Posts Tagged ‘command’

Howto install Google Chrome web browser on CentOS Linux 7

Friday, December 11th, 2020

After installing CentOS 7 Linux testing Virtual Machine in Oracle Virtualbox 6.1 to conduct some testing with php / html / javascript web script pages and use the VM for other work stuff that I later plan to deploy on production CentOS systems, I came to requirement of having a working Google Chrome browser.

In that regards, next to Firefox, I needed to test the web applications in commercial Google Chrome to see what its usercan expect. For those who don't know it Google Chrome is based on Chromium Open source browser ( which is available by default via default CentOS EPEL repositories.

One remark to make here is before installing Google Chrome, I've also test my web scripts first with chromium, to install Chromium free browser on CentOS:

[root@localhost mozilla_test0]# yum install chromium
Loaded plugins: fastestmirror, langpacks
Loading mirror speeds from cached hostfile
 * base:
 * epel:
 * extras:
 * updates:
Resolving Dependencies
–> Running transaction check
—> Package chromium.x86_64 0:85.0.4183.121-1.el7 will be installed
–> Processing Dependency: chromium-common(x86-64) = 85.0.4183.121-1.el7 for package: chromium-85.0.4183.121-1.el7.x86_64
–> Processing Dependency: nss-mdns(x86-64) for package: chromium-85.0.4183.121-1.el7.x86_64
–> Processing Dependency: for package: chromium-85.0.4183.121-1.el7.x86_64
–> Running transaction check
—> Package chromium-common.x86_64 0:85.0.4183.121-1.el7 will be installed
—> Package minizip.x86_64 0:1.2.7-18.el7 will be installed
—> Package nss-mdns.x86_64 0:0.14.1-9.el7 will be installed
–> Finished Dependency Resolution


Dependencies Resolved

 Package                              Arch                        Version                                   Repository                 Size
 chromium                             x86_64                      85.0.4183.121-1.el7                       epel                       97 M
Installing for dependencies:
 chromium-common                      x86_64                      85.0.4183.121-1.el7                       epel                       16 M
 minizip                              x86_64                      1.2.7-18.el7                              base                       34 k
 nss-mdns                             x86_64                      0.14.1-9.el7                              epel                       43 k

Transaction Summary
Install  1 Package (+3 Dependent packages)

Total download size: 113 M
Installed size: 400 M
Is this ok [y/d/N]: y
Downloading packages:
(1/4): minizip-1.2.7-18.el7.x86_64.rpm                                                                               |  34 kB  00:00:00     
(2/4): chromium-common-85.0.4183.121-1.el7.x86_64.rpm                                                                |  16 MB  00:00:08     
(3/4): chromium-85.0.4183.121-1.el7.x86_64.rpm                                                                       |  97 MB  00:00:11     
(4/4): nss-mdns-0.14.1-9.el7.x86_64.rpm                                                                              |  43 kB  00:00:00     
Total                                                                                                       9.4 MB/s | 113 MB  00:00:12     
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
  Installing : minizip-1.2.7-18.el7.x86_64                                                                                              1/4
  Installing : chromium-common-85.0.4183.121-1.el7.x86_64                                                                               2/4
  Installing : nss-mdns-0.14.1-9.el7.x86_64                                                                                             3/4
  Installing : chromium-85.0.4183.121-1.el7.x86_64                                                                                      4/4
  Verifying  : chromium-common-85.0.4183.121-1.el7.x86_64                                                                               1/4
  Verifying  : minizip-1.2.7-18.el7.x86_64                                                                                              2/4
  Verifying  : chromium-85.0.4183.121-1.el7.x86_64                                                                                      3/4
  Verifying  : nss-mdns-0.14.1-9.el7.x86_64                                                                                             4/4

  chromium.x86_64 0:85.0.4183.121-1.el7                                                                                                     

Dependency Installed:
  chromium-common.x86_64 0:85.0.4183.121-1.el7            minizip.x86_64 0:1.2.7-18.el7            nss-mdns.x86_64 0:0.14.1-9.el7           


Chromium browser worked however it is much more buggy than Google Chrome and the load it puts on the machine as well as resources it consumes is terrible if compared to Proprietary G. Chrome.

Usually I don't like google chrome as it is a proprietary product and I don't even install it on my Linux Desktops, neither use as using is against any secure wise practice and but I needed this time ..

Thus to save myself some pains therefore proceeded and installed Google Chromium.
Installion  of Google Chrome is a straight forward process you download the latest rpm run below command to resolve all library dependencies and you're in:



[root@localhost mozilla_test0]# rpm -ivh google-chrome-stable_current_x86_64.rpm
warning: google-chrome-stable_current_x86_64.rpm: Header V4 DSA/SHA1 Signature, key ID 7fac5991: NOKEY
error: Failed dependencies:
    liberation-fonts is needed by google-chrome-stable-87.0.4280.88-1.x86_64 is needed by google-chrome-stable-87.0.4280.88-1.x86_64
[root@localhost mozilla_test0]# wget
–2020-12-11 07:03:02–
Resolving (…, 2a00:1450:4017:802::200e
Connecting to (||:443… connected.
HTTP request sent, awaiting response… 200 OK
Length: 72280700 (69M) [application/x-rpm]
Saving to: ‘google-chrome-stable_current_x86_64.rpm


100%[==================================================================================================>] 72,280,700  11.0MB/s   in 6.6s   

2020-12-11 07:03:09 (10.4 MB/s) – ‘google-chrome-stable_current_x86_64.rpm’ saved [72280700/72280700]

[root@localhost mozilla_test0]# yum localinstall google-chrome-stable_current_x86_64.rpm
Loaded plugins: fastestmirror, langpacks
Examining google-chrome-stable_current_x86_64.rpm: google-chrome-stable-87.0.4280.88-1.x86_64
Marking google-chrome-stable_current_x86_64.rpm to be installed
Resolving Dependencies
–> Running transaction check
—> Package google-chrome-stable.x86_64 0:87.0.4280.88-1 will be installed
–> Processing Dependency: liberation-fonts for package: google-chrome-stable-87.0.4280.88-1.x86_64
Loading mirror speeds from cached hostfile
 * base:
 * epel:
 * extras:
 * updates:
–> Processing Dependency: for package: google-chrome-stable-87.0.4280.88-1.x86_64
–> Running transaction check
—> Package liberation-fonts.noarch 1:1.07.2-16.el7 will be installed
–> Processing Dependency: liberation-narrow-fonts = 1:1.07.2-16.el7 for package: 1:liberation-fonts-1.07.2-16.el7.noarch
—> Package vulkan.x86_64 0: will be installed
–> Processing Dependency: vulkan-filesystem = for package: vulkan-
–> Running transaction check
—> Package liberation-narrow-fonts.noarch 1:1.07.2-16.el7 will be installed
—> Package vulkan-filesystem.noarch 0: will be installed
–> Finished Dependency Resolution

Dependencies Resolved

 Package                             Arch               Version                      Repository                                        Size
 google-chrome-stable                x86_64             87.0.4280.88-1               /google-chrome-stable_current_x86_64             227 M
Installing for dependencies:
 liberation-fonts                    noarch             1:1.07.2-16.el7              base                                              13 k
 liberation-narrow-fonts             noarch             1:1.07.2-16.el7              base                                             202 k
 vulkan                              x86_64                  base                                             3.6 M
 vulkan-filesystem                   noarch                  base                                             6.3 k

Transaction Summary
Install  1 Package (+4 Dependent packages)

Total size: 231 M
Total download size: 3.8 M
Installed size: 249 M
Is this ok [y/d/N]: y
Downloading packages:
(1/4): liberation-fonts-1.07.2-16.el7.noarch.rpm                                                                     |  13 kB  00:00:00     
(2/4): liberation-narrow-fonts-1.07.2-16.el7.noarch.rpm                                                              | 202 kB  00:00:00     
(3/4): vulkan-filesystem-                                                                   | 6.3 kB  00:00:00     
(4/4): vulkan-                                                                              | 3.6 MB  00:00:01     
Total                                                                                                       1.9 MB/s | 3.8 MB  00:00:02     
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
Warning: RPMDB altered outside of yum.
  Installing : vulkan-filesystem-                                                                                  1/5
  Installing : vulkan-                                                                                             2/5
  Installing : 1:liberation-narrow-fonts-1.07.2-16.el7.noarch                                                                           3/5
  Installing : 1:liberation-fonts-1.07.2-16.el7.noarch                                                                                  4/5
  Installing : google-chrome-stable-87.0.4280.88-1.x86_64                                                                               5/5
Redirecting to /bin/systemctl start atd.service
  Verifying  : vulkan-                                                                                             1/5
  Verifying  : 1:liberation-narrow-fonts-1.07.2-16.el7.noarch                                                                           2/5
  Verifying  : 1:liberation-fonts-1.07.2-16.el7.noarch                                                                                  3/5
  Verifying  : google-chrome-stable-87.0.4280.88-1.x86_64                                                                               4/5
  Verifying  : vulkan-filesystem-                                                                                  5/5

  google-chrome-stable.x86_64 0:87.0.4280.88-1                                                                                              

Dependency Installed:
  liberation-fonts.noarch 1:1.07.2-16.el7         liberation-narrow-fonts.noarch 1:1.07.2-16.el7       vulkan.x86_64 0:      
  vulkan-filesystem.noarch 0:      


