Tag: linux

bash: man: command not found
When running man command on a debian server, i get error
```
root@lab:~# man sftp
-bash: man: command not found
root@lab:~#
```
This is because man-db package not installed on the server. To fix, install man-db package with
```
apt install man-db
```
See Errors
November 13, 2020
Show git branch in terminal
When working with git, to avoid accidental commit to wrong branch, it will be better if terminal show the branch name you are currently in.

To display “git branch” in terminal, edit .bashrc
```
vi ~/.bashrc
```
Add following code to end of the file.
```
# Show git branch in command promt if git repo

show_git_branch() {
    git branch 2> /dev/null | sed -e '/^[^*]/d' -e 's/* $.*$/ (\1)/'
}

PS1='${debian_chroot:+($debian_chroot)}\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]$(show_git_branch)\$ '
```
You will need to restart terminal after making changes to ~/.bashrc file or run
```
source ~/.bashrc
```
See Git
November 12, 2020
Find Reverse-DNS/PTR using dig, nslookup, host
Reverse-DNS/PTR is used by mail servers. It is used to map an IP address to FQDN or hostname.

To find Reverse DNS record for an IP address, run
```
nslookup IP_ADDR_HERE
```
In this example IP 51.38.246.115 have a Reverse DNS record ok.serverok.in

Using dig

You can also use dig command to find PTR record for an IP address.
```
dig -x IP_ADDR_HERE
dig +short -x IP_ADDR_HERE
```
Using host
```
host IP_ADDR_HERE
```
See DNS
November 9, 2020
Disable SELinux on CentOS/RHEL
To disable SELinux

Method 1
```
sed -i 's/SELINUX=enforcing/SELINUX=disabled/' /etc/selinux/config
setenforce 0
```
Method 2

edit file
```
vi /etc/selinux/config
```
Find
```
SELINUX=enforcing
```
Replace with
```
SELINUX=disabled
```
Reboot the server with
```
reboot
```
You can disable selinux for current session by running command
```
setenforce 0
```
See SELinux
November 7, 2020

Show Hidden Files

For linux to show hidden files when you type ls command

Edit ~/.bashrc

vi /root/.bashrc

Add line

alias ls='ls -la --color'

Save and exit. Now you need to re login to SSH to see the hidden files when you type “ls”

After editing the file will look like

[root@194 ~]# cat /root/.bashrc
# .bashrc

# User specific aliases and functions

alias rm='rm -i'
alias cp='cp -i'
alias mv='mv -i'
alias ls='ls -la --color'

# Source global definitions
if [ -f /etc/bashrc ]; then
        . /etc/bashrc
fi
[root@194 ~]#

October 25, 2020

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

October 15, 2020

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
October 15, 2020

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

October 15, 2020

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
```
See telnet
October 15, 2020
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
October 14, 2020
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
October 14, 2020

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

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

October 12, 2020

Tag: linux

Using dig

Using host

Method 1

Method 2