Boot ISO from USB Stick

Many Linux distribution builders like Devuan and Debian produce hybrid ISO image that work on discs and USB sticks. You can make bootable USB sticks by simply copying the image to the USB device with this command

dd if=isoimage of=/dev/sdX bs=10240

You can find the device name (X) by looking into dmesg oder lsscsi, and looking for newly plugged scsi/block devices.

But some companies still don’t know about hybrid images and provide ISO images the need to be on disk. (like samsung SSD updater).

Workarround: You can use SysLinux to make an USB stick that can boot an ISO image

Install SysLinux using standard procedures, in my case: aptitude install syslinux (currently 3:6.03-dfsg-5-deb8u1)

Do the following steps:

  1. (optional) wipe the USB stick:  dd if=/dev/zero of=/dev/sdX
  2. make a bootable vfat partition: eg with fdisk /dev/sdX  (“n”), set the partition type to VFAT (“t” “c”) and make it active (“a”)
  3. format the partition to vfat: mkfs.vfat /dev/sdX1
  4. install SysLinux on it: syslinux /dev/sdX1
  5. mount the newly created partition: mount /dev/sdX1 /mnt/usb
  6. copy the memdisk feature to syslinux: cp /usr/lib/syslinux/memdisk /mnt/usb/
  7. make a syslinux.cfg file on the USB stick: vi /mnt/usb/syslinux.cfg
    LABEL iso
        LINUX memdisk
        INITRD image.iso
        APPEND iso
  8. copy the iso image to the usbstick: cp isoimage.iso /mnt/usb/image.iso
  9. unmount the USB strick: umount /mnt/usb

You can even put different ISO images to one stick by copying multiple ISO images and making multiple entries in syslinux.cfg.

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Source IP Address Based on User

If you want to use different source IP addresses based on the logged in user or running service on a Linux computer you can use these simple commands:

/sbin/ifconfig eth0:1 NEW-IP-ADDRESS netmask YOUR-NORMAL-NETMASK
/sbin/iptables -t nat -A POSTROUTING -m owner --uid-owner USERNAME -j SNAT --to-source NEW-IP-ADDRESS

You can use this if the source IP is necessary for remote firewall filter lists, or to separate IP traffic from services that don’t allow to configure the outgoing source IP.

Add these lines to /etc/rc.local to make it permanent.

Version: Should work on every Linux kernel of the last 10 years, tested on Linux 4.11.1

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Multi Seat Linux Workstation

Current computers are fast enough to handle more than one user at a time. So I started the project to setup my workstation to support two seats, one for me and one for my gf.


  1. Two concurrent Xorg sessions both with one keyboard, one mouse, and two monitors
  2. Separate audio for both seats
  3. Auto mounting of USB storage sticks for the secondary seat. When connected to a specific USB port the usb stick is mounted in the home directory of the logged in user of the second seat.

All this has to work while still keeping root privileges strictly separated. For security reasons I don’t use systemd polkit and other tools that allow normal users to gain root privileges. (Un)Mounting, Shutdown, Printersetup, Hardwaresetup are root tasks, normal users must not be able to do these tasks because it would compromise system security.

A normal user must not be able to shut down the system or see other users USB storages just because she is sitting in front of the local console.

Two Xorg Sessions

The workstation has two graphics cards one nvidia PEG card and an onboard Intel CPU graphics. I had to activate the onboard graphics in BIOS to be able to use it on Linux. The xorg-server-intel driver on Debian Jessie was to old to support the Intel Skylake HD530 graphics, so I upgraded the package “xserver-xorg-video-intel” from jessie-backports (“aptitude -t jessie-backports install xserver-xorg-video-intel”).

