Linux Tips and Tricks

This post is to summarize some interesting but special usage which is out of normal linux commands. For basic Linux commands, you can find it from my previous post. 

Related Posts:

• Basic Linux Commands
• Linux CentOS/Ubuntu Firewall and SELinux
• Linux Tips and Tricks

DNS Server

1. Check current dns server

nslookup

server

2. Modify DNS server

nano /etc/resolve.conf

List All Running Services

systemctl --type=service --state=running

Add user into sudo / wheel

adduser newuser

passwd newuser

On Ubuntu:

usermod -aG sudo newuser

On CentOS:

usermod -aG wheel newuser

Manually Change IP

Ubuntu:

• Before version 17.10, edit file /etc/network/interfaces
• Starting with 17.10, Netplan is the default network management tool
• ip link
• nano /etc/netplan/01-netcfg.yaml
• netplan apply

CentOS

• “nmcli d” command in your terminal for quick list ethernet card installed on your machine:
• “nmtui” command in your terminal to open Network manager.
• IPv4 configuration <Automatic>
• [x] Automatically connect
• service network restart
• ip a

Check Your Public IP Address from CLI

Following commands can show you what the public ip address is for your linux machine if it can connect to Internet.

• curl -s checkip.dyndns.org|sed -e 's/.*Current IP Address: //' -e 's/<.*$//'
• curl icanhazip.com
• curl http://ipv4.icanhazip.com
• telnet www.checkmyip.com 80 | grep confidence | grep -Eo '([0-9]{1,3}\.){3}[0-9]{1,3}'
• wget -O - -q icanhazip.com
• wget http://ipinfo.io/ip -qO -
• curl ifconfig.me

Keep Terminal Running in background (Screen)

• Install screen (Depends on the Linux Distribution if it came pre installed or not) : yum install screen
• Initiate a Screen : screen or  screen -S <screen name> <command to execute>
• Detach from the screen : "CTRL+A,D" not "CTRL+A+D"
• List all the screen currently working : screen -ls
• Reattach to a screen : screen  -r  <session number> or screen -r <screen name>
• Kill specific screen: screen -X -S <screen name> quit
• Kill all screens : pkill screen

Build SSH Trust Relationship Between Linux Machines

Become root:

sudo su - 

Change to user nsm:
su nsm 

Go to the /home/nsm directory:
cd /home/nsm 

Create the keys: (Path should be /home/nsm/.ssh/id_rsa. Leave the passphrase blank.)

ssh-keygen -t rsa

Secure copy the public key to the other server as the admin user: (use admin password)

scp /home/nsm/.ssh/id_rsa.pub admin@<ipAddressOfOtherServer>:/home/admin/authorized_keys

• or Go to the remote server. The command below will add the key that is in temp1 file to the end of the authorized_keys file.

cat temp1 >> authorized_keys

• Repeat steps 2-6 on  deviceB.   On deviceB, become root: (from user nsm, exit to root). Move the authorized_keys file that was copied to admin into nsm/.ssh:

mv /home/admin/authorized_keys /home/nsm/.ssh/authorized_keys

• Change ownership of authorized_keys: 

chown nsm:nsm /home/nsm/.ssh/authorized_keys

• At this point, you will be able to SSH between both servers without it asking for a password.

ssh [email protected]

Find Large Folders 

1. Use NCDU program to check folder size.

2. Check your application

For example: pihole folder, pihole-FTL.db, which holds long term query data. It can get quite large with years usage. 

• /root/etc-pihole/pihole-FTL.db

3. Check /var/log/journalctl

You can diminish the size of the journal by means of these commands:

sudo journalctl --vacuum-size=100M

This will retain the most recent 100M of data.

sudo journalctl --vacuum-time=10d

will delete everything but the last 10 days.

Find Big Files in Linux File System

Check folders size Command - DU :

bitnami@ip-172-26-0-136:/opt$ du -shc /opt/bitnami/apps/wordpress/htdocs/wp-content/*
1.8G    /opt/bitnami/apps/wordpress/htdocs/wp-content/ai1wm-backups
24K     /opt/bitnami/apps/wordpress/htdocs/wp-content/backup-migration
4.0K    /opt/bitnami/apps/wordpress/htdocs/wp-content/blogs.dir
24K     /opt/bitnami/apps/wordpress/htdocs/wp-content/cache
4.0K    /opt/bitnami/apps/wordpress/htdocs/wp-content/index.php
6.3M    /opt/bitnami/apps/wordpress/htdocs/wp-content/languages
132M    /opt/bitnami/apps/wordpress/htdocs/wp-content/plugins
27M     /opt/bitnami/apps/wordpress/htdocs/wp-content/themes
152K    /opt/bitnami/apps/wordpress/htdocs/wp-content/updraft
4.0K    /opt/bitnami/apps/wordpress/htdocs/wp-content/upgrade
512M    /opt/bitnami/apps/wordpress/htdocs/wp-content/uploads
68K     /opt/bitnami/apps/wordpress/htdocs/wp-content/w3tc-config
2.4G    total

Find Command:

• find . -type f -size +10000 -exec ls -lh {} \; 
• find . -type f -size +50000k -exec ls -lh {} \; | awk '{ print $9 ": " $5 }'
• Find large files (>10M) in current folder
find . -type f -size +10000k 

a. Juniper Firewall  

Sample output:

root@FW% find . -type f -size +10000 -exec ls -lh {} \; 
-rw-r--r--  1 930  929   134M Jan  5 17:34 ./cf/packages/junos-11.4R6.6-domestic
-rw-r--r--  1 root  wheel   139M Sep  8  2011 ./cf/var/log/junos-srxsme-11.2R2.4-domestic.tgz
-rw-r-----  1 root  wheel   4.9M Feb 11 17:12 ./cf/var/db/idpd/db/secdb_02.db
-rw-r-----  1 root  wheel   6.7M Feb 11 17:13 ./cf/var/db/idpd/db/secdb_03.db
-rw-r-----  1 root  wheel    64M Feb 11 17:13 ./cf/var/db/idpd/db/secdb_06.db
-rwxr-xr-x  1 admin  20    24M May 23 08:38 ./cf/var/db/idpd/nsm-download/SignatureUpdate.xml
.....

b. Checkpoint Firewall gateway:

[Expert@CP]# find . -type f -size +50000k -exec ls -lh {} \; | awk '{ print $9 ": " $5 }'
./sysimg/CPwrapper/linux/CPEndpointSecurity/EndpointSecurityServer.bin: 145M
./sysimg/CPwrapper/linux/windows/SmartConsole.exe: 194M
./sysimg/CPwrapper/linux/CPrt/CPrt-R75.40-00.i386.rpm: 53M
./sysimg/CPwrapper/linux/CPportal/CPportal-R75.40-00.i386.rpm: 59M
./var/log/db: 336M
....

Some other commands to help you find out which folder or files using your space:

df -h   //check disk usage
du -sh /* | sort -nr   // check each folder's usage
du -sh /var/* | sort -nr    //check internal folder usage inside folder /var

du -h --max-depth=1
lsof | grep deleted   //check if there is deleted file locked by process.


cat /dev/null > filename    //clear up logs
rm -rf filename

//clean up /var/log/journal files
journalctl --vacuum-time=1w   //keep one week log
journalctl --vacuum-size=500M  //keep only 500MB logs
rm -rf /var/log/journal/f9d400c5e1e8c3a8209e990d887d4ac1  //delete /var/log/journal log files

Clean all Linux History 

1. Clear systemd journal logs 

It will delete archived journal log files until usage is less 100MB 

sudo journalctl --rotate --vacuum-size=100M

2. Clear history trails

Following commands can clean most of your history trails in your linux system.  Please let me know if you found there is anything missing.  I will add the command in.