Once Chrome is installed you can either run it from gnome-terminal

[test@localhost ~]$ gnome-terminal &


Or find it in the list of CentOS programs:

Applications → Internet → Google Chrome


Last step to do is to make Google Chrome easily updatable to keep up VM level on high security and let it get updated every time when apply security updates with yum check-update && yum upgrade
for that its necessery to create new custom repo file

[root@localhost mozilla_test0]# vim /etc/yum.repos.d/google-chrome.repo

Now letes import the gpg checksum key

[root@localhost mozilla_test0]# rpmkeys –import

That's all folks google-chrome is at your disposal.

Add Zabbix time synchronization ntp userparameter check script to Monitor Linux servers

Tuesday, December 8th, 2020



How to add Zabbix time synchronization ntp userparameter check script to Monitor Linux servers?

We needed to set on some servers at my work an elementary check with Zabbix monitoring to check whether servers time is correctly synchronized with ntpd time service as well report if the ntp daemon is correctly running on the machine. For that a userparameter script was developed called userparameter_ntp.conf the script is simplistic and few a lines of bash shell scripting 
stuff is based on gresping information required from ntpq and ntpstat common ntp client commands to get information about the status of time synchronization on the servers.

[root@linuxserver ]# ntpstat
synchronised to NTP server ( at stratum 3
   time correct to within 47 ms
   polling server every 1024 s


[root@linuxserver ]# ntpq -c peers
     remote           refid      st t when poll reach   delay   offset  jitter
+timeserver1     2 u  319 1024  377   15.864    1.270   0.262
+timeserver2     2 u  591 1024  377   16.287   -0.334   1.748
*timeserver3     2 u   47 1024  377   15.613   -0.553   0.251
 timeserver4 .INIT.          16 u    – 1024    0    0.000    0.000   0.000

Below is Zabbix UserParameter script that does report us 3 important values we monitor to make sure time server synchronization works as expected the zabbix keys we set are ntp.offset, ntp.sync, ntp.exact in attempt to describe what we're fetching from ntp client:

[root@linuxserver ]# cat /etc/zabbix/zabbix-agent.d/userparameter_ntp.conf

UserParameter=ntp.offset,(/usr/sbin/ntpq -pn | /usr/bin/awk 'BEGIN { offset=1000 } $1 ~ /\*/ { offset=$9 } END { print offset }')
#UserParameter=ntp.offset,(/usr/sbin/ntpq -pn | /usr/bin/awk 'FNR==4{print $9}')
UserParameter=ntp.sync,(/usr/bin/ntpstat | cut -f 1 -d " " | tr -d ' \t\n\r\f')
UserParameter=ntp.exact,(/usr/bin/ntpstat | /usr/bin/awk 'FNR==2{print $5,$6}')

In Zabbix the monitored ntpd parameters set-upped looks like this:




!Note that in above userparameter example, the commented userparameter script is a just another way to do an ntpd offset returned value which was developed before the more sophisticated with more regular expression checks from the /usr/sbin/ntpd via ntpq, perhaps if you want to extend it you can also use another script to report more verbose information to Zabbix if that is required like ouput from ntpq -c peers command:

UserParameter=ntp.verbose,(/usr/sbin/ntpq -c peers)

Of course to make the Zabbix fetch necessery data from monitored hosts, we need to set-up further new Zabbix Template with the respective Trigger and Items.

Below are few screenshots including the triggers used.


  • ntpd.trigger


  • NTP Synchronization trigger




As you can see from history we have setup our items to Store history of reported data to Zabbix from parameter script for 90 days and update our monitor check, every 30 seconds from the monitored hosts to which Tempate is applied.

Well that's all folks, time synchronization issues we'll be promptly triggering a new Alarm in Zabbix !

Postfix copy every email to a central mailbox (send a copy of every mail sent via mail server to a given email)

Wednesday, October 28th, 2020


Say you need to do a mail server migration, where you have a local configured Postfix on a number of Linux hosts named:



all configured to send email via old Email send host ( in each linux box's postfix configuration's /etc/postfix/
Now due to some infrastructure change in the topology of network or anything else, you need to relay Mails sent via another asumably properly configured Linux host relay (

Usually such a migrations has always a risk that some of the old sent emails originating from local running scripts on Linux-host1, Linux-Host2 … or some application or anything else set to send via them might not properly deliver emails to some external Internet based Mailboxes via the new relayhost

E.g. in /etc/postfix/ Linux-Host* machines, you have below config after the migration:

relayhost = []

Lets say that you want to make sure, that you don't end up with lost emails as you can't be sure whether the new email server will deliver correctly to the old repicient emails. What to do then?

To make sure will not end up in undelivered state and get lost forever after a week or so (depending on the mail queue configuration retention period made on Linux sent MTAs and mailrelay, it is a very good approach to temprorary set all email communication that will be sent via a BCC emaills (A Blind Carbon Copy) of each sent mail via relay that is set on your local configured Postfix-es on Linux-Host*.

In postfix to achieve that it is very easy all you have to do is set on your a postfix config variable always_bcc smartly included by postfix Mail Transfer Agent developers for cases exactly like this.

To forward all passed emails via the mail server just place in the end of /etc/postfix/mail.conf after login via ssh on

Now all left is to reload the postfix to force the new configuration to get loaded on systemd based hosts as it is usually today do:

# systemctl reload postfix

Finally to make sure all works as expected and mail is sent do from do a testing via local MTAs. 

Linux-Host:~# echo -e "Testing body" | mail -s "testing subject" -r ""

Linux-Host:~# echo -e "Testing body" | mail -s "testing subject" -r ""

As you can see I'm using the -r to simulate a sender address, this is a feature of mailx and is not available on older Linux Os hosts that are bundled with mail only command.
Now go to and open the in Outlook (if it is M$ Office 365 MX Shared mailbox), Thunderbird or whatever email fetching software that supports POP3 or IMAP (in case if you configured the common all email mailbox to be on some other Postfix / Sendmail / Qmail MTA). and check whether you started receiving a lot of emails 🙂

That's all folks enjoy ! 🙂

Linux: Howto Disable logging for all VirtualHosts on Apache and NGINX Webservers one liner

Wednesday, July 1st, 2020

Did you happen to administer Apache Webservers or NGINX webservers whose logs start to grow so rapidly that are flooding the disk too quickly?
Well this happens sometimes and it also happens that sometimes you just want to stop logging especially, to offload disk writting.

There is an easy way to disable logging for requests and errors (access_log and error_log usually residing under /var/log/httpd or /var/log/nginx ) for  all configured Virtual Domains with a short one liner, here is how.

Before you start  Create backup of /etc/apache2/sites-enabled / or /etc/nginx to be able to revert back to original config.

# cp -rpf /etc/apache2/sites-enabled/ ~/

# cp -rpf /etc/nginx/ ~/

1. Disable Logging for All  Virtual Domains configured for Apache Webserver

First lets print what the command will do to make sure we don't mess something

# find /home/hipo/sites-enabled/* -exec echo sed -i 's/#*[Cc]ustom[Ll]og/#CustomLog/g' {} \;

You will get some output like

find /home/hipo//sites-enabled/* -exec echo sed -i 's/#*[Cc]ustom[Ll]og/#CustomLog/g' {} \;

find /etc/apache2/sites-enabled/* -exec sed -i 's/#*[Cc]ustom[Ll]og/#CustomLog/g' {} \;
find /etc/apache2/sites-enabled/* -exec sed -i 's/#*[Ee]rror[Ll]og/#ErrorLog/g' {} \;

2. Disable Logging for All configured Virtual Domains for NGINX Webserver

find /etc/nginx/sites-enabled/* -exec sed -i 's/#*access_log/#access_log/g' {} \;
find /etc/nginx/sites-enabled/* -exec sed -i 's/#*error_log/#error_log/g' {} \;

f course above substituations that will comment out with '#' occurances from file configs of only default set access_log and error_log / access.log, error.log 
for machines where there is no certain convention on file naming and there are multiple domains in custom produced named log files this won't work.

