Posts Tagged ‘default’

How to filter dhcp traffic between two networks running separate DHCP servers to prevent IP assignment issues and MAC duplicate addresses

Tuesday, February 8th, 2022

Tracking the Problem of MAC duplicates on Linux routers

If you have two networks that see each other and they're not separated in VLANs but see each other sharing a common netmask lets say or, it might happend that there are 2 dhcp servers for example (isc-dhcp-server running on and dhcpd running on can broadcast their services to both LANs (netmask and Local Net LAN The result out of this is that some devices might pick up their IP address via DHCP from the wrong dhcp server.

Normally if you have a fully controlled little or middle class home or office network (10 – 15 electronic devices nodes) connecting to the LAN in a mixed moth some are connected via one of the Networks via connected Wifi to others are LANned and using static IP adddresses and traffic is routed among two ISPs and each network can see the other network, there is always a possibility of things to go wrong. This is what happened to me so this is how this post was born.

The best practice from my experience so far is to define each and every computer / phone / laptop host joining the network and hence later easily monitor what is going on the network with something like iptraf-ng / nethogs  / iperf – described in prior  how to check internet spepeed from console and in check server internet connectivity speed with speedtest-cliiftop / nload or for more complex stuff wireshark or even a simple tcpdump. No matter the tools network monitoring is only part on solving network issues. A very must have thing in a controlled network infrastructure is defining every machine part of it to easily monitor later with the monitoring tools. Defining each and every host on the Hybrid computer networks makes administering the network much easier task and  tracking irregularities on time is much more likely. 

Since I have such a hybrid network here hosting a couple of XEN virtual machines with Linux, Windows 7 and Windows 10, together with Mac OS X laptops as well as MacBook Air notebooks, I have followed this route and tried to define each and every host based on its MAC address to pick it up from the correct DHCP1 server (that is distributing IPs for Internet Provider 1 (ISP 1), that is mostly few computers attached UTP LAN cables via LiteWave LS105G Gigabit Switch as well from DHCP2 – used only to assigns IPs to servers and a a single Wi-Fi Access point configured to route incoming clients via Linux NAT gateway server.

To filter out the unwanted IPs from the DHCPD not to propagate I've so far used a little trick to  Deny DHCP MAC Address for unwanted clients and not send IP offer for them.

To give you more understanding,  I have to clear it up I don't want to have automatic IP assignments from DHCP2 / LAN2 to DHCP1 / LAN1 because (i don't want machines on DHCP1 to end up with IP like or DHCP2 (to have, as such a wrong IP delegation could potentially lead to MAC duplicates IP conflicts. MAC Duplicate IP wrong assignments for those older or who have been part of administrating large ISP network infrastructures  makes the network communication unstable for no apparent reason and nodes partially unreachable at times or full time …

However it seems in the 21-st century which is the century of strangeness / computer madness in the 2022, technology advanced so much that it has massively started to break up some good old well known sysadmin standards well documented in the RFCs I know of my youth, such as that every electronic equipment manufactured Vendor should have a Vendor Assigned Hardware MAC Address binded to it that will never change (after all that was the idea of MAC addresses wasn't it !). 
Many mobile devices nowadays however, in the developers attempts to make more sophisticated software and Increase Anonimity on the Net and Security, use a technique called  MAC Address randomization (mostly used by hackers / script kiddies of the early days of computers) for their Wi-Fi Net Adapter OS / driver controlled interfaces for the sake of increased security (the so called Private WiFi Addresses). If a sysadmin 10-15 years ago has seen that he might probably resign his profession and turn to farming or agriculture plant growing, but in the age of digitalization and "cloud computing", this break up of common developed network standards starts to become the 'new normal' standard.

I did not suspected there might be a MAC address oddities, since I spare very little time on administering the the network. This was so till recently when I accidently checked the arp table with:

Hypervisor:~# arp -an     5c:89:b5:f2:e8:d8      (Unknown)    00:15:3e:d3:8f:76       (Unknown)


and consequently did a network MAC Address ARP Scan with arp-scan (if you never used this little nifty hacker tool I warmly recommend it !!!)
If you don't have it installed it is available in debian based linuces from default repos to install

Hypervisor:~# apt-get install –yes arp-scan

It is also available on CentOS / Fedora / Redhat and other RPM distros via:

Hypervisor:~# yum install -y arp-scan



Hypervisor:~# arp-scan –interface=eth1    00:16:3e:0f:48:05       Xensource, Inc.    00:16:3e:04:11:1c       Xensource, Inc.    00:15:3e:bb:45:45       Xensource, Inc.    00:15:3e:59:96:8e       Xensource, Inc.    00:15:3e:d3:8f:77       Xensource, Inc.    8c:89:b5:f2:e8:d8       Micro-Star INT'L CO., LTD     5c:89:b5:f2:e8:d8      (Unknown)    00:15:3e:d3:8f:76       (Unknown)

192.168.x.91     02:a0:xx:xx:d6:64        (Unknown)
192.168.x.91     02:a0:xx:xx:d6:64        (Unknown)  (DUP: 2)

N.B. !. I found it helpful to check all available interfaces on my Linux NAT router host.

As you see the scan revealed, a whole bunch of MAC address mess duplicated MAC hanging around, destroying my network topology every now and then 
So far so good, the MAC duplicates and strangely hanging around MAC addresses issue, was solved relatively easily with enabling below set of systctl kernel variables.

1. Fixing Linux ARP common well known Problems through disabling arp_announce / arp_ignore / send_redirects kernel variables disablement


Linux answers ARP requests on wrong and unassociated interfaces per default. This leads to the following two problems:

ARP requests for the loopback alias address are answered on the HW interfaces (even if NOARP on lo0:1 is set). Since loopback aliases are required for DSR (Direct Server Return) setups this problem is very common (but easy to fix fortunately).

If the machine is connected twice to the same switch (e.g. with eth0 and eth1) eth2 may answer ARP requests for the address on eth1 and vice versa in a race condition manner (confusing almost everything).

This can be prevented by specific arp kernel settings. Take a look here for additional information about the nature of the problem (and other solutions): ARP flux.

To fix that generally (and reboot safe) we  include the following lines into


Hypervisor:~# cp -rpf /etc/sysctl.conf /etc/sysctl.conf_bak_07-feb-2022
Hypervisor:~# cat >> /etc/sysctl.conf

# LVS tuning


Press CTRL + D simultaneusly to Write out up-pasted vars.

To read more on Load Balancer using direct routing and on LVS and the arp problem here

2. Digging further the IP conflict / dulicate MAC Problems

Even after this arp tunings (because I do have my Hypervisor 2 LAN interfaces connected to 1 switch) did not resolved the issues and still my Wireless Connected devices via network (ISP2) were randomly assigned the wrong range IPs 192.168.0.XXX/24 as well as the wrong gateway (ISP1).
After thinking thoroughfully for hours and checking the network status with various tools and thanks to the fact that my wife has a MacBook Air that was always complaining that the IP it tried to assign from the DHCP was already taken, i"ve realized, something is wrong with DHCP assignment.
Since she owns a IPhone 10 with iOS and this two devices are from the same vendor e.g. Apple Inc. And Apple's products have been having strange DHCP assignment issues from my experience for quite some time, I've thought initially problems are caused by software on Apple's devices.
I turned to be partially right after expecting the logs of DHCP server on the Linux host (ISP1) finding that the phone of my wife takes IP in 192.168.0.XXX, insetad of IP from (which has is a combined Nokia Router with 2.4Ghz and 5Ghz Wi-Fi and LAN router provided by ISP2 in that case Vivacom). That was really puzzling since for me it was completely logical thta the iDevices must check for DHCP address directly on the Network of the router to whom, they're connecting. Guess my suprise when I realized that instead of that the iDevices does listen to the network on a wide network range scan for any DHCPs reachable baesd on the advertised (i assume via broadcast) address traffic and try to connect and take the IP to the IP of the DHCP which responds faster !!!! Of course the Vivacom Chineese produced Nokia router responded DHCP requests and advertised much slower, than my Linux NAT gateway on ISP1 and because of that the Iphone and iOS and even freshest versions of Android devices do take the IP from the DHCP that responds faster, even if that router is not on a C class network (that's invasive isn't it??). What was even more puzzling was the automatic MAC Randomization of Wifi devices trying to connect to my ISP1 configured DHCPD and this of course trespassed any static MAC addresses filtering, I already had established there.

