Category: Linux

  • ping: unknown host localhost

    On a server, ping localhost did not work

    root@cloud1:~# ping localhost
ping: unknown host localhost
root@cloud1:~#

    /etc/hosts files was proper, had entry for localhost.

    root@cloud1:~# cat /etc/hosts | grep localhost
127.0.0.1 localhost
::1 ip6-localhost ip6-loopback
root@cloud1:~#

    problem was due to /etc/nsswitch.conf file missing in the server. To fix create file

    vi /etc/nsswitch.conf

    Add

    # /etc/nsswitch.conf
#
# Example configuration of GNU Name Service Switch functionality.
# If you have the `glibc-doc-reference' and `info' packages installed, try:
# `info libc "Name Service Switch"' for information about this file.

passwd:         compat systemd
group:          compat systemd
shadow:         compat
gshadow:        files

hosts:          files mdns4_minimal [NOTFOUND=return] dns myhostname
networks:       files

protocols:      db files
services:       db files
ethers:         db files
rpc:            db files

netgroup:       nis

    Set permission for the file

    chmod 644 /etc/nsswitch.conf

  • yum-config-manager

    yum-config-manager allows you to manage yum repositories.

    To add a repo, run

    yum-config-manager --add-repo https://download.docker.com/linux/centos/docker-ce.repo

    Disable a repository

    yum-config-manager --disable rpmfusion-free-updates

    See yum

  • Start MongoDB from command line

    To start MongoDB from command line, run

    mongod --auth --dbpath /path/to/mongodb

    See MongoDB

  • Remote Desktop

    Remote Desktop Software

    https://guacamole.apache.org
    Google Chome Remote Desktop
    https://rustdesk.com – Free Open source teamviewer alternative.

    Enable Remote Desktop

    See vnc

  • Ubuntu allow a normal user to edit WiFi Connection

    Ubuntu allow a normal user to edit WiFi Connection

    When a normal user edit Wi-Fi connection on Wi-Fi Settings, they will be asked to enter administrator user pasword.

    wifi ubuntu settings

    To allow a normal user to modify network connection, you can edit the connection configuration file in folder

    /etc/NetworkManager/system-connections

    This file look like

    root@sok-01:/etc/NetworkManager/system-connections# cat SOK
[connection]
id=SOK
uuid=982596a7-44c8-4e52-a696-6acf4daeec12
type=wifi
permissions=

[wifi]
mac-address=E8:DE:27:0B:39:55
mac-address-blacklist=
mode=infrastructure
ssid=SOK

[wifi-security]
auth-alg=open
key-mgmt=wpa-psk
psk=Sup3rD0nkey

[ipv4]
dns-search=
method=auto

[ipv6]
addr-gen-mode=stable-privacy
dns-search=
method=auto
root@sok-01:/etc/NetworkManager/system-connections#

    Find

    permissions=

    Replace with

    permissions=user:USER_NAME_HEARE:;

    USER_NAME_HEARE = replace this with actual username, that need to edit the network configuration.

  • Change Linux user shell

    To change shell for a user, run

    chsh --shell PATH_TO_SHELL USERNAME

    Example

    chsh --shell /bin/bash okadmin

    I will add a user, if no shell specified, it use sh as shell.

    root@lab:~# useradd okadmin
root@lab:~# su - okadmin
No directory, logging in with HOME=/
$ echo $SHELL
/bin/sh
$ exit
root@lab:~# chsh --shell /bin/bash okadmin
root@lab:~# su - okadmin
No directory, logging in with HOME=/
okadmin@lab:/$ echo $SHELL
/bin/bash
okadmin@lab:/$ logout
root@lab:~#

    You can find what shell you are using now with command

    echo $SHELL

  • CentOS add IP Address

    To add IP address on a Linux server, first find name of your network interface. This can be done with command

    ip link

    In most case, it can be enp3s0 or eth0, if you have more than one network card, this may be differnt, in such case, you need to identify which network interface you need to configure the the IP address .