This one liner was inspired from a friend's daily Martin Petrov. Martin blogged initially about this nice tip for those reading Cyrillic check out, so. Thanks Marto ! 🙂

How to check if shared library is loaded in AIX OS – Fix missing

Thursday, February 20th, 2020


I've had to find out whether an externally Linux library is installed  on AIX system and whether something is not using it.
The returned errors was like so:


# gpg –export -a

Could not load program gpg:
Dependent module /opt/custom/lib/libreadline.a( could not be loaded.
Member is not found in archive

After a bit of investigation, I found that gpg was failing cause it linked to older version of, the workaround was to just substitute the newer version of over the original installed one.

Thus I had a plan to first find out whether this libreadline.a is loaded and recognized by AIX UNIX first and second find out whether some of the running processes is not using that library.
I've come across this interesting IBM official documenation that describes pretty good insights on how to determine whether a shared library  is currently loaded on the system. which mentions the genkld command that is doing
exactly what I needed.

In short:
genkld – creates a list that is printed to the console that shows all loaded shared libraries


Next I used lsof (list open files) command to check whether there is in real time opened libraries by any of the running programs on the system.

After not finding anything and was sure the library is neither loaded as a system library in AIX nor it is used by any of the currently running AIX processes, I was sure I could proceed to safely overwrite libreadline.a ( with libreadline.a with (

The result of that is again a normally running gpg as ldd command shows the binary is again normally linked to its dependend system libraries.

aix# ldd /usr/bin/gpg
/usr/bin/gpg needs:



# gpg –version
gpg (GnuPG) 1.4.22
Copyright (C) 2015 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.


Home: ~/.gnupg
Supported algorithms:
Hash: MD5, SHA1, RIPEMD160, SHA256, SHA384, SHA512, SHA224
Compression: Uncompressed, ZIP, ZLIB, BZIP2



How to debug failing service in systemctl and add a new IP network alias in CentOS Linux

Wednesday, January 15th, 2020


If you get some error with some service that is start / stopped via systemctl you might be pondering how to debug further why the service is not up then then you'll be in the situation I was today.
While on one configured server with 8 eth0 configured ethernet network interfaces the network service was reporting errors, when atempted to restart the RedHat way via:

service network restart

to further debug what the issue was as it was necessery I had to find a way how to debug systemctl so here is how:


How to do a verbose messages status for sysctlct?


linux:~# systemctl status network

linux:~# systemctl status network


Another useful hint is to print out only log messages for the current boot, you can that with:

# journalctl -u service-name.service -b


if you don't want to have the less command like page separation ( paging ) use the –no-pager argument.


# journalctl -u network –no-pager

Jan 08 17:09:14 lppsq002a network[8515]: Bringing up interface eth5:  [  OK  ]

    Jan 08 17:09:15 lppsq002a network[8515]: Bringing up interface eth6:  [  OK  ]
    Jan 08 17:09:15 lppsq002a network[8515]: Bringing up interface eth7:  [  OK  ]
    Jan 08 17:09:15 lppsq002a systemd[1]: network.service: control process exited, code=exited status=1
    Jan 08 17:09:15 lppsq002a systemd[1]: Failed to start LSB: Bring up/down networking.
    Jan 08 17:09:15 lppsq002a systemd[1]: Unit network.service entered failed state.
    Jan 08 17:09:15 lppsq002a systemd[1]: network.service failed.
    Jan 15 11:04:45 lppsq002a systemd[1]: Starting LSB: Bring up/down networking…
    Jan 15 11:04:45 lppsq002a network[55905]: Bringing up loopback interface:  [  OK  ]
    Jan 15 11:04:45 lppsq002a network[55905]: Bringing up interface eth0:  RTNETLINK answers: File exists
    Jan 15 11:04:45 lppsq002a network[55905]: [  OK  ]
    Jan 15 11:04:45 lppsq002a network[55905]: Bringing up interface eth1:  RTNETLINK answers: File exists
    Jan 15 11:04:45 lppsq002a network[55905]: [  OK  ]
    Jan 15 11:04:46 lppsq002a network[55905]: Bringing up interface eth2:  ERROR     : [/etc/sysconfig/network-scripts/ifup-eth] Device eth2 has different MAC address than expected, ignoring.
    Jan 15 11:04:46 lppsq002a network[55905]: [FAILED]
    Jan 15 11:04:46 lppsq002a network[55905]: Bringing up interface eth3:  RTNETLINK answers: File exists
    Jan 15 11:04:46 lppsq002a network[55905]: [  OK  ]
    Jan 15 11:04:46 lppsq002a network[55905]: Bringing up interface eth4:  ERROR     : [/etc/sysconfig/network-scripts/ifup-eth] Device eth4 does not seem to be present, delaying initialization.
    Jan 15 11:04:46 lppsq002a network[55905]: [FAILED]
    Jan 15 11:04:46 lppsq002a network[55905]: Bringing up interface eth5:  RTNETLINK answers: File exists
    Jan 15 11:04:46 lppsq002a network[55905]: [  OK  ]
    Jan 15 11:04:46 lppsq002a network[55905]: Bringing up interface eth6:  RTNETLINK answers: File exists
    Jan 15 11:04:47 lppsq002a network[55905]: [  OK  ]
    Jan 15 11:04:47 lppsq002a network[55905]: Bringing up interface eth7:  RTNETLINK answers: File exists
    Jan 15 11:04:47 lppsq002a network[55905]: [  OK  ]
    Jan 15 11:04:47 lppsq002a network[55905]: RTNETLINK answers: File exists
    Jan 15 11:04:47 lppsq002a network[55905]: RTNETLINK answers: File exists
    Jan 15 11:04:47 lppsq002a network[55905]: RTNETLINK answers: File exists
    Jan 15 11:04:47 lppsq002a network[55905]: RTNETLINK answers: File exists
    Jan 15 11:04:47 lppsq002a network[55905]: RTNETLINK answers: File exists
    Jan 15 11:04:47 lppsq002a network[55905]: RTNETLINK answers: File exists
    Jan 15 11:04:47 lppsq002a network[55905]: RTNETLINK answers: File exists
    Jan 15 11:04:47 lppsq002a network[55905]: RTNETLINK answers: File exists
    Jan 15 11:04:47 lppsq002a network[55905]: RTNETLINK answers: File exists
    Jan 15 11:04:47 lppsq002a systemd[1]: network.service: control process exited, code=exited status=1
    Jan 15 11:04:47 lppsq002a systemd[1]: Failed to start LSB: Bring up/down networking.
    Jan 15 11:04:47 lppsq002a systemd[1]: Unit network.service entered failed state.
    Jan 15 11:04:47 lppsq002a systemd[1]: network.service failed.
    Jan 15 11:08:22 lppsq002a systemd[1]: Starting LSB: Bring up/down networking…
    Jan 15 11:08:22 lppsq002a network[56841]: Bringing up loopback interface:  [  OK  ]
    Jan 15 11:08:22 lppsq002a network[56841]: Bringing up interface eth0:  RTNETLINK answers: File exists
    Jan 15 11:08:22 lppsq002a network[56841]: [  OK  ]
    Jan 15 11:08:26 lppsq002a network[56841]: Bringing up interface eth1:  RTNETLINK answers: File exists
    Jan 15 11:08:26 lppsq002a network[56841]: [  OK  ]
    Jan 15 11:08:26 lppsq002a network[56841]: Bringing up interface eth2:  ERROR     : [/etc/sysconfig/network-scripts/ifup-eth] Device eth2 has different MAC address than expected, ignoring.
    Jan 15 11:08:26 lppsq002a network[56841]: [FAILED]
    Jan 15 11:08:26 lppsq002a network[56841]: Bringing up interface eth3:  RTNETLINK answers: File exists
    Jan 15 11:08:27 lppsq002a network[56841]: [  OK  ]



Another useful thing debug arguments is the -xe to do:

# journalctl -xe –no-pager


  • -x (– catalog)
    Augment log lines with explanation texts from the message catalog.
    This will add explanatory help texts to log messages in the output
    where this is available.
  •  -e ( –pager-end )  Immediately jump to the end of the journal inside the implied pager


Finally after fixing the /etc/sysconfig/networking-scripts/* IP configuration issues I had all the 8 Ethernet interfaces to work as expected

# systemctl status network




2. Adding a new IP alias to eth0 interface

Further on I had  to add an IP Alias on the CenOS via its networking configuration, this is done by editing /etc/sysconfig/network-scripts/ifcfg* files.
To create an IP alias for first lan interface eth0, I've had to created a new file named ifcfg-eth0:0

linux:~# cd /etc/sysconfig/network-scripts/
linux:~# vim ifcfg-eth0:0

with below content


Adding this IP address network alias works across all RPM based distributions and should work also on Fedora and Open SuSE as well as Suse Enterprise Linux.
If you however prefer to use a text GUI and do it the CentOS server administration way you can use nmtui (Text User Interface for controlling NetworkManager). tool.

linux:~# nmtui




Rsync copy files with root privileges between servers with root superuser account disabled

Tuesday, December 3rd, 2019



Sometimes on servers that follow high security standards in companies following PCI Security (Payment Card Data Security) standards it is necessery to have a very weird configurations on servers,to be able to do trivial things such as syncing files between servers with root privileges in a weird manners.This is the case for example if due to security policies you have disabled root user logins via ssh server and you still need to synchronize files in directories such as lets say /etc , /usr/local/etc/ /var/ with root:root user and group belongings.

Disabling root user logins in sshd is controlled by a variable in /etc/ssh/sshd_config that on most default Linux OS
installations is switched on, e.g. 

grep -i permitrootlogin /etc/ssh/sshd_config
PermitRootLogin yes

Many corporations use Vulnerability Scanners such as Qualys are always having in their list of remote server scan for SSH Port 22 to turn have the PermitRootLogin stopped with:


PermitRootLogin no

In this article, I'll explain a scenario where we have synchronization between 2 or more servers Server A / Server B, whatever number of servers that have already turned off this value, but still need to
synchronize traditionally owned and allowed to write directories only by root superuser, here is 4 easy steps to acheive it.


1. Add rsyncuser to Source Server (Server A) and Destination (Server B)

a. Execute on Src Host:


groupadd rsyncuser
useradd -g 1000 -c 'Rsync user to sync files as root src_host' -d /home/rsyncuser -m rsyncuser


b. Execute on Dst Host:


groupadd rsyncuser
useradd -g 1000 -c 'Rsync user to sync files dst_host' -d /home/rsyncuser -m rsyncuser


2. Generate RSA SSH Key pair to be used for passwordless authentication

a. On Src Host

su – rsyncuser

ssh-keygen -t rsa -b 4096


b. Check .ssh/ generated key pairs and make sure the directory content look like.


[rsyncuser@src-host .ssh]$ cd ~/.ssh/;  ls -1



3. Copy to Destination host server under authorized_keys


scp ~/.ssh/  rsyncuser@dst-host:~/.ssh/authorized_keys


Next fix permissions of authorized_keys file for rsyncuser as anyone who have access to that file (that exists as a user account) on the system
could steal the key and use it to run rsync commands and overwrite remotely files, like overwrite /etc/passwd /etc/shadow files with his custom crafted credentials
and hence hack you 🙂

Hence, On Destionation Host Server B fix permissions with:

su – rsyncuser; chmod 0600 ~/.ssh/authorized_keys
[rsyncuser@dst-host ~]$


For improved security here to restrict rsyncuser to be able to run only specific command such as very specific script instead of being able to run any command it is good to use little known command= option
once creating the authorized_keys


4. Test ssh passwordless authentication works correctly

For that Run as a normal ssh from rsyncuser

On Src Host


[rsyncuser@src-host ~]$ ssh rsyncuser@dst-host

Perhaps here is time that for those who, think enabling a passwordless authentication is not enough secure and prefer to authorize rsyncuser via a password red from a secured file take a look in my prior article how to login to remote server with password provided from command line as a script argument / Running same commands on many servers 

5. Enable rsync in sudoers to be able to execute as root superuser (copy files as root)


For this step you will need to have sudo package installed on the Linux server.

Then, Execute once logged in as root on Destionation Server (Server B)


[root@dst-host ~]# grep 'rsyncuser ALL' /etc/sudoers|wc -l || echo ‘rsyncuser ALL=NOPASSWD:/usr/bin/rsync’ >> /etc/sudoers


Note that using rsync with a ALL=NOPASSWD in /etc/sudoers could pose a high security risk for the system as anyone authorized to run as rsyncuser is able to overwrite and
respectivle nullify important files on Destionation Host Server B and hence easily mess the system, even shell script bugs could produce a mess, thus perhaps a better solution to the problem
to copy files with root privileges with the root account disabled is to rsync as normal user somewhere on Dst_host and use some kind of additional script running on Dst_host via lets say cron job and
will copy gently files on selective basis.

Perhaps, even a better solution would be if instead of granting ALL=NOPASSWD:/usr/bin/rsync in /etc/sudoers is to do ALL=NOPASSWD:/usr/local/bin/
that will get triggered, once the files are copied with a regular rsyncuser acct.


6. Test rsync passwordless authentication copy with superuser works

Do some simple copy, lets say copy files on Encrypted tunnel configurations located under some directory in /etc/stunnel on Server A to /etc/stunnel on Server B

The general command to test is like so:

rsync -aPz -e 'ssh' '–rsync-path=sudo rsync' /var/log rsyncuser@$dst_host:/root/tmp/

This will copy /var/log files to /root/tmp, you will get a success messages for the copy and the files will be at destination folder if succesful.


On Src_Host run:


[rsyncuser@src-host ~]$ dst=FQDN-DST-HOST; user=rsyncuser; src_dir=/etc/stunnel; dst_dir=/root/tmp;  rsync -aP -e 'ssh' '–rsync-path=sudo rsync' $src_dir  $rsyncuser@$dst:$dst_dir;


7. Copying files with root credentials via script

The simlest file to use to copy a bunch of predefined files  is best to be handled by some shell script, the most simple version of it, could look something like this.

# On server1 use something like this
# On server2 dst server
# add in /etc/sudoers
# rsyncuser ALL=NOPASSWD:/usr/bin/rsync




for i in $(echo ${src[@]}); do
rsync -aPvz –delete –dry-run -e 'ssh' '–rsync-path=sudo rsync' "$i" $rsyncuser@$dst_host:$dst_dir"$i";

In above script as you can see, we define a bunch of files that will be copied in bash array and then run a loop to take each of them and copy to testination dir.
A very sample version of the script 


Lets do short overview on what we have done here. First Created rsyncuser on SRC Server A and DST Server B, set up the key pair on both copied the keys to make passwordless login possible,
set-up rsync to be able to write as root on Dst_Host / testing all the setup and pinpointing a small script that can be used as a backbone to develop something more complex
to sync backups or keep system configurations identicatial – for example if you have doubts that some user might by mistake change a config etc.
In short it was pointed the security downsides of using rsync NOPASSWD via /etc/sudoers and few ideas given that could be used to work on if you target even higher
PCI standards.


Getting Console and Graphical hardware system information on Linux with cpuinfo, neofetch, CPU-X (CPU-Z Unix alternative), I-nex and inxi

Tuesday, September 17th, 2019


Earlier I've wrote extensive article on how to get hardware information on Linux using tools such as dmidecode, hardinfo, lshw, hwinfo, x86info and biosdecode but there are few other hardware reporting tools for Linux worthy to mention that has been there for historical reasons such as cpuinfo as we as some new shiny ones such as neofetch (a terminal / console hardware report tool as well the CPU-X and I-Nex  which is Linux equivalent to the all known almost standard for Windows hardware detection CPU-Z worthy to say few words about.

1. cpuinfo


Perhaps the most basic tool to give you a brief information about your Processor type (model) number of Cores and Logical Processors is cpuinfo

I remember cpuinfo has been there since the very beginning on almost all Linux distributions's repository, nowadays its popularity of the days when the kings on the Linux OS server scenes were Slackware, Caldera OpenLinux and Redhat 6.0 Linux and Debian 3.0  declined but still for scripting purposes it is handy small proggie.

To install and run it in Debian  / Ubuntu / Mint Linux etc.:


aptitude install -y cpuinfo





2. neofetch


The next one worthy to install and check is neofetch (a cross-platform and easy-to-use system information
 command line script that collects your Linux system information and display it on the terminal next to an image, it could be your distributions logo or any ascii art of your choice.)

The cool thing about neofetch is besides being able to identify the System server / desktop hardware parameters, it gives some basic info about number of packages installed on the system, memory free and in use, used kernel and exact type of System (be it Dell PowerEdge Model XX, IBM eSeries Model / HP Proliant Model etc.


neofetch info generated on my home used Lenovo Thikpad T420

neofetch info from running current machine

neofetch even supports Mac OS X and Windows OS ! 🙂

To install neofetch on Mac OS X:

/usr/bin/ruby -e "$(curl -fsSL"

or via Mac ported packages using brew

brew install neofetch


neofetch is even installable on Windows OS that has the scoop command line installer tool installer manager with below PowerShell code in cmd.exe (Command line):

powershell Set-ExecutionPolicy RemoteSigned -scope CurrentUser
iex (new-object net.webclient).downloadstring('')
scoop install git
scoop install neofetch


By the way Scoop was quite a finding for me and it is pretty handy to install plenty of useful command line Linux / UNIX tools, such as curl, wget, git etc. in the same easy straight forward way as a standard yum or apt-get on Windows (without explicitly installing things as GnuWin and CygWin).

3. CPU-X graphical user interface hardware report Linux GUI alternative to Windows CPU-Z

The packages for CPU-X are a bit outdated and even though there are rpm packages for Fedora, OpenSuSE and .deb package for Debian for Debian, Ubuntu and ArchLinux (pacman), there is no up to date version for Debian 10 and the package builds distributed for different Linux distros are a bit outdated.

Thus to install CPU-X on any Linux distribution it is perhaps best to use the portable version (static binary) of CPU-X.
It is currently available on

To install latest portable version of CPU-X


mkdir CPU-X
cd CPU-X

tar -zxvvf CPU-X_v3.2.4_portable.tar.gz
-rwxr-xr-x yohan/users 4563032 2019-01-13 22:15 CPU-X_v3.2.4_portable.bsd64
-rwxr-xr-x yohan/users 5484968 2019-01-13 22:15 CPU-X_v3.2.4_portable.linux64


cp -rpf CPU-X_v3.2.4_portable.linux64 /usr/local/bin/
ln -sf /usr/local/bin/CPU-X_v3.2.4_portable.linux64 /usr/local/bin/cpu-x

Next run as superuser (root)

hipo@jeremiah:~$ su -c 'cpu-x'


As seen from below screenshots cpu-x reports a lot of concrete specific hardware data on:

  • Processor
  • Motherboard
  • Memory
  • System
  • Graphic card
  • Performance







CPU-X can be installed also on FreeBSD very easily by just installing from BSD port tree sysutils/cpu-x/
It is also said to work on other *BSDs, NetBSD, OpenBSD Unixes but I guess this will require a manual compilation based on FreeBSD's port Makefile.

4. I-Nex another GUI alternative to CPU-Z for UNIX / Linux

I-Nex is even more useful for general hardware reporting as it reports many hardware specifications not reported by CPU-X such as Battery type and Model Name  (if the hardware report is on a laptop), info on USB devices slots or plugged USB devices brand and specifications, the available Network devices on the system (MAC Addresses) of each of it, Installed and used drivers on Hard Disk (ATA / SATA / SCSI / SSD), HW Sector size, Logical Block size, HDD Sectors count and other specific Hard Drive data as well as information on available Audio (Sound Blaster) devices (HDA-Intel), used Codecs, loaded kernel ALSA driver, Video card used and most importantly indicators on Processor reported CPU (temperature).


To install I-nex

Go to or any of the mirror links where it resides and install the respective package, in my case, I was doing the installation on Debian Linux, so fetched current latest amd64 package which as of moment of writting this article is i-nex_7.6.0-0-bzr977-20161012-ubuntu16.10.1_amd64.deb , next installed it with dpkg

dpkg -i i-nex_7.6.0-0-bzr977-20161012-ubuntu16.10.1_amd64.deb


As the package was depending on some other .deb packages, which failed to install to install the missing ones I had to further run

apt –fix-broken install




I-Nex thermal indicators about CPU temperature on a Linux Desktop notebook








There are other Hardware identification report tools such as CUDA-Z that are useful to check if you have Nvidia Video Card hardware Installed on the PC to check the status of CUDA enabled GPUs, useful if working with nVidia Geforce, Quadro, Tesla cards and ION chipsets.

If you use it however be aware that CUDA-Z is not compatible with 3rd-party linux drivers for NVidia so make sure you have the current official Nvidia version.


5. Inxi full featured system information script


Inxi is a 10000 lines mega bash script that fetches hardware details from multiple different sources in /proc /sys and from commands on the system, and generates a beautiful looking console report that non technical users can read easily.



inxi -Fx




Each of the pointed above tools has different method of collection of Hardware information from various resources e.g. – kernel loaded modules, dmesg, files like /proc/meminfo /proc/version /proc/scsi/scsi /proc/partitions.
Hence some of the tools are likely to report more info than otheres, so in case if some information you need regarding the system plugged in hardware is missing you can perhaps obtain it from another program. Most Linux distribution desktop provided GNOME package are including Hardinfo gui tool, but in many cases above mentioned tools are likely to add even more on info on what is inside your PC Box.
If you're aware of others tools that are useful not mentioned here please share it.

Scanning ports with netcat “nc” command on Linux and UNIX / Checking for firewall filtering between source and destination with nc

Friday, September 6th, 2019


Netcat ( nc ) is one of that tools, that is well known in the hacker (script kiddie) communities, but little underestimated in the sysadmin world, due to the fact nmap (network mapper) – the network exploratoin and security auditing tool has become like the standard penetration testing TCP / UDP port tool

nc is feature-rich network debugging and investigation tool with tons of built-in capabilities for reading from and writing to network connections using TCP or UDP.

Its Plethora of features includes port listening, port scanning & Transferring files due to which it is often used by Hackers and PenTesters as Backdoor. Netcat was written by a guy we know as the Hobbit <>.

For a start-up and middle sized companies if nmap is missing on server usually it is okay to install it without risking to open a huge security hole, however in Corporate world, due to security policies often nmap is not found on the servers but netcat (nc) is present on the servers so you have to learn, if you haven't so to use netcat for the usual IP range port scans, if you're so used to nmap.

There are different implementations of Netcat, whether historically netcat was UNIX (BSD) program with a latest release of March 1996. The Linux version of NC is GNU Netcat (official source here) and is POSIX compatible. The other netcat in Free Software OS-es is OpenBSD's netcat whose ported version is also used in FreeBSD. Mac OS X also comes with default prebundled netcat on its Mac OS X from OS X version (10.13) onwards, on older OS X-es it is installable via MacPorts package repo, even FreeDOS has a port of it called NTOOL.

The (Swiss Army Knife of Embedded Linux) busybox includes a default leightweight version of netcat and Solaris has the OpenBSD netcat version bundled.

A cryptography enabled version fork exists that supports that supports integrated transport encryption capabilities called Cryptcat.

The Nmap suite also has included rewritten version of GNU Netcat named Ncat, featuring new possibilities such as "Connection Brokering", TCP/UDP Redirection, SOCKS4 client and server support, ability to "Chain" Ncat processes, HTTP CONNECT proxying (and proxy chaining), SSL connect/listen support and IP address/connection filtering. Just like Nmap, Ncat is cross-platform.

In this small article I'll very briefly explain on basic netcat – known as the TCP Army knife tool port scanning for an IP range of UDP / TCP ports.


1. Scanning for TCP opened / filtered ports remote Linux / Windows server


Everyone knows scanning of a port is possible with a simple telnet request towards the host, e.g.:



The most basic netcat use that does the same is achiavable with:


220 jeremiah ESMTP Exim 4.92 Thu, 05 Sep 2019 20:39:41 +0300

Beside scanning the remote port, using netcat interactively as pointing in above example, if connecting to HTTP Web services, you can request remote side to return a webpage by sending a false referer, source host and headers, this is also easy doable with curl / wget and lynx but doing it with netcat just like with telnet could be fun, here is for example how to request an INDEX page with spoofed HTTP headers.

nc Web-Host.COM 25
GET / HTTP/1.1
User-Agent: my-spoofed-browser


2. Performing a standard HTTP request with netcat


To do so just pype the content with a standard bash integrated printf function with the included end of line (the unix one is \n but to be OS independent it is better to use r\n  – the end of line complition character for Windows.


printf "GET /index.html HTTP/1.0\r\nHost:\r\n\r\n" | nc 80


3. Scanning a range of opened / filtered UDP ports


To scan for lets say opened remote system services on the very common important ports opened from UDP port 25 till, 1195 – more specifically for:

  • UDP Bind Port 53
  • Time protocol Port (37)
  • TFTP (69)
  • Kerberos (88)
  • NTP 123
  • Netbios (137,138,139)
  • SNMP (161)
  • LDAP 389
  • Microsoft-DS (Samba 445)
  • Route BGP (52)
  • LDAPS (639)
  • openvpn (1194)


nc -vzu 25 1195


UDP tests will show opened, if no some kind of firewall blocking, the -z flag is given to scan only for remote listening daemons without sending any data to them.