Anyways there was also a good think out of tthat intermixed exercise 🙂 While playing around with the Gigabit network router of vivacom I found a cozy feature SCHEDULEDING TURNING OFF and ON the WIFI ACCESS POINT  – a very useful feature to adopt, to stop wasting extra energy and lower a bit of radiation is to set a swtich off WIFI AP from 12:30 – 06:30 which are the common sleeping hours or something like that.

3. What is MAC Randomization and where and how it is configured across different main operating systems as of year 2022?

Depending on the operating system of your device, MAC randomization will be available either by default on most modern mobile OSes or with possibility to have it switched on:

  • Android Q: Enabled by default 
  • Android P: Available as a developer option, disabled by default
  • iOS 14: Available as a user option, disabled by default
  • Windows 10: Available as an option in two ways – random for all networks or random for a specific network

Lately I don't have much time to play around with mobile devices, and I do not my own a luxury mobile phone so, the fact this ne Androids have this MAC randomization was unknown to me just until I ended a small mess, based on my poor configured networks due to my tight time constrains nowadays.

Finding out about the new security feature of MAC Randomization, on all Android based phones (my mother's Nokia smartphone and my dad's phone, disabled the feature ASAP:

4. Disable MAC Wi-Fi Ethernet device Randomization on Android

MAC Randomization creates a random MAC address when joining a Wi-Fi network for the first time or after “forgetting” and rejoining a Wi-Fi network. It Generates a new random MAC address after 24 hours of last connection.

Disabling MAC Randomization on your devices. It is done on a per SSID basis so you can turn off the randomization, but allow it to function for hotspots outside of your home.

  1. Open the Settings app
  2. Select Network and Internet
  3. Select WiFi
  4. Connect to your home wireless network
  5. Tap the gear icon next to the current WiFi connection
  6. Select Advanced
  7. Select Privacy
  8. Select "Use device MAC"

5. Disabling MAC Randomization on MAC iOS, iPhone, iPad, iPod

To Disable MAC Randomization on iOS Devices:

Open the Settings on your iPhone, iPad, or iPod, then tap Wi-Fi or WLAN


  1. Tap the information button next to your network
  2. Turn off Private Address
  3. Re-join the network

Of course next I've collected their phone Wi-Fi adapters and made sure the included dhcp MAC deny rules in /etc/dhcp/dhcpd.conf are at place.

The effect of the MAC Randomization for my Network was terrible constant and strange issues with my routings and networks, which I always thought are caused by the openxen hypervisor Virtualization VM bugs etc.

That continued for some months now, and the weird thing was the issues always started when I tried to update my Operating system to the latest packetset, do a reboot to load up the new piece of software / libraries etc. and plus it happened very occasionally and their was no obvious reason for it.


6. How to completely filter dhcp traffic between two network router hosts
IP / to stop 2 or more configured DHCP servers
on separate networks see each other

To prevent IP mess at DHCP2 server side (which btw is ISC DHCP server, taking care for IP assignment only for the Servers on the network running on Debian 11 Linux), further on I had to filter out any DHCP UDP traffic with iptables completely.
To prevent incorrect route assignments assuming that you have 2 networks and 2 routers that are configurred to do Network Address Translation (NAT)-ing Router 1:, Router 2:

You have to filter out UDP Protocol data on Port 67 and 68 from the respective source and destination addresses.

In firewall rules configuration files on your Linux you need to have some rules as:

# filter outgoing dhcp traffic from to
-A INPUT -p udp -m udp –dport 67:68 -s -d -j DROP
-A OUTPUT -p udp -m udp –dport 67:68 -s -d -j DROP
-A FORWARD -p udp -m udp –dport 67:68 -s -d -j DROP

-A INPUT -p udp -m udp –dport 67:68 -s -d -j DROP
-A OUTPUT -p udp -m udp –dport 67:68 -s -d -j DROP
-A FORWARD -p udp -m udp –dport 67:68 -s -d -j DROP

-A INPUT -p udp -m udp –sport 67:68 -s -d -j DROP
-A OUTPUT -p udp -m udp –sport 67:68 -s -d -j DROP
-A FORWARD -p udp -m udp –sport 67:68 -s -d -j DROP

You can download also with above rules from here

Applying this rules, any traffic of DHCP between 2 routers is prohibited and devices from Net: will no longer wrongly get assinged IP addresses from Network range: as it happened to me.

7. Filter out DHCP traffic based on MAC completely on Linux with arptables

If even after disabling MAC randomization on all devices on the network, and you know physically all the connecting devices on the Network, if you still see some weird MAC addresses, originating from a wrongly configured ISP traffic router host or whatever, then it is time to just filter them out with arptables.

## drop traffic prevent mac duplicates due to vivacom and bergon placed in same network –
dchp1-server:~# arptables -A INPUT –source-mac 70:e2:83:12:44:11 -j DROP

To list arptables configured on Linux host

dchp1-server:~# arptables –list -n

If you want to be paranoid sysadmin you can implement a MAC address protection with arptables by only allowing a single set of MAC Addr / IPs and dropping the rest.

dchp1-server:~# arptables -A INPUT –source-mac 70:e2:84:13:45:11 -j ACCEPT
dchp1-server:~# arptables -A INPUT  –source-mac 70:e2:84:13:45:12 -j ACCEPT

dchp1-server:~# arptables -L –line-numbers
Chain INPUT (policy ACCEPT)
1 -j DROP –src-mac 70:e2:84:13:45:11
2 -j DROP –src-mac 70:e2:84:13:45:12

Once MACs you like are accepted you can set the INPUT chain policy to DROP as so:

dchp1-server:~# arptables -P INPUT DROP

If you later need to temporary, clean up the rules inside arptables on any filtered hosts flush all rules inside INPUT chain, like that

dchp1-server:~#  arptables -t INPUT -F

How to test RAM Memory for errors in Linux / UNIX OS servers. Find broken memory RAM banks

Friday, December 3rd, 2021



1. Testing the memory with motherboard integrated tools

Memory testing has been integral part of Computers for the last 50 years. In the dawn of computers those older perhaps remember memory testing was part of the computer initialization boot. And this memory testing was delaying the boot with some seconds and the user could see the memory numbers being counted up to the amount of memory. With the increased memory modern computers started to have and the annoyance to wait for a memory check program to check the computer hardware memory on modern computers this check has been mitigated or completely removed on some hardware.
Thus under some circumstances sysadmins or advanced computer users might need to check the memory, especially if there is some suspicion for memory damages or if for example a home PC starts crashing with Blue screens of Death on Windows without reason or simply the PC or some old arcane Linux / UNIX servers gets restarted every now and then for now apparent reason. When such circumstances occur it is an idea to start debugging the hardware issue with a simple memory check.

There are multiple ways to test installed memory banks on a server laptop or local home PC both integrated and using external programs.
On servers that is usually easily done from ILO or IPMI or IDRAC access (usually web) interface of the vendor, on laptops and home usage from BIOS or UEFI (Unified Extensible Firmware Interface) acces interface on system boot that is possible as well.


An old but gold TIP, more younger people might not know is the


Prolonged SHIFT key press which once held with the user instructs the machine to initiate a memory test before the computer starts reading what is written in the boot loader.

So before anything else from below article it might be a good idea to just try HOLD SHIFT for 15-20 seconds after a complete Shut and ON from the POWER button.

If this test does not triggered or it is triggered and you end up with some corrupted memory but you're not sure which exact Memory bank is really crashing and want to know more on what memory Bank and segments are breaking up you might want to do a more thorough testing. In below article I'll try to explain shortly how this can be done.

2. Test the memory using a boot USB Flash Drive / DVD / CD 

Say hello to memtest86+. It is a Linux GRUB boot loader bootable utility that tests physical memory by writing various patterns to it and reading them back. Since memtest86+ runs directly off the hardware it does not require any operating system support for execution. Perhaps it is important to mention that memtest86 (is PassMark memtest86)and memtest86+ (An Advanced Memory diagnostic tool) are different tools, the first is freeware and second one is FOSS software.

To use it all you'll need is some version of Linux. If you don't already have some burned in somewhere at your closet, you might want to burn one.
For Linux / Mac users this is as downloading a Linux distribution ISO file and burning it with

# dd if=/path/to/iso of=/dev/sdbX bs=80M status=progress

Windows users can burn a Live USB with whatever Linux distro or download and burn the latest versionof memtest86+ from  on Windows Desktop with some proggie like lets say UnetBootIn.

2.1. Run memtest86+ on Ubuntu

Many Linux distributions such as Ubuntu 20.0 comes together with memtest86+, which can be easily invoked from GRUB / GRUB2 Kernel boot loader.
Ubuntu has a separate menu pointer for a Memtest.


