Use netconsole with syslog on Cumulus Linux Switches

netconsole is a Linux feature that allows you to redirect kernel messages output from dmesg to a location across the network using UDP. You can capture and store these messages on a syslog server to investigate issues on the Cumulus Linux switch that is generating the dmesg output. This is useful where a physical console is not connected and you need to debug kernel events, such as system crashes and unexpected reboots.

netconsole is not a replacement for a physical console. It does not provide an interactive console to the switch; it is a remote logging service only. netconsole is also not available until the network has initialized on boot. Log data from early in the boot cycle is not captured. Use netconsole whenever a physical console is not available to log data.

Configure the netconsole Module

You must reboot the switch at the end of this process to apply the changes.

To configure the netconsole module on your Cumulus Linux switch:

  1. Set up the netconsole kernel module to load on boot:

    cumulus@switch:~$ echo netconsole | sudo tee /etc/modules-load.d/netconsole.conf
  2. Configure the netconsole kernel module options to point to your syslog server.

    cumulus@switch:~$ echo 'options netconsole netconsole=[...]' | sudo tee /etc/modprobe.d/netconsole.conf

    In the above command, replace [...] with the desired configuration. The format for the options is as follows (with default values in parentheses):

             +            if present, enable extended console support
             src-port     source UDP port (6665)
             src-ip       source IP address (<dev> interface address)
             dev          network interface (eth0)
             tgt-port     UDP port of the syslog server (6666)
             tgt-ip       IP address of the syslog server
             tgt-macaddr  Ethernet MAC address of the next hop to the syslog server (broadcast)
    • The netconsole module requires only the tgt-ip parameter; other parameters use their default value if unspecified.
    • Because the netconsole module might get loaded before you configure dev, you must specify src-ip.
    • If the syslog server is not on the same Ethernet segment as the source device, you must specify tgt-macaddr.
    • It is more efficient to specify tgt-macaddr than to use the default, which is an Ethernet broadcast.

    To determine the values dev and tgt-macaddr, use the following procedure. When running the commands, replace the values between angle brackets (< >) to match your configuration.

    1. Use ip route get to determine the interface and IP address of the next hop to the syslog server.

      If the syslog server is reachable through a front port, run:

      cumulus@switch:~$ ip route get <tgt-ip>

      If the syslog server is reachable through the management port (mgmt VRF), run:

      cumulus@switch:~$ ip route get <tgt-ip> vrf mgmt

      Look at the output of the ip route get command. If it is in the following format (without a via keyword), the syslog server is on the same Ethernet segment. The value to use for the dev parameter is the dev value reported in the output, eth0 in this example. The nexthop ip is the first field. dev eth0 table mgmt src uid 1000

      If the output of the ip route get command is in the following format (with a via keyword), you reached the syslog server through a gateway. The value to use for the dev parameter is the dev value reported in the output, eth0 in this example. The nexthop ip is the via value, in this example. via dev eth0 table mgmt src uid 1000
    2. Use the arping command to determine the next hop MAC address:

      cumulus@switch:~$ sudo arping -i <dev> -c1 -r <nexthop ip>

      The value to use for the tgt-macaddr parameter is the output of the previous command.

      For example, to configure a switch with netconsole logging to a syslog server reachable at IP address and port 514, run the following commands:

      cumulus@switch:~$ ip route get vrf mgmt via dev eth0 table mgmt src uid 0
      cumulus@switch:~$ sudo arping -i eth0 -c1 -r
      cumulus@switch:~$ ip -4 addr show dev eth0
      2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master mgmt state UP group default qlen 1000
          inet brd scope global dynamic eth0
             valid_lft 6855sec preferred_lft 6855sec
      cumulus@switch:~$ echo netconsole > /etc/modules-load.d/netconsole.conf
      cumulus@switch:~$ echo 'options netconsole netconsole=@,514@' > /etc/modprobe.d/netconsole.conf
      cumulus@switch:~$ sudo reboot
  3. You can increase or decrease the amount of data you want to log.