Then I configured the Xservers. Xorg can run multiple times with some configuration tweaking. I built two simple Xorg.conf. One for the first seat

# /etc/X11/Xorg.first-desk.conf

Section “Device”

Identifier “Nvidia Graphics”
Driver “nvidia”


Section “InputClass”

Identifier “Dell Keyboard”
MatchVendor “DELL”
MatchIsKeyboard “true”
Option “Ignore” “true”


Section “InputClass”

Identifier “Logitech Mouse”
MatchVendor “Logitech”
MatchIsPointer “true”
Option “Ignore” “true”


And one for the second seat:

# /etc/X11/Xorg.second-desk.conf

Section “Device”

Identifier “Intel Graphics”
Driver “intel”
BusID “PCI:0:2:0”


Section “InputClass”

Identifier “TheRest”
Option “Ignore” “true”


Section “InputClass”

Identifier “Dell Keyboard”
MatchVendor “DELL”
MatchIsKeyboard “true”
Option “Ignore” “false”


Section “InputClass”

Identifier “Logitech Mouse”
MatchVendor “Logitech”
MatchIsPointer “true”
Option “Ignore” “false”


Xorg tries take the first graphics card. To force one Xserver to the second card you need the BusID line. You can find this BusID with lspci:

# lspci
00:00.0 Host bridge: Intel Corporation Device 191f (rev 07)
00:01.0 PCI bridge: Intel Corporation Device 1901 (rev 07)
00:02.0 Display controller: Intel Corporation Device 1912 (rev 06)
00:14.0 USB controller: Intel Corporation Device a12f (rev 31)
00:16.0 Communication controller: Intel Corporation Device a13a (rev 31)

The sepration of mouse and keyboard works by blacklisting (“Ignore”) one keyboard and one mouse on the primary Xsession and an inverted blacklist on the secondary seat that blacklists all input devices except this one keyboard and mouse.

Two start two Xorg Xservers I added changed the file /etc/X11/xdm/Xservers to this:

:0 local /usr/bin/X :0 vt7 -config /etc/X11/Xorg.first-desk.conf -novtswitch -nolisten tcp
:1 local /usr/bin/X :1 -sharevts -config /etc/X11/Xorg.second-desk.conf -novtswitch -nolisten tcp

“-sharevts” and “-novtswitch” were the magic settings that allowed to run Xorg concurrently. Without this option the Xservers could only run one at a time by switching between VT7 and VT8 (Ctrl-Alt-F7 / Ctrl-Alt-F8).

Separate Audio

The onboard sound card has 8 channel output for surround sound. ALSA can split this multichannel output to multiple soundcards with this /etc/asound.conf file:

# /etc/asound.conf

pcm_slave.fourchannels {
    pcm "hw:0"
    period_time 0
    period_size 1024
    buffer_size 8192
    channels 4

pcm.jack1 {
   type plug
   slave.pcm {
        type dmix
        ipc_key 2381
        ipc_perm 0666
        slave "fourchannels"
        bindings [ 0 1 ]

pcm.jack2 {
   type plug
   slave.pcm {
        type dmix
        ipc_key 2381
        ipc_perm 0666
        slave "fourchannels"
        bindings [ 2 3 ]

This configuration splits the front from the surround (back) speaker output. Per user you can set the default output to either jack1 or jack2 with this ~/.asoundrc file:

pcm.!default {
    type plug
    slave.pcm "jack2"

Currently I hard wired this configuration per user. If me and my GF would change seats frequently I would write a “.asoundrc” file during Xsession startup every time a users logs in on the first or second seat (DISPLAY :0 or :1).

Automounting USB Storage for Second Seat

I used udevd and a small shell script to do the job.