Other distributions RPM based distributions such as CentOS, Fedora Linux, Redhat things differ.

2.2. memtest86+ on Fedora

Fedora used to have the memtest86+ menu at the GRUB boot selection prompt, but for some reason removed it and in newest Fedora releases as of time such as Fedora 35 memtest86+ is preinstalled and available but not visible, to start on  already and to start a memtest memory test tool:

  •   Boot a Fedora installation or Rescue CD / USB. At the prompt, type "memtest86".

boot: memtest86

2.3 memtest86+ on RHEL Linux

The memtest86+tool is available as an RPM package from Red Hat Network (RHN) as well as a boot option from the Red Hat Enterprise Linux rescue disk.
And nowadays Red Hat Enterprise Linux ships by default with the tool.

Prior redhat (now legacy) releases such as on RHEL 5.0 it has to be installed and configure it with below 3 commands.

[root@rhel ~]# yum install memtest86+
[root@rhel ~]# memtest-setup
[root@rhel ~]# grub2-mkconfig -o /boot/grub2/grub.cfg

    Again as with CentOS to boot memtest86+ from the rescue disk, you will need to boot your system from CD 1 of the Red Hat Enterprise Linux installation media, and type the following at the boot prompt (before the Linux kernel is started):

boot: memtest86

memtest86+ testing 5 memory slots

As you see all on above screenshot the Memory banks are listed as Slots. There are a number of Tests to be completed until
it can be said for sure memory does not have any faulty cells. 

Pass: 0
Errors: 0 

Indicates no errors, so in the end if memtest86 does not find anything this values should stay at zero.
memtest86+ is also usable to detecting issues with temperature of CPU. Just recently I've tested a PC thinking that some memory has defects but it turned out the issue on the Computer was at the CPU's temperature which was topping up at 80 – 82 Celsius.

If you're unfortunate and happen to get some corrupted memory segments you will get some red fields with the memory addresses found to have corrupted on Read / Write test operations:


2.4. Install and use memtest and memtest86+ on Debian / Mint Linux

You can install either memtest86+ or just for the fun put both of them and play around with both of them as they have a .deb package provided out of debian non-free /etc/apt/sources.list repositories.

root@jeremiah:/home/hipo# apt-cache show memtest86 memtest86+
Package: memtest86
Version: 4.3.7-3
Installed-Size: 302
Maintainer: Yann Dirson <>
Architecture: amd64
Depends: debconf (>= 0.5) | debconf-2.0
Recommends: memtest86+
Suggests: hwtools, memtester, kernel-patch-badram, grub2 (>= 1.96+20090523-1) | grub (>= 0.95+cvs20040624), mtools
Description-en: thorough real-mode memory tester
 Memtest86 scans your RAM for errors.
 This tester runs independently of any OS – it is run at computer
 boot-up, so that it can test *all* of your memory.  You may want to
 look at `memtester', which allows testing your memory within Linux,
 but this one won't be able to test your whole RAM.
 It can output a list of bad RAM regions usable by the BadRAM kernel
 patch, so that you can still use you old RAM with one or two bad bits.
 This is the last DFSG-compliant version of this software, upstream
 has opted for a proprietary development model starting with 5.0.  You
 may want to consider using memtest86+, which has been forked from an
 earlier version of memtest86, and provides a different set of
 features.  It is available in the memtest86+ package.
 A convenience script is also provided to make a grub-legacy-based
 floppy or image.

Description-md5: 0ad381a54d59a7d7f012972f613d7759
Section: misc
Priority: optional
Filename: pool/main/m/memtest86/memtest86_4.3.7-3_amd64.deb
Size: 45470
MD5sum: 8dd2a4c52910498d711fbf6b5753bca9
SHA256: 09178eca21f8fd562806ccaa759d0261a2d3bb23190aaebc8cd99071d431aeb6

Package: memtest86+
Version: 5.01-3
Installed-Size: 2391
Maintainer: Yann Dirson <>
Architecture: amd64
Depends: debconf (>= 0.5) | debconf-2.0
Suggests: hwtools, memtester, kernel-patch-badram, memtest86, grub-pc | grub-legacy, mtools
Description-en: thorough real-mode memory tester
 Memtest86+ scans your RAM for errors.
 This tester runs independently of any OS – it is run at computer
 boot-up, so that it can test *all* of your memory.  You may want to
 look at `memtester', which allows to test your memory within Linux,
 but this one won't be able to test your whole RAM.
 It can output a list of bad RAM regions usable by the BadRAM kernel
 patch, so that you can still use your old RAM with one or two bad bits.
 Memtest86+ is based on memtest86 3.0, and adds support for recent
 hardware, as well as a number of general-purpose improvements,
 including many patches to memtest86 available from various sources.
 Both memtest86 and memtest86+ are being worked on in parallel.
Description-md5: aa685f84801773ef97fdaba8eb26436a

Tag: admin::benchmarking, admin::boot, hardware::storage:floppy,
 interface::text-mode, role::program, scope::utility, use::checking
Section: misc
Priority: optional
Filename: pool/main/m/memtest86+/memtest86+_5.01-3_amd64.deb
Size: 75142
MD5sum: 4f06523532ddfca0222ba6c55a80c433
SHA256: ad42816e0b17e882713cc6f699b988e73e580e38876cebe975891f5904828005


root@jeremiah:/home/hipo# apt-get install –yes memtest86+

root@jeremiah:/home/hipo# apt-get install –yes memtest86

Reading package lists… Done
Building dependency tree       
Reading state information… Done
Suggested packages:
  hwtools kernel-patch-badram grub2 | grub
The following NEW packages will be installed:
0 upgraded, 1 newly installed, 0 to remove and 21 not upgraded.
Need to get 45.5 kB of archives.
After this operation, 309 kB of additional disk space will be used.
Get:1 buster/main amd64 memtest86 amd64 4.3.7-3 [45.5 kB]
Fetched 45.5 kB in 0s (181 kB/s)     
Preconfiguring packages …
Selecting previously unselected package memtest86.
(Reading database … 519985 files and directories currently installed.)
Preparing to unpack …/memtest86_4.3.7-3_amd64.deb …
Unpacking memtest86 (4.3.7-3) …
Setting up memtest86 (4.3.7-3) …
Generating grub configuration file …
Found background image: saint-John-of-Rila-grub.jpg
Found linux image: /boot/vmlinuz-4.19.0-18-amd64
Found initrd image: /boot/initrd.img-4.19.0-18-amd64
Found linux image: /boot/vmlinuz-4.19.0-17-amd64
Found initrd image: /boot/initrd.img-4.19.0-17-amd64
Found linux image: /boot/vmlinuz-4.19.0-8-amd64
Found initrd image: /boot/initrd.img-4.19.0-8-amd64
Found linux image: /boot/vmlinuz-4.19.0-6-amd64
Found initrd image: /boot/initrd.img-4.19.0-6-amd64
Found linux image: /boot/vmlinuz-4.19.0-5-amd64
Found initrd image: /boot/initrd.img-4.19.0-5-amd64
Found linux image: /boot/vmlinuz-4.9.0-8-amd64
Found initrd image: /boot/initrd.img-4.9.0-8-amd64
Found memtest86 image: /boot/memtest86.bin
Found memtest86+ image: /boot/memtest86+.bin
Found memtest86+ multiboot image: /boot/memtest86+_multiboot.bin
File descriptor 3 (pipe:[66049]) leaked on lvs invocation. Parent PID 22581: /bin/sh
Processing triggers for man-db (2.8.5-2) …


After this both memory testers memtest86+ and memtest86 will appear next to the option of booting a different version kernels and the Advanced recovery kernels, that you usually get in the GRUB boot prompt.

2.5. Use memtest embedded tool on any Linux by adding a kernel variable


2.4.1. Reboot your computer

# reboot

2.4.2. At the GRUB boot screen (with UEFI, press Esc).

2.4.3 For 4 passes add temporarily the memtest=4 kernel parameter.

memtest=        [KNL,X86,ARM,PPC,RISCV] Enable memtest
                Format: <integer>
                default : 0 <disable>
                Specifies the number of memtest passes to be
                performed. Each pass selects another test
                pattern from a given set of patterns. Memtest
                fills the memory with this pattern, validates
                memory contents and reserves bad memory
                regions that are detected.

3. Install and use memtester Linux tool

At some condition, memory is the one of the suspcious part, or you just want have a quick test. memtester  is an effective userspace tester for stress-testing the memory subsystem.  It is very effective at finding intermittent and non-deterministic faults.

