Ubuntu Server: How to activate kernel dumps

If you are running ubuntu server, you can add kdump on your system to write kernel dumps in case of sudden reboots etc.

Installing is very easy:

root@ubuntuserver:/etc# apt install linux-crashdump
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following additional packages will be installed:
  binutils binutils-common binutils-x86-64-linux-gnu crash kdump-tools kexec-tools libbinutils libdw1 libsnappy1v5 makedumpfile
Suggested packages:
  binutils-doc
The following NEW packages will be installed:
  binutils binutils-common binutils-x86-64-linux-gnu crash kdump-tools kexec-tools libbinutils libdw1 libsnappy1v5 linux-crashdump makedumpfile
0 upgraded, 11 newly installed, 0 to remove and 43 not upgraded.
Need to get 2,636 B/5,774 kB of archives.
After this operation, 26.0 MB of additional disk space will be used.
Do you want to continue? [Y/n] 
Get:1 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 linux-crashdump amd64 4.15.0.46.48 [2,636 B]
Fetched 2,636 B in 0s (28.1 kB/s)     
Preconfiguring packages ...
Selecting previously unselected package binutils-common:amd64.
(Reading database ... 66831 files and directories currently installed.)
Preparing to unpack .../00-binutils-common_2.30-21ubuntu1~18.04_amd64.deb ...
Unpacking binutils-common:amd64 (2.30-21ubuntu1~18.04) ...
Selecting previously unselected package libbinutils:amd64.
Preparing to unpack .../01-libbinutils_2.30-21ubuntu1~18.04_amd64.deb ...
Unpacking libbinutils:amd64 (2.30-21ubuntu1~18.04) ...
Selecting previously unselected package binutils-x86-64-linux-gnu.
Preparing to unpack .../02-binutils-x86-64-linux-gnu_2.30-21ubuntu1~18.04_amd64.deb ...
Unpacking binutils-x86-64-linux-gnu (2.30-21ubuntu1~18.04) ...
Selecting previously unselected package binutils.
Preparing to unpack .../03-binutils_2.30-21ubuntu1~18.04_amd64.deb ...
Unpacking binutils (2.30-21ubuntu1~18.04) ...
Selecting previously unselected package libsnappy1v5:amd64.
Preparing to unpack .../04-libsnappy1v5_1.1.7-1_amd64.deb ...
Unpacking libsnappy1v5:amd64 (1.1.7-1) ...
Selecting previously unselected package crash.
Preparing to unpack .../05-crash_7.2.1-1ubuntu2_amd64.deb ...
Unpacking crash (7.2.1-1ubuntu2) ...
Selecting previously unselected package kexec-tools.
Preparing to unpack .../06-kexec-tools_1%3a2.0.16-1ubuntu1_amd64.deb ...
Unpacking kexec-tools (1:2.0.16-1ubuntu1) ...
Selecting previously unselected package libdw1:amd64.
Preparing to unpack .../07-libdw1_0.170-0.4_amd64.deb ...
Unpacking libdw1:amd64 (0.170-0.4) ...
Selecting previously unselected package makedumpfile.
Preparing to unpack .../08-makedumpfile_1%3a1.6.3-2_amd64.deb ...
Unpacking makedumpfile (1:1.6.3-2) ...
Selecting previously unselected package kdump-tools.
Preparing to unpack .../09-kdump-tools_1%3a1.6.3-2_amd64.deb ...
Unpacking kdump-tools (1:1.6.3-2) ...
Selecting previously unselected package linux-crashdump.
Preparing to unpack .../10-linux-crashdump_4.15.0.46.48_amd64.deb ...
Unpacking linux-crashdump (4.15.0.46.48) ...
Processing triggers for ureadahead (0.100.0-20) ...
Setting up libdw1:amd64 (0.170-0.4) ...
Setting up kexec-tools (1:2.0.16-1ubuntu1) ...
Generating /etc/default/kexec...
Setting up binutils-common:amd64 (2.30-21ubuntu1~18.04) ...
Processing triggers for libc-bin (2.27-3ubuntu1) ...
Setting up makedumpfile (1:1.6.3-2) ...
Setting up libsnappy1v5:amd64 (1.1.7-1) ...
Processing triggers for systemd (237-3ubuntu10.12) ...
Processing triggers for man-db (2.8.3-2ubuntu0.1) ...
Setting up libbinutils:amd64 (2.30-21ubuntu1~18.04) ...
Setting up kdump-tools (1:1.6.3-2) ...

Creating config file /etc/default/kdump-tools with new version
Sourcing file `/etc/default/grub'
Sourcing file `/etc/default/grub.d/50-curtin-settings.cfg'
Sourcing file `/etc/default/grub.d/kdump-tools.cfg'
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-4.15.0-45-generic
Found initrd image: /boot/initrd.img-4.15.0-45-generic
done
Created symlink /etc/systemd/system/multi-user.target.wants/kdump-tools.service → /lib/systemd/system/kdump-tools.service.
Setting up linux-crashdump (4.15.0.46.48) ...
Setting up binutils-x86-64-linux-gnu (2.30-21ubuntu1~18.04) ...
Setting up binutils (2.30-21ubuntu1~18.04) ...
Setting up crash (7.2.1-1ubuntu2) ...
Processing triggers for ureadahead (0.100.0-20) ...
Processing triggers for libc-bin (2.27-3ubuntu1) ...
Processing triggers for systemd (237-3ubuntu10.12) ...

Within the installation you have to answer these questions:

After the installation the following parameter is added to the kernel cmdline:

grep -r crash /boot* |grep cfg
/boot/grub/grub.cfg:        linux    /boot/vmlinuz-4.15.0-46-generic root=UUID=a83c2a94-91c4-461a-b6a4-c7a81422a857 ro  maybe-ubiquity crashkernel=384M-:128M
/boot/grub/grub.cfg:            linux    /boot/vmlinuz-4.15.0-46-generic root=UUID=a83c2a94-91c4-461a-b6a4-c7a81422a857 ro  maybe-ubiquity crashkernel=384M-:128M

with

crashkernel=:[,:,...][@offset]
    range=start-[end] 'start' is inclusive and 'end' is exclusive 

The configuration is done via /etc/default/kdump-tools. Here the parameter to control the directory to dump the core into:

cat /etc/default/kdump-tools  |grep DIR
# KDUMP_COREDIR - local path to save the vmcore to.
KDUMP_COREDIR="/var/crash"

Next step is to reboot and verify the kernel cmdline.

#cat /proc/cmdline 
BOOT_IMAGE=/boot/vmlinuz-4.15.0-46-generic root=UUID=a83c2a94-91c4-461a-b6a4-c7a81422a857 ro maybe-ubiquity crashkernel=384M-:128M

To get a coredump just use the following commands:

root@ubuntuserver:/etc# sysctl -w kernel.sysrq=1
kernel.sysrq = 1
root@ubuntuserver:/etc# echo c > /proc/sysrq-trigger