4. Port Scanning TCP listening ports with Netcat


As prior said using netcat to scan for remote opened HTTP Web Server on port 80 an FTP on Port 23 or a Socks Proxy or MySQL Database on 3306 / PostgreSQL DB on TCP 5432 is very rare case scenario.

Below is example to scan a Local network situated IP for TCP open ports from port 1 till 7000.


# nc -v -n -z -w 5 1-7000

           nc: connect to 80 (tcp) failed: Connection refused
           nc: connect to 20 (tcp) failed: Connection refused
           Connection to port [tcp/ssh] succeeded!
           nc: connect to 23 (tcp) failed: Connection refused


Be informed that scanning with netcat is much more slower, than nmap, so specifying smaller range of ports is always a good idea to reduce annoying waiting …

The -w flag is used to set a timeout to remote connection, usually on a local network situated machines the timeout could be low -w 1 but for machines across different Data Centers (let say one in Berlin and one in Seattle), use as a minimum -w 5.

If you expect remote service to be responsive (as it should always be), it is a nice idea to use netcat with a low timeout (-w) value of 1 below is example:

netcat -v -z -n -w 1 scanned-hosts 1-1023


5. Port scanning range of IP addresses with netcat

If you have used Nmap you know scanning for a network range is as simple as running something like nmap -sP -P0 192.168.0.* (to scan from IP range 1-255 map -sP -P0 (to scan from local IPs ending in 1-150) or giving the network mask of the scanned network, e.g. nmap -sF – for more examples please check my previous article Checking port security on Linux with nmap (examples).

But what if nmap is not there and want to check a bunch 10 Splunk servers (software for searching, monitoring, and analyzing machine-generated big data, via a Web-style interface.), with netcat to find, whether the default Splunk connection port 9997 is opened or not:


for i in `seq 1 10`; do nc -z -w 5 -vv splunk0$ 9997; done


6. Checking whether UDP port traffic is allowed to destination server


Assuring you have access on Source traffic (service) Host A  and Host B (remote destination server where a daemon will be set-upped to listen on UDP port and no firewall in the middle Network router or no traffic control and filtering software HUB is preventing the sent UDP proto traffic, lets say an ntpd will be running on its standard 123 port there is done so:

– On host B (the remote machine which will be running ntpd and should be listening on port 123), run netcat to listen for connections


# nc -l -u -p 123
Listening on [] (family 2, port 123)

Make sure there is no ntpd service actively running on the server, if so stop it with /etc/init.d/ntpd stop
and run above command. The command should run as superuser as UDP port 123 is from the so called low ports from 1-1024 and binding services on such requires root privileges.

– On Host A (UDP traffic send host


nc -uv remote-server-host 123



If the remote port is not reachable due to some kind of network filtering, you will get "connection refused".
An important note to make is on some newer Linux distributions netcat might be silently trying to connect by default using IPV6, bringing false positives of filtered ports due to that. Thus it is generally a good idea, to make sure you're connecting to IPV6


$ nc -uv -4 remote-server-host 123


Another note to make here is netcat's UDP connection takes 2-3 seconds, so make sure you wait at least 4-8 seconds for a very distant located hosts that are accessed over a multitude of routers.

7. Checking whether TCP port traffic allowed to DST remote server

To listen for TCP connections on a specified location (external Internet IP or hostname), it is analogous to listening for UDP connections.

Here is for example how to bind and listen for TCP connections on all available Interface IPs (localhost, eth0, eth1, eth2 etc.)

nc -lv 12345


Then on client host test the connection with


nc -vv 12345
Connection to 12345 port [tcp/*] succeeded!


8. Proxying traffic with netcat

Another famous hackers use of Netcat is its proxying possibility, to proxy anything towards a third party application with UNIX so any content returned be printed out on the listening nc spawned daemon like process.
For example one application is traffic SMTP (Mail traffic) with netcat, below is example of how to proxy traffic from Host B -> Host C (in that case the yandex current mail server

linux-srv:~# nc -l 12543 | nc 25

Now go to Host A or any host that has TCP/IP protocol access to port 12543 on proxy-host Host B (linux-srv) and connect to it on 12543 with another netcat or telnet.

to make netcat keep connecting to MX (Mail Exchange) server you can run it in a small never ending bash shell while loop, like so:


linux-srv:~# while :; do nc -l 12543 | nc 25; done

 Below are screenshots of a connection handshake between Host B (linux-srv) proxy host and Host A (the end client connecting) and Host C (



Host B netcat as a (Proxy)

that is possible in combination of UNIX and named pipes (for more on Named pipes check my previous article simple linux logging with named pipes), here is how to run a single netcat version to proxy any traffic in a similar way as the good old tinyproxy.

On Proxy host create the pipe and pass the incoming traffic towards and write back any output received back in the named pipe.

# mkfifo backpipe
# nc -l 8080 0<backpipe | nc 80 1>backpipe

Other useful netcat proxy set-up is to simulate a network connectivity failures.

For instance, if server:port on TCP 1080 is the normal host application would connect to, you can to set up a forward proxy from port 2080 with

    nc -L server:1080 2080

then set-up and run the application to connect to localhost:2080 (nc proxy port)

    /path/to/application_bin –server=localhost –port=2080

Now application is connected to localhost:2080, which is forwarded to server:1080 through netcat. To simulate a network connectivity failure, just kill the netcat proxy and check the logs of application_bin.

Using netcat as a bind shell (make any local program / process listen and deliver via nc)


netcat can be used to make any local program that can receive input and send output to a server, this use is perhaps little known by the junior sysadmin, but a favourite use of l337 h4x0rs who use it to spawn shells on remote servers or to make connect back shell. The option to do so is -e

-e – option spawns the executable with its input and output redirected via network socket.

One of the most famous use of binding a local OS program to listen and receive / send content is by
making netcat as a bind server for local /bin/bash shell.

Here is how

nc -l -p 4321 -e /bin/sh

If necessery specify the bind hostname after -l. Then from any client connect to 4321 (and if it is opened) you will gain a shell with the user with which above netcat command was run. Note that many modern distribution versions such as Debian / Fedora / SuSE Linux's netcat binary is compiled without the -e option (this works only when compiled with -DGAPING_SECURITY_HOLE), removal in this distros is because option is potentially opening a security hole on the system.

If you're interested further on few of the methods how modern hackers bind new backdoor shell or connect back shell, check out Spawning real tty shells article.


For more complex things you might want to check also socat (SOcket CAT) – multipurpose relay for bidirectional data transfer under Linux.
socat is a great Linux Linux / UNIX TCP port forwarder tool similar holding the same spirit and functionality of netcat plus many, many more.

On some of the many other UNIX operating systems that are lacking netcat or nc / netcat commands can't be invoked a similar utilitiesthat should be checked for and used instead are:

ncat, pnetcat, socat, sock, socket, sbd

To use nmap's ncat to spawn a shell for example that allows up to 3 connections and listens for connects only from network on port 8081:

ncat –exec "/bin/bash" –max-conns 3 –allow -l 8081 –keep-open


9. Copying files over network with netcat

Another good hack often used by hackers to copy files between 2 servers Server1 and Server2 who doesn't have any kind of FTP / SCP / SFTP / SSH / SVN / GIT or any kind of Web copy support service – i.e. servers only used as a Database systems that are behind a paranoid sysadmin firewall is copying files between two servers with netcat.

On Server2 (the Machine on which you want to store the file)

nc -lp 2323 > files-archive-to-copy.tar.gz

On server1 (the Machine from where file is copied) run:

nc -w 5 2323 < files-archive-to-copy.tar.gz


Note that the downside of such transfers with netcat is data transferred is unencrypted so any one with even a simple network sniffer or packet analyzier such as iptraf or tcpdump could capture the file, so make sure the file doesn't contain sensitive data such as passwords.

Copying partition images like that is perhaps best way to get disk images from a big server onto a NAS (when you can't plug the NAS into the server).

10. Copying piped archived directory files with netcat


On computer A:

export ARIBTRARY_PORT=3232
nc -l $ARBITRARY_PORT | tar vzxf –

On Computer B:

tar vzcf – files_or_directories | nc computer_a $ARBITRARY_PORT


11. Creating a one page webserver with netcat and ncat

As netcat could listen to port and print content of a file, it can be set-up with a bit of bash shell scripting to serve
as a one page webserver, or even combined with some perl scripting and bash to create a multi-serve page webserver if needed.