The advantage of memtester "live system check tool is", you can check your system for errors while it's still running. No need for a restart, just run that application, the downside is that some segments of memory cannot be thoroughfully tested as you already have much preloaded data in it to have the Operating Sytstem running, thus always when possible try to stick to rule to test the memory using memtest86+  from OS Boot Loader, after a clean Machine restart in order to clean up whole memory heap.

Anyhow for a general memory test on a Critical Legacy Server  (if you lets say don't have access to Remote Console Board, or don't trust the ILO / IPMI Hardware reported integrity statistics), running memtester from already booted is still a good idea.

3.1. Install memtester on any Linux distribution from source

# tar zxvf memtester-4.2.2.tar.gz
# cd memtester-4.2.2
# make && make install

3.2 Install on RPM based distros


On Fedora memtester is available from repositories however on many other RPM based distros it is not so you have to install it from source.

[root@fedora ]# yum install -y memtester


3.3. Install memtester on Deb based Linux distributions from source

To install it on Debian / Ubuntu / Mint etc. , open a terminal and type:

root@linux:/ #  apt install –yes memtester

The general run syntax is:

memtester [-p PHYSADDR] [ITERATIONS]

You can hence use it like so:

hipo@linux:/ $ sudo memtester 1024 5

This should allocate 1024MB of memory, and repeat the test 5 times. The more repeats you run the better, but as a memtester run places a great overall load on the system you either don't increment the runs too much or at least run it with  lowered process importance e.g. by nicing the PID:

hipo@linux:/ $ nice -n 15 sudo memtester 1024 5


  • If you have more RAM like 4GB or 8GB, it is upto you how much memory you want to allocate for testing.
  • As your operating system, current running process might take some amount of RAM, Please check available Free RAM and assign that too memtester.
  • If you are using a 32 Bit System, you cant test more than 4 GB even though you have more RAM( 32 bit systems doesnt support more than 3.5 GB RAM as you all know).
  • If your system is very busy and you still assigned higher than available amount of RAM, then the test might get your system into a deadlock, leads to system to halt, be aware of this.
  • Run the memtester as root user, so that memtester process can malloc the memory, once its gets hold on that memory it will try to apply lock. if specified memory is not available, it will try to reduce required RAM automatically and try to lock it with mlock.
  • if you run it as a regular user, it cant auto reduce the required amount of RAM, so it cant lock it, so it tries to get hold on that specified memory and starts exhausting all system resources.

If you have 8 Gigas of RAM plugged into the PC motherboard you have to multiple 1024*8 this is easily done with bc (An arbitrary precision calculator language) tool:

root@linux:/ # bc -l
bc 1.07.1
Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006, 2008, 2012-2017 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'. 

 for example you should run:

root@linux:/ # memtester 8192 5

memtester version 4.3.0 (64-bit)
Copyright (C) 2001-2012 Charles Cazabon.
Licensed under the GNU General Public License version 2 (only).

pagesize is 4096
pagesizemask is 0xfffffffffffff000
want 8192MB (2083520512 bytes)
got  8192MB (2083520512 bytes), trying mlock …Loop 1/1:
  Stuck Address       : ok        
  Random Value        : ok
  Compare XOR         : ok
  Compare SUB         : ok
  Compare MUL         : ok
  Compare DIV         : ok
  Compare OR          : ok
  Compare AND         : ok
  Sequential Increment: ok
  Solid Bits          : ok        
  Block Sequential    : ok        
  Checkerboard        : ok        
  Bit Spread          : ok        
  Bit Flip            : ok        
  Walking Ones        : ok        
  Walking Zeroes      : ok        
  8-bit Writes        : ok
  16-bit Writes       : ok



4. Shell Script to test server memory for corruptions

If for some reason the machine you want to run a memory test doesn't have connection to the external network such as the internet and therefore you cannot configure a package repository server and install memtester, the other approach is to use a simple memory test script such as

# Downloaded from
echo "ByteOnSite Memory Test"
cpus=`cat /proc/cpuinfo | grep processor | wc -l`
if [ $cpus -lt 6 ]; then
threads=$(($cpus / 2))
echo "Detected $cpus CPUs, using $threads threads.."
memory=`free | grep 'Mem:' | awk {'print $2'}`
memoryper=$(($memory / $threads))
echo "Detected ${memory}K of RAM ($memoryper per thread).."
freespace=`df -B1024 . | tail -n1 | awk {'print $4'}`
if [ $freespace -le $memory ]; then
echo You do not have enough free space on the current partition. Minimum: $memory bytes
exit 1
echo "Clearing RAM Cache.."
sync; echo 3 > /proc/sys/vm/drop_cachesfile
echo > dump.memtest.img
echo "Writing to dump file (dump.memtest.img).."
for i in `seq 1 $threads`;
# 1044 is used in place of 1024 to ensure full RAM usage (2% over allocation)
dd if=/dev/urandom bs=$memoryper count=1044 >> dump.memtest.img 2>/dev/null &
echo $i
for pid in "${pids[@]}"
wait $pid

echo "Reading and analyzing dump file…"
echo "Pass 1.."
md51=`md5sum dump.memtest.img | awk {'print $1'}`
echo "Pass 2.."
md52=`md5sum dump.memtest.img | awk {'print $1'}`
echo "Pass 3.."
md53=`md5sum dump.memtest.img | awk {'print $1'}`
if [ “$md51” != “$md52” ]; then
elif [ “$md51” != “$md53” ]; then
elif [ “$md52” != “$md53” ]; then
if [ $fail -eq 0 ]; then
echo "Memory test PASSED."
echo "Memory test FAILED. Bad memory detected."
rm -f dump.memtest.img
exit $fail

Nota Bene !: Again consider the restults might not always be 100% trustable if possible restart the server and test with memtest86+

Consider also its important to make sure prior to script run,  you''ll have enough disk space to produce the dump.memtest.img file – file is created as a test bed for the memory tests and if not scaled properly you might end up with a full ( / ) root directory!


4.1 Other memory test script with dd and md5sum checksum

I found this solution on the well known sysadmin site nixCraft, I think it makes sense and quicker.

First find out memory site using free command.

# free
             total       used       free     shared    buffers     cached
Mem:      32867436   32574160     293276          0      16652   31194340
-/+ buffers/cache:    1363168   31504268
Swap:            0          0          0

It shows that this server has 32GB memory,

# dd if=/dev/urandom bs=32867436 count=1050 of=/home/memtest

free reports by k and use 1050 is to make sure file memtest is bigger than physical memory.  To get better performance, use proper bs size, for example 2048 or 4096, depends on your local disk i/o,  the rule is to make bs * count > 32 GB.

# md5sum /home/memtest; md5sum /home/memtest; md5sum /home/memtest

If you see md5sum mismatch in different run, you have faulty memory guaranteed.
The theory is simple, the file /home/memtest will cache data in memory by filling up all available memory during read operation. Using md5sum command you are reading same data from memory.

5. Other ways to test memory / do a machine stress test

Other good tools you might want to check for memory testing is mprime – 

  •  (mprime can also be used to stress test your CPU)

Alternatively, use the package stress-ng to run all kind of stress tests (including memory test) on your machine.
Perhaps there are other interesting tools for a diagnosis of memory if you know other ones I miss, let me know in the comment section.

Install and configure rkhunter for improved security on a PCI DSS Linux / BSD servers with no access to Internet

Wednesday, November 10th, 2021


rkhunter or Rootkit Hunter scans systems for known and unknown rootkits. The tool is not new and most system administrators that has to mantain some good security servers perhaps already use it in their daily sysadmin tasks.

It does this by comparing SHA-1 Hashes of important files with known good ones in online databases, searching for default directories (of rootkits), wrong permissions, hidden files, suspicious strings in kernel modules, commmon backdoors, sniffers and exploits as well as other special tests mostly for Linux and FreeBSD though a ports for other UNIX operating systems like Solaris etc. are perhaps available. rkhunter is notable due to its inclusion in popular mainstream FOSS operating systems (CentOS, Fedora,Debian, Ubuntu etc.).

Even though rkhunter is not rapidly improved over the last 3 years (its last Official version release was on 20th of Febuary 2018), it is a good tool that helps to strengthen even further security and it is often a requirement for Unix servers systems that should follow the PCI DSS Standards (Payment Card Industry Data Security Standards).

Configuring rkhunter is a pretty straight forward if you don't have too much requirements but I decided to write this article for the reason there are fwe interesting options that you might want to adopt in configuration to whitelist any files that are reported as Warnings, as well as how to set a configuration that sets a stricter security checks than the installation defaults. 