    Now create file

    vi /etc/sysconfig/network-scripts/ifcfg-enp3s0:1

    Here i used enp3s0 as network inferface name, replace it with whatever network interface name you use.

    Add following content

    DEVICE="enp3s0:1"
IPADDR=IP_ADDR_HERE
NETMASK=255.255.255.255

    IP_ADDR_HERE = replace with the IP address you need to add.

    enp3s0 = replace with actual network interface name.

    Restart network service with command

    systemctl restart network

    If you need to add another IP, create file with name

    INTERFACE_NAME:2

    for example “enp3s0:2”, you need to use same alias name in DEVICE entry (first line) in the file.

    Adding IP Range in CentOS 6

  • gonit golang replacement for monit

    gonit is a replacent for monitoring software monit. monit was removed from Debian 10 due to security issues and the devloper will not provide a patch for the security vlunerabilty in time for Debian 10 release.

    You can download gonit from

    https://github.com/bitnami/gonit

    root@ip-172-26-14-184:~# gonit status

Uptime                         22h5m41s
Last Check                     2019-08-05 08:23:57.612314368 +0000 UTC m=+79440.116643915
Next Check                     2019-08-05 08:25:57.612314368 +0000 UTC m=+79560.116643915
Pid                            3162
Pid File                       /var/run/gonit.pid
Control File                   /etc/gonit/gonitrc
Socket File                    /var/run/gonit.sock
Log File                       /var/log/gonit.log
Process 'apache'
  status                                        Running
  pid                                              2020
  uptime                                       22h5m41s
  monitoring status                           monitored

Process 'mysql'
  status                                        Running
  pid                                              1911
  uptime                                       22h5m41s
  monitoring status                           monitored

Process 'php-fpm'
  status                                        Running
  pid                                              2014
  uptime                                       22h5m41s
  monitoring status                           monitored


root@ip-172-26-14-184:~#

  • Check if you are using 32 or 64 bit Linux

    To identify if you are using 32 bit or 64 bit Linux OS, run command arch.

    boby@sok-01:~$ arch
x86_64
boby@sok-01:~$

    You can also use uname command

    boby@sok-01:~$ uname -i
x86_64
boby@sok-01:~$

    Another way is to use hostnamectl command and look for “Architecture”.

    boby@sok-01:~$ hostnamectl
   Static hostname: sok-01
         Icon name: computer-desktop
           Chassis: desktop
        Machine ID: 70486772aac1410c9a8031851ab60a0d
           Boot ID: 46ff31881cca4e7090edb35f2af89a4a
  Operating System: Ubuntu 18.04.2 LTS
            Kernel: Linux 4.15.0-55-generic
      Architecture: x86-64
boby@sok-01:~$

  • linux sound

    aplay

    sudo alsa force-reload 

    vi /etc/modprobe.d/alsa-base.conf

    Show Sound Devices

    boby@fwhlin:~$ cat /proc/asound/cards
 0 [Intel          ]: HDA-Intel - HDA Intel
                      HDA Intel at 0xfb9f8000 irq 75
 1 [HDMI           ]: HDA-Intel - HDA ATI HDMI
                      HDA ATI HDMI at 0xfbcfc000 irq 76
boby@fwhlin:~$

    boby@boby-pc ~ $ cat /proc/asound/card0/codec* | grep Codec
Codec: Realtek ALC892
boby@boby-pc ~ $ 

    boby@boby-pc ~ $ cat /proc/asound/card0/pcm0c/info
card: 0
device: 0
subdevice: 0
stream: CAPTURE
id: ALC892 Analog
name: ALC892 Analog
subname: subdevice #0
class: 0
subclass: 0
subdevices_count: 1
subdevices_avail: 1
boby@boby-pc ~ $

  • aplay

    aplay is a command line sound player for ALSA soundcard driver.

