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KERNEL PANIC VFS Unable to mount root fs on unknown-block(0,0)
Stefan Helander

Fixing KERNEL PANIC! VFS: Unable to mount fs on unknown-block(0,0)

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I had this on an Ubuntu 24.04 LTS which had updated the kernel. The problem is that there is no initramfs for the kernel version you are trying to boot.

  • Boot on a USB-stick with a live image of Ubuntu, using the Try Ubuntu boot option
  • List the devices in the system:
    sudo fdisk -l
  • Your disk is probably something like /dev/sda1 or /dev/sda2. In my case it was /dev/sda2. Do:
    sudo mount /dev/sda2 /mnt
    sudo mount --bind /dev /mnt/dev
    sudo mount --bind /dev/pts /mnt/dev/pts
    sudo mount --bind /proc /mnt/proc
    sudo mount --bind /sys /mnt/sys
    sudo chroot /mnt
  • List the kernels installed:
    dpkg --list | grep linux-image
  • Generate initramfs for the latest version, in my case 6.17.0-14-generic:
    update-initramfs -u -k 6.17.0-14-generic
  • Update grub:
    update-grub
  • Shutdown, remove the live USB-stick and restart the computer.
Stefan Helander

Prevent Mac OSX ssh from disconnecting (also on any Linux/BSD/*nix system)

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To prevent ssh from disconnecting while idle, add the following to ~/.ssh/config:

Host *
    ServerAliveInterval 30
    TCPKeepAlive no

This solution is alse useable in any Linux/BSD/*nix environment. If you want to implement this not only on your own user, as a sysadmin, add the above to /etc/ssh/ssh_config instead.

Stefan Helander

rm unable to delete file even as root or sudo [Solved]

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When it is not possible to delete a file using rm even when logged in as root or by using sudo, check the file attributes with:

lsattr filename.ext

If it has the “i” attribute, this is preventing the file from being deleted, even by super users. When you have verified the reason for this and still want to delete the file then remove the “i” attribute with:

chattr -i filename.ext

Then delete the file using rm as you normally would.

Stefan Helander

/etc/cron.daily/amavisd-new: Please run this cronjob as user amavis

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After a system update I started to receive cron messages saying “/etc/cron.daily/amavisd-new:
Please run this cronjob as user amavis”.

It turns out that this happens because the cron script is replaced by a new one located in /etc/cron.d and the one in /etc/cron.daily is left behind giving this error message.

Solution: After checking that the new script exists in /etc/cron.d, just delete /etc/cron.daily/amavisd-new

More information about this here (bug report).

Stefan Helander

Meltdown and Spectre vulnerabilities – what to do?

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Meltdown and Spectre are two vulnerabilities present in hardware making it potentially possible for programs to steal information, like passwords etc.

Meltdown affects only Intel processors while Spectre, which is more complex, also partly affects AMD and ARM based processors.

It is not yet known if these vulnerabilities has been exploited by anone. It can affect personal computers, servers, tablets and mobile phones, i.e. more or less any device containing a processor.

More information on: https://spectreattack.com/

What can you do?

  • Check your operating system for updates the upcoming weeks (this is normal good security practice, but make sure you do it frequently)
  • Install and update your virus protection. Even if the antivirus program can’t protect you from the attack it might be able to inform you that your device has got malicious code onboard

You can find security bulletins, security advisorys, faq:s etc for your operating system here: https://meltdownattack.com/#faq-advisory

Stefan Helander

postgrey whitelisting outlook.com

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Greylisting is a way of reducing the amount of incoming spam and virus emails, in addition to other counter measures. Postgrey is a good choice if you are running postfix.

However, to be able to recieve emails from senders using outlook.com you need to whitelist it. This is because outlook.com is using a different host on each try to deliver the mail, causing delays up to hours, days or the mail not being delivered at all within time limits.

Create/edit the file /etc/postgrey/whitelist_clients.local and add:

# outlook.com
/^.*\.outbound\.protection\.outlook\.com$/
/^.*\.prod\.outlook\.com$/

Then execute:

service postgrey reload

Edit December 2021: The rule is changed from /^mail-.*\.outbound\.protection\.outlook\.com$/ to /^.*\.outbound\.protection\.outlook\.com$/ as Microsoft has changed their server naming standard.

Stefan Helander

How to make a floppy file image and mounting it in Linux

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To make a file image of a floppy disk in Linux (like Ubuntu), you need (of course) a computer with a floppy drive (/dev/fd0). Insert the floppy you want to copy to an image file and issue the command:

$ sudo dd bs=512 count=2880 if=/dev/fd0 of=floppy.img 

Block size (bs) and count above is for a 1,44 MB 3.5" floppy disk.

The image file can be copied back to another floppy disk with the command: 

$ sudo dd bs=512 count=2880 if=floppy.img of=/dev/fd0

The image file can also be mounted directly from the image file without the need of a physical floppy disk: 

$ sudo mkdir /media/fd
$ sudo mount -o loop floppy.img /media/fd/

I use the above method to move the content from a number of floppy disks to my hard drives. Computers today are rarely seen with a floppy drive and I wanted to secure the content before it is too late 🙂 

 

Stefan Helander

How to find out what Ubuntu version a system is running

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There are two ways:

# cat /etc/issue
Ubuntu 9.10 \n \l

or

# lsb_release -a
Distributor ID:    Ubuntu
Description:    Ubuntu 9.10
Release:    9.10
Codename:    karmic

The latter is slower but gives more information.

Stefan Helander

Pimp my router! Linksys WRT54GL on steroids… or at least on Tomato

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I while ago I got my hands on a Linksys WRT54GL broadband router. This little fellow runs on Linux. Nice, I thought. After fiddling around with it for a while I found that the firmware had bugs.

Linksys WRT54GLI added timed access restrictions (to cut off my teenagers Internet access automatically in the evening). This worked fine until I added another rule that had nothing to do with the first and suddenly my teenagers had Internet access all night long. I also want an incoming VPN connection (PPTP) that I forward by using port forward. This worked fine for a week and of course, it stopped working when I was abroad and needed it the most. Apart from that, there is no telnet or ssh login to the router (it is running on Linux you know).

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