1. Install rkhunter .deb / .rpm package depending on the Linux distro or BSD

  • If you have to place it on a Redhat based distro CentOS / Redhat / Fedora

[root@Centos ~]# yum install -y rkhunter


  • On Debian distros the package name is equevallent to install there exec usual:

root@debian:~# apt install –yes rkhunter

  • On FreeBSD / NetBSD or other BSD forks you can install it from the BSD "World" ports system or install it from a precompiled binary.

freebsd# pkg install rkhunter

One important note to make here is to have a fully functional Alarming from rkhunter, you will have to have a fully functional configured postfix / exim / qmail whatever mail server to relay via official SMTP so you the Warning Alarm emails be able to reach your preferred Alarm email address. If you haven't installed postfix for example and configure it you might do.

– On Deb based distros 

[root@Centos ~]#yum install postfix

– On RPM based distros

root@debian:~# apt-get install –yes postfix

and as minimum, further on configure some functional Email Relay server within /etc/postfix/

# vi /etc/postfix/
relayhost = []

2. Prepare rkhunter.conf initial configuration

Depending on what kind of files are present on the filesystem it could be for some reasons some standard package binaries has to be excluded for verification, because they possess unusual permissions because of manual sys admin monification this is done with the rkhunter variable PKGMGR_NO_VRFY.

If remote logging is configured on the system via something like rsyslog you will want to specificly tell it to rkhunter so this check as a possible security issue is skipped via ALLOW_SYSLOG_REMOTE_LOGGING=1. 

In case if remote root login via SSH protocol is disabled via /etc/ssh/sshd_config
PermitRootLogin no variable, the variable to include is ALLOW_SSH_ROOT_USER=no

It is useful to also increase the hashing check algorithm for security default one SHA256 you might want to change to SHA512, this is done via rkhunter.conf var HASH_CMD=SHA512

Triggering new email Warnings has to be configured so you receive, new mails at a preconfigured mailbox of your choice via variable


# vi /etc/rkhunter.conf




# Needed for corosync/pacemaker since update 19.11.2020


# enabled ssh root access skip



# Email address to sent alert in case of Warnings


Optionally if you're using something specific such as corosync / pacemaker High Availability cluster or some specific software that is creating /dev/ files identified as potential Risks you might want to add more rkhunter.conf options like:

# Allow PCS/Pacemaker/Corosync
# Needed for corosync/pacemaker since update 19.11.2020

# tomboy creates this one
# created by libv4l
# created by spice video
# created by mdadm
# 389 Directory Server
# squid proxy
# squid ssl cache
# Allow podman


3. Set the proper mirror database URL location to internal network repository


Usually  file /var/lib/rkhunter/db/mirrors.dat does contain Internet server address where latest version of mirrors.dat could be fetched, below is how it looks by default on Debian 10 Linux.

root@debian:/var/lib/rkhunter/db# cat mirrors.dat 

As you can guess a machine that doesn't have access to the Internet neither directly, neither via some kind of secure proxy because it is in a Paranoic Demilitarized Zone (DMZ) Network with many firewalls. What you can do then is setup another Mirror server (Apache / Nginx) within the local PCI secured LAN that gets regularly the database from official database on (by installing and running rkhunter –update command on the Mirror WebServer and copying data under some directory structure on the remote local LAN accessible server, to keep the DB uptodate you might want to setup a cron to periodically copy latest available rkhunter database towards the http://mirror-url/path-folder/)

# vi /var/lib/rkhunter/db/mirrors.dat


A mirror copy of entire db files from Debian 10.8 ( Buster ) ready for download are here.

Update entire file property db and check for rkhunter db updates


# rkhunter –update && rkhunter –propupdate

[ Rootkit Hunter version 1.4.6 ]

Checking rkhunter data files…
  Checking file mirrors.dat                                  [ Skipped ]
  Checking file programs_bad.dat                             [ No update ]
  Checking file backdoorports.dat                            [ No update ]
  Checking file suspscan.dat                                 [ No update ]
  Checking file i18n/cn                                      [ No update ]
  Checking file i18n/de                                      [ No update ]
  Checking file i18n/en                                      [ No update ]
  Checking file i18n/tr                                      [ No update ]
  Checking file i18n/tr.utf8                                 [ No update ]
  Checking file i18n/zh                                      [ No update ]
  Checking file i18n/zh.utf8                                 [ No update ]
  Checking file i18n/ja                                      [ No update ]



4. Initiate a first time check and see whether something is not triggering Warnings

# rkhunter –check


As you might have to run the rkhunter multiple times, there is annoying Press Enter prompt, between checks. The idea of it is that you're able to inspect what went on but since usually, inspecting /var/log/rkhunter/rkhunter.log is much more easier, I prefer to skip this with –skip-keypress option.

# rkhunter –check  –skip-keypress

5. Whitelist additional files and dev triggering false warnings alerts

You have to keep in mind many files which are considered to not be officially PCI compatible and potentially dangerous such as lynx browser curl, telnet etc. might trigger Warning, after checking them thoroughfully with some AntiVirus software such as Clamav and checking the MD5 checksum compared to a clean installed .deb / .rpm package on another RootKit, Virus, Spyware etc. Clean system (be it virtual machine or a Testing / Staging) machine you might want to simply whitelist the files which are incorrectly detected as dangerous for the system security.

Again this can be achieved with


Some Cluster softwares that are preparing their own /dev/ temporary files such as Pacemaker / Corosync might also trigger alarms, so you might want to suppress this as well with ALLOWDEVFILE


If Warnings are found check what is the issue and if necessery white list files due to incorrect permissions in /etc/rkhunter.conf .


Re-run the check until all appears clean as in below screenshot.


Fixing Checking for a system logging configuration file [ Warning ]

If you happen to get some message like, message appears when rkhunter -C is done on legacy CentOS release 6.10 (Final) servers:

[13:45:29] Checking for a system logging configuration file [ Warning ]
[13:45:29] Warning: The 'systemd-journald' daemon is running, but no configuration file can be found.
[13:45:29] Checking if syslog remote logging is allowed [ Allowed ]

To fix it, you will have to disable SYSLOG_CONFIG_FILE at all.


Change Windows 10 default lock screen image via win registry LockScreenImage key change

Tuesday, September 21st, 2021


If you do work for a corporation on a Windows machine that is part of Windows Active Directory domain or a Microsoft 365 environment and your Domain admimistrator after some of the scheduled updates. Has enforced a Windows lock screen image change.
You  might be surprised to have some annoying corporation logo picture shown as a default Lock Screen image on your computer on next reoboot. Perhaps for some people it doesn't matter but for as a person who seriously like customizations, and a valuer of
freedom having an enforced picture logo each time I press CTRL + L (To lock my computer) is really annoying.

The logical question hence was how to reverse my desired image as  a default lock screen to enkoy. Some would enjoy some relaxing picture of a Woods, Cave or whatever Natural place landscape. I personally prefer simplicity so I simply use a simple purely black

To do it you'll have anyways to have some kind of superuser access to the computer. At the company I'm epmloyeed, it is possible to temporary request Administrator access this is done via a software installed on the machine. So once I request it I become
Administratof of machine for 20 minutes. In that time I do used a 'Run as Administartor' command prompt cmd.exe and inside Windows registry do the following Registry change.

The first logical thing to do is to try to manually set the picture via:

Settings ->  Lock Screen

But unfortunately as you can see in below screenshot, there was no way to change the LockScreen background image.


In Windows Registry Editor

I had to go to registry path


And from there in create a new "String Value" key


so full registry key path should be equal to:


The value to set is:



If you want to set a black background picture for LockScreen like me you can download my black background picture from here.

That's all press CTRL + L  key combination and the black screen background lock screen picture will appear !

Hopefully the Domain admin, would not soon enforce some policty to update the registry keys or return your old registry database from backup if something crashs out with something strange to break just set configuration.

To test whether the setting will stay permanent after the next scheduled Windows PC update of policies enforced by the Active Directory (AD) sysadmin, run manually from CMD.EXE

C:\> gpupdate /force

The command will download latest policies from Windows Domain, try to lock the screen once again with Control + L, if the background picture is still there most likely the Picture change would stay for a long.
If you get again the corporation preset domain background instead,  you're out of luck and will have to follow the same steps every, now and then after a domani policy update.

