How to inspect network traffic on your cell phone / mobile device

Tags: analyze traffic, mobile traffic investigation, mitm proxy attack
Last update: Feb 2021

In this tutorial we will use a regular computer with two USB wifi adapters: one for the connection to our internet/wan and the other one for creating our local wifi hotspot. You can substitute one of the wifi adapters with an ethernet adapter of course. I’m using two wifi adapter in order to not mess around with cables and because they were laying on my shelf. Pretty all others should work, too.

1. Used hardware / software

  • some computer with Ubuntu 20.04 LTS installed (Linux Mint also worked)
  • usb wifi adapter #1: TP-Link TL-WN321G (chipset RT2501/2573)
  • usb wifi adapter #2: TP-Link TL-WN722N (chipset AR9271)
  • you’ll need to install the proxy tool: sudo apt-get install mitmproxy

2. Setting up the wifi adapters

I’m using Ubuntu or Mint for this first steps because it’s quite easy to set it up with GUI’s. For other distributions this way might differ a bit.

Turn on the wifi hotspot for the first wifi adapter.

For the sake of this tutorial we’ll keep things easy and use “monitored” as SSID and “internet” as password.

You should see something like this in the end.

The second adapter will be connected with the internet/wan. Namely called “guest”.

3. Checking the connections and parameters

It’s important to check the output of “ifconfig” just to make sure everything has been set up correctly.

wlx6470022a323f: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet  netmask  broadcast
        inet6 fe80::1927:4d17:ceef:b47a  prefixlen 64  scopeid 0x20<link>
        ether 64:70:02:2a:32:3f  txqueuelen 1000  (Ethernet)
        RX packets 6106  bytes 6409739 (6.4 MB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 4896  bytes 1197861 (1.1 MB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

wlx940c6de46be3: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet  netmask  broadcast
        inet6 fe80::d6fe:b25f:3863:244e  prefixlen 64  scopeid 0x20<link>
        ether 94:0c:6d:e4:6b:e3  txqueuelen 1000  (Ethernet)
        RX packets 183  bytes 27080 (27.0 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 384  bytes 60515 (60.5 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

What do you learn from this console output? Several important things:

  • wifi adapter #1 is accessible by its name “wlx6470022a323f”
  • wifi adapter #1 owns the local IP from the guest-testing network with access to the internet
  • wifi adapter #2 is accessible by its name “wlx940c6de46be3”
  • wifi adapter #2 own a local IP without any further access right now

4. Enabling routing and setting up the firewall rules

Now we need to enable IP forwarding.

sudo sysctl -w net.ipv4.ip_forward=1

And add some firewall rules for wifi adapter #1: wlx6470022a323f

sudo iptables -t nat -A PREROUTING -i wlx6470022a323f -p tcp --dport 80 -j REDIRECT --to-port 8080
sudo iptables -t nat -A PREROUTING -i wlx6470022a323f -p tcp --dport 443 -j REDIRECT --to-port 8080

Afterwards we need some additional rules for wifi adapter #2: wlx940c6de46be3

sudo iptables -t nat -A PREROUTING -i wlx940c6de46be3 -p tcp --dport 80 -j REDIRECT --to-port 8080
sudo iptables -t nat -A PREROUTING -i wlx940c6de46be3 -p tcp --dport 443 -j REDIRECT --to-port 8080

5. Starting mitmproxy

In order to be able to watch the traffic you need to start the sniffing host/proxy.

Start the proxy with: mitmproxy --mode transparent --showhost

You’ll find an empty window because no device is connected with our hotspot named “monitored”.

6. Connect with a mobile phone

Take your mobile phone and connect to our hotspot “monitored”. As soon as we connect, the network traffic should be visible. Is this case we can see some default Android behavior.

If you try to connect to an encrypted website on your mobile phone, you’ll get some error message at the bottom, that a (TLS) Client Handshake failed. In order to make this encryption visible we need to install an intermediate encryption certificate on our phone which is provided by our tool mitmproxy: Open up the browser on your phone and open the website “”. You’ll be redirected automatically to a page where you can choose to download and install the certificate (depending on your device).

How to install a privacy friendly Jitsi Meet on Ubuntu and run it securely

Tags: jitsi installation, secure, ubuntu, linux
Last update: Jan 2021


  • a fresh installation of Ubuntu 21.04 LTS or Ubuntu 18.04 LTS
  • verify all updates are installed by issuing
sudo apt-get update && sudo apt-get dist-upgrade && sudo apt-get autoremove
  • If you intend to run Jitsi on your new publicly accessible server, reachable under, you should change the name of your host accordingly in both files and reboot your machine:
sudo nano /etc/hostname
sudo nano /etc/hosts
  • These instructions also take care of the TLS certificate. If the server is meant to be in your own infrastructure, you should ensure that at least ports 80 and 443 can be reached from outside for creating the TLS certificate. That is easily overlooked.

Configuring the firewall

# allow HTTP, HTTPS and UDP datagram for Jitsi communication packets
sudo ufw allow 80/tcp
sudo ufw allow 443/tcp
sudo ufw allow 4443/tcp
sudo ufw allow 10000/udp

# allow SSH if you need it
sudo ufw allow 22/tcp

# enable firewall and check status
sudo ufw enable
sudo ufw status

Installing Jitsi packages

First we need to import the official Jitsi repository:

# importing the repo
echo 'deb stable/' | sudo tee /etc/apt/sources.list.d/jitsi-stable.list
# importing the encryption key for the repo
wget -qO - | sudo apt-key add -

First we install the required packages:

# rebuildung the package list
sudo apt-get update
# installing needed packages
sudo apt-get install apt-transport-https jitsi-meet

You’ll be asked for the hostname of the current installation. Use

Afterwards choose to generate a self-signed Let’s Encrypt certificate.

