslogd.

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/kernel-parameters.txt 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.