echo > /var/log/wtmp
echo > /var/log/btmp
echo >/var/log/lastlog
echo > /var/log/secure
echo > /var/log/messages
echo >/var/log/syslog
echo >/var/log/xferlog
echo >/var/log/auth.log
echo >/var/log/user.log
cat /dev/null > /var/adm/sylog
cat /dev/null > /var/log/maillog
cat /dev/null > /var/log/openwebmail.log
cat /dev/null > /var/log/mail.info
echo >/var/run/utmp
echo > ~/.bash_history
history -c
echo > .bash_history
history -cw

3. To Clean all log files

For CentOS 6:

cat /dev/null > /var/log/boot.log
cat /dev/null > /var/log/btmp
cat /dev/null > /var/log/cron
cat /dev/null > /var/log/dmesg
cat /dev/null > /var/log/dracut.log
cat /dev/null > /var/log/lastlog
cat /dev/null > /var/log/maillog
cat /dev/null > /var/log/messages
cat /dev/null > /var/log/secure
cat /dev/null > /var/log/spooler
cat /dev/null > /var/log/tallylog
cat /dev/null > /var/log/wtmp
cat /dev/null > /var/log/yum.log

For CentOS 7:

cat /dev/null > /var/log/boot.log
cat /dev/null > /var/log/btmp
cat /dev/null > /var/log/cron
cat /dev/null > /var/log/dmesg
cat /dev/null > /var/log/firewalld
cat /dev/null > /var/log/grubby
cat /dev/null > /var/log/lastlog
cat /dev/null > /var/log/mail.info
cat /dev/null > /var/log/maillog
cat /dev/null > /var/log/messages
cat /dev/null > /var/log/secure
cat /dev/null > /var/log/spooler
cat /dev/null > /var/log/syslog
cat /dev/null > /var/log/tallylog
cat /dev/null > /var/log/wpa_supplicant.log
cat /dev/null > /var/log/wtmp
cat /dev/null > /var/log/yum.log

Use ssh key to encrypt / decrypt files

Create a file:
echo ‘This is a sekret’ >/tmp/msg.txt

Export public key:
openssl rsa -in ~/private.pem -out /tmp/public.pub -outform PEM -pubout

Encrypt file with public key (anyone can have this key):
openssl rsautl -encrypt -inkey /tmp/public.pub -pubin -in /tmp/msg.txt -out /tmp/file.enc

Decrypt the file with private key (only you should have the private key):
openssl rsautl -decrypt -inkey ~/private.pem -in /tmp/file.enc -out /tmp/decrypted.txt

Check decoded message:
cat /tmp/decrypted.txt

AWS Amazon Linux Instance Commands

sudo yum update -y
sudo yum install -y httpd24 php70 mysql56-server php70-mysqlnd
sudo service httpd star


sudo chkconfig httpd on
chkconfig --list httpd
curl http://localhost

sudo usermod -a -G apache ec2-user
groups
sudo chown -R ec2-user:apache /var/www
sudo chmod 2775 /var/www
find /var/www -type d -exec sudo chmod 2775 {} \;
find /var/www -type f -exec sudo chmod 0664 {} \;
echo "<?php phpinfo(); ?>" > /var/www/html/phpinfo.php
sudo yum list installed httpd24 php70 mysql56-server php70-mysqlnd
sudo service mysqld start
sudo chkconfig mysqld on
sudo service httpd restart

Validate UDP ports reachability

Ensure, that your ports are reachable through your external IP.

To validate UDP connection the simplest way, run this on your server:

nc -ul 52101

And this on your local client:

nc -u [server ip] 52101

Then try to type on one end, you should see characters on the other side.

If it does not work for you, then most likely your port forwarding is not working correctly. Or your ISP is blocking traffic.

If you get Command 'nc' not found. error, you can install netcat package using:

sudo apt-get install netcat