Obtaining a TLS certificate

We will use the builtin shell script for getting our certificate but it depends on the certbox package:

# installing lets encrypt's certbot
sudo apt-get install certbot

For reasons of simplicity and avoiding some common errors we use the prepared script for fetching the encryption certificate automatically. All we have to do is to provide our mail address.

sudo /usr/share/jitsi-meet/scripts/

Accessibility test

Point your preferred browser to and verify the installation. The page should be TLS encrypted and not showing any errors.

Avoiding abuse / configuring access control

By default the current installation of Jitsi is allowing everybody to enter our just created virtual meeting room without any access control. If you leave it in that state, anyone can connect to the server and transmit audio and video steams – even if no one else is inside the meeting. This inevitably leads to the server operator sending you an abuse notice and blocking your server due to high traffic. In a self-experiment, I experienced around 3000 GB of data running through my unsecured server. Don’t try this at home 😉

To restrict this abuse we’ll

  • enable accounts for us (authenticated users) and for all others (guests)
  • allow guests to join only if we have entered a certain room before

Turning on access control
We need to edit 3 files.

1. We turn access control on by changing the authentication type:

sudo nano /etc/prosody/conf.avail/

by changing the block:

VirtualHost ""
    -- enabled = false -- Remove this line to enable this host
    authentication = "anonymous"


VirtualHost ""
    -- enabled = false -- Remove this line to enable this host
    authentication = "internal_hashed"

and allowing guests to log in by adding at the end of the file:

VirtualHost "" 
	authentication = "anonymous" 
	c2s_require_encryption = false

2. We edit a file belonging to Jitsi:

sudo nano /etc/jitsi/meet/

by changing the block:

var config = {
    // Connection

    hosts: {
        // XMPP domain.
        domain: '',

        // When using authentication, domain for guest users.
        // anonymousdomain: '',


var config = {
    // Connection

    hosts: {
        // XMPP domain.
        domain: '',

        // When using authentication, domain for guest users.
           anonymousdomain: '',

3. We edit the Jicofo element:

sudo nano /etc/jitsi/jicofo/

by adding a second line at the end of the file. From:


Adding XMPP users
Now the access control is configured and we have to add users to XMPP/Prosody in order to authenticate ourselves and to create rooms:

sudo prosodyctl register <username> <password>

Restart everything
because we have finished configuration:

sudo systemctl restart prosody && sudo systemctl restart jicofo && sudo systemctl restart jitsi-videobridge2

Test your Jitsi installation

Point your browser again to and try to create a conference room. Usually (before our efforts above) a room would be created instantly and you could join. Now you are getting the hint that your room hasn’t been opened yet. Only you can open it by hitting the button to authenticate in order to allow others in.

Customizing your Jitsi installation

If you want to change the default branding you can edit the interface configuration file:

sudo nano /usr/share/jitsi-meet/interface_config.js

You might find the following line/settings useful:

# line 5 changes the page title
APP_NAME: 'Jitsi Meet',

# line 49 changes the logo within the room
DEFAULT_LOGO_URL: 'images/watermark.svg',

# line 50 changes the default name for guests

# line 51 changes the logo on the first page
DEFAULT_WELCOME_PAGE_LOGO_URL: 'images/watermark.svg',

# line 100 changes the animation of generic room names

# line 113 changes the logo link

# line 167 changes the visibility of existing rooms

Credits & further reading

Replacing a faulty disk in a software RAID1 array on Linux

Tags: linux, raid1, software raid, mdstat, md0
Last update: Jan 2021


We have a running software RAID1 at moint point /dev/md0 consisting of the following partitions:

  • /dev/sda1
  • /dev/sdb1

Showing details

You can get some detailed information about the fitness of /dev/md0 by issuing:

cat /proc/mdstat

An exemplary entry looks like this:

Personalities : [raid1]
md0 : active raid1 sdb1[1] sda1[0]
      136448 blocks [2/2] [UU]

and tells us that

  • it’s a raid1 array
  • md0 consists of sda1 (first part) and sdb1 (second part)
  • the raid1 is running synchronously > [UU]

If one partition of your raid fails then you will see underscores instead of the character ‘U’: [U_] or [_U] (depends on which partition is faulty).

Replacing a failed partition

For explanatory purposes we assume /dev/sdb1 to be faulty. If you want to replace the disk you have to follow the following steps:

(Optional) If your partition isn’t already marked failed you have to mark it failed:

sudo mdadm --manage /dev/md0 --fail /dev/sdb1

Then you can remove the faulty partition from the array md0:

sudo mdadm --manage /dev/md0 --remove

Remember: In our example we just have one partition per disk. If you have more than one partition belonging to multiple arrays, be sure to remove all partitions of that one particular disk which you want to replace. Example: another additional /dev/sdb2 could belong to /dev/md1.

Once all your partitions from the faulty disk are removed from all raid arrays you can replace the disk by replacing it with a new one. You should shutdown the system before 😉 Since this new one won’t be partitioned like the old one, we need to copy the partition structure from the disk which is still active in our array (/dev/sda). For copying a GTP partition structure from /dev/sda to our new /dev/sdb and creating a new UUID we can use:

# copy sda to sdb (GPT)
sudo sgdisk -R /dev/sdb /dev/sda
# create UUID
sudo sgdisk -G /dev/sdb

If you use classical MBR for disks below 2TB:

# copy sda to sdb (MBR)
sudo sfdisk -d /dev/sda | sudo sfdisk /dev/sdb

After having copied the partition structure we need to add the partition /dev/sdb1 back again to our array /dev/md0:

sudo mdadm --manage /dev/md0 --add /dev/sdb1

Now you can watch the progress of rebuilding your software array which is running completely transparent in the background until the target state of [UU] is reached again:

Personalities : [raid1]
md0   : active raid1 sdb1[1] sda1[0]
      1464725760 blocks level 5, 64k chunk, algorithm 2 [6/5] [U_]
      [==>..................]  recovery = 12.6% (37043392/292945152) finish=127.5min speed=33440K/sec

If you replaced a bootable disk (for instance /dev/sda) think about installing GRUB again. It might be helpful to copy the full boot partition (let’s say /dev/sda0) to /dev/sdb0 and to install GRUB also on /dev/sdb in order to allow booting from both disks.

# installing GRUB
sudo grub-install /dev/sda

Useful Linux commands

Tags: Linux
Last update: Mar 2018

  1. Compare binary files within the console and highlight those areas which differ. Vim and Colordiff have to be installed before.
diff -y <(xxd <(xxd file.two) | colordiff

2. Using a harddisk device for storing encrypted data:

sudo cryptsetup luksFormat -c aes-xts-plain64 -s 512 -h sha512 -y /dev/sdX
sudo cryptsetup luksOpen /dev/sdX usb-backup
sudo mkfs.btrfs -L backupharddisk /dev/mapper/backup
sudo mount /dev/mapper/backup /usb-backup

3. Mirroring with rsync from /source to /destination. Compression is great when backing up over the Internet.

rsync --compress --compress-level=9 --human-readable -av --progress --delete --log-file=/path/to/log.file /source /destination

4. Wipe a file with zeros:

shred -fvzun 0 file

5. Wipe a directory recursively with zeros:

srm -rvs directory

6. Backup of harddisks into compressed image file. Due to its multithreading capability pigz is much faster than gzip during compression.

sudo dd if=/dev/sdX status=progress bs=1M | pigz > harddisk.img.gz

7. Restore this prior zipped image file and write it directly to disk:

sudo gunzip -c image.gz | sudo dd of=/dev/sdX status=progress bs=1M

8. Recover a Linux Soft-RAID md/5:

sudo mdadm --assemble --scan
sudo cryptsetup luksOpen /dev/md/5 temp
sudo mount /dev/mapper/temp /your_path

9a. Mount a Linux Soft-RAID manually:

sudo mdadm --examine /dev/sdX
sudo mdadm -A -R /dev/md9 /dev/sdX
sudo mount /dev/md9 /openedRaidPartition

9b. And close it again:

sudo umount /openedRaidPartition
sudo mdadm -S /dev/md9

10. List all loadable kernel modules:

find /lib/modules/$(uname -r) -type f -name \*.ko

11. Template for configuring a fresh manjaro xfce installation:

#update system
sudo pacman -Syyu
#install zshell
sh -c "$(curl -fsSL"
sudo pacman -S powerline-fonts
#install virtualbox
sudo pacman -S virtualbox
#copy fonts which you need into /usr/share/fonts and run
sudo fc-cache
#fixing the Manjaro Linux XFCE text shadow problem on the desktop
xfconf-query -c xfce4-desktop -p /desktop-icons/center-text -n -t bool -s false

12. Nice to have Manjaro packages:

gufw (firewall gui)
chromium (browser)
synapse (launcher)
psensor (temperature)
ether (pendrive tool)
p7zip zip unzip unrar
tilda (background terminal)

13. Laravel development quickstart:

git clone project.git
cd project
composer install
composer update
cp .env.example .env
php artisan key:generate
php artisan serve --port=8000

14. Restricting sFTP:

Add to /etc/ssh/sshd_config:

Subsystem sftp internal-sftp
  Match Group sftpusers
  ChrootDirectory %h
  ForceCommand internal-sftp
  AllowTcpForwarding no
  X11Forwarding no

Adding user and change configuration:

useradd newuser
usermod -s /bin/false newuser
chown root:root /home/newuser
chmod 755 /home/newuser
mkdir /home/newuser/writablefolder
chown newuser:newuser /home/newuser/writeablefolder
chmod 755 /home/newuser/writeablefolder
groupadd sftpusers
usermod -G sftpusers newuser
service sshd restart
service vsftpd restart

15. Replacing a broken harddisk (/dev/sda) with two partitions with RAID1 (md0 + md1):

#after having replaced the harddisk; copying MBR partition table
sfdisk -d /dev/sdb | sfdisk /dev/sda
#adding partitions to existing RAID-array md0 and md1:
sudo mdadm /dev/md0 -a /dev/sda1
sudo mdadm /dev/md1 -a /dev/sda2
#reinstalling grub on both disks again, leave options empty, select two disks (/dev/sda + /dev/sdb)
sudo dpkg-reconfigure grub-pc

16. Rsync command which doesn’t copy everything every time while preserving times:

rsync -avP source destination
#a = archive mode
#v = verbose
#P = show progress

17. Bash: iterate through all files within a specific folder and count its files

for i in /folderpath /folderpath ; do 
    echo -n $i": " ; 
    (find "$i" -type f | wc -l) ; 

18. copying KVM to PROXMOX

# converting image
qemu-img convert -f qcow2 -O raw image.qcow2 image.img

# creating VM in proxmox
qm create 120 --bootdisk scsi0

# importing disk to someStorage (as named in "pvesm status")
# importdisk adds the image as unused disk
qm importdisk 120 someImage.img someStorage

# attaching the image to the VM. still needs to be marked as boot device by hand
qm set 120 --scsi0 someStorage:vm-120-disk-0