Enjoy your new smooth LockScreen Image 🙂


Disable NetworkManager automatic Ethernet Interface Management on Redhat Linux , CentOS 6 / 7 / 8

Friday, February 5th, 2021


Most of Linux distributions had introduced the NetworkManager service and are slowly trying to push out the old ways and use entirely it to manage network configs. Though at times this is very helpful stuff especially if you have Linux running on Laptop on servers is a guarantee for troubles.

If you are a system administrator like me and you need that needs to configure a New server with lets say 8 (Ethernet interface) LAN cards each to be configured with different IPs and you have a mixture of configuration where some eth1,eth2 etc. (4 of the interface IPs has to be static IPs and others has to be taken from a DHCP lease. NetworkManager is not something that you will want as usually you don't expect soon a network IP topology change. Below is example from a Living Hypervisor server machine that has 8 Network Interfaces configured together with few Virtual Interfaces used by the running KVM Virtual Machines.

[root@redhat :~ ]# ip address show |grep ": <"
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
2: ens1f0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master team0 state UP group default qlen 1000
3: eno1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master team0 state UP group default qlen 1000
4: ens1f1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master br2 state UP group default qlen 1000
5: ens1f2: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
6: eno2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master br1 state UP group default qlen 1000
7: ens1f3: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
8: eno3: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
9: eno4: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
10: venet0: <BROADCAST,POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN group default
11: br1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
12: br2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
13: team0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master br0 state UP group default qlen 1000
14: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
15: host-routed: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
16: virbr0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default qlen 1000
17: virbr0-nic: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast master virbr0 state DOWN group default qlen 1000
18: virbr1: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default qlen 1000
19: virbr1-nic: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast master virbr1 state DOWN group default qlen 1000
26: vme52540019e701: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master br0 state UNKNOWN group default qlen 1000
27: vme52540081868b: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master br1 state UNKNOWN group default qlen 1000
28: vme525400a13f03: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master br2 state UNKNOWN group default qlen 1000

Having a NM managing so many LAN connected Ethernets can create you A LOT of surprises even if your servers are in a Highly Secured data center where chance of sudden IP change or network misbehaves are minimal. Even minimal some in Housing might do something wrong on the Rack mixing up with another server or switch andyour server might end up easily with unexplainable Network problems because of this NM service which is trying 'to balance' any network issues according to some algorithms …

Thus to save yourlself the troubles and completely disable NetworkManager (NM) Ethernets handling.

As a hint some of the troubles you might get especially if the System Hardware has issues with the Integrated Motherboard LAN Controllers such as of Dell PowerEdge R640 Rack Server.
I've recently observed one such Dell Rack mounted machine I had to configure from scratch which has out of the box 
NM preinstalled by a colleague and was doing strange stuff with the routings causing it to become remotely inacessible after reboot.
Even though I have started configuring the IPs and have double and triple check the configuration and machine had proper set of /etc/sysconfig/network-scripts/ifcfg-* configuration it still failed to boot with a network properly brought up and become unreachable via remote SSH connection immediately after sending machine to init 6 with /usr/sbin/init 6 (alias for shutdown -r now or reboot -f now :)

On Redhat 8 / CentOS 8 to Disabling permanently NM you have to disable NM systemd services permanently and add NM_CONTROLLED=no to each of the Ethernet configurations listed in network-scripts/ifcfg-eno3 eno4 eno1np0 etc. ifaces.

1. Disable completely Network Manager service and mask it

[root@redhat :~ ]# systemctl mask NetworkManager.service
[root@redhat :~ ]# systemctl stop NetworkManager.service
[root@redhat :~ ]# systemctl disable NetworkManager.service

2. Check if all systemd networkmanager components scripts are really disabled

# systemctl list-unit-files | grep NetworkManager

NetworkManager-dispatcher.service disabled
NetworkManager-wait-online.service enabled
NetworkManager.service disabled

NetworkManager-wait-online.service seems to be also enabled so we have to disable it.

[root@redhat :~ ]#  systemctl mask NetworkManager-wait-online.service
[root@redhat :~ ]#  systemctl disable NetworkManager-wait-online.service

Double check NM services

[root@redhat :~ ]#  systemctl list-unit-files | grep NetworkManager

3. Install / Enable old (legacy) network-scripts 

network-scripts is disabled by default due to it doesn't play well with NM.
Install the rpm package to enable it back

[root@redhat :~ ]#  yum install -y network-scripts 

4. Test if network-scripts is really enabled

Use Redhat's nmcli command for controlling network manager if it reports NM not running then you're fine

[root@redhat :~ ]#  nmcli device
Error: NetworkManager is not running.

5. Disable legacy use network-scripts print outs

Bring down some interface with ifdown Redhat script frontend to ifconfig and bring it up with ifup iface-name

# ifup eno4
WARN      : [ifup] You are using 'ifup' script provided by 'network-scripts', which are now deprecated.
WARN      : [ifup] 'network-scripts' will be removed in one of the next major releases of RHEL.
WARN      : [ifup] It is advised to switch to 'NetworkManager' instead – it provides 'ifup/ifdown' scripts as well.

Notice the warnings they're harmless and safe to ignore however it is pretty annoying to see them, to disable them:

[root@redhat :~ ]#  touch /etc/sysconfig/disable-deprecation-warnings

6. Use network.service old-fashioned systemd service

From now on you can start using the good old well known and properly working network.service

[root@redhat :~ ]#  systemctl status network

To enable the network service to start after boot:

[root@redhat :~ ]#  systemctl enable network

7. Disable NetworkManager use from Network configuration scripts ifcfg-* for all server available configured ethernet cards

Open with text editor every network script and append NM_CONTROLLED="no" to the end of the file.

[root@redhat :~ ]#  vi /etc/sysconfig/network-scripts/ifcfg-ethernetX

To save yourself the time if you want to disable NetworkManager use for all /etc/sysconfig/network-scripts/ifcfg-* use a simple shell loop:

[root@redhat :~ ]# cd /etc/sysconfig/network-scripts/
[root@redhat :/etc/sysconfig/network-scripts ]# for i in *ifcfg*; do echo NM_CONTROLLED="no" >> $i; done

To load the new network settings do another network reload / restart

[root@redhat :~ ]# systemctl restart network

To disable NetworkManager on older CentOS 6 / Redhat 6 / SuSE / Fedora Linux where the OS still not systemd enabled instead of using systemctl you can straight do it with old and well known chkconfig redhat script.

[root@centos6 :~ ]# service NetworkManager stop
[root@centos6 :~ ]# chkconfig NetworkManager off

How to Avoid the 7 Most Frequent Mistakes in Python Programming

Monday, September 9th, 2019


Python is very appealing for Rapid Application Development for many reasons, including high-level built in data structures, dynamic typing and binding, or to use as glue to connect different components. It’s simple and easy to learn but new Python developers can fall in the trap of missing certain subtleties.

Here are 7 common mistakes that are harder to catch but that even more experienced Python developers have fallen for.


1. The misuse of expressions as function argument defaults

Python allows developers to indicate optional function arguments by giving them default values. In most cases, this is a great feature of Python, but it can create some confusion when the default value is mutable. In fact, the common mistake is thinking that the optional argument is set to whatever default value you’ve set every time the function argument is presented without a value. It can seem a bit complicated, but the answer is that the default value for this function argument is only evaluated at the time you’ve defined the function, one time only.  



2. Incorrect use of class variables

Python handles class variables internally as dictionaries and they will follow the Method Resolution Order (MRO). If an attribute is not found in one class it will be looked up in base classes so references to one part of the code are actually references to another part, and that can be quite difficult to handle well in Python. For class attributes, I recommend reading up on this aspect of Python independently to be able to handle them.



3. Incorrect specifications of parameters for exception blocks

There is a common problem in Python when except statements are provided but they don’t take a list of the exceptions specified. The syntax except Exception is used to bind these exception blocks to optional parameters so that there can be further inspections. What happens, however, is that certain exceptions are then not being caught by the except statement, but the exception becomes bound to parameters. The way to get block exceptions in one except statement has to be done by specifying the first parameter as a tuple to contain all the exceptions that you want to catch.


4. Failure to understand the scope rules

The scope resolution on Python is built on the LEGB rule as it’s commonly known, which means Local, Enclosing, Global, Built-in. Although at first glance this seems simple, there are some subtleties about the way it actually works in Python, which creates a more complex Python problem. If you make an assignment to a variable in a scope, Python will assume that variable is local to the scope and will shadow a variable that’s similarly named in other scopes. This is a particular problem especially when using lists.


5. Modifying lists during iterations over it


When a developer deletes an item from a list or array while iterating, they stumble upon a well known Python problem that’s easy to fall into. To address this, Python has incorporated many programming paradigms which can really simplify and streamline code when they’re used properly. Simple code is less likely to fall into the trap of deleting a list item while iterating over it. You can also use list comprehensions to avoid this problem.



