DHCP Relay on Linux

DHCP relaying is used to forward DHCP requests to a DHCP server if the client and the server are not on the same network. One standard implementation of this is isc-dhcp-relay which is part of the isc-dhcp package.

Problem: The dhcrelay is forwarding the dhcp request to the dhcp server and the dhcp server is responding correctly, but the dhcrelay ignores the response.

Discussion: the option “-i” tells the dhcrelay which interface to listen on for dhcp requests. But at the same time “-i” must be set for the interface that receives the dhcp server response. In newer versions (eg 4.4.2) there are even two options “-id”, “-iu” for downstream and upstream. The problem is: Both interfaces must be ethernet interfaces and have broadcasts, even though the dhcp server response is a simple unicast packet. If the connection to your dhcp server is a “tun” interface like openvpn or something not ethernet like, isc-dhcp-relay cannot receive the dhcp server response.

Solution: Don’t use isc-dhcp-relay but dhcp-helper. dhcp-helper is much simpler and works as expected. dhcp-helper is in the main debian/devuan repositories.

Versions: Devuan 3, isc-dhcp-relay 4.4.1-2, dhcp-helper 1.1-1

LS Style After Devuan 3 or Debian 10 Update

After Devuan 3 update ls output showed characters, that are not really in the directory listing. The reason is that “ls” draws quotes around filenames with spaces. This is a bug from my point of view, ls should never change the actual filenames. If a filename has quotes or double quotes it’s even weirder . “ls” adds the other quotes or even backslash quoting and closes and opens string within the string.

alex@workstation:~/test$ ls -l 
total 0
-rw-r--r-- 1 alex alex 0 Jul  9 17:55 '"'\''withbothquotes'\''"'
-rw-r--r-- 1 alex alex 0 Jul  9 17:53 '"withdoublequotes"'
-rw-r--r-- 1 alex alex 0 Jul  9 17:54 '"withdoublequotesandquotes'\'''
-rw-r--r-- 1 alex alex 0 Jul  9 17:55 "'file'"
-rw-r--r-- 1 alex alex 0 Jul  9 17:52 'some file'

To fix this bug sysadmins like me have to set the following environment variable (eg. in /etc/profile, ~/.bashrc, etc)

export QUOTING_STYLE=literal

After this setting you get the real filenames back:

alex@workstation:~/ttd$ ls -l 
total 0
-rw-r--r-- 1 alex alex 0 Jul  9 17:55 "'withbothquotes'"
-rw-r--r-- 1 alex alex 0 Jul  9 17:53 "withdoublequotes"
-rw-r--r-- 1 alex alex 0 Jul  9 17:54 "withdoublequotesandquotes'
-rw-r--r-- 1 alex alex 0 Jul  9 17:55 'file'
-rw-r--r-- 1 alex alex 0 Jul  9 17:52 some file

Time Format after Devuan 3 and Debian 10 Update

After updating to Devuan 3 the date command shows 12hours am/pm but my days have 24 hours. The locale was always en_US.UTF8 to keep sane command and error output.

Debian 10 thinks they had to fix the correct hour display to the complicated one.

Therefor all sysadmins like me have to apply the following workaround, to keep both sane command output and reasonable time format.

update-locale LC_TIME=C.UTF-8

This changed the locale for time output to “C” in /etc/default/locale. Date looks correct again:

# date
Mon Jun  8 21:20:33 CEST 2020

Linux Live-boot Fails after Debian/Devuan Update

Problem: after updating from Debian 8 to Devuan 2 the overlay live-boot failes with “no such device”

Discussion: I use a bootable USB stick combined with live-boot. In this case the USB stick partition 3 is a normal ext4 file system used as read only “plainroot” filesystem. Live-boot overlays this with an ramfs.
As I don’t know the /dev/sdaX file on the target system I use “root=LABEL=KROOT” to find the USB root image. This worked before but it does not any more. The reason is the following line in /lib/live/boot/9990-overlay.sh in the “plain root system” section:

mount -t $(get_fstype "${image_directory}") -o ro,noatime "${image_directory}" "${croot}/filesystem"

get_fstype “LABEL=KROOT” results in “unkown” and this mount command fails.

Solution: I removed the get_fstype part -t $(get_fstype “${image_directory}”) in /lib/live/boot/9990-overlay.sh. Mount guesses the filesystem type automatically.

After that you have to rebuild initramdisk with update-initramfs.

Version: tested with devuan 2.1, and this kernel boot options: “read-only boot=live root=LABEL=KROOT rootdelay=10 ignore_uuid plainroot”

APT sources list

Problem: when debian goes from “testing” to “stable” to “oldstable” the package sources change. eg. jessie-updates are remove, same happened to jessie-backports

The current file /etc/apt/sources.list for jessie (currently oldstable) could look like this

deb http://ftp.debian.org/debian/ jessie main contrib non-free
deb http://security.debian.org/ jessie/updates main contrib non-free

Configure WLAN Statically in Debian/Linux

If you want to configure WLAN settings on a Linux machine statically you can use the normal /etc/network/interfaces configuration method of Debian. For WPA-PSK you can use this 3 steps:

Install the “wpasupplicant” package

Generate a psk line with “wpa_passphrase” and copy the hex string after “psk=”

root@server:~# wpa_passphrase WLANNAME
# reading passphrase from stdin

Add some lines to /etc/network/interfaces using this hex string

allow-hotplug wlan0
iface wlan0 inet dhcp
metric 4
wpa-essid WLANNAME
wpa-scan-ssid 1
wpa-psk fe5409c4831b3daafff41fe2e6ed15ba7ed18c87bab254315e0be5f9180573d3

The line “wpa-scan-ssid 1” allows to use hidden WLAN that are not broadcasted. With “metric 4” you can make WLAN less preferred if there is a second LAN connection that should be preferred (default is “metric 1”).

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

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

Check Raid Status for Dell Raids on Linux

Linux support from dell is still very poor. They still support only RedHat$ and SuSE$.

But there are ways to check the Raid status of Dell server on debian. http://hwraid.le-vert.net/ is doing a good job in collecting information and building Debian style packages.


A Dell server “PowerEdge T130” with “LSI Logic / Symbios Logic MegaRAID SAS-3 3008”  also called “PERC H330” running Debian 8.7.

The kernel uses the megaraid_sas driver. At http://hwraid.le-vert.net/debian/pool-jessie you can find a package called “megacli_8.07.14-1_amd64.deb”

You can check your raid status with:

megacli -LDInfo -Lall -a0

or add this to your crontab file, to receive mails when not all raids are in “optimal” state:

7 * * * *    /usr/sbinmegacli -LDInfo -Lall -a0 | grep “^State” | grep -v ” Optimal$”