19. repair a degraded ZFS pool/raid1 in PROXMOX

# "zpool status" output during normal operation

  pool: rpool
 state: ONLINE
  scan: none requested

	NAME                                             STATE     READ WRITE CKSUM
	rpool                                            ONLINE       0     0     0
	  mirror-0                                       ONLINE       0     0     0
	    ata-SanDisk_SDSSDH3_2T00_193732800276-part3  ONLINE       0     0     0
	    ata-SanDisk_SDSSDH32000G_192970802134-part3  ONLINE       0     0     0

errors: No known data errors
# "zpool status" of degraded state if we unplug /dev/sdb manually

  pool: rpool
 state: DEGRADED
status: One or more devices could not be used because the label is missing or
	invalid.  Sufficient replicas exist for the pool to continue
	functioning in a degraded state.
action: Replace the device using 'zpool replace'.
  scan: none requested

	NAME                                             STATE     READ WRITE CKSUM
	rpool                                            DEGRADED     0     0     0
	  mirror-0                                       DEGRADED     0     0     0
	    ata-SanDisk_SDSSDH3_2T00_193732800276-part3  ONLINE       0     0     0
	    ata-SanDisk_SDSSDH32000G_192970802134-part3  UNAVAIL      3   212     0

errors: No known data errors

We see that the disk with ID ata-SanDisk_SDSSDH32000G_192970802134-part3 is not available any more. We have to do the following steps:

  • shutdown the server
  • plug in a brand new replacement disk
  • boot the server up again

You’ll see now another status:

  pool: rpool
 state: DEGRADED
status: One or more devices could not be used because the label is missing or
	invalid.  Sufficient replicas exist for the pool to continue
	functioning in a degraded state.
action: Replace the device using 'zpool replace'.
  scan: none requested

	NAME                                             STATE     READ WRITE CKSUM
	rpool                                            DEGRADED     0     0     0
	  mirror-0                                       DEGRADED     0     0     0
	    ata-SanDisk_SDSSDH3_2T00_193732800276-part3  ONLINE       0     0     0
	    14464946144281260868                         UNAVAIL      0     0     0  was /dev/disk/by-id/ata-SanDisk_SDSSDH32000G_192970802134-part3

We need to prepare the new drive and replace it:

# copy partition table from good /dev/sda to new /dev/sdb
sgdisk -R /dev/sdb /dev/sda
sgdisk -G /dev/sdb

# copy the bios and efi boot partitions
dd if=/dev/sda1 of=/dev/sdb1 bs=512
dd if=/dev/sda2 of=/dev/sdb2 bs=512

# replace command
zpool replace rpool ata-SanDisk_SDSSDH32000G_192970802134-part3 /dev/sdb3

Keep in mind: Never use the detach command to remove a disk from a pool, because by adding it again you might be not careful enough to add it in mirror function. The default behaviour is to add a disk in striping mode (raid0)! You’ll have to backup everything and to reinstall a mirrored array again from scratch. Useful commands are:

# show raid status
zpool status

# show all your disk IDs
ls -l /dev/disk/by-id/

Backup/sync files with 7-Zip CLI

Tags: Windows | Linux | 7-Zip Command Line Interface
Last update: Dec 2017

1. Foreword / Setting / Requirements

We want a quick and tiny script to initially backup our files secondarly to update our archive for example on a daily basis. We want to modify our archive only in case a file/folder has been changed.

As of today virtualization is in fashion because VM’s are quite easy to handle and they support nice things like snapshots and an easy migration for instance. So the possibility is quite high we have already a machine which is running an hypervisor somewhere at home or in the office. Therefore this cheatsheet is dealing with a VM.

Mandatory for this tutorial:
• 7-Zip CLI executable running on a Windows machine (7za.exe)
• basic knowledge about Windows’ Command Line

2. The single-line command

Short and beautiful:

path/to/7za.exe a -up0q0r2x1y1z1w2 -mx1 "path/to/archive.7z" "path/to/source"

3. Explaining the parameters

• path/to/7za.exe: Your place where 7za.exe is stored.
• a: Command to ADD files/folders to an archive.
• -up0q0r2x1y1z1w2: Options which actually enable 7-Zip to work as expected. This holy string consists of:
• -u parameter UDPATE
• p0 condition P: What to do if file exists in archive, but is not matched with wildcard? 0 = Ignore file (don’t create item in new archive for this file).
• q0 condition Q: NOT in source, but IN archive > 0 = Ignore file.
• r2 condition R: IN source, but NOT in archive > add and compress file.
• x1 condition X: in source OLDER, than in archive > copy file from old to new archive.
• y1 condition Y: in source NEWER, than in archive > copy file from old to new archive.
• z1 condition Z: file is identical > copy file from old to new archive.
• w2 condition W: same time but different size > add and compress file.

By the way: There is a very good documentation for 7-Zip out there.

-mx1 I prefer to determine the level of compression. It’s saving some time for data which is already highly compressed like movies, pictures and music.
Possible Values: 0 (none) | 1 (fast) | 3 (fast) | 5 (normal) | 7 (maximum) | 9 (ultra). Newer benchmarks are showing the time/compression ratio is best at level 3.

“path/to/archive.7z” Where the archive has to be stored. If your path contains any spaces, be sure to use quotation marks.

“path/to/source” Your file/folder which should be copied (recursively). Use quotation marks if there are spaces inside your path.

How to securely wipe your flash drive with Debian/Ubuntu Linux

Tags: Secure wipe, Linux
Last update: Dec 2017

First we want to fill up our pendrive with zeroes:

sudo cat /dev/zero > /dev/your_pendrive

After completion we can check if our flash drive is really wiped with zeros by installing and using a text-based Hex-editor:

sudo apt-get install ht

Remember: We trust nobody so we check manually if zeros have been written to the drive. If you see weird characters something went wrong.

sudo hte /dev/your_pendrive

How to setup a bridged OpenVPN VM on Windows 2012 R2 using Ubuntu 16.04.1 LTS

Tags: Windows 2012 R2 Standard | Hyper-V | Ubuntu 16.04.1 LTS | Bridged OpenVPN
Last update: May 2017

1. Foreword / Setting / Requirements

Sometimes we are travelling through the globe and imagine that on a particular day we are working onsite with our mobile device. Suddenly we recognize there are files missing which are quite important and we believe them to be on a shared drive in the office. Would be great to have a solution now – kind of remote access to our files…

As of today virtualization is in fashion because VM’s are quite easy to handle and they support nice things like snapshots and an easy migration for instance. So the possibility is quite high we have already a machine which is running an hypervisor somewhere at home or in the office. Therefore this cheatsheet is dealing with a VM.

Mandatory for this tutorial:
• installed Ubuntu 16.04.1 LTS Server
• installed Microsoft Server with Hyper-V (hypervisor for virtual machines)
• basic knowledge about Linux, Windows Server and IP networks, because we won’t explain every step too detailed

2. Creating the VM

• Create a second generation virtual machine with the following specs: 1 GB vRAM, 1 vCore, at least 10GB vSpace.
• Create a virtual switch which is associated to the external network, so your ethernet adapter of your server.
• Choosing a static MAC address for this VM will make it easier to remember. We need is in the following step.
• activate MAC spoofing for this virtual switch (important!)

3. Router configuration

• Bind your static MAC in the router to a parmanent IP address.
• You can already configure needed port forward to this IP. In this tutorial we are using ports 1194 and 5000.
• Choosing a static MAC address for this VM will make it easier to remember. We need it in the following step.

4. Ubuntu server

• Usual installation from image, tick just the SSH server in order to allow us to connect.
• We want to make sure all updates are installed:

sudo apt-get update && sudo apt-get upgrade
  • You can do a distribution upgrade if applicable:
sudo apt-get dist-upgrade
  • Now we need the packages for OpenVPN and the certs
sudo apt-get install openvpn easy-rsa

5. Generating certificates

• easy-rsa can be found at one of the following directories:
– /etc/openvpn/easy-rsa
– /usr/share/openvpn/easy-rsa
– /usr/share/easy-rsa
• Let’s generate a keyset for our server. Change into the prior mentioned easy-rsa directory and follow the instructions to generate our keys:

./build-key-server HOSTNAME_OF_YOUR_SERVER
  • Now we need a cert for the client:

You should find a subfolder “keys” with all just now generated files inside.

6. Setting up the bridge

• Create a new file: /etc/openvpn/openvpn-bridge

# Define Bridge Interface
# Define list of TAP interfaces to be bridged,
# for example tap="tap0 tap1 tap2".
# Define physical ethernet interface to be bridged
# with TAP interface(s) above.
case "$1" in
    for t in $tap; do
        openvpn --mktun --dev $t
    brctl addbr $br
    brctl addif $br $eth
    for t in $tap; do
        brctl addif $br $t
    for t in $tap; do
        ifconfig $t promisc up
#    sleep ?
    ifconfig $eth promisc up
#    sleep ?
    ifconfig $br $eth_ip netmask $eth_netmask broadcast $eth_broadcast
    sleep 1
    route add default gw $eth_gateway
    ifconfig $br down
    brctl delbr $br
    for t in $tap; do
        openvpn --rmtun --dev $t
    ifconfig $eth $eth_ip netmask $eth_netmask broadcast $eth_broadcast
    route add default gw $eth_gateway
    echo "Usage:  openvpn-bridge {start|stop}"
    exit 1
exit 0
  • Make it executable
chmod +x openvpn-bridge

7. Adding service to autostart

• Create or open file /lib/systemd/system/openvpn@.service and add two lines at the end of [Services] to make the service run at boot time:

ExecStartPre=/etc/openvpn/openvpn-bridge start
ExecStopPost=/etc/openvpn/openvpn-bridge stop

• OpenVPN status can now be checked by:

systemctl status openvpn@server.service
  • Restarting the service is of course possible via:
service openvpn restart


systemctl restart openvpn@server.service

8. OpenVPN server configuration

• Create /etc/openvpn/server.conf with the following content:

mode server
port 993
proto tcp
dev tap0
ca /etc/openvpn/keys/ca.crt
cert /etc/openvpn/keys/vpn.crt
key /etc/openvpn/keys/vpn.key
# Der zweite Parameter zu tls-auth lautet 0 beim Server und 1 beim Client
tls-auth /etc/openvpn/keys/static.key 0
dh /etc/openvpn/keys/dh2048.pem
push "dhcp-option DNS YOUR_GATEWAY_IP"
max-clients 3 #max concurrent connections
comp-lzo #should transfer be compressed?
cipher AES-256-CBC #choose encryption algorithm
keepalive 10 120 #keeping connection alive
client-to-client #authorises client to client connectivity
verb 3 #verbosity

9. Symbolic link to folder “keys”

• The syntax is: ln -s TARGET SOURCE

ln -s /usr/share/easy-rsa/keys keys

10. Reboot the VM

sudo reboot

11. Configuration of a windows client

• Config files of OpenVPN are located at C:\Program Files\OpenVPN\config
• Create a file in this folder called client.ovpn
• Fill it with the following:

