Create Amazing QR Codes with QRCode Monkey

Posted on July 27, 2017 by Ahmed Tawfik

And it’s FREE! I wouldn’t have written a blog entry for something except if it was such an amazing one:

https://www.qrcode-monkey.com/

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Fix Slow SFTP/SCP File Upload Rate and Stalling caused by TCP SACK

Posted on July 26, 2017 by Ahmed Tawfik

In brief, sometimes a conflict occurs between some firewalls and the TCP SACK. In my case, the upload was severely affected and reached upload speeds of a few kilobytes over a LAN!

The following command disables TCP SACK and fixes the slow upload on all Ethernet cards:

sudo sysctl -w net.ipv4.tcp_sack=0

To make it permanent, edit /etc/sysctl.conf and add this line

net.ipv4.tcp_sack = 0

Then run:

sudo sysctl -p

Enjoy the ultra super duper fast uploads! I can now upload at above 40 MB/sec!

Sources:

  1. http://kb.pert.geant.net/PERTKB/SelectiveAcknowledgements
  2. http://linuxsecure.blogspot.qa/2008/05/scp-stalled-through-firewall-ssh-no.html
  3. https://stackoverflow.com/questions/11985008/sending-a-large-file-with-scp-to-a-certain-server-stalls-at-exactly-2112-kb
  4. https://www.cyberciti.biz/faq/making-changes-to-proc-filesystem-permanently/
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Reclaim storage space from qcow2 image after moving

Posted on July 23, 2017 by Ahmed Tawfik

After it is moved to another host, since it is a sparse file, the qcow2 takes the full space, e.g. if it was dynamically set to 100GB, it will take 100GB, even if only 50 GB were actually used.

I could get back more than 20 GB from my Win7 VM (from a 50 GB file) using this:

sudo qemu-img convert -O qcow2 win7.qcow2 win7_small.qcow2

Enjoy!

Source: http://www.tuxfixer.com/how-to-shrink-openstack-qcow2-image-with-qemu-img/

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Securely delete files/folders in Ubuntu

Posted on July 19, 2017 by Ahmed Tawfik

In brief, you should NEVER trust your HDD before handing it over to someone else!

The solution to this would be either wipe the whole HDD or at least wipe the files/folders that you need to keep out of reach.

There are two quick ways to do this on Ubuntu; shred and wipe. I prefer wipe because it can accept folders as well.

The point is to fill the place of your data with random bits multiple times, as follows:

  1. sudo apt install wipe
  2. wipe -rq folder_name

r: for recursive

q: for quick mode, with only 4 iterations of random data.

Enjoy!

Note: This process is very time-consuming, even in quick mode, depending on how large your data is, and how fast the HDD you are wiping is.

Source: https://askubuntu.com/questions/57572/how-to-delete-files-in-secure-manner

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Add an additional physical volume to your encrypted LVM volume group

Posted on July 18, 2017 by Ahmed Tawfik

With a default Linux Mint installation, do the following after connecting the new hard disk to your computer, we will assume the new HDD is /dev/sda:

  1. sudo pvcreate /dev/sda
  2. sudo vgextend mint-vg /dev/sda
  3. sudo lvextend -l +100%FREE /dev/mint-vg/root
  4. sudo cryptsetup resize /dev/mapper/mint–vg-root
  5. Enjoy!

Source: https://blog.jamiebaldanza.org/2014/01/23/extend-encrypted-lvm-by-adding-new-physical-disk/

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