    • To increase the amount of data the kernel logs (see Introduction to the Linux kernel log levels at, adjust the log level. By default, a Cumulus Linux switch logs kernel data at level 3 (KERN_ERR). It might be useful to log all the data when trying to debug an issue. To do this, increase the kernel printk value to 7 in the /etc/systctl.d/99-sysctl.conf file:
    $ echo 'kernel.printk = 7 4 1 7' | sudo tee -a /etc/sysctl.d/99-sysctl.conf
    • To limit the data to just kernel panic logs, set the kernel module option oops_only to 1 by appending oops_only=1 to the options netconsole line you used in /etc/modprobe.d/netconsole.conf.

      cumulus@switch:~$ echo 'options netconsole netconsole=@,514@ oops_only=1' | sudo tee /etc/modprobe.d/netconsole.conf
  4. Reboot the switch. The boot sequence applies the settings.

    cumulus@switch:~$ sudo reboot

Create a Running Configuration

The following procedure only impacts the running kernel (otherwise known as a non-persistent configuration) on the switch. After the switch reboots, you lose these settings.

To create a running configuration on a Cumulus Linux switch:

  1. Increase the kernel logging level (optional).

    cumulus@switch:~$ sudo dmesg -n 7
  2. Load the netconsole kernel module with the appropriate options.

    If the syslog server is reachable through the management VRF, when loading the netconsole module at runtime, <dev> must be the name of the management VRF interface, mgmt.

    cumulus@switch:~$ sudo modprobe netconsole netconsole=@,6666@

    Using the same configuration as above:

    cumulus@switch:~$ sudo modprobe netconsole netconsole=@,514@

    To use the oops_only setting, append this option to the modprobe command:

    cumulus@switch:~$ sudo modprobe netconsole netconsole=@,514@ oops_only=1

Configure an rsyslog Server to Receive Console Log Data

The following steps show how to configure an rsyslog server to receive UDP traffic on port 6666 from two devices and create separate log files for each. You can add this to your existing rsyslog configuration.

You must be the root (super) user on your server to perform these steps.

  1. Create a specific configuration file with your favorite editor:

    cumulus@switch:~$ sudo vi /etc/rsyslog.d/remote-netconsole.conf
  2. Add the following content to the file. Change the IP addresses to match the IP addresses of your switches and the appropriate destination log files.

    $ModLoad imudp
    $RuleSet remote
    # Modify the following template according to the devices on which you want to
    # store logs. Change the IP address and subdirectory name on each
    # line. Add or remove "else if" lines according to the number of your
    # devices.
    if $fromhost-ip=='' then /var/log/remote/spineswitch1/console.log
    else if $fromhost-ip=='' then /var/log/remote/leafswitch1/console.log
    else if $fromhost-ip=='' then /var/log/remote/leafswitch2/console.log
    else /var/log/remote/other/console.log
    & stop
    $InputUDPServerBindRuleset remote
    $UDPServerRun 6666
    $RuleSet RSYSLOG_DefaultRuleset
  3. Create a directory to store the log files. The following example creates a directory called /var/log/remote.

    # mkdir /var/log/remote
  4. Restart rsyslog.

    # systemctl restart rsyslog.service

Test the Setup

You can test this setup in one of two ways:

  • Append data to the kernel log
  • Intentionally crash the switch (which causes a catastrophic failure of the switch)

Append Data to the Kernel Log

To create a new kernel log message and verify that the syslog server recorded it, run the following command on the switch configured with netconsole:

cumulus@switch:~$ echo "<0>test message $(date +%s)" | sudo tee /dev/kmsg

Confirm that the same message output by this command is also recorded on the syslog server.

Crash a Switch

This causes a catastrophic failure of the switch and results in an immediate reboot. Ensure your network is ready for this to occur and you understand the consequences.

To invoke a kernel panic to test the process, log in to the switch you want to crash and run the following command:

cumulus@switch:~$ echo c | sudo tee /proc/sysrq-trigger

If the process is working correctly, you see log data sent to the rsyslog server.

Log File Sample Output

Here is some sample output from the rsyslog server:

May 12 17:13:59 [17593.272492] sysrq: SysRq :
May 12 17:13:59 Trigger a crash
May 12 17:13:59 [17593.277181] BUG: unable to handle kernel
May 12 17:13:59 NULL pointer dereference
May 12 17:13:59  at           (null)
May 12 17:13:59 [17593.285951] IP:
May 12 17:13:59  [<ffffffff81496256>] sysrq_handle_crash+0x16/0x20
May 12 17:13:59 [17593.292773] PGD 4cb06067
May 12 17:13:59 PUD 4ca44067
May 12 17:13:59 PMD 0
May 12 17:13:59
May 12 17:13:59 [17593.297566] Oops: 0002 [#1]
May 12 17:13:59 SMP