# setting client or server mode
# providing IP of your VPN server (and a port if needed)
# if no port is set the default is UDP 5000
# port of your client. if nobind is set, default port 1194 will be used
# using the UDP or TCP protocol
proto tcp
# setting up the working mode: bridging (TAP device) oder routing (TUN device)
dev tap
# which adapter should be used? please provide the correct windows' name of the
# adapter to use. in this case we renamed it to "tap-bridge" before.
#dev-node tap-bridge
# which IP address should be used within our bridged VPN network?
# where are the keys located?
# please keep in mind to use double-backslashes for a windows environment!
# for windows 7 additional quotation marks are required!
ca ca.crt
# the second parameter is: 0 = server, 1 = client
tls-auth static.key 1
# shall compression be used? if yes:
# which encryption cipher has to be used?
cipher AES-256-CBC
# accept data pushed by the server
# setting a verbose level of 1
verb 1

• Copy your generated cert files from the server (/usr/share/easy-rsa/keys) into the client folder (C:\Program Files\OpenVPN\config), since they are needed for the client profile to work:



How to build a more secure personal computer

Tags: Coreboot, F2A85-M, F2A85-M PRO, A10-5800K
Last Update: Jan 2021

This guide is based on the BIOS UEFI replacement called Coreboot which runs on an ASUS F2A85-M or F2A85-M PRO mainboard with an AMD A10-5800K CPU with integrated graphics.

1. Introduction and aim

This initial idea came up with the uprising problems and vulnerabilities of Intel ME and AMD Secure Technology (PSP). After having researched several alternatives without running those (in)secure technologies I was looking for very specific hardware to meet the requirements on security as mentioned in the pinetree of open and secure classical computing.

The aim is to provide a comprehensive guide which allows the daily user to gain a higher level of security. The daily user shall be empowered to get back the control about every piece of hardware within the computer system. In general hardware is always run by the so-called firmware. Firmware is software which is in the closest control of the underlying hardware to make it work as intended. So firmware is needed by the all BIOS/UEFI computer systems we know. Based on these hardware firmwares the central BIOS/UEFI unit is able to tell other computer parts how to cooperate. The newer the hardware, the worse the availability of open firmware. In fact in most cases the availability of open code is zero, because vendors assume that keeping the source code secret (closed source code) increases the security of the product. Nowadays we’ve learned by many lessons that this conviction is wrong. It’s a fact that every source contains errors which might be exploitable if they are found. The more people access a source code, the better this code is reviewed and the fewer errors are included.

Conclusion: Due to this circumstance sometimes developers have to use vendor’s original binary code (firmware) to get the hardware to work at all. That means that wide limitations are accepted by almost all vendors. Security can only be guaranteed by open source code. This leads to a conflict of objectives: On one hand we want a secure system and on the other hand we don’t want to lose comfort by leaving out the functionality which the vendor has placed within his (closed) firmware.

Maybe you take this into account before you support a certain vendor by buying his hardware the next time.

Since it’s practically almost impossible to use every piece of hardware out there by open code, we need to find the best compromise between functionality and security:

  • free and open source code
  • running Coreboot or even Libreboot
  • running Windows and GNU/Linux
  • as little as possible of (closed) firmware
  • enough performance to get work done

2. Software we will need

  • For booting the system we will use Coreboot instead of the vendor’s BIOS/UEFI. Libreboot is even a “better” option but – as mentioned in other posts – unfortunately very few and old boards are supported.
  • As an operating system you can choose one from the recommendations. I prefer using Manjaro GNU/Linux with the free driver stack option which you can choose while booting the installation disk from a pen-drive. In my view this is the best mix between bleeding edge development, security and user comfort.
  • If you prefer to build the Coreboot ROM on your own, you need a Linux on which you can compile the Coreboot sources. Ubuntu is usually a good choice.
  • If you prefer to extract the VGA BIOS on your own, you will also need to run a live Linux from a pen-drive for extraction.

3. Hardware you will need (mostly second hand)

Hardware usedNotes$
Both the ASUS F2A85-M or F2A85-M PRO are currently the best choice for a desktop platform, because it is widely supported by Coreboot.40
AMD A10-5800K
As we found out before AMD’s Piledriver and Steamroller CPU architectures (codenames Trinity, Richland and Kaveri) are the latest ones which does NOT include those (in)secure technologies. The AMD A10-5800K or AMD A10-6800K are the most compatible and powerful CPU/APU you can run on with Coreboot on this mainboard.40
CPU Cooler
Xilence A250PWM
Pick any adequate CPU cooler you want which is supporting the FM2 socket. You can use a good old Arctic Freezer A30 with a silent 120mm fan (~ 30 $). If you want something new you can take the Xilence A250PWM (XC035).10
Kingston KHX1600C10D3B1K2
DDR3-1866 RAM is the fastest RAM speed our CPU is supporting. The amount depends just on your personal demands. With a minimum of 8 GB we should be good to go. I’ve been successful with Corsair CML16GX3M2A1600C100 and Kingston KHX1600C10D3B1K2; but there are many many others that will work, too.50
HDD/SSDWe need a SATA hard-drive for the operating system. Think about using a SSD if you want a fast system. Remember to encrypt your partitions and/or files – especially if you are using a SSD.50
GPUIn the CPU above the graphics are onboard and included within the CPU. So there is no need to buy a dedicated one.0
Power SupplyAs a power supply any standard ATX AC adaptor will work.30
Bitfenix Prodigy M
Choose a computer case according to your personal preferences. I like the Bitfenix Prodigy M. Remember: Not all micro ATX cases are suitable for taking bigger/higher CPU coolers. This one takes and bigger CPU cooler like the Freezer A30 perfectly.50
In order to flash the BIOS chip via SPI we need a CH341A USB Programmer.15
BIOS chip
I recommend strongly to buy a separate Winbond W25Q64FVAIG or W25Q64BVAIG spare chip in order to flash the Coreboot build onto it. There are people who flash the onboard chip during a running system but this process can fail and in my opinion it’s better to have a fallback – just in case flashing or extraction have failed.10
total cost, approx.295

4. Assembling your computer

I assume you know how to assemble a computer so now it’s a good time to do so. Make sure your DDR3 RAM is placed into the BLUE SLOTS. Otherwise your computer won’t boot up.