6. Name clash with Python standard library


Python has so many library modules which is a bonus of the language, but the problem is that you can inadvertently have a name clash between your module and a module in the standard library. The problem here is that you can accidentally import another library which will import the wrong version. To avoid this, it’s important to be aware of the names in the standard library modules and stay away from using them.



7. Problems with binding variables in closures

Python has a late binding behavior which looks up the values of variables in closure only when the inner function is called. To address this, you may have to take advantage of default arguments to create anonymous functions that will give you the desired behavior – it’s either elegant or a hack depending on how you look at it, but it’s important to know.




Python is very powerful and flexible and it’s a great language for developers, but it’s important to be familiar with the nuances of it to optimize it and avoid these errors.

Ellie Coverdale, a technical writer at Essay roo and UK Writings, is involved in tech research and projects to find new advances and share her insights. She shares what she has learned with her readers on the Boom Essays blog.

How to change default Text editor in Linux

Saturday, August 31st, 2019

This is a very trivial question but, as I thought someone that is starting with Linux basics Operating might be interested I will shortly explain in this small article how to change default text editor on Linux.

Changing default text editor is especially helpful if you have to administer a newly purchased dedicated servers, that comes with default Operating System preinstalled.

By default many Linux distributions versions such as Debian / Ubuntu comes with nano comes with a default text editor nano (ANOther enhanced free Pico editor clone) many people as me are irritated and prefer to use instead vim (Vi Improved), mcedit (the Midnight Commander), joe or emacs as a default.

1. Changing default console text editor on Debian based Linux

On Debian / Ubuntu / Mint and other deb based distributions the easiest way to change text editor is with update-alternatives cmd.

update-alternatives –config editor


Using Debian update-alternatives is useful as it makes the change OS global wide and the default mc viewer program mcview will also understand the change in the default text editor, which makes it the preferrable way to do it on deb based OS family.

An alternative way to set the default programs for the OS system wide is to create the respective symbolic link in /etc/alternatives actually what update-alternatives wrapper script does is exactly this it creates the required symlink.

2. Changing default text editor on any Linux

To change the text editor for only a single system existing user in /etc/passwd you need to edit $HOME/.bashrc, e.g. ~/.bashrc on Debian based Linux or on Fedora / RHEL / CentOS by adding to ~/.bash_profile

vim ~/.bashrc

And add

alias editor=vim


export EDITOR='/path/to/text-editor/program'
export VISUAL='/path/to/text-editor/program'

To change to mcedit for example when opening in any program that triggers to run default text editor

export EDITOR='/usr/bin/mcedit'
export VISUAL='/usr/bin/mcedit'

To make the change system wide on any Linux distribution you have to add the export EDITOR / export VISUAL at the end of /etc/bash.bashrc

To load the newly included .bashrc* instructions use source command

source ~/.bashrc


3. Changing the default text editor for mcview if all else fails


Once mc is running, use following menu keys order (also visible from Midnight Commander) menus:

    F9 Activates the top menu.
    o Selects the Option menu.
    c Opens the configuration dialog.
    i Toggles the use internal edit option.
    s Saves your preferences.


That's all folks Enjoy !


Check weather forecast from console (terminal) on GNU / Linux and FreeBSD howto

Friday, August 23rd, 2019

how to get weather forecast prognosis from command line text terminal / console on Linux and FreeBSD

Doing everything in Linux console / terminal is a question perhaps every Linux / BSD hacker wants to do as Graphical user interface and using web search or using Graphical Environment plugins is an unneded complexity + googling or duckduckgoing for weather to check your next vacation destination city has been more and more of a terrible experience (for me) as I'm not a big fan of using the OS in a GUI.
In that manner of thoughts, as a Linux console geek and hard core ASCII art fan. I was recently happy to find that  possible to check weather forecast in tty console or Linux terminal in a beautiful ascii art way easily through a Web service – a web application weather forecast service that supports displaying the current and few days in future, weather forecast either in browser as a plain text or from the command line by simply accessing it with your favourite web access / transfer tool such as;
wget / curl or any of your favourite text browser elinks / lynx / w3m or if on *BSDs use fetch command.


Install Curl data transfer tool if it is not already

Wget is installed by default across most Linux distributions and fetch is present by default on BSDs, displaying it in text browser would perhaps be never used but if you decide to give it a try maybe try with elinks (to get colorful output), w3m and lynx will display a black and white results.

In case if you miss curl, install it:

On Debian distro


aptitude install -y curl

or Fedora

yum install -y curl

Of course to use as it is Internet based Weather Forecast service the minimum you need to have is to have Internet connection to your Linux / BSD desktop computer.

Text based Weather Forecast Web App currently supports:

display the current weather as well as a 3-day weather forecast, split into morning, noon, evening and night

  • Temperature is displayed for morning, noon, evening and night (includes temperature range, wind speed and direction, viewing distance, precipitation amount and probability)
  • Provide results for Weather based on City / town / village location
  • Supports display of Moon Phases Forecast in calendar days
  • Supports multilingual names (Bulgarian Phonetic cyrillic / Russian and other exotic UTF-8 encodings such as Chineese and Japanese),  50+ languages are currently supported
  • Has ability for prognosis for hostname (domain) location based on an its IP GeoIP location on the Globe
  • Geographical locations / landmarks such as Lakes / Mountains etc. can be easily queried
  • Query results metrics could be configured, e.g. USCS units or EU and rest of world accepted ones (SI) metric
  • Displayed result could be either in ANSI (if from terminal / console / HTML if queried from browser or in PNG – if needed)


Where could be useful ?

The best applications use, I can think of are for server (shell) / perl scripting automation purposes, it could be useful especially in TOO HOT, TOO, COLD, TOO WET location in Small and Middle sized Data Centers Green Energy (Sun Panel) Parks / Wind Energy situated Linux monitoring hosts to track possible problems of overheats or overcolding of servers due to abnormal excessive temperatures such as the ones we experienced this summer here All across in Europe or in too Cold DC locations such as heat locations Deserts in African Countries, Saudi Arabia or Chukotka or Siberia in Russia.
Other application is as a backup option to other normal Weather report services by PHP or Python scripts that fetch data, from multiple places.
Of course since this is a third party controlled service, the downtime is due to excessive connection requests, the service could get flooded and stopped working, but I guess for any Commercial use, creator Igor Chubin would be happy to sell a specific crafted service for any end user candidates.

Here is few examples of the beautiful returned ASCII art formatted output of

1. Getting a three days Weather Forecast prognosis for city / town location

To get what is current weather in my current city of Living, Sofia Bulgaria just pass the city to the URL address






Default links (Linux) www text browser produces ugly black and white

2. Displaying Weather forecast with wget


wget -O- -q


If you're lazy you can even omit the http:// as wget will look for HyperText Transmission Protocol by itself


wget -O- -q


3. Getting Forecast results for a Tourist Destination

Lets get the weather forecast for the popular tourist Bulgarian destination of the Seven Rila Lakes (near Rila Monastery), situated in the Rila Mountain BG.







4. Display Forecast for a specific server IP

Displaying information on specific server IP address current situated in GeoIP database, of course could be not really true, as the IP could be just a Load Balancer a router that does NAT to some internal DMZ-ed location server, but anyways it is a cool feature.