To make netact serve a page to any connected client run in a screen / tmux session following code:


while true; do nc -l -p 80 -q 1 < somepage.html; done


Another interesting fun example if you have installed ncat (is a small web server that connects current time on server on connect).

ncat -lkp 8080 –sh-exec 'echo -ne "HTTP/1.0 200 OK\r\n\r\nThe date is "; date;'


12. Cloning Hard disk partitions with netcat

rsync is a common tool used to clone hard disk partitions over network. However if rsync is not installed on a server and netcat is there you can use it instead, lets say we want to clone /dev/sdb
from Server1 to Server2 assuming (Server1 has a configured working Local or Internet connection).


On Server2 run:

nc -l -p 4321 | dd of=/dev/sdb


Following on Server2 to start the Partition / HDD cloning process run


dd if=/dev/sdb | nc 4321


Where is the IP address listen configured on Server2 (in case you don't know it, check the listening IP to access with /sbin/ifconfig).

Next you have to wait for some short or long time depending on the partiiton or Hard drive, number of files / directories and allocated disk / partition size.

To clone /dev/sda (a main partiiton) from Server1 to Server2 first requirement is that it is not mounted, thus to have it unmounted on a system assuming you have physical access to the host, you can boot some LiveCD Linux distribution such as Knoppix Live CD on Server1, manually set-up networking with ifconfig or grab an IP via DHCP from the central DHCP server and repeat above example.

Happy netcating 🙂

Howto Configure Linux shell Prompt / Setup custom Terminal show Prompt using default shell variables PS1, PS2, PS3, PS4

Tuesday, August 27th, 2019


System Console, Command Operation Console  or Terminal is a Physical device for text (command) input from keyboard, getting the command output and monitoring the status of a shell or programs I/O operations generated traditionally with attached screen. With the development of Computers, physical consoles has become emulated and the input output is translated on the monitor usually via a data transfer  protocol historically mostly over TCP/IP connection to remote IP with telnet or rsh, but due to security limitations Consoles are now accessed over data (encrypted) network protocols with SHA2 / MD5 cryptography algorithm enabled such as over SSH (Secure Shell) network protocol..
The ancestors of physical consoles which in the past were just a Terminal (Monitoring / Monitor device attached to a MainFrame system computer).


What is Physical Console
A classical TTY (TeleTYpewriter) device looked like so and served the purpose of being just a communication and display deivce, whether in reality the actual computing and storage tape devices were in a separate room and communicating to Terminal.

TTYs are still present in  modern UNIX like GNU / Linux distrubions OSes and the BSD berkley 4.4 code based FreeBSD / NetBSD / OpenBSD if you have installed the OS on a physical computer in FreeBSD and Solaris / SunOS there is also tty command. TTY utility in *nix writes the name of the terminal attached to standard input to standard output, in Linux there is a GNU remake of same program part called GNU tty of coreutils package (try man tty) for more.

The physical console is recognizable in Linux as it is indicated with other tree letters pts – (pseudo terminal device) standing for a terminal device which is emulated by an other program (example: xterm, screen, or ssh are such programs). A pts is the slave part of a pts is pseudo there is no separate binary program for it but it is dynamically allocated in memory.
PTS is also called Line consle in Cisco Switches / Router devices, VTY is the physical Serial Console connected on your Cisco device and the network connection emulation to network device is creates with a virtual console session VTL (Virtual Terminal Line). In freebsd the actual /dev/pts* /dev/tty* temporary devices on the OS are slightly different and have naming such as /dev/ttys001.
But the existence of tty and pts emulator is not enough for communicating interrupts to Kernel and UserLand binaries of the Linux / BSD OS, thus to send the commands on top of it is running a System Shell as CSH / TSH / TCSH or BASH which is usually the first program set to run after user logs in over ptty or pseudo tty virtual terminal.



Setting the Bash Prompt in Terminal / Console on GNU / Linux

Bash has system environments to control multiple of variables, which are usually visible with env command, one important variable to change in the past was for example USER / USERNAME which was red by IRC Chat clients  such as BitchX / irssi and could be displayed publicly so if not changed to a separate value, one could have known your Linux login username by simple /whois query to the Nickname in question (if no inetd / xinetd service was running on the Linux box and usually inetd was not running).

Below is my custom set USER / USERNAME to separate

hipo@pcfreak:~$ env|grep USER

There is plenty of variables to  tune email such as MAIL store directory, terminal used TERM, EDITOR etc. but there are some
variables that are not visible with env query as they're not globally available for all users but just for the single user, to show this ones you need to use declare command instead, to get a full list of All Single and System Wide defined variables and functions type declare in the bash shell, for readability, below is last 10 returned results:


hipo@pcfreak:~$ declare | tail -10
    local quoted=${1//\'/\'\\\'\'};
    printf "'%s'" "$quoted"
quote_readline ()
    local quoted;
    _quote_readline_by_ref "$1" ret;
    printf %s "$ret"


PS1 is present there virtually on any modern Linux distribution and is installed through user home's directory $HOME/.bashrc , ~/.profile or .bash_profile or System Wide globally for all existing users in /etc/passwd (password database file) from /etc/bash.bashrc
In Debian / Ubuntu / Mint GNU / Linux this system variable is set in user home's .bashrc but in Fedora / RHEL Linux distro,
PS1 is configured from /home/username/.bash_profile to find out where PS1 is located for ur user:

cd ~
grep -Rli PS1 .bash*

Here is one more example:

hipo@pcfreak:~$ declare|grep -i PS1|head -1
PS1='\[\e]0;\u@\h: \w\a\]${debian_chroot:+($debian_chroot)}\u@\h:\w\$ '

hipo@pcfreak:~$ grep PS1 /etc/bash.bashrc
[ -z “$PS1” ] && return
# but only if not SUDOing and have SUDO_PS1 set; then assume smart user.
if ! [ -n “${SUDO_USER}” -a -n “${SUDO_PS1}” ]; then
  PS1='${debian_chroot:+($debian_chroot)}\u@\h:\w\$ '

Getting current logged in user shell configured PS1 variable can be done with echo:

hipo@pcfreak:~$ echo $PS1
\[\e]0;\u@\h: \w\a\]${debian_chroot:+($debian_chroot)}\u@\h:\w\$

So lets observe a little bit the meaning of this obscure line of (code) instructions code which are understood by BASH when being red from PS1 var to do so, I'll give a list of meaning of main understood commands, each of which is defined with \.

The ${debian_chroot} shell variable is defined from /etc/bash.bashrc

Easiest way to change PS1 is to export the string you like with the arguments like so:


root@linux:/home/hipo# export PS1='My-Custom_Server-Name# '
My-Custom_Server-Name# echo $PS1


  •     \a : an ASCII bell character (07)
  •     \d : the date in “Weekday Month Date” format (e.g., “Tue May 26”)
  •     \D{format} : the format is passed to strftime(3) and the result is inserted into the prompt string; an empty format results in a locale-specific time representation. The braces are required
  •     \e : an ASCII escape character (033)
  •     \h : the hostname up to the first ‘.’
  •     \H : the hostname
  •     \j : the number of jobs currently managed by the shell
  •     \l : the basename of the shell's terminal device name
  •     \n : newline
  •     \r : carriage return
  •     \s : the name of the shell, the basename of $0 (the portion following the final slash)
  •     \t : the current time in 24-hour HH:MM:SS format
  •     \T : the current time in 12-hour HH:MM:SS format
  •     \@ : the current time in 12-hour am/pm format
  •     \A : the current time in 24-hour HH:MM format
  •     \u : the username of the current user
  •     \v : the version of bash (e.g., 2.00)
  •     \V : the release of bash, version + patch level (e.g., 2.00.0)
  •     \w : the current working directory, with $HOME abbreviated with a tilde
  •     \W : the basename of the current working directory, with $HOME abbreviated with a tilde
  •     \! : the history number of this command
  •     \# : the command number of this command
  •     \$ : if the effective UID is 0, a #, otherwise a $
  •     \nnn : the character corresponding to the octal number nnn
  •     \\ : a backslash
  •     \[ : begin a sequence of non-printing characters, which could be used to embed a terminal control sequence into the prompt
  •     \] : end a sequence of non-printing characters

The default's PS1 set prompt on Debian Linux is:

echo $PS1
\[\e]0;\u@\h: \w\a\]${debian_chroot:+($debian_chroot)}\u@\h:\w\$

As you can see \u (print username) \h (print hostname)  and \W (basename of current working dir) or \w (print $HOME/current working dir)
are the most essential, the rest are bell character, escape character etc.