5. Extracting the VGA BIOS binary blob

Unfortunately the internal GPU won’t work unless we integrate its binary into the Coreboot ROM.

You can download now the which contains the vgabios.bin which I have extracted already from an AMD A10-5800K. Or you download it on your own following the Coreboot guide:

Boot up a linux live Distro like Ubuntu from your prepared pendrive.

Check the ID by issueing “lspci -tvnn”. As you can see my ID of the GPU is 1002:9901. Write the ID down. We will need it later to build the Coreboot ROM.

lspci -tvnn
-[0000:00]-+-00.0 Advanced Micro Devices, Inc. [AMD] Family 15h (Models 10h-1fh) Processor Root Complex [1022:1410]
           +-00.2 Advanced Micro Devices, Inc. [AMD] Family 15h (Models 10h-1fh) I/O Memory Management Unit [1022:1419]
           +-01.0 Advanced Micro Devices, Inc. [AMD/ATI] Trinity [Radeon HD 7660D] [1002:9901]
           +-01.1 Advanced Micro Devices, Inc. [AMD/ATI] Trinity HDMI Audio Controller [1002:9902]

For enabling to read the ROM we need to set a enablement bit 1 to the corresponding address 0000:00:01.0 which we found above:

echo 1 > /sys/devices/pci0000:00/0000:00:01.0/rom

Now we can extract the VGA BIOS into a binary. Copy the vgabios.bin file onto a pendrive.

cp /sys/devices/pci0000:00/0000:00:01.0/rom vgabios.bin

6. Building the Coreboot ROM

If you want to skip this step or encounter unsolvable errors prohibiting you to generate the ROM you can download one of my compiled ROMs:

Since the old method isn’t working anymore you can decide now if you want to prepare Ubuntu Linux on your own or want to download my already prepared Virtual Machine I prepared for you.

6a. Preparing Ubuntu 20.04 LTS for building Coreboot

Get yourself a fresh copy of Ubuntu 20.04 LTS. Supply your Ubuntu with needed packages:

sudo apt-get install git build-essential gnat flex bison libncurses5-dev wget zlib1g-dev python

Let’s clone the Coreboot repository and change into that folder:

git clone
cd coreboot

Now copy your vgabios.bin from chapter 5 into the root directory of your Coreboot folder. Alternatively you can use my extracted vgabios.bin from a 5800K:

wget https:/

Now you can set up the Coreboot environment for building the Coreboot ROM, which we’ll transfer to our BIOS chip.

6b. Setup your settings for the Coreboot build

Run the configuration wizard with:

make nconfig

Set up the following parameters:

  • Submenu Mainboard: Choose ASUS as vendor.
  • Submenu Mainboard: Choose F2A85-M or F2A85-M PRO as model.
  • Submenu Mainboard: Choose the DDR3 memory voltage. Refer to your vendor’s website if you are unsure. Possible values are 1.35 / 1.5 / 1.65 V. Some tester said 1.5 V is the failsafe value if you can’t figure it out.
  • Submenu Chipset: Enable Hudson XHCI Controller
  • Submenu Chipset: Add XHCI firmware. The default path is already set correctly.
  • Submenu Devices: Add your VGA BIOS image (your extracted one, or mine) and make sure the PCI ID is matching yours by typing in the right values. For our CPU 5800K the value is always the same: 1002,9901.

Save your configuration by hitting “Save”, leave the filename “.config” and exit the tool.

Build the Compiler. CPUS=4 means it will use 4 cores to compile. Adjust this value to meet your number of cores. Be patient, this process takes some time. With my older notebook I had to wait around 10-20 minutes.

make crossgcc-i386 CPUS=4

Finally we are able to build the Coreboot ROM:


You will find the generated coreboot.rom in a subfolder called build.

7. Flashing the Coreboot ROM

  • Place your spare chip on the USB programmer and plug it into a USB port on your Linux machine.
  • Install flashrom under Ubuntu by
sudo apt-get install flashrom

Flash your spare chip now with your generated coreboot.rom:

sudo flashrom -n -w build/coreboot.rom --programmer ch341a_spi

If you get the following notifications your chip is ready:

Found Winbond flash chip "W25Q64.V" (8192 kB, SPI).
Erase/write done.
Verifying flash... VERIFIED.

Replace your old BIOS chip by the new flashed one during your system is switched off. Be careful not to bend a pin. So-called DIP8 pliers might help you to release the chip from the board.

Just in case you mess something up you can download a fresh vendor’s copy of the original BIOS/UEFI image and flash it again:

8. Operating System

Now, equipped with Coreboot, you can choose to install any operating system you wish. Since I am very familiar with it I’d go with something Debian-based: PureOS. If you want to follow me, you’ll notice that after installation of PureOS 9.x your screen will remain black due to the lack of AMD graphics firmware packages within PureOS. (Actually it’s a good sign because now you see that binary blobs like the AMD firmware really weren’t loaded during bootup). To fix this we need to install a package into our PureOS installation:

8a. Prepare the pendrive

Get a pendrive formatted with FAT and use another computer to prepare it. Download the archive from and extract firmware-amd-graphics*.deb from it. If you use Linux you can use the following lines:

unzip firmware-amd-graphics*.deb

Copy this .deb package into your pendrive, unmount it and plug it into your secure computer system.