Lets get information on what is the weather on Google Global's Public DNS server IP so commonly used to guarantee a Windows and Linux Desktop client machines Internet connectivity.

curl Google Public DNS location weather forecast

5. Download PNG image picture from service


Lets say you want to get a 3 days standard Weather forecast for the popular Black Sea Resort town in Bulgaria Pomorie (a beautiful sea city which has even a functioning 5 Monks Monastery Pomorie Monastery situated near sea coast)




–2019-08-22 20:15:51–
Resolving (…
Connecting to (||:80… connected.
HTTP request sent, awaiting response… 200 OK
Length: 42617 (42K) [image/png]
Saving to: ‘Pomorie.png’

Pomorie.png                                     100%[=======================================================================================================>]  41.62K  –.-KB/s    in 0.07s   

2019-08-22 20:15:52 (586 KB/s) – ‘Pomorie.png’ saved [42617/42617]


Note: The generated .png is again the ASCII art produced by a direct text fetch bug in pic format


6. Displaying Current Moon Phase

If you want to enjoy a text based Moon phase picture through 🙂

wget -O- -q

You can also get a Moon Phase prognosis for a current future date or get a previous date phase



Full Moon Madness !! – Vampires are out beaware and Enjoy the ultra kewl ASCII Colorful Art 🙂

7. Getting help for terminal Waether Forecast results



$ curl


    $ curl          # current location
    $ curl      # weather in the Munich airport

Supported location types:

    /paris                  # city name
    /~Eiffel+tower          # any location
    /Москва                 # Unicode name of any location in any language
    /muc                    # airport code (3 letters)
    /     # domain name
    /94107                  # area codes
    /-78.46,106.79          # GPS coordinates

Special locations:

    /moon                   # Moon phase (add ,+US or ,+France for these cities)
    /moon@2016-10-25        # Moon phase for the date (@2016-10-25)


    m                       # metric (SI) (used by default everywhere except US)
    u                       # USCS (used by default in US)
    M                       # show wind speed in m/s

View options:

    0                       # only current weather
    1                       # current weather + 1 day
    2                       # current weather + 2 days
    A                       # ignore User-Agent and force ANSI output format (terminal)
    F                       # do not show the "Follow" line
    n                       # narrow version (only day and night)
    q                       # quiet version (no "Weather report" text)
    Q                       # superquiet version (no "Weather report", no city name)
    T                       # switch terminal sequences off (no colors)

PNG options:

    /paris.png              # generate a PNG file
    p                       # add frame around the output
    t                       # transparency 150
    transparency=…        # transparency from 0 to 255 (255 = not transparent)

Options can be combined:

    /Paris_0pq.png          # in PNG the file mode are specified after _
    /Rome_0pq_lang=it.png   # long options are separated with underscore


    $ curl
    $ curl
    $ curl -H "Accept-Language: fr"

Supported languages:

    af da de el et fr fa hu id it nb nl pl pt-br ro ru tr uk vi (supported)
    az be bg bs ca cy cs eo es fi ga hi hr hy is ja jv ka kk ko ky lt lv mk ml nl fy nn pt pt-br sk sl sr sr-lat sv sw th te uz zh zu he (in progress)

Special URLs:

    /:help                  # show this page
    /:bash.function         # show recommended bash function wttr()
    /:translation           # show the information about the translators




8. Comparing two cities weather from command line


One useful use of if you plan to travel from Location city A to Location city B is to compare the temperatures with a simple bash one liner script:




diff -Naur <(curl -s ) <(curl -s )



9. Using ansiweather command to get Weather Temperature / Wind / Humidity in one line beuatiful text


If you go and install answeather Linux package


apt-get install –yes ansiweather

You will get a shell script wrapper with ANSI colors and Unicode symbols support. Weather data comes from OpenWeatherMap, this is useful if is not working due to some URL malfunction (due to service is DoS-ed) etc.


ansiweather -l Atina



Lets use ansiweather to print the weather prognosis for upcoming 5 days for near port of Burgas, BG

ansiweather -F -l Burgas



10. Get all Weather current forecast for each Capital in the world

You can download and use this simple plain text file list of All Country Capitals in the World (country-capitals-all-world.txt) with ansiweather and a bash loop to get displayed each and every current day Weather Forecast in the World, here is how:


while read line; do ansiweather -l $line; sleep 3; done < country-capitals-all-world.txt


As you can see some of the very exotic third world capitals does not return data so 'ERROR: Cannot fetch weather data' is returned.

You can also substitute ansiweather with curl$line to do get the beautiful ASCII art 3 days weather forecast via


while read line; do curl$line; sleep 3; done < country-capitals-all-world.txt

I'll be happy to know other nice ASCII Art supporting Web service to enjoy from text terminal on Linux (nomatter useful or) just funny joyful prank maniacal pranks such as Watching text ASCII version remake of Star Wars Classic Movie by simply telnetting to (if you haven't so just telnet and enjoy the streamed ASCIIs ! 🙂






Talking about fun and ASCII, its worthy to mention hollywood Linux package

hipo@jeremiah:~/Desktop$ apt-cache show hollywood|grep -i desc -A 3
Description-en: fill your console with Hollywood melodrama technobabble
 This utility will split your console into a multiple panes of genuine
 technobabble, perfectly suitable for any Hollywood geek melodrama.
 It is particularly suitable on any number of computer consoles in the

Description-md5: 768f44c76220ea2b35f855ea34c8bc35
Section: games
Priority: optional

Once installed on Debian with:

aptitude install -y hollywood

You can get in a rapid manner plenty of tmux (screen like – virtual console emulator) split screen statistics about your notebook / workstation / server CPU usage, mlocate.db status, info about plugged in machine voltage, Speedometer (statistics about Network bandwidth usage), System load avarage (CPU Count, Memory Utilization) and some other random info coming out of dmesg kernel log and more. The information displayed in splitted windows changes rapidly and (assuming you run it at home Desktop with a soundblaster) and not remotely, a james bond Agent 007 soundtrack is played on the back, that brings up one's adrenaline and makes it look even cooler.


To give you an idea what to expect, here is shot of /usr/games/hollywood (the program start binary location) on Debian GNU / Linux running, Enjoy! 🙂

Putty load as default session another session – Save other Putty session configuration to default howto

Thursday, November 29th, 2018


Recently I had to use PuTTY which I haven't used for years to open a number of SSH Pernanent Tunnels necessery for my daily work as a SAP Consultant.

I've saved them under a certain new profile and saved the set SSH Tunnel configuration not in the default Session but in separate named one, therefore had to press Load button every time after clicking over my Putty shortcut icon. 

That was annoying and took few seconds out of my life every next morning for about a week, so finally I found osme time to google it and it seemed it is pretty easy to have any Putty sessoin loaded you like.

Here is how:

1. Create a new Putty Shortcut



Click over Putty icon while holding CTRL + SHIFT (Control SHIFT keys simultaneously ) and move the mouse somewhere on the desktop to create the shortcut.

2. Right click on Putty Shortcut




"C:\Program Files\PuTTY\putty.exe" -load "your_saved_session" "username@your_server_address" -pw "your_password"

fill out "target" field of shortcut using above code (alter to your own properties).
click Apply button.

If you need to pass a user and password from Shortcut itself (which is a bad practice for security but sometimes useful, for not so important Tunnels – for example a tunnel to an Open Proxy), do it by typing in the target field like so:

"C:\Program Files\PuTTY\putty.exe" -load "your_saved_session" "username@your_server_address" -pw "your_password"


And Hooray !!! After that when you click on PuTTy shortcut it loads your session automatically using given username and password.

How to enable Control Alt Backspace to Kill X server on Debian / Ubuntu Linux

Thursday, September 28th, 2017

Being a long time GNU / Linux user, I've been quite dissatisfied for the fact that in latest Debian and Ubuntu Linux, the default Key combination to Kill X (CTRL + ALT + BACKSPACE) is no longer working.

Though nowdays Xorg (XServer) is pretty stable it still happens from time to time for some application to overload the PC badly and make Gnome or KDE environment little or no responsive at all and here comes the goody CTRL + ALT + BACKSPACE it is pretty much  like  (CTRL + ALT + DEL) did restarted the computer in DOS and earlier Windows OS-es once the environment became unusable with the only difference that just Xorg server is restarted and the other using programs that are in background work just like they used to. 

CTRL + ALT + Backspace is a great thing to use especially if you're running some homebrew server and you use it both as a Server with some few little websites and as a Desktop environment to browse the net and do basic stuff.

So here comes the question how to make the CTRL + ALT + BACKSPACE keyboard combination be killing Xserver like in the good old days?

The easiest way to do it interactively in ncurses interface is by  running:


root@noah:~# dpkg-reconfigure keyboard-configuration


If somehow on the machine you don't have dpkg-reconfigure or you prefer to do set CTRL + ALT + BACKSPACE Kill Switch manually edit /etc/default/keyboard 

inside change value of




like shown below file:

root@noah:/home/hipo# cat  /etc/default/keyboard


# Consult the keyboard(5) manual page.



The configuration should be working across Debian 7, 8, 9 as well as Ubuntu 12 ..14 .. 16 and hopefully in future releases too, just as many other Linux distributions like Mint etc. the Xserver Kill Switch setting should be located in same file.

Finally if even after that change the Control Alt BackSpace Kill Switch sequence refuses to work in GNOME Desktop environment, it might be due to a local setting typical for GNOME and this should be fixed via the good known gnome-tweak-tool

So GNOME users should run it from command line and check the setting there, e.g.:


$ gnome-tweak-tool

You should check:


Typing -> Kill Sequence to Kill the X Server


it should look like shown in below screenshot: