Category: Linux

  • apt-cache

    To search for available packages, run

    apt-cache search PKG_NAME

    Example

    boby@sok-01:~$ apt-cache search wget
devscripts - scripts to make the life of a Debian Package maintainer easier
wget - retrieves files from the web
abcde - A Better CD Encoder
apt-mirror - APT sources mirroring tool
axel - light command line download accelerator
filetea - Web-based file sharing system
getdata - management of external databases
libcupt4-2-downloadmethod-wget - flexible package manager -- wget download method
libwget0 - Download library for files and recursive websites
ow-shell - shell utilities to talk to an 1-Wire owserver
puf - Parallel URL fetcher
pwget - downloader utility which resembles wget (implemented in Perl)
python-wget - pure Python download utility for Python 2
python3-wget - pure Python download utility for Python 3
snarf - command-line URL grabber
tcllib - Standard Tcl Library
texlive-latex-extra - TeX Live: LaTeX additional packages
wget2 - file and recursive website downloader
wget2-dev - development file for libwget2
wput - tiny wget-like ftp-client for uploading files
boby@sok-01:~$

    Ubuntu find Dependencies for a package
    apt

  • curl using cookie

    Here is some example of using curl to login to site, use cookie to do further requests.

    curl -k --cookie-jar ./cookies_hotfile --data "returnto=%2F&user=USERNAME_HERE&pass=PASSWORD_HERE" http://site.com/login.php
curl -L -O --cookie ./cookies_hotfile http://site.com/dl/5222/4444/file.zip.html

    See curl

  • chown

    chown command is ued to change ownership of a file or folder

    In this example ownership of folder public_html to username and group specified.

    chown -R username:group public_html

    -R used for recursively change ownership, that is all files and folders inside the folder also get the new ownership.

    See Linux Commands

  • at

    yum install at

    Enable atd

    [root@server12 ~]# service atd status
atd is stopped
[root@server12 ~]# chkconfig --list | grep atd
atd            	0:off	1:off	2:off	3:on	4:on	5:on	6:off
[root@server12 ~]# service atd start
Starting atd:                                              [  OK  ]
[root@server12 ~]#

    Setting Job

    [root@server12 ~]# echo "/sbin/shutdown -h now"  |  at  21:15  16.02.2017
job 1 at 2017-02-16 21:15
[root@server12 ~]# atq
1	2017-02-16 21:15 a root
[root@server12 ~]#

    Deleting at job

    [root@server12 ~]# atq
1	2017-02-16 21:15 a root
[root@server12 ~]# man atq
[root@server12 ~]# atrm 1
[root@server12 ~]# atq
[root@server12 ~]#

    See linux commands

  • Testing POP3 mail box with telnet

    To test if pop3 mailbox work using telnet, use

    telnet MAIL_SERVER_IP 110
user YOUR_EMAIL_ADDRESS
pass YOUR_EMAIL_PASSWORD

    Once logged in, you can use commands like

    STAT
LIST
RETR
DELE
RSET
TOP
QUIT

    telnet pop3

    See telnet

  • Auto upgrade software in Ubuntu/Debian

    To auto upgrade software packages in Ubuntu/Debian, install

    apt install -y unattended-upgrades

    Edit

    vi /etc/apt/apt.conf.d/50unattended-upgrades

    In this file, you can configure various settings.

    See apt

  • rpmquery

    rpmquery command allow you to check if a specific rpm package installed on your computer.

    rpmquery PKG_NAME

    Example

    root@server20 [~]# rpmquery nmap
nmap-6.40-19.el7.x86_64
root@server20 [~]#

    See rpm

  • Install PowerDNS on Ubuntu 20.04

    PowerDNS is an OpenSource DNS server.

    Update apt repo

    apt-get update && apt-get upgrade -y

    On Ubuntu, systemd-resolved listen to port 53. This is a local DNS resolver, we need to stop this service before we can install PowerDNS.

    systemctl disable systemd-resolved
systemctl stop systemd-resolved
rm -f /etc/resolv.conf
echo "nameserver 1.1.1.1" > /etc/resolv.conf
echo "nameserver 8.8.8.8" >> /etc/resolv.conf

    To install PowerDNS with MySQL backend, run

    apt install pdns-server pdns-backend-mysql

    Since we are using MySQL backend to store DNS zones, we need to install MySQL database.

    apt install mariadb-server

    Login to MySQL, create a database and user.

    mysql
CREATE DATABASE powerdns;
GRANT ALL ON powerdns.* TO 'powerdns'@'localhost' IDENTIFIED BY 'YOUR_MYSQL_PW_HERE';

    Restore database scheme provided by powerdns

    mysql powerdns < /usr/share/pdns-backend-mysql/schema/schema.mysql.sql

    You can see this scheme in PowerDNS documentation.

    Configure PowerDNS to use MySQL backend

    vi /etc/powerdns/pdns.d/mysql.conf

    Add following content

    # MySQL Configuration
# Launch gmysql backend
launch+=gmysql
# gmysql parameters
gmysql-host=localhost
gmysql-port=3306
gmysql-dbname=powerdns
gmysql-user=powerdns
gmysql-password=YOUR_MYSQL_PW_HERE
gmysql-dnssec=yes
# gmysql-socket=

    Restart PowerDNS

    systemctl restart pdns

    If PowerDNS started properly, you will see it listening to port 53. You can verify with command netstat -lntp

    PowerDNS netstat

    In above picture, you will see PowerDNS listen to port 53 and 8081. Port 8081 is API port, you won't see it unless you enabled it.