8b. Installing the AMD graphics firmware

First login into the system on another TTY by pressing Crtl + Alt + F3 by providing your user and password. Afterwards we will create a new folder as mounting point for your pendrive, mount it, and install the .deb package. You will need the identifier for your pendrive partition. You can find it out by using “lsblk” for instance. In my case it’s /dev/sdb1.

sudo mkdir /pendrive
sudo mount /dev/sbd1 /pendrive
cd /pendrive
sudo apt install ./fireware-amd-graphics*.deb
cd /
sudo umount /dev/sdb1

After having installed the graphics firmware you can now unplug your pendrive, reboot your system by issuing “reboot” and enjoy PureOS welcoming you.

How to set up Postfix on Debian to use a mailhoster with SMTP

Tags: Postfix, Mailhoster, Linux, SMTP
Last update: Feb 2021

First we need to install packages:

sudo apt-get install postfix libsasl2-modules sasl2-bin libsasl2-2

If it’s not opening up automatically you can configure Postfix with “sudo dpkg-reconfigure postfix”.
Confige Postfix by choosing:

  • satellite system
  • system mail name = your-computername
  • smtp relay host = [your-mailhost]:port
  • root mail recipient = empty
  • other destinations = leave default
  • yes
  • local networds = leave default
  • procmail = yes
  • mailbox size limit = 0
  • local address extension = +
  • internet protocols to use = all

Edit Postfix config file:

sudo nano /etc/postfix/

and add the following:

smtp_sasl_auth_enable = yes
smtp_sasl_security_options = noplaintext noanonymous
smtp_sasl_password_maps = hash:/etc/postfix/sasl_password
smtp_tls_wrappermode = yes
smtp_tls_security_level = encrypt
smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt
sender_canonical_maps = hash:/etc/postfix/sender_canonical

In the end your should look like:

# See /usr/share/postfix/ for a commented, more complete version
smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)
biff = no

# appending .domain is the MUA's job.
append_dot_mydomain = no

# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h

alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
mydestination = hostname.your.domain, localhost.your.domain, localhost
relayhost = []:465
mynetworks =
inet_interfaces = loopback-only
recipient_delimiter = +
compatibility_level = 2
mailbox_command = procmail -a "$EXTENSION"
mailbox_size_limit = 0
inet_protocols = all

# custom settings for using a mailhoster
smtp_sasl_auth_enable = yes
smtp_sasl_security_options = noplaintext noanonymous
smtp_sasl_password_maps = hash:/etc/postfix/sasl_password
smtp_tls_wrappermode = yes
smtp_tls_security_level = encrypt
smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt
sender_canonical_maps = hash:/etc/postfix/sender_canonical

Providing authentication details by editing this file:

sudo nano /etc/postfix/sasl_password

and adding:

[]:465 username:password

Secure the file and generate a database for it:

sudo chmod 600 /etc/postfix/sasl_password
sudo postmap hash:/etc/postfix/sasl_password

Let’s create a file to specify our sender addresses:

sudo nano /etc/postfix/sender_canonical

Fill it with all users who will be sending mails:


And process it:

sudo postmap /etc/postfix/sender_canonical

Restart Postfix and send yourself a test mail:

sudo service postfix restart
echo "test" | mailx -s "test"

You can also send a mail with attachment using mpack:

mpack -d yourmessagebody.txt -s "subject" file


In case something goes wrong you might find these command helpful:

# show the mail log and follow
tail -f /var/log/mail.log

# list all mails which are in the queue
postqueue -p

# flush all mails
postsuper -d ALL



Change history

  • Dec 2018: initial post based on raspbian
  • Feb 2021: adding packages and fixing various mailhost settings

How to create and open an encrypted Ubuntu Linux 18.04 server with Dropbear through SSH

Tags: Full Disk Encryption, Ubuntu Linux, Dropbear, SSH
Last update: Dec 2019

Some of you are security-aware and would like to install a full disk encryption on a new system. But you’ll face a problem: Once the machine reboots and you’re not around to type in the password, the machine will simply not boot up. For come over that issue you can use a small SSH listener called Dropbear which loads up during the very beginning of the boot process. Only if you authenticate with your correct RSA key, you can log on to the machine, provide the password for uncrypting the filesystem and will be kicked out again while the machine will finish its bootup.

– Freshly installed and encrypted Ubuntu 18.04 server (headless installation – no GUI).
– You should know how to use the linux terminal and the text editor nano.
– I advice you to tell your DHCP router to provide always the same IP to the machine. Otherwise the IP will change/increment every time you boot it up.

1. Let’s ensure we have all updates installed:

sudo apt update
sudo apt dist-upgrade

Install the required additional dropbear package:

sudo apt-get --assume-yes install dropbear-initramfs

You’ll notice that during package installation some keys will be automatically generated:
– DSA 1024 key for root
– RSA 2048 key for root
– ECDSA521 key for root

2. Edit the config file:

sudo nano /etc/dropbear-initramfs/config

Uncomment the DROPBEAR_OPTIONS line and add a second port where SSH should listen on:


3. Since only public key authentication is allowed we need to create and fill up our authorized_keys file:

sudo nano /etc/dropbear-initramfs/authorized_keys

Fill it with the only allowed command and append your ssh-rsa key:

command="/bin/cryptroot-unlock" ssh-rsa YOUR_KEY

4. Apply the changes and reboot:

sudo update-initramfs -u
sudo reboot

5. Now you can login to your machine for instance with:

ssh root@MACHINE_IP -p YOUR_PORT -i ~/.ssh/YOUR_KEY_FILE

– (Hetzner specific)