Udevd can start scripts on USB events:

# /etc/udev/rules.d/10-multiseat-usb.rules
# filter on SD* (scsi events) of the blockdevice subsystem
# filter on events with the sub device tree (ATTRS) of the second seat's USB Hub idVender==05e3 named "USB2.0 Hub"
# for these events start: /root/user_usb_mounter
# which mounts the device for the logged in user and opens a filebrowser
KERNEL=="sd*", SUBSYSTEM=="block", ACTION=="add", ATTRS{idVendor}=="05e3", ATTRS{product}=="USB2.0 Hub", RUN+="/root/user_usb_mounter"

The script /root/user_usb_mounter looks like this:


# logfile output
echo "================================" 

if [ "$ID_FS_USAGE" != "filesystem" ]; then
    echo "ignoring udev event without FS_USAGE == filesystem"
    echo "new files system"

    # look which user is logged in on seat :1
    second_user=`who | grep " :1 " | cut -d " " -f 1`

    if [ "$second_user" == "" ]; then
        echo "No user Session on :1 found, giving up"
        # get userid of logged in user
        muid=`grep -- "^$second_user:" /etc/passwd | cut -d ":" -f 3`
        if [ "0$muid" -le 99 ]; then
            echo "No Userid for User $second_user on :1 found, giving up"
            # find an non existant directory mountpoint and create it
            while [ -e /home/$second_user/media/usb$i ]; do 
               i=$(( $i + 1 ))
            mkdir /home/$second_user/media/usb$i
            chown $second_user /home/$second_user/media/usb$i

            #  mount the filesystem in the users home directory
            echo mount -o noatime,nodev,noexec,nosuid,uid=$muid,gid=100 "$DEVNAME" "/home/$second_user/media/usb$i"
            mount -o noatime,nodev,noexec,nosuid,uid=$muid,gid=100 "$DEVNAME" "/home/$second_user/media/usb$i" || exit 

            echo "usbstick mounted to /home/$second_user/media/usb$i"
            echo "starting xfe for $second_user"
            # Starting xfe for the user and wait for xfe close. unmount the usb device, inform the user
            (    su "$second_user" -l -c "DISPLAY=:1 xfe /home/$second_user/media/usb$i" 
                 umount "/home/$second_user/media/usb$i" && rmdir "/home/$second_user/media/usb$i" && sync && 
                 su "$second_user" -l -c "DISPLAY=:1 xmessage \"USB Stick is save to remove!\"" && exit
                 su "$second_user" -l -c "DISPLAY=:1 xmessage \"USB Stick umount failed. DANGER!\""
            ) &

) >> /tmp/udevtest.log 2>&1

This script checks if the udev event is from a filesystem. Then it checks which user is logged in, gets it’s user ID. Then it mounts the USB device in the users context and home directory. Then it opens a file browser for the user and waits until it’s closed. Then it unmounts the stick and informs the user. This script is not very pretty but it’s a quick and working hack.

Versions: Skylake Intel CPU i5-6500 64bit mode, on ASUS motherboard Z170, Debian 8 (Nov 2017), NVidia GT 640 Nvidia Drivers 375.66, Xorg Intel Drivers 2:2.99.917+git20161206

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Android Battery Drain Riddle!

Problem: an android phone is loosing battery very quickly. The battery settings don’t show any app, the display or other reasons for the battery drain.

Discussion: I found out that the battery drain is only when connected to WLAN. Then I recognized that it only happens when connected to my WLAN. The mobile phone never sleeps when connected to my WLAN. So I checked if there are many broadcasts in the network, which wasn’t. Then I moved the IP of my mobile phone to a Linux and checked for unicast traffic using tcpdump. There was it !

Solution: I have an Inverto Multibox SAT>IP server in my network and used the SAT>IP server from my mobile phone several days ago. The bug in the Inverto box is (or was) that the box has a broken idle timer implemented which means the SAT>IP stream never stops if not shut down correctly. As a result the box pushed a SAT transport stream to the mobile phone every time I am connected to WLAN, and never stopped. I rebooted and upgraded the SAT>IP box. Battery drain was gone.

Version: Inverto SAT>IP Multibox 1.17

Battery usage before and after the bugfix.

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CPU Bug on Intel Skylake and Kabylake

I had two or three system crashes on my Linux workstation after upgrading to a new mother board and CPU within some months. This is very unusual for me because stability is the main objective when I build a new a workstation. So I tried to find the reason.

Some weeks ago I found this bug report:

Hyperthreading on Skylake and Kabylake CPUs is buggy!

If your processor model (listed in /proc/cpuinfo) is 78 or 94, and the stepping is 3 you are lucky because Intel already provides a microcode update. My workstation is processor level 94 which is Intel Core i5 6500. So I installed the debian packages intel-microcode 3.20170511.1 from jessie-backports.

Since this update I had no System crash and hang up.

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FlexFabric 5700 Backup Config to TFTP in MGMT VPN-Instance

Problem: if you separate the management from the normal traffic on a switch you will usually configure the swtich via this mgmt vpn-instance and also backup and restore config files via this mgmt vpn-instance. But if you use the “backup startup-configuration to ..” it always tries to find the tftp server on the normal network Even after changing the tftp configuration with “tftp client source interface M-GigabitEthernet 0/0/0” tftp still does not work.

Solution: The backup command has no vpn-instance parameter, but the “tftp put” command has. So you can use:

tftp put startup.cfg switchbackup.cfg vpn-instance MGMT

Version: HP/HPE FlexFabric System image version: 7.1.045, Release 2422P02

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NVidia Driver on Linux-4.11.1

Problem: after installing linux kernel 4.11.1, the dkms package of the nvidia driver does not compile. You find some cryptic error message about ./Kbuild in  /var/lib/dkms/nvidia-current/375.39/build/make.log

Solution: I uninstalled the debian nvidia package and installed the newer driver from the NVidia homepage:

aptitude remove nvidia-installer-cleanup

After this upgrade nvidia and linux 4.11.1 works again

Versions: before upgrade jessie backport of nvidia-driver 375.39, after upgarde nvidia-dirver from nvidia homepage 375.66

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Ubiquiti UniFi the Next Botnet ?

I tested a Ubiquiti access point today. UAP-AC-Lite seems to be a very good and cheap access-point.

When you take it out of the box and connect it to the network it gets an IP address using DHCP and waits for a configuration. In this mode it sends broadcasts to find a controller and listens on port 22 (ssh) with standard login/password of ubnt/ubnt.
That’s not best practice but very usual for devices of this kind.

I tried two configuration modes:

    1. MobileApp based using my Android Phone:
      This App looks good, and works great, if you need just one SSID and not VLAN. Thumbs up, well done ubiquiti.
      But I guess this method will not work if this is you first access point in the network, because you will end with a chicken and egg problem.
    2. UniFi Controller based:
      UniFi runs on Win/Mac and Linux. The Debian package is far to big but it installs cleanly (Why does this webapp need 27MB of fonts?).
      With this webapp you can configure everything and it works good. But then I checked the security…

First I checked what new ports are open on my server:

tcp6 0 0 :::8443            :::* LISTEN 1373/java 
tcp6 0 0 :::6789            :::* LISTEN 1373/java 
tcp6 0 0 :::8843            :::* LISTEN 1373/java 
tcp6 0 0 :::8880            :::* LISTEN 1373/java 
tcp6 0 0 :::8080            :::* LISTEN 1373/java 
udp6 0 0 MYPUBLICIP:50880   :::* 1373/java 
udp6 0 0 :::10001           :::* 1373/java 
udp6 0 0 :::3478            :::* 1373/java 
udp6 0 0 MYINTERNALIP:58426 :::* 1373/java 

That’s to much for a Linux box with a public IP interface.
The documentation tells a little bit what these ports are used for, but some are not explained or not needed for normal operation.
I tried to strip down the open ports for security reasons, but I found no way to disable unused services or at least bind only to one IP. My minimum requirement would be to bind only to an internal interface and block the public interface.

But no way (officially: )

Shure I could write an iptables filterlist to block these ports, but that’s risky. Today they use these 9 ports, but what will happen on the next update ?

Then I checked what services are actually running on these ports. It’s a tomcat server !
A java/tomcat server that listens in all directions IPv4/6 and no easy way to limit this access. What can possibly go wrong?