A very good way to make your life easier and learn the abbreviations / generate exactly the PS1 PROMPT you want to have is with Easy Bash PS1 Generator Web Utility
with which you can just click over buttons that are capable to produce all of the PS1 codes.

1. How to show current hour:minute:seconds / print full date in Prompt Shell (PS)

Here is an example with setting the Bash Shell prompt  to include also the current time in format hour:minute:seconds (very useful if you're executing commands on a critical servers and you run commands in some kind of virtual terminal like screen or tmux.

root@pcfreak:~# PS1="\n\t \u@\h:\w# "
14:03:51 root@pcfreak:/home#



export PS1='\u@\H \D{%Y-%m-%d %H:%M;%S%z}] \W ] \$ '



Make superuser appear in RED color (adding PS1 prompt custom color for a User)

root@pcfreak:~$  PS1="\\[$(tput setaf 1)\\]\\u@\\h:\\w #\\[$(tput sgr0)\\]"



In above example the Shell Prompt Color changed is changed for administrator (root) to shebang symbol # in red, green, yellow and blue for the sake to show you how it is done, however this example can be adapted for any user on the system. Setting different coloring for users is very handy if you have to administer Mail Server service like Qmail or other Application that consists of multiple small ones of multiple daemons such as qmail + vpopmail + clamd + mysql etc. Under such circumstances, coloring each of the users in different color like in the example for debugging is very useful.

Coloring the PS1 system prompt on Linux to different color has been a standard practice in Linux Server environments running Redhat Enterprise Linux (RHEL) and SuSE Enterprise Linux and some Desktop distributions such as Mint Linux.

To make The Root prompt Red colored only for system super user (root) on any Linux distribution
, add the following to /etc/bashrc, e.g.

vim /etc/bashrc

# If id command returns zero, you've root access.
if [ $(id -u) -eq 0 ];
then # you are root, set red colour prompt
  PS1="\\[$(tput setaf 1)\\]\\u@\\h:\\w #\\[$(tput sgr0)\\]"
else # normal
  PS1="[\\u@\\h:\\w] $"



2. How to make the prompt of a System user appear Green

Add to ~/.bashrc  following line



PS1="\\[$(tput setaf 2)\\]\\u@\\h:\\w #\\[$(tput sgr0)\\]"


3. Print New line, username@hostname, base PTY, shell level, history (number), newline and full working directory $PWD


export PS1='\n[\u@\h \l:$SHLVL:\!]\n$PWD\$ '


4. Showing the numbert of jobs the shell is currently managing.

This is useful if you run and switch with fg / bg (foreground / background) commands
to switch between jobs and forget some old job.


export PS1='\u@\H \D{%Y-%m-%d %H:%M;%S%z}] \W \$]'


Multi Lines Prompt / Make very colorful Shell prompt full of stats info

PS1="\n\[\033[35m\]\$(/bin/date)\n\[\033[32m\]\w\n\[\033[1;31m\]\u@\h: \[\033[1;34m\]\$(/usr/bin/tty | /bin/sed -e ‘s:/dev/::’): \[\033[1;36m\]\$(/bin/ls -1 | /usr/bin/wc -l | /bin/sed ‘s: ::g’) files \[\033[1;33m\]\$(/bin/ls -lah | /bin/grep -m 1 total | /bin/sed ‘s/total //’)b\[\033[0m\] -> \[\033[0m\]"




5. Set color change on command failure

If you have a broken command or the command ended with non zero output with some kind of bad nasty message and you want to make, that more appearing making it red heighlighted, here is how:


PROMPT_COMMAND='PS1="\[\033[0;33m\][\!]\`if [[ \$? = “0” ]]; then echo “\\[\\033[32m\\]”; else echo “\\[\\033[31m\\]”; fi\`[\u.\h: \`if [[ `pwd|wc -c|tr -d ” “` > 18 ]]; then echo “\\W”; else echo “\\w”; fi\`]\$\[\033[0m\] “; echo -ne “\033]0;`hostname -s`:`pwd`\007"'


6. Other beautiful PS1 Color Prompts with statistics


PS1="\n\[\e[32;1m\](\[\e[37;1m\]\u\[\e[32;1m\])-(\[\e[37;1m\]jobs:\j\[\e[32;1m\])-(\[\e[37;1m\]\w\[\e[32;1m\])\n(\[\[\e[37;1m\]! \!\[\e[32;1m\])-> \[\e[0m\]"





7. Add Muliple Colors to Same Shell prompt


function prompt { local BLUE="\[\033[0;34m\]” local DARK_BLUE=”\[\033[1;34m\]” local RED=”\[\033[0;31m\]” local DARK_RED=”\[\033[1;31m\]” local NO_COLOR=”\[\033[0m\]” case $TERM in xterm*|rxvt*) TITLEBAR=’\[\033]0;\u@\h:\w\007\]’ ;; *) TITLEBAR=”” ;; esac PS1=”\u@\h [\t]> ” PS1=”${TITLEBAR}\ $BLUE\u@\h $RED[\t]>$NO_COLOR " PS2='continue-> ' PS4='$0.$LINENO+ ' }


8. Setting / Change Shell background Color



export PS1="\[$(tput bold)$(tput setb 4)$(tput setaf 7)\]\u@\h:\w $ \[$(tput sgr0)\]"


tput Color Capabilities:

  • tput setab [1-7] – Set a background color using ANSI escape
  • tput setb [1-7] – Set a background color
  • tput setaf [1-7] – Set a foreground color using ANSI escape
  • tput setf [1-7] – Set a foreground color

tput Text Mode Capabilities:

  • tput bold – Set bold mode
  • tput dim – turn on half-bright mode
  • tput smul – begin underline mode
  • tput rmul – exit underline mode
  • tput rev – Turn on reverse mode
  • tput smso – Enter standout mode (bold on rxvt)
  • tput rmso – Exit standout mode
  • tput sgr0 – Turn off all attributes

Color Code for tput:

  • 0 – Black
  • 1 – Red
  • 2 – Green
  • 3 – Yellow
  • 4 – Blue
  • 5 – Magenta
  • 6 – Cyan
  • 7 – White


9. Howto Use bash shell function inside PS1 variable

If you administrate Apache or other HTTPD servers or any other server whose processes are forked and do raise drastically at times to keep an eye while actively working on the server.


function httpdcount { ps aux | grep apache2 | grep -v grep | wc -l } export PS1="\u@\h [`httpdcount`]> "

10. PS2, PS3, PS4 little known variables

I'll not get much into detail to PS2, PS3, PS4 but will mention them as perhaps many people are not even aware they exist.
They're rarely used in the daily system administrator's work but useful for Shell scripting purposes of Dev Ops and Shell Scripting Guru Programmers.

  • PS2 – Continuation interactive prompt

A very long unix command can be broken down to multiple line by giving \ at the end of the line. The default interactive prompt for a multi-line command is “> “.  Let us change this default behavior to display “continue->” by using PS2 environment variable as shown below.

hipo@db-host :~$ myisamchk –silent –force –fast –update-state \
> –key_buffer_size=512M –sort_buffer_size=512M \
> –read_buffer_size=4M –write_buffer_size=4M \
> /var/lib/mysql/bugs/*.MYI
[Note: This uses the default “>” for continuation prompt]

  • PS3 – Prompt used by “select” inside shell script (usefulif you write scripts with user prompts)


  • PS4 – Used by “set -x” to prefix tracing output
    The PS4 shell variable defines the prompt that gets displayed.

You can find  example with script demonstrating PS2, PS3, PS4 use via small shell scripts in thegeekstuff's article Take control of PS1, PS2, PS3, PS4 read it here



In this article, I've shortly reviewed on what is a TTY, how it evolved into Pseudo TTY and how it relates to current shells which are the interface communicating with the modern UNIX like Operating systems's userland and kernel.
Also it was reviewed shortly how the current definitions of shell variables could be viewed with declare cmd. Also I went through on how to display the PS1 variable and  on how to modify PS1 and make the prompt different statistics and monitoring parameters straight into the command shell. I've shown some common PS1 strings that report on current date hour, minute, seconds, modify the coloring of the bash prompt shell, show processes count, and some PS1 examples were given that combines beuatiful shell coloring as well as how the Prompt background color can be changed.
Finally was shown how a combination of commands can be executed by exporting to PS1 to update process counf of Apache on every shell prompt iteration.
Other shell goodies are mostly welcome