    To list all available sound devices, run

    boby@sok-01:~$ aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: Generic [HD-Audio Generic], device 3: HDMI 0 [HDMI 0]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: Generic [HD-Audio Generic], device 7: HDMI 1 [HDMI 1]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: Generic [HD-Audio Generic], device 8: HDMI 2 [HDMI 2]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: Generic [HD-Audio Generic], device 9: HDMI 3 [HDMI 3]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: headset [Sennheiser USB headset], device 0: USB Audio [USB Audio]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 2: Generic_1 [HD-Audio Generic], device 0: ALC887-VD Analog [ALC887-VD Analog]
  Subdevices: 0/1
  Subdevice #0: subdevice #0
card 2: Generic_1 [HD-Audio Generic], device 1: ALC887-VD Digital [ALC887-VD Digital]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
boby@sok-01:~$

  • Nginx vs Apache

    I recently added nginx as front end for apache. Now nginx serve static content, PHP requests are peroxided to Apache.

    Nginx frontend, Apache backend

    [root@server12 ~]# ab -n 1000 -c 100 http://netfree.netfreehost.com/
This is ApacheBench, Version 2.0.40-dev <$Revision: 1.146 $> apache-2.0
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Copyright 2006 The Apache Software Foundation, http://www.apache.org/

Benchmarking netfree.netfreehost.com (be patient)
Completed 100 requests
Completed 200 requests
Completed 300 requests
Completed 400 requests
Completed 500 requests
Completed 600 requests
Completed 700 requests
Completed 800 requests
Completed 900 requests
Finished 1000 requests


Server Software:        nginx/1.1.0
Server Hostname:        netfree.netfreehost.com
Server Port:            80

Document Path:          /
Document Length:        16844 bytes

Concurrency Level:      100
Time taken for tests:   5.463353 seconds
Complete requests:      1000
Failed requests:        0
Write errors:           0
Total transferred:      17357000 bytes
HTML transferred:       16844000 bytes
Requests per second:    183.04 [#/sec] (mean)
Time per request:       546.335 [ms] (mean)
Time per request:       5.463 [ms] (mean, across all concurrent requests)
Transfer rate:          3102.49 [Kbytes/sec] received

Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:        0    0   0.8      0       5
Processing:    44  518  93.9    534     719
Waiting:       44  517  94.0    533     718
Total:         47  518  93.2    534     719

Percentage of the requests served within a certain time (ms)
  50%    534
  66%    553
  75%    566
  80%    574
  90%    592
  95%    606
  98%    642
  99%    665
 100%    719 (longest request)
[root@server12 ~]#

    Apache Only

    [root@server12 ~]# ab -n 1000 -c 100 http://netfree.netfreehost.com:81/
This is ApacheBench, Version 2.0.40-dev <$Revision: 1.146 $> apache-2.0
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Copyright 2006 The Apache Software Foundation, http://www.apache.org/

Benchmarking netfree.netfreehost.com (be patient)
Completed 100 requests
Completed 200 requests
Completed 300 requests
Completed 400 requests
Completed 500 requests
Completed 600 requests
Completed 700 requests
Completed 800 requests
Completed 900 requests
Finished 1000 requests


Server Software:        Apache/2.2.3
Server Hostname:        netfree.netfreehost.com
Server Port:            81

Document Path:          /
Document Length:        16844 bytes

Concurrency Level:      100
Time taken for tests:   7.102347 seconds
Complete requests:      1000
Failed requests:        1
   (Connect: 0, Length: 1, Exceptions: 0)
Write errors:           0
Total transferred:      17351384 bytes
HTML transferred:       16827683 bytes
Requests per second:    140.80 [#/sec] (mean)
Time per request:       710.235 [ms] (mean)
Time per request:       7.102 [ms] (mean, across all concurrent requests)
Transfer rate:          2385.69 [Kbytes/sec] received

Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:        0    0   1.1      0       6
Processing:    34  676 174.9    669    1261
Waiting:       32  675 175.0    668    1260
Total:         34  676 174.4    669    1261

Percentage of the requests served within a certain time (ms)
  50%    669
  66%    696
  75%    732
  80%    754
  90%    893
  95%    974
  98%   1081
  99%   1128
 100%   1261 (longest request)
[root@server12 ~]#

    See Apache, Nginx