    To verify PowerDNS is running, you can use command

    root@vultr:~# dig @127.0.0.1

; <<>> DiG 9.16.1-Ubuntu <<>> @127.0.0.1
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: REFUSED, id: 63898
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1
;; WARNING: recursion requested but not available

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1232
;; QUESTION SECTION:
;.				IN	NS

;; Query time: 0 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Mon Oct 12 06:53:40 UTC 2020
;; MSG SIZE  rcvd: 28

root@vultr:~#

    Back to PowerDNS

  • Find all open ports using nmap

    To find all open ports on a computer, run

    nmap -P0 -vv -sS -ff -r -n -p 1-65535 IP_ADDR_HERE

    -sS = SYN scan

    if you only need to scan most common ports, use

    nmap IP_ADDR_HERE

    Example

    nmap list ports

    To see open ports on your local computer, run

    nmap localhost
    nmap -p 22 IP_ADDR Scan a single port
    nmap -p 1-100 IP_ADDR Scan range of ports
    nmap -p- IP_ADDR Scan all 65535 ports
    nmap -F IP_ADDR Scan most popular 100 ports (fast scan)

    See nmap

  • Make hosts file editable with out sudo

    hosts file allow you to override DNS. This will allow you to point a domain to difernt IP address on your computer. It is helpful for testing sites during site migrations.

    If you edit hosts file a lot, it is better to make it owned by you or make it editable by everyone.

    sudo chmod 777 /etc/hosts

    Now onwards, you can edit the file with out sudo requirment.

    Another option is make it owned by current user.

    sudo chown USER_HERE:GROUP_HERE /etc/hosts

    See /etc/hosts

  • Migrate database from MongoDB Atlas to your server

    Migrate database from MongoDB Atlas to your server

    MongoDB Atlas is a hosted MongoDB server hosting in cloud. If you are starting with MongoDB, they have free hosting, that work for smaller sites.

    https://www.mongodb.com/cloud

    MongoDB Atlas

    On this page, you can see the version of MongoDB is Version 4.2.10. You need to install same or newer version of MongoDB on your server.

    MongoDB documentation have detailed instructions for installing MongoDB on various Operating systems.

    https://docs.mongodb.com/manual/administration/install-on-linux/

    Take MongoDB backup

    MongoDB atlas provide you command to take backup. On “Clusters” page, click the 3 dots, then select command line tools. Next page will show you command to take backup of your MongoDB database.

    mkdir /root/mongodb_backup
cd /root/mongodb_backup
mongodump --uri mongodb+srv://YOURNAME_HERE:[email protected]/DATABASE_NAME_HERE

    You can find database name in atlas, for this, click “Cluster0”, then go to Collections tab. You will see database name on this page.

    Once you run the mongodump command, it will create a folder “dump” and save backup inside.

    backup mongodb database using mongodump

    Restart MongoDB backup

    To restart MongoDB backup locally, run

    mongorestore --username=USER_NAME_HERE --password=PW_HERE --authenticationDatabase "admin"  /root/mongodb_backup/dump/

    USER_NAME_HERE and PW_HERE is MongoDB user and password. You can enable MongoDB authentication following instructions at

    https://docs.mongodb.com/guides/server/auth/

    Update Application to use local MongoDB

    The application used following connection string

    mongoose.connect("mongodb+srv://USER:[email protected]/DB_NAME?retryWrites=true&w=majority", {

    I replaced it with

    mongoose.connect("mongodb://USER:[email protected]:27017/DB_NAME?retryWrites=true&w=majority&authSource=admin", {
  • Creating Software RAID 5

    Creating Software RAID 5

    RAID 5 Requires 3 or more physical disks. It provides the redundancy of RAID 1 combined with the speed and size benefits of RAID 0. RAID 5 uses striping, like RAID 0, but also stores parity blocks distributed across each member disk. In the event of a failed disk, these parity blocks are used to reconstruct the data on a replacement disk. RAID 5 can withstand the loss of one member disk.

    I have a server with 4 * 4 TB Disks. Here is parted -l result on the server.

    On this server /dev/sda, /dev/sdb, /dev/sdc and /dev/sdd are 4 TB disks and are not in use. Lets format these disk to be used as RAID 5.

    Following commands need to be executed for each of the disks.

    parted -a optimal /dev/sda
mklabel gpt
mkpart primary ext4 0% 100%
set 1 raid on
align-check optimal 1
print
quit

    This will partition the disks, set file system as raid.

    Repeat the steps for /dev/sdb, /dev/sdc and /dev/sdd.

    Now you have all disks formatted, ready to be used in raid array.

    Create RAID 5 array with command

    mdadm --create --verbose --level=5 --chunk=64 --raid-devices=4 --layout=left-symmetric /dev/md0 /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1

    Once raid created, you can see status with command

    cat /proc/mdstat

    Before we can use raid array, we need to create a file system on the raid with command

    mkfs.ext4 /dev/md0

    formatting raid 5 array

    Create mdadm.conf file with following command

    mkdir /etc/mdadm/
cat /etc/mdadm/mdadm.conf
mdadm --detail --scan >> /etc/mdadm/mdadm.conf
cat /etc/mdadm/mdadm.conf

    Mount Raid 5 Array

    I want to mount the new raid 5 array as /home. For this i edited /etc/fstab, added

    /dev/md0        /home           ext4    errors=remount-ro 0       1

    After rebooting, i have 11 TB RAID 5 drive mounted as /home