Most people will never update this controller software, and tomcat had and will have security problems.

Hopefully ubiquiti will provide a smaller footprint configuration tool, with a bit more settings than the app, and add some security settings to the controller software.
Then I would really recommend this nice piece of hardware: Vendor Link  ,  Amazon Link

Version: UAP-AC-Lite, unifi 5.4.11-9184

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Firefox Audio is Broken due to PulseAudio Dependency

Problem: Firefox 52 audio does not work without pulseaudio.

Discussion: pulseaudio is a bloated audio daemon that often fails. In the last years, when ever audio failed on linux, the quickest solution was to uninstall pulseaudio. But the Firefox developers made the mistake to remove support for the underlying audio driver alsa, and insist on the middle layer pulseaudio.
The second problem with distribution packages of pulseaudio is that it depends on lots of things itself like dbus, systemd, consolekit,…
Which leads to the problem: you can’t use firefox on most linux distributions when you prefer a better startup system than systemd.

Solution1: Don’t use Firefox until they fix this.

Solution2: You can compile pulseaudio without dependencies and use it in pure user context:

Download pulseaudio packages, configure it with:

./configure –prefix=/your/homedir/pulseaudio –disable-systemd-daemon –disable-systemd-login –disable-systemd-journal –without-caps –disable-dbus

You might need to install some -dev packages for this configure to work: libsndfile-dev, libspeex-dev, libspeexdsp-dev, ..

make install

And add this to your ~/.bashrc (or similar startup script):

export LD_LIBRARY_PATH=/your/homedir/pulseaudio/lib/
/your/homedir/pulseaudio/bin/pulseaudio -D

Then you can start firefox with a local, limited and secured pulseaudio server.

With standard settings pulseaudio grabs the soundcard completely and blocks audio for other alsa software. This egoism is typical for a Lenn* Poett* tool. But it can be changed. Change the following lines in /your/homedir/pulseaudio/etc/pulse/

#load-module module-alsa-sink
load-module module-alsa-sink device=dmix

#load-module module-detect

With these settings pulseaudio uses the normal asla mixing features, and allows other software to use audio.

The pulseaudio developer think it’s a good idea to terminate the pulseaudio server after the last client quits, at let the next client “magically” restart the daemon. (I repeat: “let a client application start a daemon”). This remembers me of the days when we all thought inetd was a good idea. Now we no better. There is an option that prevents the pulseaudio service from  dying.

Change this line in ~/pulseaudio/etc/pulse/daemon.conf:

exit-idle-time = -1

This keeps the daemon running.

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Sophos UTM BGP Announces 0 Prefixes

Problem: A simple bgp upstream configuration. A small AS with one IP prefix wants to connect to its upstream using BGP. The BGP peerings are up. Sophos receives the expected routes from its upstream, but the upstream router does not receive the expected single prefix.

Discussion: an outbound filter list is set to prevent sending other than the local prefix x/24. Therefor ae ip filter list is configured in the web gui and connected to the bgp neighbor settings as filter list out. The web gui should generate a quagga bgpd.conf from this and it should look like this:

router bgp 2222

neighbor remote-as 1111
neighbor prefix-list REF_BgpFilBgpfiltero_4 out

ip prefix-list REF_BgpFilBgpfiltero_4 seq 5 permit le 32
ip prefix-list REF_BgpFilBgpfiltero_4 seq 10 deny le 32

But looking into the actual config file in /var/sec/chroot-quagga/etc/quagga showed that the prefix list was missing. The backup config file bgpd.conf.sav showed the correct prefix list was there before. The only change in the meantime was that I removed an other (not used) “filter list in” in the gui. It seems there is a bug in Sophos UTM that the web gui removes all prefix lists from the config file, when you actually want to remove only one.

Workarround: configure a new fillter list and attach it to the neighbor config.

It’s very good that I actually could log into the Sophos box, because I would have never found this bug without ssh access.

Version: 9.408-4


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